The no load test and the blocked rotor test are two main induction motor tests, which are performed on induction motor to know the different losses, power factor and efficiency of the induction motor.
 

No Load Test of Induction Motor

 
no load test of induction motor
 
The no load test of induction motor is performed on induction motor when it is running without load. This test tells us the magnitude of constant losses occurring in the motor.
 
The set up for no load test of induction motor is shown in the figure. The machine is started in the usual way and runs unloaded from normal voltage mains. On the mains side suitable instruments are connected between supply mains and motor terminals to measure power, line current and line voltage.
 
For power and power factor measurement, two single phase watt meters are used. Since the motor is running without load, p.f. of the motor is low less than 0.5, so one of the watt meters will give negative reading. Total power drawn by the motor is the difference of the two wattmeter readings.
 
To understand the concept properly, you should read also the power measurement in three phase circuits by two watt meter method.
 
The signs of readings are taken with various values of applied voltage and then curve is plotted against power and input voltage. From this curve windage and friction losses are determined.
 
Since the motor is not loaded so input power absorbed by the motor is providing losses only. Losses are occurring in iron core of the stator as well as the rotor which are called core losses. A small amount of copper loss is also occurring in stator winding. This can be neglected since the stator current is very small.
 
Therefore, total power consumed = rotor iron loss + stator iron loss + copper loss in stator + friction and windage loss
 
The readings noted at normal voltage, and at rated frequency are considered to find out the fixed core losses.
 

Calculation from No-Load Test of Induction Motor

 
The procedure to find out the separate losses from no-load test of an induction motor is as follows:
 
Total power by two watt meter = Po watts.
 
Copper losses in the stator = 3Io2R1.
 
Where Io is the no-load current measured by an ammeter at normal voltage and rated frequency of supply, R1 is the stator winding resistance and V per phase is the applied voltage.
 
Therefore, total constant losses = (Po – 3Io2R1) watts.
 
no-load test of induction motor image
Now to determine the friction and windage losses the curve drawn between applied voltage and input power is extended until it cuts, the vertical axis.
 
The point where it intersects, is the zero applied voltage. When applied voltage is zero the core losses and stator copper losses are zero.
 
Therefore, power input at no load and zero voltage applied represents the windage and friction losses.
 
Other calculations are as follows:
 
No-load power factor cos φo = Po /3IoV
 
No-load resistance Ro= V/Iocos φo
 
No-load reactance Xo = V/Iosin φo
 

Blocked Rotor Test on Induction Motor

 
Blocked Rotor Test on Three Phase Induction Motor
 
The set up for the blocked rotor test on induction motor is shown in the figure. It is performed by locking the rotor (by keeping the rotor not to rotate).
 
This is carried out to know the copper losses, power factor at short circuit current; total equivalent resistance and reactance.
 
This test is just similar to short circuit test of the transformer.
 
Starting with zero voltages across the stator, the applied voltage is gradually increased in steps until the full load current is circulated. The readings of voltmeter ammeter and watt meters are noted. While performing this test the following points are taken care of:

  • Means of holding tight (not to rotate) the rotor should be of proper strength.
  • The direction of rotation of a rotor should be established prior to start the test and direction of force, which is to keep the rotor blocked (unmoved) should be in opposite direction.
  • As the windings get heated the test should be carried out quickly.
  • The short circuit current should not be more than the full load current.

Calculations from Blocked Rotor Test of Induction Motor

 
Let Psc is the total power measured when Isc/phase is the current circulating and Vsc/phase is the voltage applied.
 
Then equivalent impedance/phase, Z1’ = Vsc/Isc  ohm
 
Power factor, cos φsc = Psc/3VscIsc
 
Let R1’ be the equivalent resistance then
R1’ = Psc/3I2sc ohm
 
Therefore, equivalent reactance,
X1’ = √ [(Z1’)2– (R1’)2].
 
The whole power input to the motor when the rotor is locked is absorbed as full copper losses in the motor as well as minute iron losses. A small voltage only (10 to 15% of normal voltage) is applied to circulate full load current in the motor.
 
Since iron losses depend on supply voltage and very small supply voltage is applied in load test of induction motor are therefore these losses are very small and hence these are neglected.
 
Therefore, Psc = total copper losses in motor.
 
If Isc = full load current of motor, then Psc is total copper loss on full load.
 

Load Test of Induction Motor

 
To determine how speed, efficiency, power factor, stator current, torque and slip of an induction motor vary with load, the load test on an induction motor is performed.
 
The motor is loaded with break pully arrangement and the effect of increasing load on the above quantities observed and a graph is plotted between the load (on X-axis) above quantities (on Y-axis).
 
Thanks for reading about no load test of induction motor.
 

Induction Motors — 5| Objective Type Question Answers

 

#1 Two of the power supply terminals to a 3-phase induction motor get interchanged during reconnection after maintenance of the motor. When put back into service, the motor will

rotate in the reverse direction to that prior to maintenance

#2 In a three-phase induction motor, the relative speed of stator flux with respect to is ………… is zero.

rotor flux

#3 In a 3-phase induction motor reactance under running conditions is less than its standstill value because of decrease in

frequency of rotor E.M.F.

#4 In case of a 3-phase induction motor, an increase in rotor resistance affects the motor performance in the following way

starting current decreases

#5 A squirrel cage induction motor running on no-load is loaded, which of the following statements is incorrect ?

#6 Which of the following statements about 3-phase induction motor is incorrect ?

It is capable of operation under a wide range of power factors both legging and leading.

#7 When a double squirrel cage motor is started, the current induced in the rotor

flows mostly through the upper winding.

#8 Regarding single-phasing of a 3-phase induction motor under running conditions which of the following statement is incorrect ?

It will stop and blow the remaining fuses.

#9 In a Induction motor if ‘P’ is the power delivered to a rotor and ‘s’ is the slip, then the power lost in the rotor as copper loss will be

sP

#10 The frame of an induction motor is made of

closed grained cast iron

#11 Slip rings for induction motors are made of

phosphor bronze

#12 If ‘s’ is the slip and ‘f’ is the supply frequency, the frequency of rotor current is given by

sf

#13 Type of bearing used for 25 H.P. motor is

roller bearing

#14 When an induction motor is switched on the rotor frequency is

same as supply frequency

#15 The shaft, on which the rotor of an induction motor is mounted is made of

mild steel

#16 Which of the following type of bearing is generally used to support the rotor of an induction motor ?

Ball bearing

#17 The stator of 5 H.P. induction motor is provided with

semi-closed slots with parallel teeth

#18 A wound rotor induction motor is usually not selected when

cost  is the main consideration

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