Page 415 - Power Electronics Handbook

404 Power semiconductor circuit applications

machine is identical to a conventional motor with slip rings, although since
the efficiency of the rectifier is higher the overall motor efficiency is also
greater. However, there is now random synchronism so that pull-in
characteristics are inferior to those obtained with slip ring motors and slip
frequency relay synchronisation. In the majority of applications this loss of
pull-in torque is not critical, and this scheme finds use in systems such as
reciprocating motor drives, cement mill motors which start unclutched,
and synchronous condensers.

14.3.4.4 Braking
As with d.c. motors, there are three methods for electrically braking a.c.
machines: plugging, dynamic braking and regeneration. During plugging
the direction of rotation of the motor is reversed, which is done by
reversing the direction of the rotating field by interchanging two of the
stator winding connections. An induction and reluctance motor can be
plugged in this manner, but a synchronous motor can only be plugged if it
is fitted with induction windings so that during plugging it behaves as an
induction motor. Since the rotor and stator fields are moving in opposite
directions, large currents are induced in the motor, resulting in heat loss.
The rate of plugging can be varied by controlling the supply voltage or
frequency, so as to change its torque, which can be done by using thyristor
phase-controlled circuits or frequency changers.
In dynamic braking the motor must act as a generator. For an induction
motor to behave as a generator it must be driven above the speed of the
stator field, which is not possible during braking if the normal a.c. supply is
maintained to the stator. If the stator is supplied with d.c. the excitation
necessary to induce currents in the rotor is present and at the same time the
rotor exceeds the stator field speed. The operation is as in Figure 14.16,
where since the stator poles are now stationary the rotor flux is also
stationary. The system no longer behaves as a transformer and load current
is not present in the stator. Therefore all the mechanical input is used in
generating heat in the rotor, no power being fed back to the d.c. supply.
Dynamic braking in a synchronous motor is similar to that used for d.c.
machines. Since the rotor field is independent of the stator, all that is
required is for the stator supply to be disconnected and braking resistors to
be connected across the motor terminals, the rotor field still being
maintained .
To regenerate in a synchronous motor the frequency of the supply must
be reduced gradually so that the motor, now acting as a generator, is
continually in step, the loss in rotor kinetic energy being fed back to the
supply. The motor can also be disconnected from the source and its
generated output, which will be a.c. whose frequency changes with the
motor speed, can be rectified and fed into a d.c. source. An induction
motor will generate so long as the rotor speed exceeds that of the stator
field, provided this is not d.c. The motor inertia is converted partly into
heat in the rotor and is partly fed back to the supply. The larger the
difference in rotor and stator field speeds, the smaller the ratio of
generated output to input power, and the greater the heat loss in the rotor.
Clearly, regeneration into an a.c. source for all types of a.c. motors
requires a controlled-frequency source.