Why is Synchronous Motor not self starting?

 Published On: 13, Jul, 2023
 Posted by: Mechtex

What is a synchronous motor?

A synchronous motor is an electric motor that operates at a constant speed determined by the frequency of the power supply. It is called "synchronous" because the speed of the motor's rotor is synchronized with the frequency of the alternating current (AC) power source. Synchronous motors are widely used in various industrial applications due to their efficiency and precise control capabilities. 

Synchronous motors are commonly used in applications where constant speed is required, such as in power plants, industrial processes, and certain electrical equipment. They are also used in applications where precise speed control is essential, like high-efficiency synchronous motors used in clocks and timing devices. Mechtex is a leading synchronous motor manufacturer known for high-quality motors globally.

How does a synchronous motor work?

Here's a simplified explanation of how a permanent magnet synchronous motor works:

1. Stator: The stator is the stationary part of the motor and consists of a set of electromagnets, typically evenly spaced around the inner circumference. These electromagnets are energized with AC voltage, creating a rotating magnetic field.

2. Rotor: The rotor is the rotating part of the motor and contains a set of permanent magnets or electromagnets. In the case of permanent magnets, they establish a fixed magnetic field. For electromagnets, the rotor field can be excited using direct current (DC) to create a magnetic field.

3. Synchronisation: The synchronous motor's rotor must be designed to rotate at the same speed as the rotating magnetic field of the stator. The speed of the rotating magnetic field (synchronous speed) is determined by the frequency of the AC supply and the number of pole pairs on the stator.

4. Magnetic interaction: When the stator's rotating magnetic field interacts with the fixed magnetic field of the rotor, they try to align with each other due to the magnetic attraction and repulsion forces. This alignment exerts torque on the rotor, causing it to rotate.

5. Constant speed: As long as the synchronous motor is connected to a stable AC power supply and the load on the motor is within its capabilities, the rotor will maintain synchronization with the rotating magnetic field of the stator. Thus, the motor will operate at a constant speed, the synchronous speed, regardless of the load.

6. Starting: Synchronous motors need some method to bring them up to synchronous speed when they are initially turned on since they cannot start on their own. This can be done using auxiliary devices like a small induction motor (synchronous motor starting as an induction motor) or external means like DC excitation to establish the rotor's magnetic field before synchronization.

Why is a synchronous motor not self-starting?

Unlike induction motors, which are self-starting due to the rotating magnetic field produced by the stator, synchronous motors require external means to start. Synchronous motors consist of a stator with three-phase windings and a rotor that is excited by a DC supply. The three-phase windings in the stator create a rotating magnetic field, while the DC supply provides a constant flux. However, the torque produced on the rotor of a synchronous motor is pulsating and not unidirectional. 

This pulsating torque is a result of the varying polarity of the rotor and stator poles. At any given instant, the rotor and stator poles can have the same polarity (N-N or S-S), causing a repulsive force on the rotor. In the next instant, the polarity can be N-S, resulting in an attractive force. However, due to the inertia of the rotor, it remains in a standstill condition and is unable to rotate in any direction. The direction of the instantaneous torque on the rotor reverses after a half cycle, further preventing the motor from starting on its own.

Methods for Starting Synchronous Motors

One key limitation of synchronous motors is that they are not self-starting. Unlike induction motors, synchronous motors require external means to bring the rotor to near synchronous speed before they can be synchronized to the power supply. In this article, we will explore the reasons behind this limitation and examine the different methods used to start synchronous motors. 
To overcome the self-starting limitation of synchronous motors, various methods can be employed.

Let's explore some of the most commonly used methods:

Using Pony Motors:

The pony motor method involves using a separate motor, known as a pony motor, to drive the main synchronous motor up to near synchronous speed. The pony motor is initially used to bring the rotor to a speed close to the synchronous speed. Once the rotor is near synchronous speed, the main motor is synchronized to the power supply. The DC excitation is then switched on, causing the rotor field and stator fields to "lock in" and establish magnetic locking. At this point, the pony motor can be disconnected, and the main motor continues to run at synchronous speed.

Utilizing Damper Windings:

Another method to start a synchronous motor is by utilizing damper windings. When the three-phase supply is switched on, the motor initially runs up as a normal squirrel cage induction motor. The damper windings, also known as amortisseur windings, consist of short conductors embedded in the rotor. These windings provide starting torque to the motor and help it reach synchronous speed. Once the motor reaches synchronous speed, the DC excitation is applied to the rotor, and the motor operates as a synchronous motor.

Coupling with a Small DC Machine:

In some cases, a small DC machine can be coupled with the synchronous motor to enable starting. The DC machine is initially made to act as a DC motor, driving the synchronous motor. This helps bring the synchronous motor to near synchronous speed. Once the rotor is close to synchronous speed, the DC excitation is switched on, and the synchronous motor continues to run at synchronous speed.

Using External Means to Rotate the Rotor:

Another approach to starting a synchronous motor is by using external means to rotate the rotor at a speed near or equal to the synchronous speed. This can be achieved by using an external device such as a diesel engine to drive the rotor. Once the rotor is rotating at the desired speed, the DC excitation is applied, and the motor enters synchronous operation.

Starting Procedure for Synchronous Motors

To start a synchronous motor, a specific procedure needs to be followed. Here is a general step-by-step guide:

1. Provide a three-phase AC supply to the stator windings of the synchronous motor. This creates a rotating magnetic field rotating at synchronous speed (Ns).
2. Use an external means, such as a diesel engine, to drive the rotor at a speed very close to or equal to the synchronous speed. This ensures that the rotor is rotating in the same direction as the rotating magnetic field.
3. Switch on the DC supply to the rotor, which creates rotor poles. At a particular instant, the stator and rotor, unlike poles, will face each other, and their magnetic axes will be near each other. This results in a force of attraction between the poles, pulling them into magnetic locking conditions.
4. Once magnetic locking is established, the rotor and stator poles continue to occupy the same relative positions. The rotor experiences a unidirectional torque in the direction of the rotating magnetic field, causing it to rotate at synchronous speed.
5. Once synchronism is achieved, the external device used to rotate the rotor can be removed. The rotor will continue to run at the same speed as the rotating magnetic field, thanks to magnetic locking.
It is important to maintain the DC excitation of the rotor from a DC power supply as long as the motor is operating to ensure the magnetic locking is maintained.

Synchronous motors offer several advantages in terms of efficiency and control. However, their self-starting limitation necessitates the use of external means to bring the rotor to near synchronous speed before synchronization with the power supply can occur. Methods such as using pony motors, damper windings, coupling with a small DC machine, or employing external means to rotate the rotor are employed to start synchronous motors. By following the proper starting procedure, synchronous motors can be successfully brought to synchronous speed and operate efficiently in various industrial applications.

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