Single Phase Motor : No Load & Block Rotor Test - Electrical4u

Single Phase Motor : No Load & Block Rotor Test

No-load Test

Without connecting any load on the motor shaft, full volt age is applied across the winding terminals. Since output of the motor at no-load is zero, the whole of the input power is wasted as various losses. At no-load, the speed of the rotor is very nearly equal to synchronous speed. The emf induced in the rotor and the rotor current is negligibly small. The rotor can, therefore, be approximately considered as an open circuit. No-load test of an induction motor therefore, similar to no-load test on a transformer.

The losses at no-load are:

• 1²R- loss in the stator winding.

• Core losses in the stator and rotor.

• Friction and windage losses.

No-load current drawn by an induction motor is much higher than that of a transformer and, therefore, cannot be assumed as negligible. From the total input at no-load, the 1²R- loss  in the stator winding can be subtracted to get core loss plus friction and windage losses. These losses at no-load are nearly the same as would occur under full-load condition. This is because core loss depends on applied voltage whereas friction and windage losses depend upon speed of rotation of the rotor. Applied voltage is assumed to be constant and the variation of speed of an induction motor from no-load to full-load is negligibly small.

• The Sum of the Wattmeter reading gives the Non-Load Power input to the motor.

Read Also - Loading Effect Of Voltmeter 

Blocked Rotor Test

• In this test the rotor of the motor is blocked, i.e. the rotor is not allowed to rotate. Low voltage is applied across the stator terminals through a three-phase autotransformer. Voltage is gradually increased to a value so that full rated current flows through the windings. Since the rotor circuit is closed and is not rotating, this test is similar to short-circuit test on a transformer. The voltage needed to circulate full-load current under blocked rotor condition is low. The power input to the stator is mainly wasted as 1² R-loss in the stator and the rotor windings. The core-loss at reduced volt age can he neglected.. 

• The sum of the two wattmeter readings gives the total input power. Since full-load current is allowed to flow through the stator and rotor windings, the input power can be considered approximately equal to full-load I²R- losses. From the input power it is possible to calculate equivalent resistance of the motor referred to the stator terminals. By knowing the value of this resistance, we can calculate the value of 1²R-losses at no-load. As mentioned earlier, if we sub tract 1²R- losses at no-load from the no-load power input of no-load test, we get the constant losses of the induction motor.

Efficiency

The efficiency of an induction motor. Efficiency is defined as the ratio of the output power to the input power of the motor expressed as

η = (Pout / Pin) = Pout / (Pout + P loss).100%

As for a motor, the efficiency indicates how well the motor converts electrical power into mechanical power. The input power of an induction motor is given by

Pin =3VsIs CosΦ

• The losses in induction motors are typically grouped into five significant origins: the stator copper loss Ps, the core loss Pf, the rotor copper loss P,, the windage and friction loss P, and the stray-load loss. The stray-load loss indicates residual losses that arc difficult to determine by a direct measurement or a calculation. The stator copper loss accounts for a large part of the motor loss. Fig. shows the loss distribution rating by rating.

The efficiency of induction motors also dependent on the slip. If all the losses that have little or no relationship to the slip are neglected, then the efficiency can be expressed as a function of the slip.

η = (Pout / Pin) = Pmech / Pag = (Pag - Pr) / Pag = (1 - s)