Working Principle of a Synchronous Motor

Edwiin
05/26/2025

Structure and Excitation of Synchronous Motors
The synchronous motor consists of two primary components: the stator (stationary part) and the rotor (rotating part). The stator is energized by a three-phase AC supply, while the rotor is excited by a DC supply.
Excitation Principle:
Excitation refers to the process of inducing magnetic fields in both the stator and rotor, transforming them into electromagnets. This magnetic coupling is essential for converting electrical energy into mechanical rotation.

Magnetic Field Generation in Synchronous Motors
The three-phase AC supply induces alternating north and south poles in the stator. As the supply is sinusoidal, its wave polarity (positive/negative) reverses every half-cycle, causing the stator's north and south poles to alternate. This generates a rotating magnetic field in the stator.
The rotor's magnetic field is established by a DC supply, which fixes the polarity and creates a stationary magnetic field—meaning its north and south poles remain constant.
The rotational speed of the stator's magnetic field is called the synchronous speed, determined by the supply frequency and the motor's number of poles.

Magnetic Pole Interaction in Synchronous Motors
When opposite poles of the stator and rotor align, an attractive force arises between them, generating counterclockwise torque. Torque, as the rotational equivalent of force, drives the rotor to follow the stator's magnetic poles.
Following each half-cycle, the stator's pole polarity reverses. However, rotor inertia—its tendency to resist changes in motion—maintains its position. When like poles (north-north or south-south) align, a repulsive force creates clockwise torque.
To visualize this, consider a 2-pole motor: in the figure below, opposite stator-rotor poles (N-S or S-N) induce attractive forces, as shown.

After the half cycle, the poles on the stator reverse. The same pole of the stator and rotor face each other, and the force of repulsion develops between them.

The non-unidirectional torque pulsates the rotor only in one place and because of this reason the synchronous motor is not self-starting.

Starting Mechanism of Synchronous Motors
To initiate operation, the rotor is first spun by an external drive, aligning its polarity with the stator's rotating magnetic field. As the stator and rotor poles interlock, a unidirectional torque is generated, pulling the rotor to rotate at the synchronous speed of the stator's field.
Once synchronized, the motor runs at a constant speed equal to the synchronous speed, which is fixed by the supply frequency and number of poles.
Edwiin

Generator Protection – Types of Faults & Protection Devices
Generator Protection – Types of Faults & Protection Devices
Common Generator Faults and Protection SystemsClassification of Generator FaultsGenerator faults are primarily categorized into internal and external types:Internal Faults: Arise from issues within generator components.External Faults: Stem from abnormal operating conditions or external network issues.Faults in prime movers (e.g., diesel engines, turbines) are mechanical in nature and defined during equipment design, though they must integrate with generator protections for tripping purposes.Typ
Edwiin
06/05/2025
Short Circuit Ratio of a Synchronous Machine
Short Circuit Ratio of a Synchronous Machine
The Short Circuit Ratio (SCR) of a Synchronous MachineThe Short Circuit Ratio (SCR) of a synchronous machine is defined as the ratio of the field current needed to generate rated voltage under open-circuit conditions to the field current required to sustain rated armature current during a short-circuit condition. For a three-phase synchronous machine, the SCR can be derived from its Open-Circuit Characteristic (O.C.C) at rated speed and Short-Circuit Characteristic (S.C.C), as illustrated in the
Edwiin
06/04/2025
Equivalent Circuit of a Transformer
Equivalent Circuit of a Transformer
The equivalent circuit diagram of any device can be extremely useful for predicting how the device will behave under different operating conditions. It is essentially a circuit - based depiction of the equations that describe the device's performance.The simplified equivalent circuit of a transformer is constructed by representing all of the transformer's parameters on either the secondary side or the primary side. The equivalent circuit diagram of the transformer is presented below:Let the equi
Edwiin
06/03/2025
Transformer On Load Condition
Transformer On Load Condition
Transformer Operation Under Load ConditionsWhen a transformer is under load, its secondary winding connects to a load, which can be resistive, inductive, or capacitive. A current I2 flows through the secondary winding, with its magnitude determined by the terminal voltageV2and load impedance. The phase angle between the secondary current and voltage depends on the load characteristics.Explanation of Transformer Load OperationThe operational behavior of a transformer under load is detailed as fol
Edwiin
06/03/2025
Inquiry
Download
IEE-Business is dedicated to serving the personnel in the global power industry.
Join IEE-Business, not only can you discover power equipment and power knowledge, but also canhnd like - minded friends!