Page 292 - Introduction to Marine Engineering

266 Electrical equipment

practice, very small, which makes the shunt motor an ideal choke for
constant-speed variable-load duties.
The series motor has field windings connected in series with the
armature windings (Figure 14,10). With this arrangement an increase in
load will cause a reduction in speed and a fall in back e.m.f. The
increased load current will, however, now increase the magnetic field
and therefore the back e.m.f. The motor will finally stabilise at some
reduced value of speed. The series motor speed therefore changes
considerably with load.
Control of d.c. motors is quite straightforward. The shunt wound
motor has a variable resistance in the field circuit, as shown in Figure
14.9. This permits variation of the current in the field coils and also the
back e.m.f., giving a range of constant speeds. To reverse the motor the
field current supply is reversed, as shown in Figure 14.9.
One method of speed control for a series wound motor has a variable
resistance in parallel with the field coils. Reverse operation is again
achieved by reversing the field current supply as shown in Figure 14.10.
In operation the shunt wound motor runs at constant speed
regardless of load. The series motor runs at a speed determined by the
load, the greater the load the slower the speed. Compounding—the use
of shunt and series field windings—provides a combination of these
characteristics. Starting torque is also important. For a series wound
motor the starting torque is high and it reduces as the load increases.
This makes the series motor useful for winch and crane applications. It
should be noted that a series motor if started on no-load has an infinite
speed. Some small amount of compounding is usual to avoid this
dangerous occurrence. The shunt wound motor is used where constant
speed is required regardless of load; for instance, with fans or pumps.
The starting of a d.c. motor requires a circuit arrangement to limit
armature current. This is achieved by the use of a starter (Figure 14.11).
A number of resistances are provided in the armature and progressively
removed as the motor speeds up and back e.m.f. is developed. An arm,
as part of the armature circuit, moves over resistance contacts such that a
number of resistances are first put into the armature circuit and then

Figure 14,10 Series wound d.c. motor