Induction Motor Testing

Induction Motor Testing:

 Motor testing is divided into two categories:

1.      Routine Testing

2.      Type Testing

1.    Routine Test: In routine testing following test are carried out on motor.

A.    Measurement of Resistance: Resistance meter is used for measurement of resistance of motor winding ( Stator ) terminal between  UV,VW,UW at ambient temperature. These values are used for the calculation of temperature rise. The resistances are measured by the Voltammetric method at ambient temperature.The test is performed by a current generator (greater than 10 Adc) and a voltmeter. The acceptance criteria shall be based on manufacturer approved procedure but usually phase to phase deviation should not be more than 2%, and max deviation from theoretical value should not be more than 5%.

B.     Insulation  Resistance ( IR ) Measurement : Insulation resistance measured between  winding & frame ( Earth ). For insulation measurement Megger is used & it having test voltage 500 Volt DC. This test is perform before & after H.V. Test. The test is one of important test in the induction motor routine testing and is done to measure the winding insulation resistance of the armature, field poles, probes, space heater and bearings (if applicable).A DC voltage is applied to the winding and the frame to which the magnetic circuit and other windings are connected. In the case the stator capacitance is too high for the instrument the measurement can be performed phase by phase neutral open. For main windings, this measurement is performed twice, before and after the dielectric test. The measured resistance values shall be verified aginst acceptance criteria provided in the test procedure

C.    High Voltage Test: This high voltage is applied between winding & frame (earth) of motor by using High Voltage Tester .  For first time 415 Volt Motor H.V. taken @ 2 KV as IS:4029 (table no. 1) & second time when we will take H.V. it will reduce to 80 % of 2 KV. This test is performing one minute. This test is used to determine the insulation failure or any touching between motor body and winding.

D.    Reduce Voltage Test : This test is perform to confirm that motor run at reduce voltage & it pick up the  synchronous speed (as per available frequency ) . Motor R.P.M. is measure .Voltage applied for reduce voltage test is  [ Rated Voltage / √ 3 ] . For Example. If motor rated voltage is 415 Volt then reduce voltage is 240 Volt.

E.     No Load Test : This test is intended to find out motor no load current , core loss & windage & friction loss. In this test three phase supply given to motor terminal & motor start at rated voltage. No load power, current & R.P.M is record.

F.     Lock Rotor or Short Circuit test: This test is to carried out to determined motor starting current & starting torque.  In this test motor shaft is lock with some suitable arrangement & reduce voltage is given to the motor . Motor short circuit reduce voltage ,power, current is record. It should recognized that testing in induction motor under lock rotor condition involves unusual stresses & high heating. Therefore this test ids perform as rapidly as possible. This test is not perform on wound rotor motor.

      The purpose of the test in the motor routine testing is to calculate power factor, starting current and starting torque. The test is done on the locked rotor condition. The starting current might be high, and the test normally is done at lower voltage and result of the test extrapolated to the rated voltage. A power analyzer is used for measurement. After measurement, the ratio of starting current to the full load current and starting torque to full load torques are calculated (SC/FLC & ST/FLT). The calculated values shall be verified against acceptance criteria provided in the approved in the induction motor test procedure.

G.    Additional Test :

a.       Noise Measurement: This test is carried out to determine the motor noise level . Noise level meter or d B meter used to measure the motor noise level at different direction from 1 meter or 1.5 meter distance. ( Std. IS :12065 ).

b.      Vibration Measurement: This test is carried out to measurement of motor vibration (mm/sec. ) in axial  / horizontal / vertical direction on DE side of motor. For measurement of vibration Vibration Meter or Vibration sensor is used.( std . IS :12075 / IEC60034-14 ).

c.       Shaft Voltage Measurement: This test is perform to measure shaft voltage of motor. One end of multimetre or millivolt meter connect to motor shaft one end & other is other end of motor (i.e. DE & NDE ends of rotor ) or one end to motor shaft to motor earth terminal to  measure shaft voltage. 

d.       No Load Bearing Temperature Rise : The induction motor shall run at the rated voltage and frequency, and each bearing temperature is measured periodically.  Naturally, the bearing temperature will increase by passing the time. The temperature rise should not be  more than specified value in the induction motor testing procedure or motor datasheet. The procedure determines the running time and temperature reading intervals.

2. Type Test:

 Note : Routine test is also a part of type testing .

A.    Heat Run Test or Temperature Rise Test  : This test is intended to determine the temperature rise of motor while running at rated condition . In this test motor loading either direct method or equivalent loading method. If motor  continuous rating temperature rise test is continue till thermal equilibrium has been reach. In this test temperature of running motor & ambient temperature is measure of every half hour .Whenever the thermal equilibrium has reach motor stop & hot resistance is measure between particular time. Motor temperature rise is calculated by resistance method.The duration of temperature rise test depend upon type of duty of motor & its polarity. In  this test, the machine is coupled with an appropriate rotating equipment such as the pump, fan, compressor, etc. then the load is applied to the rotor.All variable such as current, voltage, power, stator temperature, bearing temperature are measured at the start and also in each 30 min based on the test procedure. A power analyzer and temperature device are used for measurements. 

       After thermal stabilization (bearing temperatures), the motor is stopped, and hot stator resistances is measured, and temperature rise is calculated based on following parameters:

•      Cold state winding temperature 
•      Winding temperature at test end 
•      Coolant temperature at test end 
•      Cold state stator resistance between phases 
•      Stator resistance between phases at the end of the test

       The stator temperature rise level shall be verified based on acceptance criteria addressed in the induction motor routine testing protocol /test procedure. This test is a key test in the induction motor testing.

B.     Load Test : This test is carried out to determine motor performance such as efficiency , power factor ,speed. In this test load characteristic is obtain by taking reading at higher load to followed lower load. i.e. upto 150 % or 125% ,100%,75%,50% & 25%.

C.    Coupled No Load Test : This test is perform to determine the dynamo motor correction factor which is used to load the test motor. In this test motor is running at rated voltage in coupled condition without any load & power, current, speed & torque is measured.

D.    Voltage Characteristic test : This test is optional test for only new development or special application motor. This test is similar to load test but only motor voltage is increases upto 110 % to minimum 75 % or less at constant full load . 

E.     Short Circuit or Lock Rotor Test :  This test is similar test as mention in routine test. This test is performing to determine motor starting current & starting torque.

F.     No Load Characteristic Test :  This test is carried out to determine motor friction & windage loss. In this test motor is run at  no load in decoupled condition & motor voltage is take  up to 110 % to minimum 25 % or less. Motor Voltage, current, power & power factor is recorded.
The voltage between phases, current per phase and absorbed power shall be measured at each load point. The measurement can be done by a power analyzer. The electrical calculation is made to obtain the actual losses.

   Overspeed Test  : The overspeed test is performed to make sure that rotor speed can reach to the 1.2 times of the rated speed in 2 minutes.  A speedometer is used to measure the rotor speed, and no particular noise, excess vibration and abnormal temperature rise shall be observed.

   Bearing Vibration Test at No - Load : Bearing Vibration test is performed when the rotor running at the no-load condition and the velocity vibration amplitude shall be measured in horizontal, vertical and axial directions. The velocity probe or accelerometer is used for measurement. The accelerometer measures vibration acceleration and not velocity, if used, the software converts the acceleration to the velocity by applying the calculation.For measurement of bearing vibration SPM meter (Shock Pluse Modulation ) is used.

Reference Standard : IEC60034-1 / IEC60034-2 (IS15999)