7.7. Types of D.C. Motors
There are three main types of motors characterised by the connection of the field winding in relation to the armature. These are:
1. Shunt wound motor or the shunt motor, in which the field winding is connected in parallel with the armature.
2.Series motor, in which the armature and field windings arc connected in series.
3.Compound motor, which has two field windings, one of which is connected in parallel with the armature and the other in series with it.
Shunt wound motor. Fig. 57 shows the connections of a shunt motor. From these connections it may be observed at once that the field current is constant, since it is connected directly to the supply which is assumed to be at constant voltage. Hence the flux is approximately constant and, since also the back e.m.f. is almost constant under normal conditions the speed is approximately constant. This is not strictly true, but nevertheless, it is usual for all practical purposes to regard the shunt motor as a constant speed machine. It is, therefore, employed in practice for drives, the speeds of which are required to be independent of the loads. The speed can, of course, be varied when necessary and this is done by the inclusion of a variable resistor in series with the field winding, as shown in Fig. 57. Fig. 57. Connections of a shunt molar.
Fig. 58. Connections of 11 series motor.
Series motor. Fig. 58 shows the connections for the series motor. The current passing through the field winding is the same as that in the armature, since the armature current increases as the mechanical load on the shaft increases, so also does the field current. The resultant increase in magnetic flux causes a reduction in the speed, as can be observed from a consideration of the formula:
This is a useful property for many drives in which it is desirable that a heavy increase in the load should automatically bring about a compensating reduction in speed. As with the shunt motor, the speed may also be varied independently of the load by the inclusion of a variable resistor in the field circuit. In this case, however, it is connected in parallel with the series winding as shown in Fig. 58, and is called a divertor compound motor.
Refer Fig. 59. The compound motor has a shunt field winding in addition to the series winding so that the number of magnetic lines of force produced by each of its poles is the resultant of the flux produced by the shunt coil and that due to the series coil. The flux so produced depends not only on the current and number of turns of each coil, but also on the winding direction of the shunt coil ill relation to that of the series coil. When the two fluxes assist each other the machine is a cumulative compound motor, while if they oppose each other, it is said to be a differential compound motor.