Synchronous Motors: Applications, Starting Methods & Working Principle

What is a Synchronous Motor?

A synchronous motor is an AC machine whose rotor turns in step with the stator’s rotating magnetic field during steady operation. Mechanical speed is related to electrical frequency through the number of poles.

This operating principle distinguishes a synchronous motor from an induction motor, whose rotor requires slip to produce steady torque.

Electrical motors convert electrical input into mechanical torque and motion. They can be classified by supply, phase count, rotor excitation, commutation method, construction and control method.

Common three-phase AC machines include synchronous motors and induction motors. Balanced currents in spatially displaced stator windings produce an electrical field and, through the winding magnetomotive force, a rotating magnetic field. Its mechanical rotation rate is the synchronous speed.

The rotor can use a DC-excited winding, permanent magnets or magnetic saliency. Electromagnetic torque aligns the rotor field or preferred reluctance axis with the rotating magnetic field. Under stable load, the rotor follows that field with a load angle.

The term “synchronous motor” refers to this equality between rotor mechanical speed and stator-field mechanical speed in synchronism.

On a fixed-frequency supply, a motor with a fixed pole count has one synchronous speed. A variable-frequency drive changes the electrical frequency and therefore provides controlled variable speed. Synchronous speed is:

Where:

• N = synchronous speed (in RPM, or revolutions per minute)
• f = electrical supply frequency (in Hz)
• p = number of magnetic poles

Construction of Synchronous Motor

The stator commonly resembles a three-phase induction-motor stator. Rotor construction defines the synchronous-machine type: wound-field, permanent-magnet or synchronous-reluctance.

The diagram shows a conventional wound-field motor supplied with three-phase AC at the stator and DC field excitation at the rotor. It does not represent permanent-magnet or reluctance rotors, which need no rotor DC supply.

Main Features of Synchronous Motors

1. Line-connected wound-field synchronous motors need a starting method because their field alone produces no average torque at standstill. A converter-fed synchronous motor can start from zero speed under controlled frequency and current.
2. While the rotor remains in synchronism, speed follows electrical frequency and pole count. Increasing load changes torque angle rather than steady speed until the stability or current limit is reached.
3. A wound-field motor can vary stator power factor by changing excitation within its capability limits. An unloaded machine used mainly for electrical power factor control is called a synchronous condenser. This adjustment does not apply in the same way to every permanent-magnet or reluctance motor.

Principle of Operation Synchronous Motor

A wound-field synchronous motor is doubly excited: the stator receives AC and the rotor field winding receives DC. Permanent-magnet motors obtain rotor excitation from magnets, while reluctance motors produce torque from rotor saliency.

Balanced three-phase stator currents produce rotating air-gap flux. A wound-field rotor produces a field fixed relative to the rotor. At 50 Hz, a two-pole field rotates at 3000 rpm; a four-pole field rotates at 1500 rpm. Frequency alone does not determine mechanical speed without pole count.

With a stationary DC-excited rotor on a fixed-frequency supply, successive stator poles produce alternating attractive and repulsive torque as the rotating field passes.

The average torque from this alternating action is zero at standstill, so a conventional line-connected field winding cannot accelerate the rotor by itself. A separate starting mechanism must create average starting torque.

A pony motor or damper cage can bring a line-started rotor near synchronous speed before field application and pull-in. A variable-frequency drive instead raises field speed from zero while controlling torque. After pull-in, synchronising torque maintains the load angle; excessive load can cause pole slipping and loss of synchronism.

Methods of Starting of Synchronous Motor

1. Motor starting with an external prime Mover: A pony motor turns the synchronous machine close to synchronous speed. The starter can be an induction motor or a DC shunt motor. The main stator is synchronised and field excitation is established under the specified sequence, then the pony motor is disconnected or unloaded.
2. Damper winding Conductive bars in the rotor pole faces form a cage. Slip between the cage and rotating stator field induces current and induction torque at start. As speed approaches synchronous speed, slip and cage starting torque fall. Field excitation then pulls the rotor into synchronism. The cage’s induction torque becomes zero at synchronous speed, but the bars can still damp rotor oscillations. During acceleration, the motor operates like a three phase induction motor.

Applications of Synchronous Motors

The applications of synchronous motors include:

1. An unloaded wound-field synchronous machine can operate as a synchronous condenser. Excitation control lets it supply or absorb reactive power for voltage and power-factor control within its capability curve.
2. Synchronous motors serve constant-speed and variable-speed pumps, compressors, mills, conveyors, extruders, servos and high-power process drives. The choice against an induction motor depends on speed, torque, efficiency, excitation, converter, maintenance and lifecycle cost; no universal power range makes one type preferable.