figure 5. Torque versus speed curves for the four types of dc motors. Because of the large amount of armature current, the series field is much stronger than the similar shunt field. The armature's 92 ieee power & energy magazine counter EMF is developed at a significantly lower speed than with the other motors. This large speed droop is a char- acteristic of the series motor, making it unsuitable for many applications. On the other hand, the series motor draws less current to produce a given amount of torque and produces maximum torque at zero speed. This makes it the ideal motor for traction applications. The fourth motor is known as the differential compound motor, and it operates quite differently than the others. With an increase in load torque and the resulting initial increase in armature current, the series field opposes and weakens the shunt field. This means that the counter EMF is reduced without (much of) a speed drop. The counter EMF is actually decreased by a reduction in field strength rather than by a drop in speed. This lower counter EMF allows an increase in armature current, bringing the motor torque to a point where it matches the load torque. Differential compound motors are seldom used. They tend to be july/august 2019http://WWW.ELECTRICAL-STUDIES.COM