Click on Download Button to Download 22523 Industrial AC Machines Unit 4 Notes PDF
Synchronous Motors
1. Introduction:
A synchronous motor is an AC motor in which the rotation of
the shaft is synchronized with the frequency of the AC supply. This means the
rotation period is precisely equal to an integral number of AC cycles.
2. Construction:
- Stator: It's the stationary part of
the motor that contains three-phase windings placed in slots.
- Rotor: The rotating part can be of
two types: salient pole and non-salient (cylindrical) pole.
- Field
Windings:
Positioned on the rotor to produce the necessary magnetic field when
excited by a DC supply.
- Slip
Rings: Used to
provide DC excitation to the rotor field windings.
3. Working Principle:
Synchronous motors operate on the principle of magnetic
locking. When the rotor is rotated at a certain speed, it gets locked with the
rotating magnetic field produced by the three-phase supply of the stator, and
both rotate at the same synchronous speed.
4. Characteristics:
- Torque-Speed
Characteristic:
A synchronous motor can run at only synchronous speed. Below this speed, it
does not develop any torque.
- V-Curves: They depict the relationship
between field current and armature current for various power factors.
5. Starting Methods:
Synchronous motors cannot start on their own. Common methods
include:
- Using
a small auxiliary motor
- Using
a damper winding (like in squirrel cage induction motors)
- Starting
as an induction motor and then providing DC excitation to pull into
synchronism
6. Advantages:
- Power
factor can be adjusted over a wide range by varying its excitation, making
it suitable for power factor correction.
- It
operates at a constant speed irrespective of the load.
- Suitable
for driving loads at a constant speed like alternators and compressors.
7. Disadvantages:
- Requires
a separate DC source for its excitation.
- More
complex starting procedure compared to induction motors.
- Not
self-starting.
8. Applications:
- In
power factor correction (as synchronous condensers)
- In
places where constant speed is required, regardless of load conditions,
like large compressors, pumps, and fans.
- In
precision tools and instrumentation applications.
9. Maintenance:
Regular inspections should include checking rotor and stator
windings, ensuring bearings are lubricated, and examining slip rings and
brushes for wear and tear.
10. Conclusion:
Synchronous motors offer the advantage of constant speed
operation and power factor control, making them invaluable in specific
industrial scenarios. However, their requirement for external excitation and
the complexities in starting make them less preferable for general-purpose
applications.
This is a basic overview of synchronous motors. Each section
can be elaborated upon further for deeper understanding.