Synchronous motor features, applications and excitation

A synchronous motor is an AC motor whose rotor, excited by DC, locks with the stator rotating field and runs at exactly synchronous speed $N_s = 120f/P$.

Characteristic features

1. Runs only at synchronous speed; speed does not change with load (zero speed regulation) as long as supply frequency is constant.
2. Not self-starting; needs damper winding, pony motor or variable frequency supply to start.
3. Power factor can be controlled by field current: lagging, unity or leading.
4. When over-excited it supplies reactive power, so it can work as a synchronous condenser.
5. If load torque exceeds the pull-out torque, it falls out of step and stops.
6. Needs a DC source for excitation, so it is costlier and more complex than an induction motor.
7. Efficiency is high, especially in large and low-speed ratings.
8. May hunt (oscillate about $N_s$) under sudden load changes; damper windings reduce this.

Applications

• Power factor correction in industries and substations (synchronous condenser).
• Constant speed drives: compressors, large fans, blowers, centrifugal pumps, rolling mills, cement mills.
• Low-speed high-power drives: reciprocating compressors, ball mills.
• Motor-generator sets, frequency changers; small versions (reluctance, hysteresis) in clocks and recorders.
• Voltage regulation at the end of long transmission lines.

Effect of excitation on armature current and pf

At constant load the input $3VI_a\cos\phi$ is fixed, so the active component $I_a\cos\phi$ is constant. Changing the field current changes $E_f$ and only the reactive component of $I_a$.

 Ia (A)
   |\                 /
   | \  lagging      / leading
   |  \             /
   |   \___________/  <- min Ia, upf
   +----------------------- If
    under   normal   over-excited

• Under-excitation ($E_f < V$): resultant $E_r = V - E_f$ makes $I_a$ lag $V$; large current, lagging pf.
• Normal excitation ($E_f$ roughly equal to $V$): $I_a$ in phase with $V$; current minimum, unity pf.
• Over-excitation ($E_f > V$): $I_a$ leads $V$; current rises again, leading pf.

         Ia(lead)
           \      Ia(upf)
            \       |
  -----------\------+------> V
              \     |
            Ia(lag) |
     tip of Ia moves on vertical line

The plot of $I_a$ against $I_f$ is the V-curve; joining the minimum points gives the unity pf compounding curve. Curves for higher load lie above, and the pf curve has an inverted-V shape.