Page 710 - Basic Electrical Engineering

The blocked-rotor test is performed by blocking the rotor so that it is not
allowed to rotate. Low voltage is applied to the rotor through a three-phase
auto-transformer (Variac). The Output voltage of the variac is adjusted so that

full-load-rated current flows through the windings. The core loss which is
proportional to this low input voltage is small. There is no friction and

windage loss as the rotor is not allowed to rotate. The sum of the wattmeter
2
readings can approximately be taken as equal to full-load I R loss in the
windings.
Thus, we observe that the no-load test and the blocked-rotor test together

provide us the account for losses that would take place when the motor is
fully loaded with full voltage applied across its terminals. By knowing the
losses, we will be able to calculate the efficiency of the motor.

In these two tests, we have created a loading condition that would happen
when the motor is actually loaded. That is why this method of finding out the

efficiency without actually applying load on the motor shaft is called an
indirect method.

8.17 APPLICATIONS OF INDUCTION MOTORS

Around 90 per cent of the electrical motors used in industry and domestic
appliances are either three-phase induction motors or single-phase induction

motors. This is because induction motors are rugged in construction requiring
hardly any maintenance, that they are comparatively cheap, and require

supply only to the stator. No supply is required to be given to the rotor. The
rotor gets excited by virtue of electromagnetic induction. Further, there is no

requirement of brush, slip rings, or commutator. However, slip-ring-type
induction motors where extra resistance is added to the rotor circuit are used

in applications where high starting torque is required.
Three-phase induction motors are used as drive motors in pumps, lifts,
cranes, hoists, lifts, compressors, large capacity exhaust fans, driving lathe

machines, crushers, in oil extracting mills, textile and paper mills, etc.

8.18 SOLVED NUMERICAL PROBLEMS