Page 294 - Introduction to Marine Engineering

268 Electrical equipment

current-carrying conductor in a magnetic field produces the motor
effect which turns the rotor. The motor speed builds up to a value just
less than the speed of rotation of the magnetic field.
The motor speed depends upon the e.m.f. induced in the rotor and
this depends upon the difference in speed between the conductors and
the magnetic field. If the load is increased the rotor slows down slightly,
causing an increase in induced e.m.f. and thus a greater torque to deal
with the increased load. The motor is almost constant speed over all
values of load. It will start against about two times full load torque but
draws a starting current of about six times the normal full load current.
The starting current can be reduced by having a double cage
arrangement on the rotor. Two separated cages are provided, one below
the other in the rotor. When starting, the outer high-resistance cage
carries almost all the rotor current. As the motor accelerates the
low-resistance inner winding takes more and more of the current until it
carries the majority.
A number of different fixed speeds are possible by pole changing.
The speed of an induction motor is proportional to frequency divided
by the numbers of pairs of poles. If therefore a switch is provided which
can alter the numbers of pairs of poles, then various fixed speeds are
possible. The number of poles affects the starting characteristics such
that the more poles the less the starting torque to full load torque ratio.
Only the induction type of a.c. motor has been described, since it is
almost exclusively used in maritime work. Synchronous motors are
another type which have been used for electrical propulsion systems but
not auxiliary drives.
A number of different arrangements can be used for starting an
induction motor. These include direct on-line, star delta, auto
transformer and stator resistance. Direct on-line starting is usual where
the distribution system can accept the starting current. Where a slow
moving high inertia load is involved the starting time must be considered
because of the heating effect of the starting current. The star delta
starter connects the stator windings first in star and when running
changes over to delta. The star connection results in about half of the
line voltage being applied to each phase with therefore a reduction in
starting current. The starting torque is also reduced to about one-third
of its direct on line value. A rapid change-over to delta is required at
about 75% of full load speed when the motor will draw about
three-and-a-half times its full load current. The auto transformer starter
is used only for large motors. It uses tappings from a transformer to
provide, for example, 40%, 60% and 75% of normal voltage (Figure
14.12). The motor is started on one of the tappings and then quickly
switched to full voltage at about 75% full speed. The tapping chosen will
depend upon the starting torque required with a 60% tapping giving