Applied Electricity

Starting Methods of Motors

NAME : Jayasinghe W.G.P.

INDEX NO : 050180T

FIELD : Material Science and Engineering.

COURSE : B.Sc. Engineering

PRU. DATE : 21/11/2010

SUB. DATE : 08/11/2010

Introduction:Electric motors are the devices which convert the electric energy to mechanical

energy. There are three main types of motors which are dc motor, induction motor and synchronous motors. Also electric motors are categories on motor energy supplier such as dc motor, single phase ac motor and three phase ac motors.

Some motors such as single phase induction motors do not have starting torque so it is necessary to have some starting arrangement for such motors. As well as some time use a starting method for safety, durability, proper working condition and controllable starting torque. Different types of mechanisms are used such as capacity induction, split phase induction.etc.

During this lab session basically focus on methods of starting on single phase induction motors, three phase induction motors and dc shunt motors.

Theory:

Starting Methods of Single Phase Induction Motors

Unlike poly-phase induction motors, single phase induction motors do not have a starting torque. So it is necessary to have some staring arrangements are;

1. Split phase induction motors.2. Capacitor induction motors.3. Shaded pole induction motors.

1. Split phase induction motors:In split phase type induction motors, the starting torque is provided by as

auxiliary winding, which produces a flux nearly 900 out of phase with the flux produced by the main winding. The Phase difference in time is achieved by making one winding more resistive than the other. Thus the motor starts as a two phase motors.

2.Capacitor induction motors:In capacity motors a capacity is connected in series with one of the winding to

obtain the necessary phase difference in time.

3. Shaded pole motors.In shaded pole motor a heavy copper ring is inserted in these rings shifts the

resultant magnetic field from un-shaded portion periodically. Thus imparting is a small starting torque to the motor.

Starting Methods of Three-Phase Induction Motors The method of staring a three phase induction motor depends to a great extent on

starting torque required, starting current limitations and the type of motor.The basic methods available are

1. Direct on Line (D.O.L.)2. Star-Delta.3. Auto Transformer.4. Power Electronic Starter.5. Line Impedance Starter.6. Rotor Resistance Starter.

1. Direct on Line:The D.O.L. method is the direct connection of the motor terminals to the supply

terminals through a suitable starter having contactors operated by a coil. This method id restricted to motor below few kilo watts. Direct online starting causes a high starting current

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at lagging power factor. It also can cause voltage fluctuation on the supply network. Motors supplied at a high voltage may be started by D.O.L. method.

2. Start Delta:In the start-delta method the stator windings are start connected at start and when

it reaches a substantial speed, the stator windings are delta connected. At start the phase

(winding) voltage is reduced by a factor of , and hence starting torque would be of the

normal. At the same time the starting current is also reduced by a factor of . However

during the change over period from start to delta, the air gap flux decreases to zero. The speed also drops. When the switches are reclosed the transient current can be still high. Thus this method does not necessarily reduce the peak value of the starting current. Since the rotor has gained some speed the duration of the existence of high current is reduced.

3. Auto Transformer:In the auto transformer method, the starting voltage to the motor can be reduced

by an arbitrary factor K unlike the fixed factor in the start-delta method to limit the

starting current. The torque is then reduced a factor K2. With this method, there is no surge current by like that present in the star-delta method during the star to delta transition.

4. Power Electronic Starter:In the power electronic transformer methods use an electronic power control

mechanism.

5. Line Impedance Starter:

6. Rotor Resistance Starter:Wound rotor motors could be started by rotor resistance method. In the staring

wound rotor induction motors, additional rotor resistances are introduced at start. This method gives a higher starting torque and a lower starting current. As the machine speed up, the rotor resistances are gradually reduced and finally short circuited.

Starting Method of DC shunt Motor

3 point starter:

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Spring

OffOn

Supply

Field winding

HC

OC Relay

+

-

With intention of minimizing starting current a series of resistances are connected in series with the armature during normal operation. When the starting arm is held up by the No volt coil (HC), if the supply disconnects or the current through the field become zero then HC release the starting arm. In addition to that if the motor is overloaded, overload relay operates and short –circuits the HC. Hence even in overload condition the supply is disconnected.

Procedure:

Practical 1: Starting Methods of Single Phase Induction Motors

1. Apply 100V to the rotor winding of the fan motor. Give a small push in the clockwise direction. Measure the speed using the stroboscope. Bring the voltage zero and allow the motor stop. Apply 100V to the rotor winding and measure the speed in the anti clockwise direction.

2. Connect the two windings in parallel and adjust the supply voltage to 100V. Motor will now run as split phase motor. Changing the connection Change of stator winding to reverse the machine.

3. Connect the 2µF capacitor in series with stator winding and start the motor with 100Vsupply. Now machine run as a capacity motor. Reverse the motor by changing the connection.

4. Change C from 2µF to 12µF and note the minimum voltage necessary to just run the motor without speeding up. Note the current in the stator winding (Is) and the value of the capacitor used.

5. Study the shade pole motor by connecting it at 130V supply. Measure the speed using a tachometer. Note the number of poles & directions of motion. (From shaded portion to un-shaded portion or other war round.

Practical 2: Starting Methods of Three Phase Induction Motors.

1. Study the construction of wound rotor and squirrel cage induction machines. Write down their operating speed and various other nameplate data and determine the synchronous speed.

2. Study the manually operated star-delta switch available on the panel. Start the 3 phase using this switch.

3. Study the manually operated star-delta starter using magnetic relays, push button and a timer, the block diagram of which is given in below. Adjust the timer setting and see the difference.

4. Study the rotor resistance starter and start wound rotor induction motor using the rotor resistance method.

5. Start a squirrel cage induction motor using auto transformer method.

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Armature

Result:

Practical 1: Starting Methods of Single Phase Induction Motors

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rpm Supply Voltage for WindingOnly winding 0 rpm 100 VWinding and Stator 215 rpm 100V

Applied Capacity Staring Voltage Current in Winding

3.00 µF 19 V 0.008 A4.00 µF 22 V 0.015 A6.36 µF 24 V 0.017 A7.00 µF 19 V 0.012 A9.30 µF 26 V 0.018 A10.30 µF 28 V 0.021 A13.30 µF 32 V 0.024 A

R1, R2, R3, - Magnetic RelaysT - Electric Timer.M - Motor

On Off

Wound Rotor ( three phase):ASEAMotor ~ 50 HzType : MKD11 N : 594859718 kW 2.5hp 1390 rpm

Prim. 380 V Prim. Y 4.5 A ∆

SEN – 260402. SSEN 26001 IEC 34-1 Made in Swedeen.

Practical 2: Starting Methods of Three Phase Induction Motors.

Calculations:

Synchronous speed , where p is total no of poles (4), f supplier frequency (50 Hz).

Slip , where motor rotating speed.

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Squirrel Cage:Model : EMDBN IGN CLASS ERating : CONT JTS C4004/ BODY JP225Rotor : C BEARING G202.6202Frame No: DBN 71 SER N0: 8401170

0.2 kW - 4 poles

Hz V A rpm Code50 200 1.4 1440 J60 200 1.2 1735 H60 200 1.3 1745 K

Connection:

Masturta Electric Industrial Co. Ltd.

Wound Rotor (single phase):ASEAMotor : Shunt 5881.262Type : 2.0 kW 3000 rpm

0.4 AMargin: 260 V Margin: 0.3 ASEN 2601TEC Is 1 kgMade is Weden Fabrigue EN Sadee Nepana Power

U V W

Minimum current requiement for start motor = 0.0053 A

Minimum voltage requiement for staer = 15 .1V

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Discussion:

1. Why the shaded pole motor rotates from un-shaded portion to shaded portion.

Motors are worked by forces which are work between the stator and rotor. Generally generate magnetic force repulse which are the same poles. When consider this phenomenon the repulse force acting from magnetic poles area to none magnetic poles.

When considering shaded pole motors they are having a high magnetic repulsion forces in the un-shaded poles compare to the shaded poles, so rotor rotate from high magnetic repulse force area to low

repulsion area. Another word form un-shaded poles to shaded poles.

A shaded-pole motor is a type of AC single-phase induction motor. It is basically a small squirrel cage motor in which the auxiliary winding is composed of a copper ring surrounding a portion of each pole. This auxiliary winding is called a shading coil. Currents in this coil delay the phase of magnetic flux in that part of the pole enough to provide a rotating magnetic field. The direction of rotation is from the un-shaded side to the shaded side of the pole. The effect produces only a low starting torque compared to other classes of single-phase motors.

These motors have neither only one winding on capacitor nor starting switch, so making them, economical and reliable. Because their starting torque is low they are best suited to driving fans or other loads that are easily started. They are built in power sizes up to about 1/6 hp or 125 watts output. For larger motors, other designs offer better characteristics.

2. The construction and working principle of the Direct On Line (D.O.L) starter.

A direct on line (DOL) or across the line starter starts electric motors by applying the full line voltage to the motor terminals. A DOL motor starter also contain protection devices, and in some cases, condition monitoring. Smaller sizes of direct on-line starters are manually operated and larger sizes use an electromechanical contactor (relay) to switch the motor circuit. Solid-state direct on line starters also exist.

The switch may be a manually operated load break switch or circuit breaker, but more commonly it would be an electromagnetic contactor which can be opened by the thermal overload relay under fault conditions. Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact.

To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, to establish the magnetic field in the iron, and then the current will be limited to the locked rotor current of the motor. The motor will develop locked rotor Torque and begin to accelerate towards full speed. As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load. The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.

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Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will cease accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque. The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia. DOL starting results in maximum start current and maximum start torque. This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load.

3. Shortcoming of the D.O.L. starter. Starters using magnetic contactors usually derive the power supply for the contactor coil

from the same source as the motor supply. An auxiliary contact from the contactor is used to maintain the contactor coil energized after the start command for the motor has been released. If a momentary loss of supply voltage occurs, the contactor will open and not close again until a new start command is given. This prevents restarting of the motor after a power failure. This connection also provides a small degree of protection against low power supply voltage and loss of a phase. However since contactor coils will hold the circuit closed with as little as 80% of normal voltage applied to the coil, this is not a primary means of protecting motors from low voltage operation.

4. The suitability of star-delta and auto transformer starting methods for starting of three phase induction motors.

Star-Delta StarterThe Star Delta starter can only be used with a motor which is rated for

connection in delta operation at the required line voltage, and has both ends each of the three windings available individually. At Start, the line voltage is applied to one end of each of the three windings, with the other end bridged together, effectively connecting the windings in a star connection. Under this connection, the voltage across each

winding is of line voltage and so the current flowing in each winding is also reduced

by this amount. The resultant current flowing from the supply is reduced by a factor of

as is the torque. This type of starter is an open transition starter and so the switch to

delta is accompanied by a very high torque and current transient. In most situations, there would be less damage to the equipment and less interference to the supply employed. The star delta starter does get around the regulations in some countries where there is a requirement for a reduced voltage starter. The main benefits of the star delta starter are more economical and less complicate.

Auto TransformerAn Auto transformer starter uses an auto transformer to reduce the voltage applied

to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter. There are two ways of connecting an auto transformer starter, the most obvious way is to apply full voltage to the transformer via contactor, and connect the motor to the tap by means of contactor. When the motor has accelerated to full speed, or has run out of acceleration torque, the tap contactor opens, disconnecting the motor from the transformer and another contactor closes connecting the motor to the supply. The transformer can now be disconnected from the supply. This format is known as an open transition starter and is less than ideal due to the fact that the motor is disconnected for a short period of time during the start period. While the motor is connected and accelerating, there is a rotating magnetic field in the stator which causes flux in the rotor and thus a rotor current to flow. At the instant the motor is disconnected, there is a magnetic field in the

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rotor which is spinning with-in the stator winding. The motor acts as a generator until the rotor field decays. The voltage generated by the motor is not synchronized to the supply, and so on reconnection to the supply, the voltage across the contactor at closure can be as much as twice the supply voltage resulting in a very high current and torque transient. This open transition switching is often known as the auto-reclose effect as it yields similar characteristics to opening and closing a breaker on a supply to one or more motors. The consequences of open transition switching can be as bad as broken shafts and stripped gears.By a rearrangement of the power circuit, it is possible, at no extra cost, to build a closed transition starter and thereby eliminate the current and torque transients. The closed transition auto transformer starter is known as the Korndorffer starter. The open transition switching is achieved by reconnecting the tap contactor between the transformer and motor, to the star connection of the transformer, hard wiring the motor to the tap, and altering the sequence of contactor control. To start the machine, the main contactors and the star contactors are closed applying reduced voltage to the motor. When the motor has reached full speeds, (or run out of acceleration torque) the star contactor is opened effectively converting the auto transformer starter into a primary reactance starter. Next the primary reactance is bridged by a contactor applying full voltage to the motor. At no time does the motor become disconnected from the supply.

The transformer is generally only intermittent rated for the starting duty, and so the frequency and duration of the starts is limited. With a transformer starter, it is relatively easy to change taps and thereby increase the starting voltage if a higher torque is required. The auto transformer starter is a constant voltage starter, so the torque is reduced by the voltage reduction squared over the entire speed range, unlike the primary resistance or primary reactance starters which are constant impedance starters and where the start voltage is dependent on the ratio of the motor impedance to the motor plus starter impedance. As the motor accelerates, its impedance rises and consequently, the terminal voltage of the motor also rises, giving a small torque increase at higher speeds. Unlike the primary resistance and primary reactance starter, the current flowing into the motor is different from that flowing from the supply. The supply current flows into the primary circuit of a transformer, and the secondary current is applied to the motor. The transformer reduces the primary current by the same ratio as the voltage reduction. This would suggest that the lowest starting current will be achieved by the use of an auto transformer starter. In most instances, the load will require an increasing torque as it accelerates, and so often a higher tap must be selected in order to accelerate the load to full speed before the step to full voltage occurs. If a multistage transformer starter is employed, then the primary current will certainly be lower than other forms of induction motor starter

5. The advantage of rotor resistance starting method for wound rotor induction motor compound with the other alternatives.

The Primary Resistance starter will have one or more sets of resistors which, during start, are connected in series with the supply to the motor. The series resistors limit the starting current drawn by the motor, and thus reduce the starting torque of the motor. Once the motor is up to full speed (or after a period of time) the resistors are bridged by a contactor to apply full voltage to the motor. If the full details of the motor starting characteristics are known, and the starting characteristics of the load are also known, it is practical to determine the correct value of the resistors to provide enough start torque for the load while minimizing the starting current. A primary resistance starter correctly designed and constructed will cause the motor to accelerate the load to almost full speed with the resistors in circuit before they are bridged out. In a poorly designed system, the transition to full voltage will occur at less than 80% full speed, and the current will then step up to almost DOL current, resulting in little gain from the use

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of the primary resistance starter other than the increased cost of the starter. Advantageous is the starter supplier, not to the end user. Improved starting characteristics with some loads can be achieved by the use of several stages of resistance and bridging out increasing amounts of resistance as the motor accelerates. With the primary resistance starter, it is not easy to alter the resistance and hence the starting characteristics once the starter is built. Therefore, it is important that the correct resistors are selected in the first place.

The primary resistance starter reduces the voltage applied to the motor terminals while passing the full starting current to the motor. Consequently, there is very high power dissipation in the resistors, resulting in the requirement for very high power rated resistors. Typically, the resistors will dissipate as much as 150% - 200% the power rating of the motor for the duration of the start. The resistors may be either metallic resistors, or liquid resistors. Metallic resistors have a positive temperature coefficient and as a result, as they heat up, their resistance increases. Liquid resistors, such as saline solution, have a negative temperature coefficient and so consequently, as they heat up, their resistance reduces. The heat build up in the resistors during start, and their temperature dependant resistance characteristics, make it essential the resistors are allowed to fully cool between starts. This restricts the starting frequency and the minimum time between the starts.

6. Reverse the direction of a three phase induction motor. The rotation direction of a 3 phase induction motor can be reversed by inter changing

any two of the three phase-supply lines.

7. The advantage and disadvantage of single phase induction motor. This method allows external resistance to be connected to the rotor through slip rings

and brushes. Initially, the rotor resistance is set to maximum and is then gradually decreased as the motor speed increases, until it becomes zero.

The rotor impedance starting mechanism is usually very bulky and expensive when compared with other methods. It also has very high maintenance costs. Also, a considerable amount of heat is generated through the resistors when current runs through them. The starting frequency is also limited in this method.

However the rotor impedance method is known to be the smoothest and least stressful method of accelerating an induction motor. It also allows the motor to be startrd while on load.

8. The importance of starting method of a DC motor. At starting the motor back emf isn’t present; also resistance of the motor coil is small,

so when starting the motor get high current. Due to that current motor can be damage to avoid this damage there must be required a safe starting method,

9. The starting methods of a DC motor. i.Ward-Leonard Arrangement.

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If armature and field of dc shunt motor are energized together, large current is drawn at start but the torque builds up gradually as the field flux increases gradually. To improve the torque per ampere of line current drawn it is advisable to energize the field first. The starting current is given by V/Ra and hence to reduce the starting current to a safe value, the voltage V can be reduced or armature circuit resistance Ra can be increased. Variable voltage V can be obtained from a motor generator set. This arrangement is called Ward-Leonard arrangement. A schematic diagram of Ward-Leonard arrangement is shown in figure below. By controlling the field of the Ward-Leonard generator one can get a variable voltage at its terminals which is used for starting the motor.

ii.Increased Armature Circuit ResistanceThe second method of starting with increased armature circuit resistance can be

obtained by adding additional resistances in series with the armature, at start. The current and the torque get reduced. It can be unloaded machine reaches its final speed but a loaded machine may crawl at a speed much below the normal speed. Also, the starting resistance wastes large amount of power. Hence the starting resistance must be reduced to zero at the end of the starting process. This has to be done progressively, making sure that the current does not jump up to large values. Starting of series motor and compound motors are similar to the shunt motor. Better starting torques are obtained for compound motors as the torque per ampere is more.`

Reference:1. Lecture note and presentation.2. Book:

i. Basic Electrical Engineering, P S Dhogal.3. Web Resources:

i.www.wikipedia.org/ii.www.tpub.com/

iii.www.lmphotonics.com/4. Softer Tools,

i.AutoCAD 2007ii.Microsoft Exceliii.Microsoft Word

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