Page 150 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

3 A synchronous motor is inherently not self-starting.
4 The changes in applied voltage do not affect synchronous motor torque as much as they affect the induction
motor torque. The breakdown torque of a synchronous motor varies approximately as the first power of applied
voltage whereas that of an induction motor depends on the square of this voltage.
5 A D.C. excitation is required by synchronous motor but not by induction motor.
6 Synchronous motors are usually costlier and complicated than induction motors, but they are particularly
attractive for low-speed drives (below 300 rpm.) because their power factor can always be adjusted to 1.0 and
their efficiency is high. However, induction motors are excellent for speeds above 600 rpm.
7 Synchronous motors can be run at ultra-low speeds by using high power electronic converters which generate
very low frequencies. Such motors of 10 MW range are used for driving crushers, rotary kilns and variable-
speed ball mills etc.
Application
Synchronous motors find extensive application for the following classes of service:
1 Power factor correction
2 Constant-speed, constant-load drives
3 Voltage regulation

a Power factor correction
Overexcited synchronous motors having leading power factor are widely used for improving power factor of those
power systems which employ a large number of induction motors and other devices having lagging power factor
such as welders and fluorescent lights etc.
b Constant-speed applications
Because of their high efficiency and high-speed, synchronous motors (above 600 rpm.) are well-suited for loads
where constant speed is required such as centrifugal pumps, belt-driven reciprocating compressors, blowers, line
shafts, rubber and paper mills etc. Low-speed synchronous motors (below 600 rpm.) are used for drives such as
centrifugal and screw-type pumps, ball and tube mills, vacuum pumps, chippers and metal rolling mills etc.
c Voltage regulation
The voltage at the end of a long transmission line varies greatly especially when large inductive loads are present.
When an inductive load is disconnected suddenly, voltage tends to rise considerably above its normal value
because of the line capacitance. By installing a synchronous motor with a field regulator (for varying its excitation),
this voltage rise can be controlled. When line voltage decreases due to inductive load, motor excitation is
increased, thereby raising its power factor which compensates for the line drop. If, on the other hand, line voltage
rises due to line capacitive effect, motor excitation is decreased, thereby making its power factor lagging which
helps to maintain the line voltage at its normal value.
Stopping of synchronous motors

Owing to the inertia of the rotor and its load, a large synchronous motor may take several hours to stop after being
disconnected from the line. To reduce the tome following braking methods are used:
1 Maintain full DC excitation with the armature in short circuit.

2 Maintain full DC excitation with the armature connected to three external resistors.
3 Apply mechanical braking.
In methods 1 & 2, the motor slows down because it functions as generator, dissipating its energy in the resistive
elements of the circuit. Mechanical brake is usually applied only after the motor has reached half speed or less.
A lower speed prevents undue wear of the brake shoes.

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CITS : Power - Electrician & Wireman - Lesson 76-85 CITS : Power - Electrician & Wireman - Lesson 76-85