A 3 phase, salient pole synchronous motor is fed from an infinite bus and is running at no load. Now if the field current of the motor is reduced to zero then the

A 3 phase, salient pole synchronous motor is fed from an infinite bus and is running at no load. Now if the field current of the motor is reduced to zero then the

Right Answer is:

Motor will run at synchronous speed


An infinite bus is a system that imposes its own voltage and frequency upon any apparatus connected to its terminals. Once connected to a large system (infinite bus), a synchronous generator becomes part of a network comprising hundreds of other generators that deliver power to thousands of loads. It is impossible, therefore, to specify the nature of the load (large or small, resistive or capacitive) connected to the terminals of this particular generator.

  • For constant power output, a synchronous motor can be made to operate at either a lagging power factor, unity power factor or leading power factor by varying the field excitation.
  • At normal excitation, the motor draws minimum armature current at unity power factor.
  • At under excitation the armature current increases and the power factor becomes lagging.
  • When the motor is overexcited it draws a current at the leading power factor.
  • An overexcited synchronous motor acts as a power factor correction device and is also known as a synchronous condenser. The variation of armature current and power factor as a function of field current is plotted to give a better insight.

When filed current is reduced to zero, reluctance torque comes into play. If the field winding of an unloaded salient pole synchronous motor is open-circuited the motor field current becomes zero and the synchronous motor runs as a reluctance motor.

Reluctance Torque:

  • Reluctance torque is the torque generated because the motor is moving to a position where the reluctance seen by the armature flux is declining.
  • Reluctance exists only when there is an unsymmetrical air gap.
  • Therefore it exists only for Salient pole synchronous machines and does not exist for the cylindrical machine.
  • Reluctance torque comes into play when any winding is disconnected i.e. filed winding or armature winding in the running conditions
  • Reluctance torque also produces reluctance power which makes the machine more stable.
  • Variable reluctance motor behaves as similar to silent pole synchronous motor unexcited.
  • And after the loss of residual flux, it will run as a reluctance motor due to the generation of reluctance torque.
  • The machine will continue to run at synchronous speed due to reluctance torque (i.e. even when field winding or armature windings are disconnected).

In a salient pole synchronous motor, the power flow is given by,

$P = frac{{EV}}{{{X_d}}}sin delta + frac{{{V^2}}}{2}left[ {frac{1}{{{X_q}}} – frac{1}{{{X_d}}}} right]sin 2delta$


When the field winding gets open Eb = 0

$P = frac{{{V^2}}}{2}left[ {frac{1}{{{X_q}}} – frac{1}{{{X_d}}}} right]sin 2delta$


From the above equation, it is clear that even when the field winding is open the power in the synchronous motor is generated due to which the motor continuously runs at a synchronous speed.

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