Variable Voltage Made Easy - Elevator Books · shunt-field motor control. ... Source of steady DC to supply power for fields: ... were tried, such as “reversed series

April 2007 | ELEVATOR WORLD | 77

Continuing Education: Variable Voltage

Learning Objectives After reading this article, youshould have learned about u the theory of variable-voltage

control applied to elevators,using the Ward-Leonard system.

u methods of obtaining control ofacceleration, deceleration andleveling.

u how to circumvent residual cur-rents.

u the best way to obtain smoothcontrol of elevator motion.

u the use of damping-field andshunt-field motor control.

u the design and operation of aself-excited generator.

u the methods designed to preventthe hoist motor from pullingthrough the brake.

u what happens when motor loadand slip increase.

u resistance and Star-Delta meth-ods of AC-motor starting.

u troubleshooting noises in thedriving motor on a motor-generator set.

u causes of sparking in genera-tors and hoist motors.

u how to check a DC machine ormotor-generator set for electricalneutral.

u how to trace various variable-voltage problems to parts of thesystem.

u how to determine the synchro-nous speed of a motor.

u determining the causes andfinding remedies for rough stops.

Variable Voltage Made Easy Using Print No. C-1469-Aby Joseph C. TamsittEdited by Robert S. Caporale, MSc

The following article is an editedversion of the one originally publishedin the October 1959 issue of ELEVATORWORLD. It was so well received thatELEVATOR WORLD sold out all extracopies of the magazine within twoweeks of publication. Over the past 50

years, thousands of copies of the arti-cle have been sold as reprints and inthe ELEVATOR WORLD EducationalPackage. The subject matter of the article is still pertinent to elevator fieldpersonnel today as a great deal of thisvintage elevator equipment is still inservice.

Long accepted, the variable-voltagemethod of control has been widelyused for elevator control. Knownunder several names, such as unitmultivoltage, generator field control(a very apt name), variable voltage,Ward-Leonard Control and manyothers, it has a long history. Inventedin Germany, it was brought to the U.S.and first used for propulsion controlon marine craft, primarily ferry boats.It is used in marine control, machinetool drives, conveyors, textile drivesand many other places where a rapidchange of speed, precise control, anda definite and powerful low speedmay be required. The attemptedscope of this article will be to explainvariable-voltage operation and theoryas applied to elevators so that it canbe understood. The writer hopes todo this without resorting to languagethat would require an electrical engi-neer to interpret its meaning.

In Figure 1 you will see a constantsource of DC power. This power maybe from any source, such as a smallgenerator called an “exciter,” rectifiersor vacuum tubes. The left wire ismarked +, or positive, for convenience.The right wire is marked -, or negative.The amount of power required issmall – only about enough to lightthree 100-watt lamps. Accordingly,the associated switch gear need notbe large. You will also note in Figure

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Continuing Education: Variable Voltage Continued

1 that the elevator motor field is be-tween + and -. Thus, it has power ap-plied at all times in our simplified di-agram. We say it is “constantlyexcited.”

The generator field, between ter-minals GF1 and GF2, is interruptedby the up and down switches. It hasresistors R1, R2, R3 and R4 that arecut out of the circuit by the speedswitches 1, 2, 3 and 4. As the genera-tor field has a fixed amount of elec-trical resistance, these resistors willcause a reduced voltage to be appliedto the field. As the speed switches areoperated, the voltage rises. Thus, forillustration, we have 15 volts appliedto GF1-GF2. When all the switches areopen, we will have minimum excita-tion. This is true when the car is lev-eling into the floor, or when it isstarting out.

When switch 1 closes, the fieldvoltage rises. This voltage continuesto rise as switches 2, 3 and 4 areclosed. At this point, the field has itshighest applied voltage, having R5 inthe circuit. R5 is for the purpose ofregulating the top speed of the car,as will be explained later. You willalso note that when the up switch isclosed, the + line is connected to GF2and the - line is connected to GF1.However, when the down switch isclosed, the + line is to GF1 and the -line is to GF2. This effectively reversesthe flow of current through the field,or “reverses its polarity.” In the lower

section of Figure 1, we see the gen-erator armature and the elevator motorarmature. The brushes of the gener-ator were connected solidly by largewires to the elevator motor brushes.This is, again, a simplified circuit, andwill be expounded upon later.

The armature of the generator isconstantly revolved by a suitablemotor, which is usually on a com-mon shaft with the generator. At thispoint, we will absorb a little verysimple theory: the output of a gener-ator armature (that is, the power itwill generate at a fixed speed) is rel-ative to the current applied to itsfields. Thus, with R1, R2, R3, R4 andR5 in the circuit to the generatorfield, the output in voltage generatedwill be small. However, with R1, R2,R3 and R4 shorted out by the speedswitches, its output will be at almostmaximum. The only drop from max-imum will be that occasioned by resistor R5.

At this point, the meaning of the“generator field control” and “variablevoltage,” etc. becomes apparent. Byagain inserting the resistors in the fieldcircuit, the output of the generatorwill be reduced and, if the directionswitches up and down are open, itsoutput will be zero or very near to zero.

Now, a little more theory: the po-larity of the generated current will bein relation to the polarity of the gen-erator fields. If the up switch is closed,the current will flow through the fieldsin one direction, and if the downswitch is closed, the current will flowin reverse. As this current flow re-verses, the “polarity” of the fieldchanges. As the polarity of the fieldchanges, the polarity of the genera-tor changes. Thus, we may have wire1 as positive (or +) when the upswitch is closed, and wire 2 as -.When the down switch is closed, thereverse is true: wire 1 will be - andwire 2 will be +.

The elevator (or “hoist”) motorwill now be examined. The field ofthis motor, as noted before, has a

constantly applied voltage. We havedetermined, however, that the volt-age applied to the armature of thismotor by wires 1 and 2 will vary involtage. Another dose of theory: aDC motor with constantly excitedfields will vary in speed relative tothe voltage applied to the armature.

Now we have the whole picture.By changing the amount of resistorsin the generator field, we have changedthe speed of the elevator. Varyingthis resistance means handling smallcurrents. A 2-ampere current cancontrol a car of 3000 pounds at 300fpm, for example. We have noted thatour direction switches change the po-larity of wires 1 and 2 by reversingthe generator field current directionflow. As the polarity of these wireschange, the direction of rotation ofthe hoist motor will also change. Witha 2-ampere switching current, we canreverse our 40-HP hoisting motor.

We will discuss explicit methodsof obtaining good control of acceler-ations, decelerations and leveling,together with methods of circumvent-ing residual currents. As this controlis old, many methods have beenused to obtain the desired results.Obviously, if we closed switches 1, 2,3 and 4 simultaneously, the gener-ated voltage would shoot up rapidly.The hoist motor would be almost ashort circuit when at rest, so severedamage would result to the genera-tor, motor brushes and commutators.It is imperative that the time requiredfor the voltage rise on the generatorfields be spread over a definite timeinterval to prevent excess currentbeing required by the hoist motor. Ifwe successively close 1, 2, 3 and 4with a time interval between each,we will still get sudden (but not aslarge) surges as each switch closes.This will give “stairstep” control, andwould be objectionable to the pas-sengers.

As many readers are engaged inthe servicing of all types of elevator

EF1 EF2

UP

UP

DOWN

DOWN

GF2

GF1

1 2 3 4

R1 R2 R3 R4R5

Wire 1

Wire 2

Source of steady DC to supply power for fields:this does not need to exceed 3 amperes for theaverage job – about enough power to operatethree 100-watt lamps

Elevator motor field (This has a predeterminedvoltage applied.)

GF1 & GF2 are the terminals on the generator field.1, 2, 3 & 4 are the speed-control contacts.

Generator armature

Elevator motorarmature

Figure 1


control, we will discuss the older (aswell as the newer) methods of“smooth control.”

One of the earliest methods wasthe damping field, as shown in Fig-ure 2. We have the same shunt fieldas used in Figure 1, GF1-GF2. Wealso have another field, wound withheavy wire, on the same pole piecesas GF1 and GF2. This wire was usu-ally wound directly on the iron poles,with GF1-GF2 wound over it. Theterminals of the damping field wereconnected, causing the field to short-circuit itself. The purpose of the fieldwas to resist sudden changes in themagnetic strength of the poles. Adashpot or other timed switch closedfour contacts: 1, 2, 3 and 4. Insteadof “stairsteps,” the dampening fieldsmoothed out the steps to a large de-gree. It was used until leveling camein, then it defeated itself: when theleveling switch opened the circuit tothe shunt field, the dampening fieldwould not let the generator stop itsoutput quickly, so the car would notstop accurately. Many expedientswere tried, such as “reversed series”or “differential field,” but they wereawkward to handle, requiring largeswitches and complex generators.These “reversed fields” will be dis-cussed later in the article.

Another method was large, iron-core reactors placed across theshunt field. These were, in effect,much the same as damping fields,but more flexible. Their results couldbe varied by moving the position ofthe iron core. These were costly andhad somewhat the same “hold-on”characteristics as the damping field.

A third method (used by a largecompany) was the self-excited gen-erator. In Figure 3 you will see F1-F2.This is a high-resistance field thatwould give leveling speed. Its initialpurpose was to determine the polar-ity of the generator. When F1-F2 wasexcited, the generator would put outabout 15 volts. When the HS contact

closed, the generated current wouldbe applied to GF1-GF2. This flow ofcurrent through GF1-GF2 would fur-ther increase the output of the gen-erator, and this rise would continueto a point where the rated outputwould be reached. To slow down, HSwould be opened, R1 would be in-serted and the voltage would decayto the leveling voltage.

Another large company used ourfamiliar 1, 2, 3 and 4 switch idea,only it used a large number – some-times as many as 20 – closed by amotor-driven cam switch. Slowdownwas achieved by reversing the cammotor.

Another method was the pilotmotor generator set (see Figure 4).This was a small (about one-third-HP) motor generator with the gener-ator connected to the big generator’sfield, as in Figure 4. The AC drivingmotor had a very small adjustable

reactor in each phase. By screwingin the iron core, the applied voltageto the AC motor would be reducedwhile accelerating, and the rate ofacceleration would be reduced. Back-ing out the iron core would increasethe rate of acceleration. This wasvery easy to adjust. Deceleration wassimple – the pilot generator continuedto generate as it slowed down. Whenthe slowdown (SD) contact closed, aresistor would be placed across thepilot generator so that it deceleratedmore rapidly and the car slowed ac-cordingly. Leveling was from a fixedlow-voltage source.

The writer worked on the develop-ment of the following control, and onecompany still used it and had a patenton it. General Electric pioneered thedamping motor control in the mid1920s. In the writer’s opinion, it wasone of the most dependable, easiestadjusted and most stable types of

Generator shunt field

GF1 GF2

Damping field

UP

UP

DOWN

DOWN

F1

F2

Generator shunt field

GF1 GF2

HS

R1

Generator Motor

UP

DOWN

GF1

GF2

SD

Smallgenerator

Small-generatordriving motor

DOWN

UP

Reactor

Reactor

Reactor

L1

L2

L3

Largegeneratorshunt field

Figure 2

Figure 3

Figure 4

Continued



control. With this biased opinion, weshall describe this method in detail.

In Figure 5, we have the schemeoutlined. The field of the dampingmotor is connected across the powersource. The armature is across thegenerator field. We have the same top-speed control resistor as in Figure 1(R2 in this diagram). R3 is the low-speed or leveling resistor; AR is avoltage-controlled resistor. A littletheory: a DC motor’s armature is al-most a short circuit when at rest, andif under no load, it will draw less andless current as it accelerates to itsrated speed. When the car is to start,the direction and high-speed (HS)switch close; no timing is required.The damping motor armature, beingalmost a short, will allow very littlecurrent to flow into GF1-GF2. The“lost voltage” will be dropped by R2and AR resistors.

As the damping motor starts to revolve, it will pass more and morevoltage to GF1-GF2 until full voltageis obtained. Thus, we have an infi-nite number of points of speed con-trol – much smoother than our 1, 2, 3and 4. To adjust the rate (time) of ac-celeration, we increase or decreasethe amount of flywheel on the motorshaft: the heavier the wheel, themore gradual the acceleration. Once

set (with a motor of goodquality) there is no change, asthe inertia of the wheel doesnot vary. The coil AR is con-nected across the generatorarmature. The AR contact willclose when the voltage ap-plied to the armature reachesa predetermined value. Thepurpose of the resistor/relaycombination is to preventexcessive voltage from beingapplied to the damping motorarmature, as the AR resistorhelps the R2 resistor dissipatethe applied voltage.

To slow down, the HSswitch opens, dropping the

applied voltage to the leveling volt-age. However, we do not get a sud-den slowdown – our damping motorbecomes a generator, driven by itsmass and the stored energy of theflywheel. The leveling speed isreached gradually, and only after thedamping motor has decreased itsspeed. To change the rate of deceler-ation, we move the top on resistorR1. The more of this resistor that isbypassed (or shorted out by the con-tact on the slowdown relay), themore rapid the rate of deceleration.

This control requires no timers,tubes, switches or trick fields in thegenerator. There are methods ofcontrol that were used on high speedjobs called Rototrol (Westinghouse)and Amplidyne (General Electric).Amplidyne has been described in ELEVATOR WORLD (October 1964).Both of these are speed-reference,feedback systems, but would not, inour opinion, fit the subject of this article. We have accelerated and decelerated the car, so we will nowdiscuss varying loads, overloads andoverhauling loads, and the methodof compensating for them.

Figure 6 is a typical generator sat-uration curve (“output in relation tofield current” is another way of sayingit). These curves are furnished by themanufacturer of the rotating equip-ment to the control designer so thathe may more accurately predict thecar speeds under varying conditions.From this curve, we see that the arma-ture voltage (represented in numbersfrom 0-300 in the left margin) is af-fected by the amperes of the field cir-cuit. We can thus determine that 0.1ampere will give us an armature volt-age of about 10 volts, enough for agood leveling speed. An amperage of2.8 will give us an armature voltageof 225 volts, or full speed. A genera-tor will, as we see from this, put outa voltage in proportion to the fieldsaturation. The field strength (or sat-uration) is expressed in “ampere turns.”The ampere turns in a field are deter-mined by multiplying the amperes ofcurrent flowing through the field bythe number of turns of wire in the field.For example, a current of 1 amperethrough a field of 1,000 turns willgive 1,000 ampere turns. A current of100 amperes through a field of oneturn will give 100 ampere turns.

Most generators used in elevatorcontrols are compound wound. Thismeans that we have a “shunt field.”This consists of a coil of relativelysmall wire wound on each pole

Hot Resistances (75οC)

ARM 4 interpoles .054 ΩFull series field (8) .009 ΩShunt field 32.8 Ω

Saturation Curve

30 KW 1750 RPM (115-V excitation)Variable-voltage generator

Volt

s30

0

Field amperes1 2 3

Rated loop volts

4

UP

DOWN

DOWN

UP

AR HS

AR

GF1

GF2

R1

R2 AR R3

8D

D.M.ARM.

D.M. Field

Figure 5

Figure 6


piece. This is the “1 ampere at 1,000turns” field in the example above.We also have a “series field.” Thisconsists of a very few turns of largewire – often less than five turns. It isin series with the armature, hence itsname. All main power of the genera-tor flows through this field. It is the“100 ampere at one turn” field in theexample. We will now discuss thepurpose of this field. (See Figure 7 forthe series-field location.)

If the current in the generatorshunt field were adjusted to give anexact output at a no-load condition,the voltage would not remain con-stant. As the load increases, the cur-rent (or amperage) will rise. As allparts of the system have a fixed elec-trical resistance, the voltage willtend to drop in accordance withOhms’ Law. This is expressed by theequation I = E/R. I = current in am-peres, R = resistance and E = voltage.Thus, if we require 10 amperes at abalanced load, we have a very smallcurrent flow. The resistance as shownon the curve is as follows: armatureand interpoles, .054 ohms; seriesfield, .009 ohms; and the hoist motorarmature and interpoles (now shown),about .054 ohms. The wire resistancewill be approximately .07 ohms onthe average job. This is a total resist-ance of .187 ohms. Using the equationof Ohms’ Law, we have, at a full loadof 100 amperes, 100 = E/O .187. Theanswer is a voltage drop of 18.7volts. As the car speed varies withthe voltage, the speed would be 225- 18.7 = 207.3 volts full speed, or fullrated speed. If, for example, therated car speed is 225 fpm, the full-load speed would drop to 207 fpm.

The speed drop ofthe generator is ag-gravated by the speeddrop of the hoistmotor. As the hoistmotor in our vari-able-voltage systemsare shunt machines

(i.e., they have only shunt fields andno series fields), we have a speed dropas outlined in Figure 8. By means ofthe series field, we can raise the armature voltage 18.7. As it is notpractical to divide the series-field intofractions of turns, we bypass the se-ries field coils with a large adjustableresistor. This resistor is in the form ofstrips of resistance material that doesnot change its value too greatly witha rise in temperature. These strips areconnected so that they are in parallelwith the series field. If a strip is re-moved, the series-field effect isgreater, because more current willflow through the series field and lessthrough the resistor. Thus, it is sim-ple for the adjustor to compound thegenerator so that, with full load, theupward and downward leveling speedsare exactly equal. If he does this, hisfull-load speeds should have less thana 5% variation. Usually the variation isfrom 1% to 2%.

Some older control systems usedtaps on the series field to obtain thisadjustment. This was very difficulton small generators, and it was veryeasy for the adjustor to have a coilconnected in reverse, with a bad effecton the commutator. Let us assumethat the shunt field has 1,000 turns.The current, according to the curve,is 2.8 amperes. This results in 2,800ampere turns, as our voltage “sags”18.7 or 8%. If we raise our saturation,8% of 2,800 is 224 ampere turns. Ifthe current is 100 amperes and weuse three turns, we will have 300ampere turns. A shunt of .027 ohmswill give the required results. This isdetermined by the series-field resist-ance. The .027 shunt will give us a

means of adjustment to compensatefor the motor-speed drop. Naturally,the worker in the field does not con-cern himself with these figures. Thecontrol designer has worked out theproblems, and the adjustor merely“trims” for the slight variations of theequipment.

We can now run the car with abalanced load at 225 fpm, and with afull load at the same speed. When welower the load, the series field againhelps us. When lowering a load, themotor, because of its excited field andthe fact that it is driven by the load onthe car, becomes a generator. Thevoltage at the motor and generatorbrushes rises in proportion to the degree of overspeed caused by theoverdriving effect and the loaded car.This causes the generator to becomea motor as more voltage is presentthan it generates. The generatorspeed cannot rise, for the AC motordriving it has as much power to pre-vent overspeed as it does to drive theset up to speed. The current forcedinto the generator flows through theseries in the reverse direction. Thistends to reverse the polarity of thegenerator pole pieces. The amountof magnetism generated by the seriesfield will be subtracted from the shunt-field magnetic strength, and this willlower the loop (or armature voltage),causing the speed to remain constant.

We now come to “circulating cur-rent.” Once magnetized, all steel oriron will retain some flux of magnet-ism. How much the material retainsin a given volume of steel is relativeto its hardness. The harder the metal,the more flux it will retain.

When we open the circuit to thegenerator field by dropping out thedirection switch, the generator doesnot completely stop generating of itsown accord, but must receive somehelp from the control system. The ar-mature revolving in the magnetic fieldcreated by the residual magnetismwill continue to produce a certain

Series field

Elevator hoisting-motor armature

Generatorarmature

Figure 7

Continued





voltage. Of course, when the direc-tion switch drops out, the brake setson the hoist motor. Oddly enough,this aggravates the condition. Whenthe brake sets, the armature of thehoist motor becomes very close to ashort circuit. While the generatedvoltage may be low, the stalled ar-mature makes the amperage (flow)of the current become rather high.

This brings about a vicious circle:All the current flows through the se-ries fields of the generator. We havea large number of ampere turns ineffective work. Our generator putsout more power due to these ampereturns, and this adds to the initial am-pere turns. The current will very rap-idly rise to such a point that the hoistmotor will pull through the brakeand run away. It may do this with thehoistway and car doors open. Therehave been many methods workedout to prevent this condition, and wewill outline a few of them.

One method was to short out theseries field when the car stopped.This required a large contact, withalmost perfect contact surfaces – andthey must stay that way. Refer backto our sample generator and you willsee the very small resistance of theseries field involved. The contactwould have to have far less, for ifany resistance was present in thecontact, the generator would gener-ate in proportion of the contact re-sistance to the resistance of the se-ries field. Small contact leakagecould allow enough generation notto pull the brake, but still enough toburn out the coil under the brushesof the hoist motor.

Another method was to have a“killer,” or a magnetizing field. Thiswas a small shunt field that was ex-cited in the reverse direction to killthe residual. This was rather difficultto balance out in practice.

The third method was to open theloop circuit between the generator

and the motor. This had its prob-lems, too. The large contact involvedwould develop some resistance. Asthe contact must pass 15 to 20 voltsin leveling, a very small voltage dropwas so large (percentage-wise) thatleveling tended to be erratic.

Probably the most widely used isthe “suicide circuit.” This is, as it im-plies, a device to cause the set to “killitself.” On Print No. C-1469-A, youwill see a complete schematic. Oneside of the loop circuit is tied to oneend of the shunt field. When thebrake switch drops out, or if both di-rection switches are out, the otherside of the loop is hooked to the re-maining shunt-field terminal. Thiscircuit is polarized so that if the cir-culating current starts to rise, theshunt field is automatically built upin the reverse magnetic direction.The circulating current is held to lessthan 1.5 volts – usually .5 to .7 volts.In this switching arrangement, wehave a double circuit: that is, two in-dependent ones, so that either couldfail and the remaining one would stilloperate. All switches are “fail safe” inthat the contacts are normally closed.An open coil will close the circuit.

We will now discuss some of theproblems met by the adjustor andservice workers. We will addressthese problems in four stages: the setdriving motor, the generator, thehoist motor and the control system.

The synchronous (no-load) speedof a motor is determined by the fre-quency of the current (usually 60 cycles) and the number of poles. Intheory, a four-pole machine wouldrun at 1,800 rpm. In practice, how-ever, it runs at a slower speed. Thedifference between the synchronousspeed and the actual speed underload is the slip. This figure is given asa percentage figure. As a DC genera-tor’s output voltage varies directlywith its armature’s speed, we desire

the driving motor to have as little slipas possible.

When the slip increases as torqueincreases, it approaches a point of“no return.” To understand this, wewill review what happens as the loadand slip increase. The speed will drop,due to the applied load. The rate atwhich the secondary conductors – thesquirrel-cage rotors – cut (or slip back-ward through) the revolving field willincrease. The induced electro-motiveforce will increase, and the currentand the torque will increase until thetorque just meets the demand of theload. However, this can only go sofar. As the slip increases, the frequencyinduced in the squirrel cage increases,as does the leakage reactance in di-rect proportion. This results in a lagin the secondary (the squirrel cage)current behind the flux of the revolv-ing field. This results in a decrease ineffective torque.

A point is reached where the motorslips badly and starts to rapidly slowdown. This is the “pull-out” point.Due to the overloads imposed duringacceleration of heavy loads, we musthave a high pull-out torque on ourrotor design. Also, due to the fact thatwe desire a steady output as describedabove, we must have a low slip. Ele-vator MG sets are started unloaded.This fact allows us to use a motor oflow starting torque. The result of thisis low starting current and less heat inthe machine. The rotor used to satisfythese requirements is, therefore, alow-resistance (low-slip) squirrel cagewith deep section bars (low startingcurrent, high pull-out torque), and itusually has slots not parallel to theaxis to reduce noise.

The AC motor may be started inseveral ways: 1) Resistance starts: In this method,

resistance is inserted in each phaseto reduce the voltage to the motorterminals. As the motor reaches

One supplier of rotating equipmentsecures the bars by melting downthe projected ends by means of acarbon arc. This is inexpensive, butleads to one of our troubles. As thecopper is raised to the fusion point, itpicks up oxygen and becomesporous and brittle. When the stressesof starting the set and the loads ofacceleration and deceleration areapplied, these embrittled points willsometimes fail, and we will have anoisy set. This damage may be re-paired by carefully silver-solderingfrom the rear or inboard side of theend-rings.

Mechanical noises are usually easyto locate and correct. Examples are aloose fan, a dynamically unbalancedset that causes vibration or a badbearing. One particular noise thatshould be mentioned in passing iscaused by a periodic vibration of highfrequency. It is apparent at times asthe set is accelerating, and quiteoften it is evident when the set iscoasting to a stop. At other times itwill slowly build up and slowly diedown while the set is running. Youwill find that this will only occur iftwo conditions are met: 1) A pressedsteel end bell and 2) a ball bearing.Various remedies can be used. A shotof light oil in the bearing will tem-porarily alleviate the condition. Amore permanent remedy is to bendone arm of the pressed steel end bell,or to weld or bolt a substantial pieceof metal in the end bell to break upthe harmonics.

If high circulating current is en-countered, we have a condition thatmust be corrected. All sets should beperiodically checked for this condi-tion. A voltmeter connected to adja-cent brushes immediately after astop is a good check. The stop that ischecked should be one after up travelunder load or after down travel empty.This will usually show the highest

motor terminals at the motor junctionbox. Each pair of leads should betested. The voltage should be equalon each pair of leads, and should notdrop appreciably – from 208 to 200volts is not excessive. If the voltagedrop is excessive or unequal, lookfor trouble external to the motor. Ifthe voltage drop is equal and withinacceptable limits, we must assumethat the noise is occasioned by themotor.

Using an ammeter (preferably aclamp-on type), check the amperagein each motor lead. If the current isequal, we may assume it is not thestator. If they are unequal, we may beassured the trouble is in the stator, forwe have determined that the appliedvoltage is equal on each terminal.

If the current is unequal, we mayhave a motor that is wound withseveral circuits in parallel – eithertwo, three or four Star or DoubleDelta, with one circuit opened. Thiswill unbalance the machine so that itwill be noisy under load.

If it is not the stator and the trou-ble is in the motor, we have the rotorleft to check. To further determine thecause, we must disassemble the ma-chine. Inspect the outer diameter ofthe rotor carefully to see if the bear-ings are failing and if the rotor is rub-bing the stator when the load surgesoccur.

If no apparent rubbing is noted,we are usually faced with trouble inthe squirrel cage. In a motor withcopper or copper alloy bars, we havethe bars inserted through an end-ringof copper or copper alloy. These barsare fused to the end-ring. If one ormore of these bars is not securelyfastened, we have a noisy motor thatwill slip excessively under load. Themost common point of failure in thisrespect is in the middle of the end-ring. The broken bars are located bylight, by tapping or by gently pryingthem to detect motion.

its rated speed, the resistance isshorted, and full voltage is appliedto the terminals. This method re-quires high starting currents:

2) Star-Delta start and run: In thisvery popular scheme, the motor isconnected (in Star or Wye) to start,and when full speed is obtained,the windings are connected in Delta.The voltage impressed in eachphase when Star-connected is 58%of the running voltage. The currentin starting is proportionately re-duced. This means less heat in themachine, and less stress on therotor. See Figure 8, line 2.

3) Increment start: In this method, apart (usually half) of the motorwinding is connected to the sup-ply lines. When speed is almostsynchronous, the remainder of thewinding is connected. This methodis used in other industries, but theelevator industry has not used itto any great extent due to the re-luctance of the set manufacturersto furnish the proper motors. If a driving motor on an MG set is

noisy when at full speed, there maybe either a mechanical or electricalcondition causing this trouble, so thefirst procedure is to determine whichit is. This can be most difficult. Let usassume that the noise level is lowwhen the car is at rest, and that it ishigh when the car accelerates anddecelerates. Usually this noise is thehum or “howl” of an AC characteristic.

Remove the end plug of the machine and attach a tachometer.Observe the speed at varying loads.The motor should run very close toits rated speed (assume 1,800 rpm)when under no load. If the slip orspeed loss is excessive when the loadpeaks are reached, we have a clue.

The 1,800-rpm machine shouldnot drop below 1,750 rpm underpeaks of load. Following our clue, avoltmeter should be applied to the

April 2007 | ELEVATOR WORLD | 85Continued



voltage, but checks should be madeunder the reverse conditions. Mostmanufacturers recommend that witha nominal loop voltage of 230 volts,the circulating current is not over 2.5%.Practice, however, indicates this figureto be too high, as a continued voltageof this value will cause hoist-motorheating, and there is a very directdanger of the car running away if thecondition should slightly change forthe worse. High circulating currentcan be caused by a defect in the sui-cide circuit or, if other means areused, a defect in the demagnetizingcircuit.

High circulating current may alsobe caused by too much series field.Make the full-load leveling check. Itmay also be caused by the brushesbeing out of neutral. Usually, the se-ries field effect is not strong enough.Make the full-load leveling test, cor-rect the series field and adjust thepoint of slowdown. Of course, theleveling speed must not be too high.

It may be of interest to note onecase in particular. The customer, a veryable elevator adjuster, complainedthat at intervals the car would befound in the overhead with the top-floor door open, or it would be in thepit with the bottom-floor door open.This would occur about five or sixtimes a week, and never when it wasunder observation. Careful checkswere made on compounding and thesuicide circuits. It was a new set – thebrush yoke was pinned at the factory.

With no load on the car, it was runto the top floor and the circulatingcurrent was 1 volt. On successiveruns to the top floor it was 0.5 volts,0.75 volts, 0.5 volts and 1.4 volts. Asan experiment, when the circulatingcurrent was 1 volt at the top floor,the brushes were pushed hard againstthe commutator and the voltagewent up to 28 volts, and the carpulled through the brake and landed

the weights. It was determined thatthe brushes were so far out of neutralthat the interpole effect had changed.If the commutator film broke down,the generator would become a seriesmachine and the car would run away.Setting the machine on neutral cor-rected the trouble.

An uncommon trouble in genera-tors is a reluctance to change theirmagnetic polarity. These troubles aremore common and, in many cases,easier to cure. Sparking at the com-mutator is one difficulty often encoun-tered. It may be caused by one ormore conditions; we will discuss each,with the reasons therefore.

If the sparking occurs only underheavy loads of accelerating and de-celerating, then the generator may beoverloaded. The peak currents in theloop circuit should be noted, and theymust not exceed 2.5 times the full-load rating of the generator. If theydo, reduce the rate of speed changeby control adjustment until they areless than this value. If sparking occursunder all load conditions and is exces-sively heavy but with little or no spark-ing under no load, check for reversedinterpoles. If sparking occurs underone brush in one direction and movesto another brush when the directionof travel is reversed, we either have asingle reversed or shorted interpole,or a reversed or shorted shunt field.

If a shunt field exists, the carspeed will be less than it should beand a resistance check of the fieldcoils will reveal the short, if that isthe trouble. To determine the polar-ity of the shunt fields, a visual checkof the connecting wire will often reveal the reversed unit. The wireshould enter the opposite side of eachalternate field. A positive check maybe made by means of two pieces ofsteel, such as large nails or saw blades.Excite the fields with direct current,and place the bars on adjacent poles

with their ends touching. They shouldattract each other. A reversed polewill cause them to repel. This, ofcourse, must be done with the arma-ture removed. Interpoles may be vi-sually checked in the same way, andthey may be magnetically checkedby passing current through them bymeans of a storage battery.

Sparking of varying intensity maybe occasioned under all load condi-tions. This is often a result of an in-correct brush grade and/or a roughcommutator. The wrong brush willoften roughen the commutator.Sparking can occur at all times, evenwhen the set is running idle. Thismay be caused by the generator beingout of electrical neutral, or it may beoccasioned by uneven air gap. Badball bearings that would cause thebrushes to bounce or oscillate wouldalso cause this trouble.

To check a DC machine, eithermotor or generator, for electricalneutral is a very simple procedure.All that is required is a source of direct current and a millimeter. Withthe machine fully assembled, discon-nect at least one armature lead.Also, disconnect the two shunt fieldleads, GF1 and GF2. Make sure theAC motor cannot start. Connect themillimeter to two adjacent brushholders: see Figure 9.

Connect GF1 solidly to one side ofthe DC supply. Hold GF2 to the othersource. If the meter rises when thefield is excited, reverse the twometer wires. Hold GF2 on the DCsource until all motion in the meterhas stopped. Rapidly remove GF2from the circuit, and note carefullyhow much the meter rises. Move thebrush yoke in either direction – wehave no clue as to the proper rota-tion. Try the application and removalof current again. If the meter riseshigher, the brushes were moved thewrong way. If the meter direction is


reversed, they were moved too far inthe correct direction. When in theexact neutral position, no appreciablemotion will be noted on the meter.

Be sure to check again after tight-ening the yoke. Some manufacturersuse a split yoke, which will hold po-sition. One uses a cup-point setscrew, and the yoke often moveswhen this is tightened. If this occurs,grind off the end of the set screwuntil it is square and smooth. Notethat any brush shift will change thecompounding or series field effect ofthe generator. Movement against ro-

tation will increase the effect,and movement with rotationwill decrease the effect. Thesechanges must be compensatedfor by series field adjustments.If it is not possible to get thebrushes on exact neutral, andif the meter kicks one waywhen current application rises

over zero, and kicks the same wayand again crosses zero, the brushesare not evenly divided. That is, if foursets of brushes are used, they are noton 90º spacing. This should be cor-rected.

Other conditions that can causesparking are shorted commutatorsegments, shorted turns in the arma-ture, an open armature circuit orpartially shorted fields. These condi-tions call for a competent re-windshop. Another generator trouble maybe occasioned by variation in level-ing speeds with varying loads. This is

caused by incorrect compounding,by being off neutral or both. To checkthe compounding, put a full load onthe car and run up and down at leveling speed. If the hoisting speedis lower than the lowering speed, in-crease the series field. If hoisting isfaster than lowering, decrease theseries field.

If “spotting” occurs in slowingdown (that is, if the car pauses andthen continues in its direction, oreven reverses itself before continu-ing in its direction), we are eitherslowing too rapidly or have toomuch series. Make the full-load lev-eling speed check outlined above. Ifthe compounding is correct, eitherstart the slowing earlier or do thisand decrease the rate of slowdown.

If “overshoot” occurs at the floor(that is, if the car runs through thefloor at some loads), we have the op-posite condition of spotting. This is

DCSource

Fiel

d

Series Field

Milliammeter

ARM. Inter-pole

Method of Checking Electrical NeutralFigure 8

Continued



evidenced by several things. The mostapparent is that when the car goesthrough the floor, it will not releveluntil either a high voltage is appliedor until considerable time has elapsed.

This trouble can be one of threethings. The first is hard or high car-bon iron in the frame. If this condi-tion occurs, expert help must be ob-tained, as the pole pieces must beisolated with brass shims and bolts.So many design factors enter intothis condition that it should not belightly undertaken. The second con-dition that might cause this is too-low leveling speed and/or incorrectseries adjustment. These conditionsare evident, as are their cures. Thethird condition is field-pole design. Iftoo high a voltage is required to ex-cite the main poles, we have a clue.This means we have a pole of verymany turns of fine wire for its shuntwinding. The series firld is wound onthe outside of this, making a reactorof very slow operation.

One manufacturer uses 85-voltfields. This means relatively fewturns of wire. The releveling is rapid.Another manufacturer uses a muchhigher voltage, and the releveling isslower. In passing, it might be well tonote that the “low-voltage” shuntfield will ensure better suiciding.

In investigating a case of slowreleveling and excessive high-speedovershoot on one of these field

structures, new poles were woundfor 75 volts, and the results werespectacular. Releveling was rapid,and the high-speed overshoot wasreduced 20 fpm on a 500-foot job

The hoist motor, being a cousin ofthe generator, has a group of ills thatare parallel to the ones of the gener-ator. Sparking troubles are identicalto the ones of the generator, andmay be cured in a like manner. Theonly variation is that we cannot havesparking when idle. Speed regula-tion, particularly third speed, can beaffected by brush position. If the gen-erator has been thoroughly checkedas to series effect and neutral, andspeed variation between up anddown full load is encountered, checkthe hoist motor and its wiring. If thewiring is of ample size and all con-nections are good (no heat at anyterminal), see if the brushes on themotor are well seated. If they are,check for neutral as described underthe method outlined in the generatordiscussion. If you desire to raise thefull-load hoisting speed, move thebrushes against the direction of rota-tion when hoisting. A very slight mo-tion is necessary for this correction.

We now approach some of thegeneral problems of variable voltagethat may be traced to various parts ofthe system. They are generally classedas rough starts, rough and/or uncer-tain stops, and speed and control

variations. Rough startscan be caused by applying too muchvoltage to the shuntfield of the generatorfor the initial start. Avoltmeter on GFH GF2will determine this.The value of the firstapplication of voltageshould generally not beover 15% of the full-speed voltage. Your

control manufacturer should be con-sulted on this point. A very commoncause of rough starts is the failure ofthe brake to fully and rapidly releasebefore the car starts to accelerate.Many factors can enter into this – toolow a voltage applied to the brake,too rapid an increase in generatorvoltage, poor brake design, partiallyshorted coils in one or more brakecoils or a dragging brake. A contact onthe brake that will close when thebrake is fully opened is an excellentdevice. This contact should be soconnected that it will hold down thegenerator excitation until the brakeis fully opened.

Another cause of rough starts thatis little understood or considered isthe shunt field of the hoisting motor.One manufacturer rates the fields ofits motors so that only about 50% excitation may be used for holding orstanding. A DC field takes a consid-erable time to saturate after voltageis applied, providing a very adversecondition. When the doors close andthe interlock circuit is made up, weraise the field from a 50% to a 100%value, voltagewise. However, themagnetic flux does not rise immedi-ately. Accordingly, the car starts witha weak motor field, which tends tomake the motor run faster with agiven loop voltage. It also requireshigher amperage to run – this in-creases the series-field effect and fur-ther raises the loop voltage. A graph ofthis condition shows an unpleasantspeed hump. (See Figure 9.) A verystartling demonstration may be madeof this condition by applying full fieldto the same motor before the doorsclose and noting the change in thestart due to the increased time of fieldbuildup.

Stalling in leveling may sometimesbe traced to the hoist motor. If it hasbeen determined that the series fieldis correctly adjusted, that the level-Time

Spee

d

A

B

A = Speed/time curve of motor with full fieldB = Speed/time curve of motor with 50% standing field

Figure 9


ing speed is as high as is desired, andthat the car will still stall under loadwhen leveling, a quick check will determine if it is the hoist motor thatis at fault. Try increasing the field current on the motor. Caution shouldbe exercised in this maneuver, andthe manufacturer’s recommendationshould not be exceeded.

It is a characteristic of a DC ma-chine that the torque at a given volt-age and amperage will increase withfield strength up to the saturationpoint of the magnetic structure. This“saturation point” is the limit of theiron to pass more magnetic lines offorce, regardless of the current pass-ing through the magnet coil (in thiscase, the pole piece). It is also acharacteristic, derived from thisstatement, that with a given flow ofcurrent (the ability of the generatorto deliver power) that a given loadcan be lifted with less amperage witha stronger motor field. Of course, withthe same loop voltage the speed willbe less with a stronger field, so tomaintain the same leveling speed, ifwe increase the shunt field of themotor, we can increase the excitingcurrent of the generator to raise theloop voltage.

Speed changes from cold to hotoperation can often be traced to thehoist motor. If the field design is suchthat field temperature rises with con-tinued use, its resistance must increaseas the temperature rises. As the re-sistance increases, the current flow,due to Ohm’s Law, decreases. We nowhave a motor running with a weakerfield, and consequently operating ata higher speed. The cure for this is inthe province of the motor designer. Ithas two choices. One would be todesign a field structure that wouldnot heat. The second would be tooperate the motor at full magneticsaturation. If he does this, a smalldrop in field current will not have anappreciable effect.

In discussing the hoist motor, itmight be well to mention the two-speed DC motor. This is a machine ofnecessarily larger dimensions than asingle-speed machine. It is largelyused on elevators that are normally forpassenger duty but on some occa-sions must hoist a heavier load. Whenoperated as a passenger unit, themotor is operated on a partially excited field and will run at its toprated speed. When it must hoist a fullload, full field is applied. This is either done automatically or by amanual switch. When operated atmaximum field, the motor will run ateither 1/2 or 1/3 speed, according tothe design. The motor will be at a con-stant horsepower. This means that asthe speed is decreased to 1/2 or 1/3,the torque will rise to two or threetimes the full-speed torque. This is avery handy application.

The control system of variablevoltage may be approached from somany directions that it is difficult togeneralize on difficulties to be encoun-tered. We have outlined the necessityfor the gradual application of currentto the generator field, and it followsthat the decrease in current to thegenerator field must be smooth, pos-itive and carefully determined as toposition as the stop is made.

One of the common complaintson variable voltage is the final stop.Due to the fact that the rest of theoperation is so inherently smooth, ajar on the final stop is very objection-able, and much work has been doneon this. In determining the causesand remedies for this rough stop, asimple procedure may be followed.This may be done with a fully loadedcar, and then checked with a balancedcar and with an empty car. The firststep is to determine the electrical stop.Assuming that the series field andleveling are properly adjusted as tocharacteristics and speed, the brakeis held off so that it will not set. Stops

are made with the brake lifted, andtheir severity is noted, as is the amountthat the floor is passed – not on driftafter stop, but on the stop. If this stopis hard, which is a rare event, it maybe necessary to somewhat cushionthe suicide circuit with a resistor. Werepeat that this is most unusual. Theresistor in the suicide circuit shouldbe added with caution, and the circu-lating current should be rechecked.

In most cases, the stop will be sat-isfactory, or the car will run throughthe floor to some extent. If it doesrun through the floor, either the sui-cide circuit is not satisfactory and isnot operating properly, the series fieldis not properly adjusted or we have adefective generator. After these fac-tors have been carefully determined,we must set up a brake operation tosuit their conditions. If the initial stopswere good and no run through wasencountered, the brake may be elec-trically controlled so that it will applyafter the car has stopped. This maybe accomplished by a delay in thedropout of the brake relay or bymaking the brake set slowly. Thiscan be done with resistance in paral-lel with the brake coil. It is evidentthat no hard-and-fast rule may belaid down for this procedure due tothe wide variation in magnetic andelectrical brake designs. If the initialstops were through the floor, wemust apply the brake in a differentmanner, for if we delay the applica-tion, leveling will obviously vary withload. Accordingly, we must work outa system whereby the passenger willnot feel the brake application.

When the leveling zone is reached,a considerable amount of resistanceis inserted in the brake circuit, and aparallel resistance is connected acrossthe brake coil. This will accomplish adual purpose. The resistance in serieswill allow the brake to partially set.The amount of the series resistanceshould be varied until the brake shoes

Continued



just rub the wheel. The parallel re-sistance should usually be equal tothe resistance of the brake coil.

The parallel resistance is to pre-vent the brake from setting rapidly. Itwill be useful at two points: 1) whenthe shoes go from a wide-open posi-tion to the initial contact with thewheel, and 2) when the current is totally removed from the brake,making the final setting not so harsh.Properly worked out, the brake ap-plication under either method willnot be felt. One manufacturer hasseven different variations of partialbrake setting. These variations areworked out according to load and di-rection.

Author:

Joseph C. Tamsitt was an engineer and educator

who served as technical editor for ELEVATOR

WORLD from 1961-1965. During an industry

career that spanned more than 50 years, he served

as chief engineer for General Elevator Co. and

Keystone Electric Co., and later as director of

Engineering, Production and Education for Horner

Elevator Co. In addition to his engineering work,

Tamsitt found the time to write numerous articles

for EW in the 1950s and 1960s, develop corre-

spondence courses for the elevator industry and

serve as director of Education for the National

Association of Elevator Contractors (NAEC) for

five years. He also chaired several ASME A17.1

committees and is credited with developing the

Type C safety for passenger elevators. After leaving

the corporate world, he was appointed executive

director of NAEC and later founded Elevator

Educators, which helped many understand the

intricacies of elevator codes and safety measures.

In 1974, he teamed with the National Association

of Elevator Safety Authorities (now NAESA Inter-

national) to create the Elevator Educators Academy,

which trained and certified elevator safety profes-

sionals. Among his numerous honors was the

1960 NAEC Man of the Year award. Tamsitt was a

graduate of Southern Methodist University. He

died in 1984.

Editor:

Robert S. Caporale is

the editor of ELEVATOR

WORLD and a member of

NAESA International, a QEI-

certified elevator inspector

and a member of the

American Society of Me-

chanical Engineers (ASME).

He also serves on several

ASME A17.1 committees

and is a founding member of the National Associa-

tion of Vertical Transportation Professionals. He

began his career in the construction industry in

1964 when he started as a draftsman at the engi-

neering firm of Jaros, Baum and Bolles (JB&B). At

JB&B he advanced to the position of associate be-

fore moving on to DTM Consulting, where he was

the director of engineering. In 1991, he joined

Syska and Hennessy Engineers as vice president

and director of the Transport System Group and,

in 1993, moved to EW as an associate editor. He

was appointed editor in 1997. Caporale earned an

AAS degree in Electrical Technology at the State

University of New York, continued his engineering

studies at the City University of New York, New

York Institute of Technology and University College

Northampton, U.K., where he earned a Master of

Science Lift Engineering degree.

Learning-Reinforcement QuestionsUse the below learning-reinforcement questions to study for the Continuing Education Assessment Exam available online at www.elevatorbooks.com or on page 143 of this issue.

u What is the operation theory of variable-voltage control as applied to the Ward-Leonard system? u What are the various methods tried to achieve good control of acceleration, deceleration and leveling with

variable-voltage control? u In the author’s opinion, what is the most dependable method of control? u Explain the operation of a self-excited generator. u How does a damping motor field work to control speed and leveling? u How are ampere turns in a field determined? u What does “shunt field” mean? u What is Ohm’s law? u How does Ohm’s law apply to an increase in load in the elevator system? u Explain “circulating current” and retained magnetized flux. u Examine the methods used to prevent rapid current rise and the hoist motor pulling through the brake (a

runaway).u What is the difference between synchronous speed and actual speed? u What is this difference in synchronous and actual speeds called? u What are some of the problems of variable-voltage control? u How do you trace these problems to various parts of the system?


ELEVATOR WORLD Continuing EducationAssessment Examination Questions

Instructions: u Read the article “Variable Voltage Made Easy (Using Print No. C-1469-A)” (page 77) and

study the learning-reinforcement questions.u To receive three hours of continuing-education credit, answer the assessment examination

questions found below online at www.elevatorbooks.com or fill out the ELEVATOR WORLDContinuing Education Reporting Form found overleaf and submit by mail with payment.

u Approved for Continuing Education by NAEC for CET and NAESAI for QEI.

1. Variable-voltage control is known as:a. Unit multivoltage.b. Generator field control.c. Ward-Leonard control.d. All of the above.

2. Variable-voltage control was invented in:a. The U.S.b. Italy.c. Germany.d. Sweden.

3. In variable-voltage control, the elevator motor field:a. Is constantly excited and has power applied at all times.b. Has power applied only when the car is in motion.c. Has power applied only when the car is running at full speed.d. None of the above.

4. The armature of a generator is constantly revolved by a motor,which is on a common shaft with the ________.a. Generatorb. Door-operator motorc. Hoist machine motord. None of the above

5. The polarity of the generator current will be in relation to the po-larity of the generator fields.a. Trueb. False

6. A direct-current (DC) motor with constantly excited fields willvary in speed relative to the voltage applied to the:a. Commutator.b. Fields.c. Brake coil.d. Starting contacts.

7. The company that pioneered the damping motor control in themid 1920s was:a. Westinghouse.b. Otis.c. Dover.d. General Electric.

8. A DC motor’s armature is almost a short circuit when at rest andwill draw less and less current as it ___________.a. Deceleratesb. Acceleratesc. Runs at full speedd. Comes to rest

9. What type of control requires no timers, tubes, switches or trickfields in the generator?a. Damping motor controlb. Two-speed alternating current (AC)c. Single-speed ACd. VVVF-AC

10. Generators that are used in elevator control that are compoundwound have a _______.a. Series fieldb. Isolated basec. Shunt fieldd. None of the above

11. With a generator properly compounded so that the levelingspeeds in the up and down directions are exactly equal the full-load speeds in both directions should have a variation of no lessthan _______.a. 10%b. 5%c. 24%d. 15%

12. When lowering a load, the motor becomes a generator because it has an excited field and is driven by the load.a. Trueb. False

13. Once magnetized, the degree to which a piece of iron or steelwill retain some flux of magnetism is dependent on the mate-rial ______.a. Hardnessb. Lengthc. Temperatured. All of the above

14. Switches that are “fail safe” normally have contacts that arenormally ___________.a. Closedb. Openc. Large in surface aread. Redundant

15. The synchronous (no-load) speed of a motor is determined bythe frequency of the current and the ________.a. Level of the voltage applied to fieldb. Number of poles of the motorc. Sound levelsd. None of the above

16. The amount of the difference between the synchronous speedand the actual speed under load of a motor is known as_______.a. Creepb. Slipc. Shiftd. All of the above

17. The point at which the motor slips badly and starts to rapidlyslow down is known as the __________ point.a. Breakingb. Trippingc. Pull-outd. Failing

Continuing Education: Variable Voltage



Article title: “Variable VoltageMade Easy” (EW, April 2007, page 77).

Continuing-education credit: Thisarticle will earn you three contacthours of elevator-industry continu-ing-education credit.

Directions: Select one answer foreach question in the exam. Com-

pletely circle the appropriate letter. A minimum score of80% is required to earn credit. You can also take this test on-line at website: www.elevatorbooks.com.

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This article, “Variable Voltage Made Easy,” is rated forthree contact hours of continuing-education credit. Certi-fication regulations require that actual study time is veri-fied with all program participants. Please answer thebelow question.

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ELEVATOR WORLD Continuing EducationReporting Form

18. The voltage impressed in each phase of an AC motor whenstar-connected is ________ of the running voltage.a. 33 1/3%b. 66 2/3%c. 58%d. 25%

19. A noisy motor on an MG set when running at full speed may becaused by a mechanical or _________ condition.a. Electricalb. Environmentalc. Overloadd. None of the above

20. Sparking of a generator commutator when running idle may becaused by:a. The generator being out of electrical neutral.b. Uneven gaps between the brushes and the commutator bars.c. Bad ball bearings.d. All of the above.

Documents

Variable Voltage Made Easy - Elevator Books · shunt-field motor control. ... Source of steady DC to supply power for fields: ... were tried, such as “reversed series