Blocked Rotor & No Load Test on Induction Motor

The no-load test and 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 on Three Phase Induction Motor

no load test on three phase induction motor
The no-load test of an 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 of Losses from No-Load Test

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 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.

Induction Motor Objective Type Questions


1. In a squirrel cage induction motor, torque with autostarter is ....... times the torque with direct-switching. Where K is the transformation ratio of the autotransformer.

2. If stator voltage of a squirrel cage induction motor is reduced to 50 per cent of its rated value, torque developed is reduced by ....... per cent of its full-load value.

3. For the purpose of starting an induction motor, a Y-Δ switch is equivalent to an auto-starter of ratio.......per cent.

4. A double squirrel-cage motor (DSCM) scores over SCIM in the matter of

5. In a DSCM, outer cage is made of high resistance metal bars primarily for the purpose of increasing its

6. A SCIM with 36-slot stator has two separate windings : one with 3 coil groups/ phase/pole and the other with 2 coil groups/phase/pole. The obtainable two motor speeds would be in the ratio of

7. A 6-pole 3-φ induction motor taking 25 kW from a 50-Hz supply is cumulatively-cascaded to a 4-pole motor. Neglecting all losses, speed of the 4-pole motor would be ....... r.p.m.

8. Two separate induction motors, having 6 poles and 5 poles respectively and their cascade combination from 60 Hz, 3-phase supply can give the following synchronous speeds in rpm

9. When a stationary 3-phase induction motor is switched on with one phase disconnected

10. If single-phasing of a 3-phase induction motor occurs under running conditions, it

11. One of the characteristics of a single- phase motor is that it

12. After the starting winding of a single- phase induction motor is disconnected from supply, it continues to run only on ............winding.

13. The direction of rotation of a single-phase motor can be reversed by

14. If a single-phase induction motor runs slower than normal, the more likely defect is

15. The capacitor in a capacitor-start induction- run ac motor is connected in series with ...... winding.

16. The starting torque of a capacitor-start induction-run motor is directly related to the angle α between its two winding currents by the relation

17. In a two-value capacitor motor, the capacitor used for running purposes is a/an

18. If the centrifugal switch of a two-value capacitor motor using two capacitors fails to open, then

19. Each of the following statements regarding a shaded-pole motor is true except

20. Compensating winding is employed in an ac series motor in order to

21. A universal motor is one which

22. In a single-phase series motor the main purpose of inductively-wound compensating winding is to reduce the

23. A repulsion motor is equipped with

24. A repulsion-start induction-run single- phase motor runs as an induction motor only when

25. If a dc series motor is operated on ac supply, it will

26. An outstanding feature of a universal motor is its

27. The direction of rotation of a hysteresis motor is determined by the

28. Speed of the universal motor is

29. In the shaded pole squirrel cage induction motor the flux in the shaded part always

30 . Which of the following motor is an interesting example of beneficially utilizing a phenomenon that is often considered undesirable ?


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One thought on “Blocked Rotor & No Load Test on Induction Motor

  • October 16, 2018 at 2:07 pm

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