2.1 Variable Speed Regulator

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VARIABLE SPEED REGULATOR

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Semiconductor devices used asswitches or rectifers that are capableo handling high currents.

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Semiconductor

2.1 VARIABLE SPEED REGULATOR


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3

Thyristors:Solid state devices that as bistableswitches, conducting when theirgates recieves a current pulse and aslong as they are orward biased.

Types:- SCR (Silicon Controlled Rectifer)-!" (ate !urn-"# !hyristor)-$C! ($ntegrated ate Commutated

!hyristor)

SCR

GTO

IGCT

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Three Terminal


Anode

Cathode


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% primary unction o a &C drive, such as theSIMOREG 6RA70 DC MASTER, is to convert%C voltage into a variable &C voltage. $t isnecessary to vary to &C voltage in order tocontrol the speed o a &C motor. % thyristor is

one type o device commonly used to convert%C to &C. % thyristor consists o an anode,cathode, and a gate.

ate Current% thyristor acts as a switch. $nitially, a thyristorwill conduct (switch on) when the anode ispositive with respect to the cathode and a

positive gate current is present. !he amount ogate current re'uired to switch on a thyristorvaries. Smaller devices re'uire only a ewmilliamps however, larger devices such asre'uired in the motor circuit o a &C drive mayre'uire several hundred milliamps

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Thyristor



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olding current reers to the amount ocurrent *owing rom anode to cathode to +eepthe thyristor turned on. !he gate current maybe removed once the thyristor has switchedon.

!he thyristor will continue to conduct as longas the anode remains suciently positive withrespect to the cathode to allow sucientholding current to *ow. i+e gate current, theamount o holding current varies rom deviceto device. Smaller devices may re'uire only aew milliamps and larger devices may re'uirea ew hundred milliamps.!he thyristor will switch o# when the anode is no

longer positive with respect to the cathode.

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Holding Current



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!he thyristor provides aconvenient method o converting%C voltage to a variable &Cvoltage or use in controlling thespeed o a &C motor. $n this

eample the gate is momentarilyapplied when %C input voltage isat the top o the sinewave. !hethyristor will conduct until theinput/s sinewave crosses 0ero. %tthis point the anode is no longerpositive with respect to the

cathode and the thyristor shutso#. !he result is a hal-waverectifed &C.

!he amount o rectifed &Cvoltage can be controlled bytiming the input to the gate.%pplying current on the gate atthe beginning o the sinewaveresults in a higher average voltageapplied to the motor. %pplyingcurrent on the gate later in thesinewave results in a loweraverage voltage applied to the

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AC to DC Conversion



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!he output o one thyristor isnot smooth enough to controlthe voltage o industrial motors.Si thyristors are connected

together to ma+e a 12 bridgerectifer.

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DC Drive Converter



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!he gating angle o a thyristorin relationship to the %C supplyvoltage, determines how muchrectifed &C voltage is available.owever, the negative and

positive value o the %C sinewave must be considered whenwor+ing with a ully-controlled12 rectifer. % simple ormulacan be used to calculate theamount o rectifed &C voltagein a 12 bridge. Converted &Cvoltage (3&C) is e'ual to 4.15

times the R6S value o inputvoltage (3R6S) times the cosineo the phase angle (cos7).

3&C 8 4.15 3R6S cos7!he value o &C voltage that can be obtained rom a 9:; 3%Cinput is -:.$t is important to note that voltage applied to the armatureshould not eceed the rated value o the &C motor.

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Gating Angle



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!he ollowing illustration approimatesthe output waveorm o a ullycontrolled thyristor bridge rectifer or;>, :;>, and @;>.

!he &C value is indicated by the heavy

hori0ontal line. $t is important to notethat when thyristors are gated at @;>the &C voltage is e'ual to 0ero. !his isbecause thyristors conduct or thesame amount o time in the positiveand negative bridge.

!he net result is ; 3&C. &C voltage willincrease in the negative direction asthe gating angle (7) is increased rom@;> to a maimum o 4?;>.

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Ahen the SCR is reverse polari0ed actsas a

common diode. (See the lea+agecurrent

characteristic shown in the fgure).

$n the region o the direct polari0ationSCR also

behaves li+e an common diode,

provided that theSCR has already been activated ("B).

SCR:Curve I%vs. V% (anode current vs.

anode-cathode voltage).

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Characteristic curve of the SCR



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&C motors are made up oseveral maDor componentswhich include the ollowingE

F Grame

F Shat

F Hearings

F6ain Gield Aindings (Stator)

F %rmature (Rotor)

F Commutator

F Hrush %ssembly

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Motor DC



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%rmature!he armature rotatesbetweenthe poles o the feldwindings.

Gield windings are mountedon pole pieces to ormelectromagnets.

Gield windings andpole pieces are bolted to the

rame.

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Basic Construction



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Hrushes ride on the side o thecommutator to provide supplyvoltage to the motor.

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Brushes



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6agnetic Gields

there are two electrical elements o a&C motor,the feld windings and the armature.

!he armature windings are made up ocurrentcarrying conductors that terminate at acommutator. &C voltage is applied tothe armature windings through carbonbrushes which ride on the commutator.

Ahen voltage is applied to statorwindings an electromagnet with northand south poles is established. !heresultant magnetic feld is static (non-rotational).

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DC Motor Oeration



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% &C motor rotates as a result o twomagneticGields interacting with each other.

!he frst feld is the main feld that eists inthe stator windings.

!he second feld eists in the armature.

Ahenever current *ows through a conductora magnetic feld is generated around theconductor.

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Magnetic !ields



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% relationship, +nown as the right-hand rule or motors, eistsbetween the main feld, the feldaround a conductor, and thedirection the Conductor tends move.

$ the thumb, inde fnger, and thirdfnger are held at right angles toeach other and placed as shown inthe ollowing illustration so that theinde fnger points in the direction othemain feld *u and the third fnger

points in the direction o electron*ow in the conductor, the thumb willindicate direction o conductormotion.

%s can be seen rom the ollowingillustration, conductors on the letside tend to be pushed up.

Conductors on the right side tend tobe pushed down. !his results in a

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Right " Hand for Motors



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Ahenever a conductor cuts through lines o*u a voltage is induced in the conductor.

$n a &C motor the armature conductors cutthrough the lines o *u o the main feld.

!he voltage induced into the armatureconductors is always in opposition to theapplied &C voltage.

Since the voltage induced into theconductor is in opposition to the appliedvoltage it is +nown as CI6G (counterelectromotive orce). CI6G reduces the

applied armature voltage.

!he amount o induced CI6G depends onmany actors such as the number o turnsin the coils, *u density, and the speedwhich the *u lines are cut.

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C#M!



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%n armature, as we have learned,is made up many coils andconductors. !he magnetic felds othese conductors combine to orma resultant armature feld with anorth and south pole.

!he north pole o the armature isattracted to the south pole o themain feld.

!he south pole o the armature isattracted to the north pole o the

main feld.

!his attraction eerts a continuoustor'ue on the armature. Iventhough the armature iscontinuously moving, the resultantfeld appears to be fed. !his isdue to commutation

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Armature !ield



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%s the conductor continues to rotate romposition < to position 1 current *ows away romthe commutator in the white hal and toward the

commutator in the blac+ hal. Current hasreversed direction in the conductor. !his is

$n position < the conductor has rotated @;>. %t thisposition the conductor is lined up with the mainfeld. !his conductor is no longer cutting main feldmagnetic lines o *u thereore, no voltage isbeing induced into the conductor. "nly appliedvoltage is present.

!he conductor coil is short-circuited by the brush

spanning the two adDacent commutator segments.!his allows current to reverse as the blac+commutator segment ma+es contact with thepositive side o the applied &C voltage and thewhite commutator segment ma+es contact withthe negative side o the applied &C voltage.

$n position 4 the blac+ hal o the conductor is incontact with the negative side o the &C appliedvoltage. Current *ows away rom the commutatoron the blac+ hal o the conductor and returns tothe positive side, *owing towards the commutatoron the white hal.

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Communication



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Jermanent 6agnet 6otorsSeries 6otors

Shunt 6otorsCompound 6otors

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Tyes of DC Motors



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Seed $ Tor%ue Curves



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&C 6otor I'uations

$n a &C drive voltage applied (3a) to thearmature circuit is received rom avariable &C source. 3oltage applied tothe feld circuit (3) is rom a separatesource.

!he armature o all &C motors containssome amount o resistance (Ra). Ahenvoltage is applied (3a), current ($a) *owsthrough the armature.

!he current *owing through thearmature conductors generates amagnetic feld. !his feld interacts withthe shunt ield (K) and rotation results.

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Seed $ Tor%ue Relationshis of Shunt Connected Motors



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3a 8 (tKn) = ($aRa)

AhereE

3a 8 %pplied %rmature 3oltaget 8 6otor &esign ConstantsK 8 Shunt Gield Glun 8 %rmature Speed$a 8 %rmature CurrentRa 8 %rmature Resistance

CI6G!he rotation o the armature through the shunt feld induces a voltage in thearmature (Ia) that is in opposition to the armature voltage (3a). !his is counterelectromotive orce (CI6G).CI6G is dependent on armature speed (n) and shunt ield (K) strength. %n increasein armature speed (n) or an increase o shunt ield (K) strength will cause a

corresponding increase in CI6G (Ia).Ia 8 tKn "r Ia 8 3a -($aRa)

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Armature &oltage



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!he relationship between 3% andspeed islinear as long as lu (K) remainsconstant.Gor eample, speed will be 5;L obase speed

with 5;L o 3% applied.!he interaction o the shunt andarmaturefeld *u produces tor'ue (6). %nincreasein armature current ($a) increasesarmature

*u, thereby increasing tor'ue.%n increase in feld current ($)increasesshunt ield lu (K), therebyincreasing tor'ue.

6otor !or'ue

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Motor Seed



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Hase speed corresponds to ullarmature voltage (3a) and ull lu (K).% &C motor can operate at ratedtor'ue (6) at any speed up to basespeed, by selecting the appropriatevalue o armature voltage. !his is oten

reerred to as the constanttor'ue region. %ctual tor'ue (6)produced, however, is determined bythe demand o the load ($a).Constant orsepower

Some applications re'uire the motorto be operated above base speed.

%rmature voltage (3a), however,cannot be higher than ratednameplate voltage.%nother method o increasing speedis to wea+en the ield (K).Aea+ening the feld reduces theamount o tor'ue (6) a motor canproduce. %pplications that operate

with feld wea+ening must re'uireless tor'ue

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Constant Tor%ue



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Va and Ia are constantly monitored. Ra ismeasured during the comissioning and tuning othe drive. Hecause Va, Ia, and Ra

are +nown values, CEMF (Ea can be preciselycalculated.

CEMF is proportional to speed and the speedcontroller uses this value to calculate actual speed.Speed control with CI6G eedbac+ can only be

used on applications where the motor operatesbetween 0ero and base speed.

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Seed Control 'ith C#M! !eed(ac)



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% tachometer can be used when a more accuratemeasurement o speed is needed, or when the motor willbe operated above base speed.

% measurement o actual speed is returned to the speed

controller.

!he speed controller will ma+e armature voltageadDustments tomaintain constant speed with variations in load.

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Seed Control 'ith Tach !eed(ac)



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!he drive monitors current, which is summed with the speedcontrol signal at the current controller.

!he drive acts to maintain current at or below rated current byreducing armature voltage i necessary.

!his results in a corresponding reduction in speed until thecause o the overcurrent is removed.

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Current Measurement



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Some applications re'uire the motor to operate with a specifc tor'ueregardless o speed.

!he outer loop (speed eedbac+) is removed and a tor'ue reerence isinput.

!he current controller is e#ectively a tor'ue controller because tor'ue isdirectly

proportional to current.

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Tor%ue Control


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2.1 Variable Speed Regulator