*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

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 = P_{o} watts.

Copper losses in the stator = 3*I*_{o}^{2}R_{1}.

Where *I*_{o} is the no-load current measured by an ammeter at normal voltage and rated frequency of supply, R_{1} is the stator winding resistance and V per phase is the applied voltage.

Therefore, total constant losses = (P_{o} – 3*I*_{o}^{2}R_{1}) watts.

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 φ_{o} = P_{o} /3*I*_{o}V

No-load resistance R_{o}= V/*I*_{o}cos φ_{o}

No-load reactance Xo = V/*I*_{o}sin φ_{o}

# 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 P_{sc} is the total power measured when *I*_{sc}/phase is the current circulating and V_{sc}/phase is the voltage applied.

Then equivalent impedance/phase, Z_{1}’ = V_{sc}/*I*_{sc} ohm

Power factor, cos φ_{sc} = P_{sc}/3V_{sc}*I*_{sc}

Let R_{1}’ be the equivalent resistance then

R_{1}’ = P_{sc}/3*I*^{2}_{sc} ohm

Therefore, equivalent reactance,

X_{1}’ = √ [(Z_{1}’)^{2}– (R_{1}’)^{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, P

_{sc}= total copper losses in motor.

If

*I*_{sc}= full load current of motor, then P

_{sc}is total copper loss on full load.

#### 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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- What is a variable frequency drive?
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