A synchronous motor is a type of AC motor that runs at a constant speed — the synchronous speed — regardless of the load applied. Unlike induction motors where speed drops with increasing load, a synchronous motor maintains exact synchronism with the supply frequency until its maximum torque limit is exceeded.
In this article, you will learn the construction, working principle, starting methods, characteristics, and applications of a synchronous motor.
Table of Contents
What is a Synchronous Motor?
A synchronous motor is a doubly excited AC motor that converts electrical energy into mechanical energy at a constant speed called the synchronous speed. It is "doubly excited" because:
- Stator: Supplied with three-phase AC (creates rotating magnetic field)
- Rotor: Supplied with DC (creates constant magnetic poles)
The synchronous speed depends on the supply frequency and number of poles:
Where:
- Ns = Synchronous speed (RPM)
- f = Supply frequency (Hz)
- P = Number of poles
For a 4-pole motor on 50 Hz supply: Ns = (120 × 50)/4 = 1500 RPM. The motor will run at exactly 1500 RPM whether at no load or full load.
Construction
A synchronous motor has two main parts — stator and rotor.
Stator (Armature)
- Carries the three-phase armature winding
- Made of thin laminated silicon steel to reduce eddy current losses
- Connected to three-phase AC supply
- Produces a rotating magnetic field at synchronous speed
Rotor (Field)
- Carries the field winding excited by DC supply
- DC is supplied through slip rings and brushes
- Produces constant magnetic poles (N and S)
Types of Rotor Construction
Working Principle
The working of a synchronous motor is based on the principle of magnetic locking between the stator's rotating field and the rotor's DC field.
Step-by-Step Operation
- Step 1: Three-phase AC supply is given to the stator winding, producing a rotating magnetic field at synchronous speed
- Step 2: DC supply is given to the rotor field winding, creating fixed N and S poles on the rotor
- Step 3: The rotor is brought near synchronous speed by an external starting method
- Step 4: The rotor poles magnetically lock with the stator's rotating field — N pole of rotor locks with S pole of rotating field
- Step 5: Once locked, the rotor rotates at exactly synchronous speed, maintaining a constant angular displacement (load angle) with the rotating field
As load increases, the load angle (δ) increases — the rotor poles lag further behind the stator field. But the speed remains constant. If the load exceeds the pull-out torque, the magnetic lock breaks and the motor stops.
Why is it Not Self-Starting?
When three-phase supply is connected, the stator field starts rotating at synchronous speed instantly (e.g., 1500 RPM for a 4-pole, 50 Hz motor). The rotor, due to its inertia, is still at rest.
At any instant:
- The stator N pole attracts the rotor S pole → rotor tries to move in one direction
- Half a cycle later (0.01 seconds at 50 Hz), the stator field has reversed → now it pushes the rotor in the opposite direction
The alternating attraction and repulsion happens so rapidly that the rotor cannot respond due to its mechanical inertia. The net average torque is zero, and the rotor remains stationary.
This is why a synchronous motor needs an external starting method to bring the rotor close to synchronous speed before the magnetic locking can occur.
Starting Methods
1. Damper Winding (Most Common)
Short-circuited copper bars are embedded in the rotor pole faces. During starting, these bars act like a squirrel cage — the motor starts as an induction motor. Once the rotor reaches about 95% of synchronous speed, DC excitation is applied and the rotor pulls into synchronism.
2. Pony Motor
A small auxiliary motor (usually an induction motor) is coupled to the synchronous motor shaft. It brings the rotor to near-synchronous speed, then DC excitation is applied and the pony motor is disconnected.
3. Variable Frequency Starting (VFD)
The supply frequency is gradually increased from zero. The synchronous speed increases slowly, and the rotor accelerates with it — maintaining synchronism throughout. This is the smoothest starting method but requires a variable frequency drive.
Power Factor Control
One of the most valuable features of a synchronous motor is its ability to operate at any power factor — leading, lagging, or unity — simply by adjusting the DC field excitation.
This behavior is shown by the V curve of synchronous motor. An over-excited synchronous motor running at no load is called a synchronous condenser — used for power factor improvement in industrial plants.
Key Characteristics
- Constant speed: Runs at exactly synchronous speed regardless of load (until pull-out torque is exceeded)
- Not self-starting: Requires external starting method
- Adjustable power factor: Can operate at leading, lagging, or unity PF
- Higher efficiency: More efficient than induction motors of the same rating
- Higher cost: More expensive due to DC excitation system (slip rings, brushes, exciter)
- Requires DC supply: Separate DC source needed for rotor excitation
- Stable operation: Operates stably as long as load angle δ < 90° (for cylindrical rotor)
Applications
As a Motor (Constant Speed Drive)
- Cement mills and ball mills
- Compressors and blowers
- Pumps in water treatment plants
- Paper and textile mills (precision speed)
- Rubber mixing mills
As a Synchronous Condenser (Power Factor Correction)
- Substations for reactive power compensation
- Large industrial plants with many induction motors
- Voltage regulation on long transmission lines
Comparison: When to Use Synchronous vs Induction Motor
For a detailed comparison of when each motor type is appropriate, see our article on induction motor vs synchronous motor.
FAQs
What happens if DC excitation is lost while the motor is running?
The motor loses synchronism and either stops or continues running at sub-synchronous speed as an induction motor (using damper windings). However, it will draw heavy reactive current from the supply, causing overheating. Protective relays typically trip the motor.
Can a synchronous motor run without load?
Yes. At no load, the load angle is nearly zero (rotor poles almost aligned with stator field). If over-excited at no load, it acts as a synchronous condenser, supplying reactive power to the grid.
Why is a synchronous motor more efficient than an induction motor?
Because the rotor field is created by DC excitation (no rotor copper losses from induced currents). In an induction motor, the rotor carries induced current which causes I²R losses. Also, a synchronous motor can operate at unity power factor, minimizing reactive current losses.
What is the typical power range of synchronous motors?
Synchronous motors are economically justified above about 150 kW (200 HP). Below this, induction motors are cheaper and simpler. Large synchronous motors can be rated up to 50 MW or more (for compressors and mills).
What is hunting in a synchronous motor?
Hunting is the oscillation of the rotor about its equilibrium position (load angle) when a sudden load change occurs. The rotor overshoots, then oscillates back and forth before settling. Damper windings help suppress hunting by producing a damping torque that opposes the oscillation.