SYNCHRONOUS MOTORS

SYNCHRONOUS  MOTORS

Synchronous motors are synchronous machines used to convert electrical

power to mechanical power. This subject explores the basic operation of

synchronous motors and relates their behavior to that of synchronous generators.

Motor Starting by Reducing Electrical Frequency

If the stator magnetic fields in a synchronous motor rotate at a low enough speed,

there will be no problem for the rotor to accelerate and to lock in with the stator

magnetic field. The speed of the stator magnetic fields can then be increased to

operating speed by gradually increasing f_e up to its normal 50- or 6O-Hz value.

This approach to starting synchronous motors makes a lot of sense, but it

does have one big problem: Where does the variable electrical frequency come

from? Regular power systems are very carefully regulated at 50 or 60 Hz, so until

recently any variable-frequency voltage source had to come from a dedicated

generator. Such a situation was obviously impractical except for very unusual

circumstances.

Today, things are different. With the development of such modern solid-state variable frequency drive packages, it is perfectly possible to continuously control the electrical frequency applied to the motor all the way from a fraction of a hertz up to and above full rated frequency. If such a variable-frequency drive unit is included in a motor-control circuit to achieve speed control, then starting the synchronous motor

is very easy- simply adjust the frequency to a very low value for starting, and then

raise it up to the desired operating frequency for normal running.

Motor Starting with an External Prime Mover

The second approach to starting a synchronous motor is to attach an external starting

motor to it and bring the synchronous machine up to full speed with the external

motor. then the synchronous machine can be paralleled with its power system

as a generator, and the starting motor can be detached from the shaft of the

machine. Once paralleling is completed, the synchronous motor can

be loaded down in an ordinary fashion.

This whole procedure is not as preposterous as it sounds, since many synchronous

motors are parts of motor-generator sets, and the synchronous machine

in the motor-generat or set may be started with the other machine serving as the

starting motor. Also, the starting motor only needs to overcome the inertia of the

synchronous machine without a load- no load is attached until the motor is paralleled to the power system. Since only the motor's inertia must be overcome, the

starting motor can have a much smaller rating than the synchronous motor it

starts.

Since most large synchronous motors have brushless excitation systems

mounted on their shafts, it is often possible to use these exciters as starting motors.

For many medium-size to large synchronous motors, an external starting

motor or starting by using the exciter may be the only possible solution, because

the power systems they are tied to may not be able to handle the starting currents

needed to use the amortisseur winding approach described next.

Motor Starting by Using Amortisseur Windings

By far the most popular way to start a synchronous motor is to employ amortisseur

or damper windings. Amortisseur windings are special bars laid into notches

carved in the face of a synchronous motor's rotor and then shorted out on each end

by a large shorting ring.