Page 291 - Introduction to Marine Engineering

Electrical equipment 265
Direct current motors

When a current is supplied to a single coil of wire in a magnetic field a
force is created which rotates the coil. This is a similar situation to the
generation of current by a coil moving in a magnetic field. In fact
generators and motors are almost interchangeable, depending upon
which two of magnetic field, current and motion are provided.
Additional coils of wire and more magnetic fields produce a more
efficient motor. Interpoles are fitted to reduce sparking but now have
opposite polarity to the next main pole in the direction of rotation,
When rotating the armature acts as a generator and produces current in
the reverse direction to the supply. This is known as back e.m.f.
(electromotive force) and causes a voltage drop across the motor. This
back e.m.f. controls the power used by the motor but is not present as
the motor is started. As a result, to avoid high starting currents special
control circuits or starters are used.
The behaviour of the d.c. motor on load is influenced by the voltage
drop across the armature, the magnetic field produced between the
poles and the load or torque on the motor. Some of these factors are
interdependent. For example, the voltage drop across the armature
depends upon the back e.m.f. which depends upon the speed of the
motor and the strength of the magnetic field. Shunt, series and
compound windings are used to obtain different motor characteristics
by varying the above factors.
The shunt wound motor has field windings connected in parallel with
the armature windings (Figure 14.9). Thus when the motor is operating
with a fixed load at constant speed all other factors are constant. An
increase in load will cause a drop in speed and therefore a reduction in
back e.m.f. A greater current will then flow in the armature windings
and the motor power consumption will rise: the magnetic field will be
unaffected since it is connected in parallel. Speed reduction is, in

Reversing
switch

Armature

Figure 14.9 Shunt wound d.c. motor