Starting Method of 3-Phase Induction Motor

Department of Electrical Engineering University of Moratuwa Electrical Machines Laboratory Experiment:

Starting Method of 3-phase Induction Motor

Apparatus:

3phase induction motor (Squirrel cage and wound rotor) 3phase Autotransformer, Contactors & Timer Direct on line starter Rheostats

Theory:

The methods of starting a 3phase induction motor depends to a great extent on starting torque required, starting current limitation and the type of motor. The basic methods available are, 1. Direct on line starter 2. Star-Delta starter 3. Auto transformer starter 4. Rotor impedance starter 5. Power electronic starter

DOL 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 is restricted to small motors below 5 hp (3.75 kW). Otherwise reduced-voltage starters are employed with motors above 5 hp. DLO method causes a high starting current at a lagging power factor. It also can causes voltage fluctuation on the supply network. DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts. In the case of an asynchronous motor, such as the 3-Phase Squirrel-Cage Motor, the motor will pull a high starting current until it has run up to full speed. This starting current is commonly around 6 times the full load current, but may as high as 12 times the full load current. In the star – delta method the stator windings are star connected at start and when it reaches a substantial speed, then the stator winding are connected delta. At start the winding votage is reduced by the factor of 1/√3, and hence starting torque wold be 1/3 of the normal. Starting current is also redused by the factor of 1/√3.How ever during the change over period the air gap flux decreases to zero. When the delta contactor is reclosed the transient current can be still high. Thus this method dose not necessarily reduse the peak valuve of the starting current.

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Fig 1: Star – Delta Starter

By connecting auto transformer, the starting voltage to the motor can be reduced by an arbitrary factor “k” unlike the fixed factor of 1/√3, in the star delta metod to limit starting current. The torque is then reduced a factor of “ k2 “.

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Fig 2: Auto-transformer Starter

The line impedance starting method is widly used for the wound rotor type 3phase induction motors. Additional rotor resistances are introduced at start and gradualy decreases with the speed & time to the zero. This method gives a higher starting torque and a lower starting current.

Procedure:

i. Study the construction of wound rotor and squirrel cage induction motors. Note down their operating speed and rated data on the name plate ii. Study the manually operated star-delta switch (rotary type) available on the panel. Start the induction motor by using this switch.

iii. Simulate an automatic star-delta starter using block diagram given at the end of the labsheet. Varying the timer setting, observe the difference.

iv. Study the rotor impedance starter. Keep the external impedance maximum valuve ( zero mark at the motor control pannel) at the beginning and star the wound rotor type induction motor. v. Start a squirrel cage induction motor using an auto transformer method.

Discussion:

i.

What is the purpose of employing a starting method for an 3phase induction motor.

ii.

Explain the construction and working principal of the DOL with a circuit schematic diagram.

iii.

Discuss of the short comings of the DOL starter.

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iv.

Comment transformer

on

the

suitability

starting

of

methods.

the

star-delta

Compare

and

advantages

autoand

disadvantages of each. v.

What are the advantages of rotor impedance starting method and compare with other alternatives?

vi.

What is the method for reversing the direction of a 3-phase induction motor?

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