Synchronous motor:
A synchronous motor is a type of AC motor whose rotor rotates at the same speed as the
rotating magnetic field. The stator’s magnetic field revolves at a speed that depends on the
supply frequency known as synchronous speed. Hence the name synchronous motor. The
rotor of the synchronous motor is synchronized with the frequency of the supplied current.
Construction of Synchronous Motors:
A Synchronous motor has two main parts
Stator
Rotor
Stator
The stator is the stationary part of the motor. Just like an induction motor, the stator core is
made of thin laminated sheets of steel or cast iron of good magnetic quality to reduce
hysteresis and Eddy current loss. The core has axial slots for holding the three-phase
alternating stator field winding called armature winding.The stator’s armature winding is
supplied with 3-phase power through its input terminal. It is responsible for generating the
rotating magnetic field (RMF).
Rotor
The rotor is the rotating part of a synchronous motor. It has a cylindrical shape and holds the
field winding, which is responsible for generating the magnetic field or poles. The rotor is
energized using slip rings and a brush assembly, typically with a DC source. Usually, a small
DC generator connected to its shaft is used for excitation.
Working Principle of Synchronous Motor:
Synchronous motor works on the principle of magnetic locking between the stator RMF
(rotating magnetic field) and the rotor magnetic field. As we know, opposite poles attract
each other, therefore the RMF poles attract the opposite rotor poles generating a rotating
motion.
A synchronous motor is a doubly excited machine i.e. it requires AC and DC supply for both
parts stator as well as rotor to achieve synchronism. A three-phase AC is supplied to the
stator’s windings to generate RMF. The stator is designed to have the same number of poles
as the rotor. These poles rotate at the speed that is in sync with the input frequency f is called
synchronous speed. It is given by
NS = 120f / p
A DC supply is provided to the rotor’s windings to generate a fixed magnetic field. As the
DC source supplies constant current, the rotor’s magnetic field does not vary. Magnetic poles
are generated at the opposite ends of the rotor. The rotor’s poles interact with the RMF of the
stator and rotate at the same speed as it attains the synchronous speed.If the rotor rotates at
the same speed as the stator RMF, there is no load torque. The rotor and stator poles align
with each other. If a mechanical load is applied, the rotor starts oscillating about its new
equilibrium position, this phenomenon is known as ‘hunting‘. The rotor lags a few degrees
behind the stator RMF and starts developing torque. As the load is increased the angle
between them is increased until the rotor field lags by 90° behind RMF. At this point, the
motor provides the maximum available torque called breakdown torque. If the load exceeds
this limit, the motor stalls.
Power Flow in Synchronous Motor:
Equivalent Circuit:
Star or Delta connection
Equivalent circuit simplified:
Measuring parameter:
1. Current and voltage measurements:We used a multimeter to measure the current and
voltage of the motor. This will help determine the motor's efficiency and power factor.
2. Power measurements : We used a wattmeter to measure the active power (kW) and
reactive power (kVAR) consumed by the motor.
3. Torque measurements: Used a torque meter to measure the torque produced by the
motor. This will help determine the motor's mechanical power output.
4. Speed measurements: We used a tachometer to measure the speed of the motor's rotor.
This will help determine the motor's rotational speed.
5. Efficiency measurements:To Calculate the motor's efficiency by dividing the output
power (kW) by the input power (kW).
6. Power factor measurements: To Calculate the motor's power factor by dividing the
reactive power (kVAR) by the active power (kW).
7. Current factor measurements:To Calculate the motor's current factor by dividing the
rotor current (A) by the rotor current at full load (A).
Effect of field current change:
GE Motor name plate details:
Name of the experiment: Determination of V curve of a Synchronous Motor.
Name of the experiment: Alternator Synchronization with infinite bus.
Name of the experiment: No load and loading characteristics of three phase alternator.
Name of the experiment: observation of under frequency over frequency protection.
Name of the experiment: observation of under/ over voltage protection.
V-Curves and Inverted V-Curves:
Applications of Synchronous Motors:
1. 1.Since a synchronous motor behaves like a variable inductor or a variable capacitor,
it can be used to improve the voltage regulation of transmission lines.
2. Synchronous motor having no load connected to its shaft is used for power factor
improvement.
3. Synchronous motor finds application where operating speed is less and high power is
required.
4. Some industrial applications of synchronous motors are such as high power and high
speed compressors, blowers, mainline traction, induced and forced draft fans, servo
drives, etc.
Conclusion: A synchronous motor is an electromechanical device that operates at
synchronous speed, which is directly proportional to the frequency of the supply voltage and
the number of poles in the motor. These motors are characterized by their ability to maintain
a constant speed under varying load conditions, making them highly efficient for applications
that require precise speed control, such as in industrial processes, pumps, compressors, and
power generation.
