Chuck Yung
EASA Senior Technical Support Specialist

For the wide variety of DC motor applications, there are those where a straight shunt motor is preferred and others which seem to require the greater starting torque of a series field. Why are there different field designs and are they interchangeable? What about the nameplates marked “stab shunt” or “str shunt?” The purpose of this article is to clear up lingering confusion about the types of fields as well as the benefits of each.

A motor with only a shunt field is called a shunt wound (or straight shunt) motor, with nameplates sometimes labeled as “str shunt.” The shunt motor allows easy speed control without requiring a sophisticated drive. The field power supply could be as basic as a variable AC supply (a vari-AC) rectified through a bridge rectifier. By varying the current supplied to the shunt fields, the strength of the field flux can be varied, providing speed control. Extruders and a multitude of similar applications utilize the simple shunt motor.

This article discusses:

• When torque is needed
• Determining ampere-turns
• Using transformer test

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