Electrical Engineering ⇒ Topic : Field control method

Field control method. Field control is the most common method and forms one of the outstanding advantages of shunt motors. The method is, of course, also applicable to compound motors. Adjustment of field current and hence the flux and speed by adjustment of the shunt field circuit resistance or with a solid-control when the field is separately excited is accomplished simply, inexpensively, and without much change in motor losses.The speed is inversely proportional to the field current

The lowest speed obtainable is that corresponding to maximum field current; the highest speed is limited electrically, by the effects of armature reaction under weakfield conditions in causing motor instability and poor commutation.Since voltage across the motor remains constant, it continues to deliver constant output. This characteristic makes this method suitable for fixed output loads. The performance curve of a D.C. motor with voltage and field control is shown in Fig. (a)

                                                                       fig (a)