When the rotor of the synchronous motor rotates an EMF is induced in its stator winding due to flux produced by the rotating rotor. This induced EMF is alternating and is given by,
Eb/ph = 4.44 KcKdφfTph
This EMF is produced due to the stator (armature conductors) cutting the rotor flux. This induced EMF always opposes the supply voltage according to the Lenz’s law hence called back EMF. In above expression, all other quantities are constant except flux φ.
Therefore, Eb α φ
But the rotor flux φ depends on the rotor excitation. Hence the back EMF Eb is proportional to or dependent on the rotor (field excitation).
Effect of Excitation on Synchronous Motor
If the excitation of a synchronous motor is changed, keeping the load constant, the motor power factor will get automatically adjusted to keep its active component constant under all excitation conditions.
The active component of the armature current drawn by the motor is constant because load, losses and applied voltage to the motor are constant.
Depending on the level of excitation, the synchronous motor is capable of operating in one of the following four conditions:
- Normally Excited
- Under Excited
- Over Excited
- Critically Excited
In this case, induced back EMF Eb is equal to the applied voltage V.
Under Excited Synchronous Motor
If the field excitation is such that back EMF Eb is less than the applied voltage V, then the motor is said to be under-excited. An under excited synchronous motor has a lagging power factor.
Over Excited Synchronous Motor
If DC field excitation of a synchronous motor is such that back EMF Eb is greater than applied voltage V, then the motor is said to be over excited. An over excited synchronous motor draws leading current.
An over excited synchronous motor running at no load is known as the synchronous capacitor or synchronous condenser. Synchronous capacitors are always totally enclosed. The shaft does not extend beyond the case of the motor. They are used for correcting the power factor of a lagging load such as transformers and induction motors in an installation.
Power factor correction means raising the power factor of the load from its low value to higher value. Some industrial loads run at very low power factors. It is advantageous to raise this power factor to unity or near about.
Any increase in power factor, increases supply capacity, efficiency and improves the operating characteristics of the system. Synchronous capacitors cancel the lagging kVARs of the installation with their leading kVARs. They are usually connected in parallel with the incoming power lines to the plant.
Critically excitation is defined as the excitation for which the power factor of the motor is unity. In this condition, with the change in excitation the power factor changes.
V Curves of Synchronous Motor
When the excitation of a three-phase synchronous motor taking a constant power, is varied, it changes the operating power factor of the motor.
For a constant input power and terminal voltage only increase in power factor, causes a decrease in armature current and vice versa.
The armature current will be minimum at unity power factor and increases when the power factor decreases on either side (lagging or leading).
Hence the variation in excitation (field current) causes the variation in armature current.
If we plot a curve between field current on X-axis and armature current on Y-axis, the curve so obtained is called V curves of synchronous motor.
The V curves at different power inputs are shown in Figure. With the increase in load, the V curves get shifted upwards as shown in Figure.
Thanks for reading about over excited synchronous motor and under excited synchronous motor.
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