Synchronous Motor: Applications, Starting Methods & Working Principle

What is a Synchronous Motor?

A Synchronous motor is an AC motor in which the Frequency of the supply current and the Rotation of the rotor are Synchronised. That is, the rotor’s Rotational period is equal to that of the rotating machine it is a part of.

An Electro-mechanical device that Transforms Electrical energy into Mechanical energy is an Electrical motor. We have divided it into single phase and three phase motors based on the input type.

Synchronous motors and Induction motors are the two most popular types of three phase motors. An Electrical field is created when Three-phase electric Conductors are arranged in specific geometrical positions (i.e., at a specific angle from one another). The Synchronous speed is the rate of rotation of the rotating magnetic field.

If there is an Electromagnet in this Rotating magnetic field, it Magnetically locks with the field and rotates at the same rate as the rotating field.

Due to the fact that the motor’s rotor’s speed is equal to that of the Rotating magnetic field, this is where the term “synchronous motor” originates.

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 Motor

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. Because their operating speed is synchronised with the supply frequency, they operate as constant speed motors regardless of the 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 doubly excited machines, meaning that they receive two electrical inputs. The stator winding on it is made up of a We supply DC to the rotor winding and three-phase supply to the three-phase stator winding.

Three-phase rotating magnetic flux is produced by a three-phase stator winding carrying three-phase currents. A constant flux is also produced by the rotor carrying the DC supply. From the relationship above, we can see that the three-phase rotating flux rotates at a rate of about 3000 revolutions per minute or 50 revolutions per second when the power frequency is 50 Hz.

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.

But due to the inertia of the rotor, it is unable to rotate in any direction due to that attractive or repulsive forces, and the rotor remains in standstill condition. Hence a synchronous motor is not self-starting.

Here, we employ some mechanical means to accelerate the rotor to speeds very close to synchronous speed by rotating it initially in the same direction as the magnetic field. After reaching synchronous speed, magnetic locking takes place, and the synchronous motor keeps spinning even when external mechanical assistance is removed.

However, the rotor’s inertia prevents it from rotating in any direction as a result of the attractive or repulsive forces, and it remains in a standstill state. Therefore, a synchronous motor cannot start on its own.

Methods of Starting of Synchronous Motor

Motor starting with an external prime Mover: 

Mechanical coupling connects synchronous motors to another motor. Either a DC shunt motor or a three-phase induction motor could be used. Here, initial DC excitation is not used. We then apply the DC excitation as the rotational speed approaches that of its synchronous rotation. When magnetic locking occurs after some time, the external motor’s supply is cut off.

Damper winding: 

The additional winding is positioned on the rotor pole face of the salient pole type synchronous motor in this instance. The required starting torque is generated initially when the rotor is not rotating due to the large relative speed between the damper winding and rotating air gap flux. Emf and torque both decrease as speed gets closer to synchronous speed, and eventually, when magnetic locking happens, torque also goes away entirely. Because of this, the synchronous motor in this instance first operates as a three-phase induction motor using an additional winding before being synchronised with the frequency.

Applications of Synchronous Motor

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.