Synchronous motor features and lagging/leading operation

A synchronous motor runs at constant synchronous speed $N_s = 120f/P$ and, unlike an induction motor, its power factor can be set by the DC field current.

Characteristic features

1. Speed is constant at $N_s$ from no load to full load; it depends only on supply frequency and number of poles.
2. Not self-starting; started by damper winding, pony motor or variable frequency.
3. Operates at lagging, unity or leading pf depending on excitation.
4. Over-excited motor on no load acts as a synchronous condenser.
5. Breaks down (pulls out of step) if load exceeds the maximum (pull-out) torque.
6. Requires a separate DC excitation source.
7. Prone to hunting on sudden load changes.
8. High efficiency, especially for large, low-speed drives.

How it draws lagging or leading current

For a motor (with $R_a$ neglected): $\vec V = \vec E_f + jX_s\vec I_a$, so

$$\vec I_a = \frac{\vec V - \vec E_f}{jX_s} = \frac{\vec E_r}{jX_s}$$

The current lags the resultant voltage $E_r$ by $90^\circ$. Keeping the load (and so $I_a\cos\phi$ and $E_f\sin\delta$) constant, only $E_f$ is changed by the field current.

1. Under-excitation, $E_f < V$ - lagging current

   V ---------------------->
    \  Er (from Ef tip to V tip)
     \ delta
      '-----> Ef (short)
   Ia = Er/jXs lags V   -> lagging pf

The resultant $E_r$ is nearly in phase with $V$, so $I_a$ (90° behind $E_r$) lags $V$. The motor takes magnetising (lagging) reactive power from the supply to make up for its weak field.

2. Normal excitation - unity pf

$E_f$ is such that $E_r$ is $90^\circ$ ahead of $V$; then $I_a$ is in phase with $V$, and current is minimum.

3. Over-excitation, $E_f > V$ - leading current

   V ---------------------->
    \          Er points above V
     \ delta
      '-----------------> Ef (long)
   Ia = Er/jXs leads V  -> leading pf

$E_r$ swings ahead of $V$ by more than $90^\circ$, so $I_a$ leads $V$. The excess field means the motor supplies lagging reactive power to the network, behaving like a capacitor.

This control is the basis of the V-curves and of using synchronous motors for power factor correction.