A synchronous motor is an AC motor that operates at synchronous speed, meaning the rotor rotates at the same speed as the rotating magnetic field produced by the stator. Unlike induction motors, the synchronous motor does not rely on slip for operation; therefore, its speed remains constant regardless of load, provided the motor remains synchronized with the supply frequency.
Table of Contents
Basic Principle of Operation
The working principle of a synchronous motor is based on the interaction between two magnetic fields:
- Rotating magnetic field produced by the stator
- Constant magnetic field produced by the rotor
When three-phase AC supply is applied to the stator winding, a rotating magnetic field (RMF) is produced at synchronous speed. The rotor, which carries a DC excitation, produces a stationary magnetic field. When the rotor field locks with the stator rotating field, the rotor begins to rotate at the same speed as the rotating magnetic field.
Synchronous Speed
The speed of a synchronous motor depends on the supply frequency and the number of poles.
\[
N_s = \frac{120f}{P}
\]
- \(N_s\) = synchronous speed (rpm)
- \(f\) = supply frequency (Hz)
- \(P\) = number of poles
Example:
For a 50 Hz supply and 4 poles:
\[
N_s = \frac{120 \times 50}{4} = 1500 \text{ rpm}
\]
The rotor always rotates at this fixed speed.
Construction of a Synchronous Motor
A synchronous motor consists of two main parts:
Stator
- Similar to the stator of an induction motor
- Contains three-phase windings
- Produces the rotating magnetic field
Rotor
The rotor carries DC excitation and produces the rotor magnetic field.
Common rotor types include:
- Salient pole rotor
- Cylindrical rotor
Starting of Synchronous Motor
A synchronous motor is not self-starting because the rotor cannot instantly follow the rotating magnetic field.
Common starting methods include:
Damper Winding Method
Damper bars are embedded in the rotor. At start-up, the motor behaves like an induction motor and accelerates close to synchronous speed.
Auxiliary Motor
An external motor brings the rotor close to synchronous speed before synchronization.
Variable Frequency Drive (VFD)
The supply frequency is gradually increased until the motor reaches synchronous speed.
Equivalent Circuit of Synchronous Motor
The per-phase equivalent circuit is similar to that of a synchronous generator and includes:
- Armature resistance \(R_a\)
- Synchronous reactance \(X_s\)
- Internal generated EMF \(E_f\)
The voltage equation is
\[
V = E_f + I_a(R_a + jX_s)
\]
- \(V\) = terminal voltage
- \(I_a\) = armature current
Power Developed by Synchronous Motor
The electromagnetic power developed is given by
\[
P = \frac{3VE_f}{X_s} \sin \delta
\]
- \(V\) = terminal voltage
- \(E_f\) = excitation voltage
- \(X_s\) = synchronous reactance
- \(\delta\) = torque angle
Maximum power occurs when
\[
\delta = 90^\circ
\]
Power Factor Control
One of the major advantages of synchronous motors is their ability to operate at different power factors depending on excitation.
Under-Excited Motor
- Draws lagging current
- Acts similar to an induction motor
Normal Excitation
- Operates at unity power factor
Over-Excited Motor
- Draws leading current
- Used for power factor correction
When used for power factor improvement, the motor is called a synchronous condenser.
Advantages of Synchronous Motors
- Constant speed operation
- Can operate at leading, lagging, or unity power factor
- Useful for power factor correction
- High efficiency in large ratings
- Suitable for high power applications
Disadvantages
- Not self-starting
- Requires DC excitation system
- More complex construction compared to induction motors
- Higher initial cost
Applications
Synchronous motors are widely used in:
- Power factor correction (synchronous condensers)
- Large compressors
- Rolling mills
- Pumps and blowers
- Industrial drives requiring constant speed