Which of the following losses, in a synchronous motor, does not vary with load?

Which of the following losses, in a synchronous motor, does not vary with load?

Right Answer is:

Windage losses


Windage losses in a synchronous motor do not vary with load.

Losses in Synchronous machine

The losses in synchronous machines are as follows:

(a) Fixed losses:-  Core loss, bearing, friction, and windage loss, brush friction loss. These losses are obtained from the no-load test. Core loss occurs because of the eddy currents and hysteresis caused by the main magnetic field. It is the difference between the power required to drive the synchronous machine with or without field excitation. This is taken at the rated voltage and speed.

Windage losses in the synchronous motor

  • These losses occur during the circulation of moving air around inside the machine.
  • The rotor “whips” air around and the air resistance cause losses.
  • Sometimes the rotor fan losses are included in windage and sometimes they are calculated separately; however, the physics of both are the same.
  • Windage losses vary with the airspeed relative to the motor surfaces squared. In low-speed machines, these are often neglected.
  • But in large high-speed machines (like flywheels and some 400Hz generators) windage losses are a dominant loss.
  • To reduce windage losses and improve cooling, large power generators are sometimes sealed and cooled with hydrogen rather than air.
  • Windage losses in a synchronous motor, does not vary with load.

(b) I2R loss in armature winding, stray loss in iron and conductors:- Armature I2R loss is current2 × dc resistance R corrected at 70°C and not the effective resistance. This can be calculated when I and R are measured. Stray load losses are caused due to changes in the flux distribution due to load. This can be found by a short-circuit test. The short-circuit current is adjusted to the value of load current at which the loss is to be determined, then the stray load loss = mechanical power input friction and windage loss I2R loss. The synchronous machine is run at the rated speed. Ventilation loss is the power required to circulate cooling air in addition to the windage loss.

(c) Excitation circuit losses:- These include field copper loss, rheostat loss. brush contact loss, exciter losses. Field copper loss = I2fRf, where If is the field current and Rf is the resistance of field winding. Rheostat loss is I2fRr, where Rr, is the resistance of the rheostat. Brush contact loss is taken as slip ring current. Exciter loss is considered when it is driven by a Synchronous machine and is part of the whole machine. Otherwise, it is changed to the plant and not to the alternator.

Note:- If the machine is not excited (zero field current) and running on no-load, the core loss will be zero and only windage and friction loss takes place. If the machine is excited (field current is supplied), both windage and friction and core losses take place. Thus, the core loss can be computed by taking the difference of the power consumed by machines with excitation and without excitation. It is common practice to consider core loss under load and no-load conditions the same.

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