Introduction to Synchronous Motors: Working, Types and Applications
A complete introduction to synchronous motors with definitions, diagrams, working principles, benefits, and real-world uses.
What is a Synchronous Motor?
A synchronous motor is an AC motor that operates at a constant speed determined by the frequency of the supply and the number of poles.
Unlike the induction motor, the rotor of a synchronous motor rotates in synchronisation with the stator’s magnetic field. In simple words, there is no relative speed between the rotor and the stator’s magnetic field.
This motor works on the interaction of the stator’s rotating magnetic field and the rotor’s magnetic field. The stator receives the three-phase AC supply, while the rotor is excited with DC supply to produce a magnetic field. Once the rotor synchronises with the stator magnetic field, the motor operates at constant speed.
The key features of synchronous motors include constant speed, high efficiency, high duty cycle and power factor correction, which makes them suitable for various applications.
Construction of Synchronous Motor
The construction of synchronous motor consists of two main components: stator and rotor. Stator is the stationary part, while the rotor rotates inside it.
These synchronous motor parts work together and produce the electromagnetic force required for operation. Let’s dive deep into each component:
A synchronous motor consists of the following parts :
- Stator
It is the stationary outer part of the synchronous motor. It consists of a laminated steel core mounted inside a rigid frame. It reduces eddy currents and improves operation. Slots are provided on the inner surface of the core to accommodate three-phase armature windings.
When a three-phase AC supply is applied to the armature windings, it produces a rotating magnetic field (RMF). The rotor locks with the rotating magnetic field and moves at a constant speed.
- Rotor
It is the rotating part of the motor placed inside the stator. It consists of field windings which produce a magnetic field when DC power is supplied.
\Synchronous motors either have a salient-pole rotor or a non-salient-pole rotor, depending upon the motor’s speed and application requirements. Salient pole rotors have projected poles and are mainly used for low and medium speed applications. Non-salient pole rotors have a cylindrical surface and are used for high-speed applications.
- Excitation System
The rotor windings require an excitation system to interact with the rotating magnetic field of the stator. DC excitation is supplied through the slips, brushes or brushless excitation system.
- Damper Windings
Synchronous motors are not self-starting. Therefore, they require an external device to start. Damper windings are available at the rotor poles. These windings generate the starting torque and help the motor to achieve the synchronous speed.
Working Principle of Synchronous Motor
The working of a synchronous motor is based on the interaction of the rotating magnetic field of the stator and the magnetic field of the rotor.
When AC power is supplied to the stator, it produces a magnetic field. When the rotor is excited with DC supply, it produces its own magnetic field. When these two magnetic fields interact with each other, they produce an electromagnetic force, which causes the AC motor to rotate at a constant speed. It is also known as synchronous speed.
The synchronous speed can be calculated using the formula Ns = 120f/P, where Ns is the synchronous speed, f is the frequency supplied, and P is the number of poles.
Check out a Video by "Lesic" for an Inside view of the Working of a Synchronous Motor
A synchronous motor is not self-starting because the rotor cannot lock itself with the rotating magnetic field of the stator. Therefore, external devices such as damper windings or starting motors are used to create a magnetic field in the rotor.
When the rotor reaches the speed of the stator, its magnetic poles lock the rotor with the stator’s magnetic field, and both rotate at synchronous speed. Therefore, we call it a “synchronous motor”
When the mechanical load changes, the rotor shifts its position slightly towards the rotating magnetic field to develop a torque. However, its speed remains constant. This characteristic allows the synchronous motor to maintain constant speed and makes it ideal for various industrial applications.
Applications of Synchronous Motor
Synchronous motors are used in various applications due to their constant speed, high duty cycle and power factor correction. The top applications of synchronous motors are:
- Compressors
Synchronous motors are used in air compressors and refrigeration systems where constant speed is required.
- Pumps
Synchronous motors are used in various industrial pumps which require stable speed to maintain flow.
- Conveyors
Synchronous motors are used in conveyor systems where constant speed and continuous operation are required for material movement.
- Industrial Fans & Blowers
Synchronous motors are used in ventilation systems and HVAC where constant speed is essential to maintain flow.
- Power Factor Correction
An overexcited synchronous motor is used to improve the power factor of an industrial electrical system.
Mechtex MTR7a is a 48V 1-phase synchronous motor. It offers operating voltage options of 12V, 24V, 48V, 110V, and 230V with 100% duty cycle.
Conclusion
A synchronous motor is an AC motor that operates at constant speed with the frequency of the supply. It consists of two main parts: stator and rotor.
It works when the rotating magnetic field of the stator and the magnetic field of the rotor interact and rotate at constant speed.
Therefore, this AC motor is used in various industrial applications such as compressors, pumps, conveyors, fans, blowers and power factor correction.
Frequently Asked Questions
Q1. What is a 3-Phase Synchronous Motor?
Ans - A 3-phase synchronous motor is an AC motor powered by a three-phase supply and operates at constant speed. Its stator produces a rotating magnetic field, while the rotor is excited with a DC supply and rotates at the same speed.
Q2. What are the two main types of synchronous motors?
Ans - The two main types of synchronous motors are salient-pole synchronous motors and non-salient pole synchronous motors.
The salient-pole synchronous motors have projected rotor poles and are used for low and medium-speed applications. The non-salient pole synchronous motors have a cylindrical surface and are used for high-speed applications.
Q3. Is a synchronous motor AC or DC?
Ans - A synchronous motor is an AC motor because its stator operates on AC power. However, its rotor requires a DC supply for excitation.
Q4. What is the difference between a synchronous motor and an induction motor?
Ans - The main difference between the synchronous motor and the induction motor is constant speed. The synchronous motor operates at synchronous/constant speed with no slip, while the induction motor operates at a slightly lower speed with slip.
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