Delving into the Principle Operation of Synchronous Motors

August 14, 2023 Jessica

By delving into the principle operation of synchronous motors, we aim to provide a comprehensive understanding of their construction, working principles, and applications. Whether you are an engineer, student, or enthusiast interested in electric machines, this article will serve as a valuable resource for comprehending the intricacies of synchronous motors and their importance in electrical engineering. So, let’s explore synchronous motors and unravel the principles behind their efficient and synchronized operation.

Characteristics of synchronous motor

A synchronous motor is a constant-speed AC motor. It takes AC power by the stator from the mains supply for its working, in addition, it utilizes a small amount of DC power which is fed to its rotor, we need two kinds of power sources that is one is AC and another is DC as shown in the Fig below.

Some important characteristics of synchronous motor

It runs at a constant speed (i.e.) at a synchronous speed determined by the number of poles and the frequency or not at all. The only way to change its speed is to vary the supply frequency.
It can operate under various power factors, such as lagging, leading, and unity. Hence it can be used for power factor correction and torque to drive loads.
The main drawback of a synchronous motor is that it is not self-starting. Hence it has to be run up to synchronous speed by some external means before it can be connected to the supply. This drawback is ignored because of its constant and variable power factor working condition.

Construction

A synchronous motor is identical in construction to an AC generator or alternator. The synchronous motor has a stator and rotor. A three-phase winding is placed in the stator, drawing current from an AC source and producing a rotating magnetic field.

The construction of synchronous motors is designed to facilitate the synchronization between the stator’s rotating magnetic field and the rotor’s magnetic field. This synchronization enables the engine to operate at a fixed, synchronous speed.

Let’s explore the key components of synchronous motors:

Stator

The stator is the stationary part of the synchronous motor and provides the primary magnetic field. It consists of the following components:

Stator Core: The stator core is typically thin silicon steel laminated to minimize eddy current losses. The laminated construction reduces energy losses due to magnetic hysteresis and eddy currents.
Stator Windings: The stator winding is a set of three-phase windings placed in slots around the stator core. These windings are distributed evenly at an angle of 120 degrees electrically apart to create a rotating magnetic field when energized with AC.
Slot: Slots are cavities or openings in the stator core where the stator windings are placed. The shape and number of slots influence the motor’s performance and efficiency.

Rotor

The rotor consists of electromagnetic poles. The construction of the rotor can be salient pole type or non-salient pole type. The rotor pole draws the current from a DC source and gives a DC electromagnetic field for locking with the rotating magnetic field.

There are two main types of rotors used in synchronous motors:

Salient Pole Rotor: In this type of rotor, the pole pieces (known as salient poles) project outward from the rotor surface. The pole pieces are made of laminated steel to reduce eddy current losses. The rotor windings are wound around these poles, and they can be excited with a DC to create the magnetic field.
Non-Salient Pole (Cylindrical) Rotor: This type of rotor has a cylindrical shape without protruding poles. The rotor is either made of a permanent magnet or has a DC-excited winding embedded within it. Permanent magnet rotors have interests that generate a constant magnetic field, while DC-excited rotors require a DC to create the magnetic field.

Excitation System

An excitation system is used to supply DC to the rotor winding for synchronous motors with a wound rotor. This excitation system ensures that the rotor’s magnetic field remains synchronized with the stator’s rotating magnetic field. The excitation can be provided by an external DC power source or a separate exciter mounted on the motor shaft.

Bearings

Synchronous motors have bearings that support the rotor shaft and allow it to rotate smoothly within the stator. Depending on the motor’s size and application, various approaches can be used, such as ball or sleeve bearings.

Frame and Enclosure

The motor’s frame is a rigid structure that houses all the internal components and provides mechanical support. The frame is usually steel and has openings for ventilation to dissipate heat generated during motor operation. The enclosure protects the motor from environmental factors, dust, and moisture, and it also contributes to the overall safety of the engine.

Principle of Operation

A rotating magnetic field is produced if a three-phase AC supply is given to a three-phase stator winding of the motor. This can be considered a north and South Pole rotating in space at synchronous speed, as shown in Fig. The two stator poles are assumed and marked as N_s and S_S. They are supposed to turn in a clockwise direction at synchronous speed.

Synchronous Speed N_s = 120f/P

Where f = frequency of the supply

P = No of poles in the stator.

DC excitation to the rotor forms the rotor poles N_r and S_r. At an instant for the rotor position indicated in the figure, the stator poles are at points X Ns and Y S_S.

Now there is a reaction between the stator and rotor poles. Like poles N_S and N_r of stator and rotor repel each other. Because the stator field moves in a clockwise direction, N_r and S_r of the rotor tend to rotate in the anticlockwise direction.

Half a cycle later, the position of the stator poles is interchanged due to RF. This reverse of stator polarity is very rapid. N_s is at point Y and S_S is at point X. Now N_s attracts S_r, and S_s attracts N_r. The rotor now tends to rotate clockwise; thus, the rotor is faced with a torque changing in quick succession in every half cycle. First, the rotor moves in an anticlockwise direction and then in a clockwise direction at every half of a process. Due to inertia, the rotor cannot move in any direction and becomes stationary. Therefore the synchronous motor has no starting torque and cannot start independently.

Synchronous motors operate based on a rotating magnetic field created by the stator windings.
AC in stator windings generates the rotating magnetic field.
The rotor turns at the same speed as the rotating magnetic field.
A consistent gap is maintained between the stator and rotor.
Rotor locks in step with the rotating field.
The interaction of stator and rotor fields generates torque.
Synchronous motors have zero slips for precise speed control.
They are used when constant speed is crucial, like power generation.
High efficiency due to minimal slip losses.
A permanent magnet or field-wound type.
I am adjusting excitation for power factor correction.
Field-wound type allows controllable parameters.
Lower starting torque compared to induction motors.
Excitation changes alter the power factor and phase angle.
Maintaining synchronization is essential.
They can operate as generators when mechanically driven faster.
Synchronous speed depends on frequency and poles.
Stable operation under varying loads.
More complex due to synchronization and excitation.
Modern control systems enhance adaptability

Conclusion

By understanding the convoluted principles, we gain insight into how synchronous motors harness the power of electromagnetic fields and synchronous speed to deliver consistent performance. Their ability to maintain synchronous operation, even under diverse loads, requires them in various industrial applications.