Synchronous Motor: Definition, Components, and Working Principles

Synchronous motors play a critical role in various industrial and mechanical applications. Unlike other motors, they operate at a constant speed from no load to full load, determined by the synchronous speed (Ns = 120f/P). These motors are unique due to their ability to maintain a fixed rotational speed regardless of load changes and their lack of slip. This article delves into the definition of synchronous motors, their primary components, operational characteristics, and common applications, providing an insightful understanding of their significance in the electrical industry.

What is a Synchronous Motor?

Synchronous Motor: A motor that rotates at a constant speed, from no load to full load, i.e., at synchronous speed, Ns=120f / P, is called a synchronous motor. This type of motor has no slip.

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The rotational speed of this motor does not change with load variation like other motors. The stator winding of this motor is supplied with three-phase power, and it rotates at synchronous speed, creating a rotating magnetic field.

On the other hand, the rotor’s field winding is supplied with power from an exciter through carbon brushes and slip rings to create the required number of stationary poles. The rotating field of the stator magnetically couples with the stationary opposite poles of the rotor, causing it to rotate at synchronous speed. This motor has no slip. It cannot self-start and can be operated at various power factors.

What are the Main Parts of a Synchronous Motor?

• Stator Winding
• Rotor Winding
• Exciter
• Slip Ring
• Carbon Brush
• Damper Winding, etc.

Why is a Synchronous Motor Not Self-Starting?

Initially, when the rotor is stationary, applying three-phase power to the stator creates a forward traveling wave of current. This wave moves at synchronous speed, and the torque generated changes direction according to the polarity of the current wave.

The average value of this torque is zero, meaning that the starting torque of a synchronous motor is zero, which is why it is not self-starting. To make it self-start, the rotor needs to be brought close to the speed of the rotating magnetic field generated in the stator. When DC excitation is provided, and the rotor’s poles align with the rotating magnetic field in the stator, magnetic coupling occurs, and the rotor rotates at synchronous speed.

Starting Methods of a Synchronous Motor

Since the starting torque of a synchronous motor is zero, simply applying power will not cause it to rotate. Therefore, to start the motor, the rotor must be brought close to the speed of the rotating magnetic field in the stator.

• Starting with the help of a DC motor.
• Starting with the help of a self-starting synchronous motor.
• Starting as a synchronous induction motor.
• Starting with the help of a pony motor.
• Starting with the help of an exciter.

Characteristics of a Synchronous Motor

The characteristics of a synchronous motor are:

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• If the speed remains constant, it rotates at synchronous speed; speed changes only by altering the supply frequency.
• It is not self-starting, meaning it cannot start on its own.
• It can be operated at both lagging and leading power factors.
• Regardless of the load factor, the rotational speed of the motor does not change.
• A static field is created in the rotor through excitation or other processes.

What is Excitation?

Excitation means providing stimulation. In a synchronous motor, DC excitation is provided in the field. This voltage is supplied from sources like batteries, DC shunt generators, rectifiers, etc.

Without field excitation, the synchronous motor cannot function. By adjusting the DC excitation using a field rheostat, the motor’s generated voltage can be increased or decreased, allowing the motor to operate at lagging, leading, or unity power factor.

How is a Synchronous Motor Operated at Unity Power Factor?

By adjusting the motor’s field excitation for a given load, it can reach a point where the motor draws the least current from the armature busbar. At this point, there is no angular difference between the armature current and the supply voltage, meaning the motor is operating at unity power factor.

Applications of a Synchronous Motor

• For improving power factor.
• For constant speed applications.

Other practical applications of synchronous motors include:

• Electric clocks
• Rubber mills
• Fans
• Compressors
• Pumps
• Cement factories
• Textile mills

What is Hunting in a Synchronous Motor, and Its Causes, Disadvantages, and Remedies?

Hunting:

When a synchronous motor operates under continuously varying loads or when the supply line frequency changes, the rotor speed fluctuates accordingly. This condition is known as hunting.

Read more about: What is hunting effect

Causes of Hunting:

• If the load on the motor shaft suddenly decreases, increases, or is removed, or if the load continuously fluctuates.
• If the speed of the prime mover changes.
• If the supply frequency fluctuates.
• If the synchronous motor is connected to an excessively long transmission line.

Disadvantages of Hunting:

• There are significant variations in the motor’s current and power consumption.
• The mechanical parts of the motor experience excessive stress and damage, such as bearing wear, shaft or pulley damage, or breakage.
• The motor windings may become dislodged.
• Excessive hunting can cause the motor to shut down.

What is Damper Winding?

The solenoid poles of a synchronous motor have thick copper wires placed transversely in slots, with both ends shorted by copper rings. This is called damper winding. Its two main functions are:

• First, it helps start the synchronous motor as an induction motor.
• Second, it helps reduce the effects of hunting in the synchronous motor.