STARTING METHORDS OF THREE PHASE INDUCTION MOTORS

INSTRUCTED BY: Mr. L.Meegahapola

NAME : H.M.S.L.G.BandaraINDEX NO : 050036JFIELD : EEDATE OF SUBMISSION: 2007.06.12

DISCUSSION:

Construction and working principle of DOL starters

A direct on line starter or DOL starter is used as a starting method of electric motors. In DOL starting method is done by directly connecting the motor to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. At the starting moment the rotating magnetic field generated in the stator winding induces a voltage drop across the rotor. At the short-circuited end there will be a current governed according to the Faradays law. And also that current will create the torque. The motor starter is used with push button switches, relays, isolators, circuit breakers, transformers and interlock switches. The DOL starter circuit is divided into two circuits called as control circuit and power circuit. The motor terminals connect with the power supply by using the power circuit. It is the higher voltage circuit of the DOL. The control circuit is the lower-voltage circuit and it is electrically isolated from the power circuit. The starter is energized by the control circuit using electro-mechanical relays and push button switches. Modern starters employ solid state relays like electronic relays (over load protection relays). The switch may be a manually operated load break switch or circuit breaker. Although more commonly it would be an electromagnetic contactor which can be opened by the thermal overload relay under fault conditions. And also the contactor will be controlled by using different start and stop buttons.

A picture of a DOL switch

Most motors can be run clockwise as well as anti-clockwise. To do that a reversing starter is used which is an electrical or electronic circuit that reverses the speed of a motor automatically. Therefore the circuit is composed of two DOL circuits that are for clockwise operation and the other one for anti-clockwise operation. These types of starters are used to start water pumps, compressors, fans and conveyor belts. The starting current may reach 3-8 times the normal current but the starting torque is tends to be 0.75 to 2 times the full load torque. The motor will draw a very high amount of current for a very short time. That is 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 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. This is the most simple and inexpensive method of starting a squirrel cage induction motor. And also by using this method can avoid excessive supply voltage drops because of large starting currents the method is restricted to small motors only.

Shortcomings of the DOL starter.

In DOL method the start time is dependant on the acceleration torque and the load inertia. In this method there is large starting current (That is between 3-8 times the full loads current). And also there is no specific method use to control that current. Because of large starting current used to start the motor there will also be a large voltage drop cross the distribution line. There fore this method is restricted to small motors only. Also due to that will affect on the other electrical consumer’s equipments or some times it may cause a mechanical problems with the driven load. There fore DOL method generally not applied relatively large machines. This method can be use below 7.5 kW. To decrease the starting current cage motors of medium and larger sizes are started at a reduced supply voltage. The reduced supply voltage starting is applied in star-delta and auto transformer methods.

Suitability of star-delta and auto transformer methods for starting of 3-phase induction motors

Star-Delta starting method.

The Star Delta starter method is another type of starting method. But it can only be used with a motor rated for connection in delta operation at the required line voltage. And also it should have both ends each of the three windings available individually. At start the winding is connected in a star connection. When the induction motor comes to its normal operation the switch is thrown quickly to the running position and the star connection is opened. When it reaches to the considerable amount of speed the starter winding is delta connected. The connection changing is done by manually or automatically. In the manual method the switch changing is done by the relays and the switches which are driven by the relays. Although the tap changing should be done before the motor obtain the speed otherwise the fuses will blow up and the system fail occur. In the Automatic mode the connection changing is done by centrifugal switch which is connected the rotating parts in the motor or the timer circuit will make a certain time delay to speed up the motor and it switch on the relays to made connection changes. The timer delay can be adjusted by the user as required. So the best method is the centrifugal one. The phase voltages of the motor in star connection are reduced to 1/√3 of the DOL values in delta. The starting torque is also reduced to 1/3 of its delta value. Because the phase voltage is proportional to the square of the applied voltage. And also the same as the starting current is also reduced by a factor. Not applicable for applications above high voltages because of the requirement of the excessive number of stator turns needed for the delta connection. The 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.

Star-Delta Starter

Auto Transformer method.

In the Auto transformer starter method normally use to reduce the voltage applied to a motor during start. There are two ways of connecting an auto transformer starter. The most obvious way is to apply full voltage to the transformer via a contactor and connect the motor to the tap by means of a contactor. This method known as an open transition starter and 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 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. As a result of the open transition switching can badly broken shafts and stripped gears. But make a rearrangement of the power circuit it can be avoid. Other method is called as the closed transition auto transformer method. The closed transition auto transformer starter is known as the Korndorfer starting method.

The auto transformer may have a number of output taps and be set up to provide a single stage starter or a multi stage charger. Typically in the starting of auto Transformer it has at least three tapings. If the transformer ratio is 1: k (if k<1) at a particular instant the starting current is reduced to k*IDOL from its full voltage current. Auto-transformer starting method also reduces the initial voltage applied to the motor and therefore the

starting current and torque. The motor can be connected permanently in delta or in star. It is switched first on reduced voltage from a 3-phase tapped auto -transformer and when it has accelerated sufficiently then it is switched to the running or full voltage position. The principle is similar to star-delta starting and has similar limitations. The advantage of the method is that the current and torque can be adjusted to the required value, by taking the correct tapping on the autotransformer. This method is more expensive because of the additional autotransformer. This method gives a better flexibility and good starting characteristics compared with the above methods. The autotransformer starter is more expensive than the two types, so far described and is generally used only for the larger types of motor. It is suitable for motors in which each end of the three phases is not brought out and which would therefore be unsuitable for star-delta starting.

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Auto Transformer

Advantages of rotor resistance starting method for wound rotor induction motors.

We can start a wound rotor induction motor by adding a rotor resistance starting method. That is by adding external resistance to the rotor circuit. By doing that starting torque up to the maximum torque can be achieved and also by gradually cutting out the resistance a high torque can be maintained throughout the starting period. Therefore starting torque in the range of 2 to 2.5 times the full load torque can be obtained. The added resistance also reduces the starting current so that a starting current of 1 to 1.5 times the full load current. Normally wound rotor motors are probably used in the more demanding applications. And also this is the cheapest method of starting wound rotor induction motor is by means of added rotor resistance with full line voltage across the stator terminals.

Reversing the direction of a 3-phase induction motor

The motors can run on either clockwise or anti-clockwise. To do this should change the direction of the rotating magnetic field and by changing the direction of the current flow. So to achieve it by reversing the connections to any two of the three motor terminals can reverse the direction of rotation of 3-phase induction motor which is shown in the below picture.

Reason for the shaded motor rotates from un-shaded portion to the shaded portion

In shaded pole motor a heavy copper ring is inserted in a slot cut in the pole piece. Part of each pole is encircled by a copper coil or strap. According to the Lenz's Law induced current in the strap opposes the change of flux through the coil. The current induced in these rings periodically shifts the resultant magnetic field from un-shaded portion. So that the maximum field intensity moves across the pole face on each cycle thus producing the required rotating magnetic field. Therefore the shaded motor rotates from un-shaded portion to the shaded portion.

Advantages and Disadvantages of Single Phase Induction Motors.

Split-phase induction motorsAdvantages

Produces a high power factor at full load provide much greater starting torque These kinds of motors available in different powers and speeds. There fore we can

select suitable motor for relevant applications.

DisadvantagesProduce a low starting torqueTake small amount of time to start due to the type of starting

Capacitor induction motorsAdvantages

Small in size so can be handled easilyCost effective Produces a high starting torque and low starting current Can be drive in different speeds

Disadvantages produces a huge noise Expensive due to the usage of capacitors

Shaded pole induction motorAdvantages

reliable very cheap

Disadvantages Has a low power factor Has low efficiency Deliver in low torque

Application of single phase Induction Motors.

Split-phase motor Small fans, blowers, any moderate starting torque loads, washing machines and

clothes dryers

Capacitor-start motor Compressors, pumps, any high starting torque loads, agricultural equipments, small

scale industrial machines

Shaded-pole motor Very small fans exhaust fans, timers, very low torque loads and other house hold

appliances

Advantages and Disadvantages of the induction motors.

Advantages High efficiency High speed capability No maintains and high reliability Compactness Light weight Lower cost Ease of cooling Simple construction

Disadvantages Has lower power factor

References

Internet (from www.wikipedia.com, www.oneondemand.com, www.freescale.com, www.disignenews.com)

Lecture Notes