MM0704, Lesson 3
value and to prevent coil overheating. Because the field is connected across the powerline, the density of the magnetic
field remains constant. Therefore, the torque of a shunt motor must vary with the current in the armature; that is, if the
armature current doubles, the torque also doubles. Since the magnetic field strength is constant, the motor speed will
be constant from no load to full load.
Actually, the motor speed may vary up to 10 percent but this is considered negligible. The shunt-wound motor is a
constant-speed type, but, because of fixed field current, it does not have as high a starting torque as the series motor.
Compound Motors. There are two kinds of compound DC motors, cumulative and differential. A typical compound
motor is in figure 3-9.
Figure 3-9. Compound Motor.
Cumulative. Cumulative compound motors employ both series and shunt windings. The windings are connected so
that the series winding aids the shunt field. Characteristics of this motor lie somewhere between those of a series and
those of a shunt. By properly proportioning the two fields, the speed of the motor can be made relatively constant
under varying load conditions.
In a differential-compound motor
, the fields are arranged
so that the series field opposes the main shunt
field. This weakens the main field and tends to increase the speed of the motor as the load is increased. Differential
compounding is not used much because it produces instability, especially when the series field is very strong.