Measurement of D.C. Resistance of a 3-phase Induction Motor

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Measurement of D. C. Resistance of a 3-phase Induction Motor

Q. State any two tests that are preferred on induction motors. Explain these tests in brief

 Measurement of D. C. Resistance of a 3 – phase Induction Motor : 

The state of the 3- phase induction motor may be star connected or delta connected. The terminals are available on the terminal box. Simple text to measure the D.C. resistance of the motor is the voltmeter-Ammeter method. The winding terminals of stator or rotor can be connected to a low voltage D.C. supply as shown in the Figure A

Circuit Diagram
Figure  A

The stator or rotor winding is internally star connected. Hence any two terminals are taken say and B whereas 3rd  terminal C is kept open D.C. supply is switched ‘ON’ and current is adjusted to a suitable value by varying rheostat. Ammeter and voltmeter readings are noted at the room temperature (t1). Resistance at room temperature t1 is found as R= V/I. This value is the combined value of two winding series. Hence resistance per phase is equal to R1 = R/2Ω. When motor works, the temperature increases and resistance also increases. If at a working temperature, the resistance is to be found, use the following relation     R2 =  R1[234.5 + t2  /  234.5 + t1]

This method gives the moderate results. For small values of resistances, the most accurate method would be Wheatstone-Bridge or Kelvin’s double bridge method A.C. resistance = 1.25 D.C. resistance. Same procedure is followed for delta connected stator. 
Circuit Diagram
Figure B

Equivalent resistance = 2R x R/2R x R = 2/3 =2R/3 =V/I = R=3V/2I     

This is R1   at temprature t1. R2  = R1[234.5 + t2  /  234.5 + t1]

High Voltage Test :

In this test specified voltage is applied between various windings and earth. This test should be carried out together with the insulation resistance test at manufacturer’s works. Generally high voltage test is applied only if insulation resistance is less than the specified limit.

Method of Testing :

The test shall be made with alternating voltage of any convenient frequency between 40Hz to 60Hz. The test voltage should be of sine waveform (approximately).

Duration :

The test is started by applying 1/3rd of the test voltage and then voltage is increased to full-test voltage in accordance with the table 1                              

Sr.
 No.

Part of Motor

Test  voltage
(r.m.s)

1

Stator winding (primary)

(1000 V ) + (Twice the rated voltage)
with minimum of 2000 volt.

2

Rotor winding (secondary) not permanently
short circuited.

 a. For non reversing motors or unidirectional
motors.

(1000 V) + (Twice the open circuit
standard voltage as measured between sliperings with rated voltage applied to
primary or a minimum of 2000 volt

 

 b. For motors to be reversed or brake by
reversing the primary while the motor is running.

(1000 V ) + (Four times the open
circuit voltage defined in (a))

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