Page 295 - Introduction to Marine Engineering

Electrical equipment 269

Auto transformer

Motor

Running Starting

Figure 14,12 Squirrel cage induction motor starting

about 70% of full load torque. A smaller percentage tapping will give a
smaller starting torque and vice-versa. The stator resistance starter has a
resistance in the stator circuit when the motor is started. An adjustable
timing device operates to short circuit this resistance when the motor has
reached a particular speed.
Modern electronic techniques enable a.c. induction motors to be used
in speed-control systems. The ship's supply, which may not be as stable
in voltage or frequency as that ashore, is first rectified to provide a d.c,
supply. This is then used as the power supply of an oscillator using
high-power electronic devices. These may be thyristors (for powers up to
1.5 M W or more) or transistors (for powers up to a few tens of kilowatts).
The high-power oscillator output is controlled in frequency and voltage
by a feedback system. The motor speed is varied by changing the
oscillator output frequency. The motor current necessary to obtain the
desired torque (at small angles of slip) is normally obtained by
maintaining the voltage almost proportional to frequency.
Certain protective devices are fitted in the motor circuit to protect
against faults such as single phasing, overload or undervoltage. Single
phasing occurs when one phase in a three-phase circuit becomes open
circuited. The result is excessive currents in ail the windings with, in the
case of a delta connected stator running at full load, one winding taking
three times its normal load current. A machine which is running when
single phasing occurs will continue to run but with an unbalanced
distribution of current. An overload protection device may not trip if the
motor is running at less than full load. One method of single phasing
protection utilises a temperature-sensitive device which isolates the
machine from the supply at some particular winding temperature.
Overload protection devices are also fitted and may be separate or
combined with the single phase protection device. They must have a
time delay fitted so that operation does not occur during the high