Synchronous Motors: Applications, Starting Methods & Working Principle

What is a Synchronous Motor?

A synchronous motor, or synchronous electric motor, is an AC motor in which the rotor’s rotation period matches the frequency of the supply current, perfectly aligning with the machine’s rotating field.

Let’s take a step back for a moment and define what an electrical motor is.

Electrical motors are an electro-mechanical device that converts electrical energy to mechanical energy. Based on the type of input we have classified it into single phase and 3 phase motors.

The most common type of 3 phase motors is synchronous motors and induction motors. When three-phase electric conductors are placed in certain geometrical positions (i.e. in a certain angle from one another) – an electrical field is generated. The rotating magnetic field rotates at a certain speed known as the synchronous speed.

If an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates at the same speed as the rotating field.

The term “synchronous motor” derives from the rotor’s speed matching the speed of the rotating magnetic field.

It is a fixed speed motor because it has only one speed, which is synchronous speed. This speed is synchronised with the supply frequency. The synchronous speed is given by:

Where:

• N= The Synchronous Speed (in RPM – i.e. Rotations Per Minute)
• f = The Supply Frequency (in Hz)
• p = The number of Poles

Construction of Synchronous Motor

Usually, its construction is almost similar to that of a 3 phase induction motor, except the fact that here we supply DC to the rotor, the reason of which we shall explain later.

Now, let us first go through the basic construction of this type of motor. From the above picture, it is clear that how do we design this type of machine. We apply three phase supply to the stator and DC supply to the rotor.

Main Features of Synchronous Motors

1. Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.
2. The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition
3. This motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement.

Principle of Operation Synchronous Motor

Synchronous motors are a doubly excited machine, i.e., two electrical inputs are provided to it. Its stator winding which consists of a We provide three-phase supply to three-phase stator winding, and DC to the rotor winding.

The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. Considering the 50 Hz power frequency, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolutions in 1 min or 50 revolutions in 1 sec.

At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the very next instant it will be N-S causing attractive force.

However, due to the rotor’s inertia and the conflicting forces acting on it, it cannot start on its own and remains stationary, highlighting why a synchronous motor is not self-starting.

We use a mechanical method to initially spin the rotor in the magnetic field’s direction, bringing it close to synchronous speed. Once it reaches this speed, magnetic locking keeps the motor spinning, even without external assistance.

Methods of Starting of Synchronous Motor

1. Motor starting with an external prime Mover: Synchronous motors are mechanically coupled with another motor. It could be either 3 phase induction motor or a DC shunt motor. Here, we do not apply DC excitation initially. It rotates at speed very close to its synchronous speed, and then we give the DC excitation. After some time when magnetic locking takes place supply to the external motor is cut off.
2. Damper winding In this case, the synchronous motor is of salient pole type, the additional winding is placed in the rotor pole face. Initially, when the rotor is not rotating, the relative speed between damper winding and rotating air gap flux is large and an emf is induced in it which produces the required starting torque. As speed approaches synchronous speed, emf and torque are reduced and finally when magnetic locking takes place; torque also reduces to zero. Hence, in this case, the synchronous motor first runs as three phase induction motor using additional winding and finally it is synchronized with the frequency.

Applications of Synchronous Motors

The applications of synchronous motors include:

1. A synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any electrical power factor, it is used in power systems in situations where static capacitors are expensive.
2. Synchronous motor finds applications where operating speed is less (around 500 rpm) and high power is required. For power requirements from 35 kW to 2500 KW, the size, weight and cost of the corresponding three-phase induction motor are very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc.