1. Torque vs. Armature current

The armature current depends directly upon the load connected to the mo­tor. When a load is applied to the motor it will tend to slow down. This slight reduction in speed reduces the counter-emf. The reduction in counter emf will produce a greater armature current. The increase in armature current results in an increase in torque to take care of the added load. Thus it can be seen that a load applied to the motor results in an increase in torque to meet the new load requirements,

Compound motor. The construction of the compound motor is identical with the compound generator. The current in the armature must also pass through the series-field winding. The series-field winding may be connected cumulatively where the flux due to the series field will aid the shunt-field flux, the series-field winding may also be connected differentially so that the series-field flux will oppose the shunt-field flux.

The torque developed by a cumulative compound motor will be larger than the torque developed by a shunt motor if the flux produced by the shunt-field of each machine is the same and the armature current is the same. Since the torque is proportional to the flux and the armature current, the flux in the compound motor is greater than in the shunt because of the additional flux pro­duced by the series winding.

Graphically, the effect of the additional flux is shown in Fig.2.

As the load on the motor increases, the armature current increases. The torque will increase in each machine because of the increase in armature cur­rent. However, the increase in the compound motor is greater than the shunt because the total flux increases and does not remain constant as it does in the shunt motor. The speed of the compound motor is also affected by the increase in flux. It is evident that the speed of the compound motor is less than that of the shunt motor, all other factors being equal. Figure 3.compares the speed of the shunt motor and cumulatively compound motor.

The differentially compound motor has characteristics opposite to those of the cumulatively compound motor as compared with the shunt motor. The de­crease in the flux produces a reduction in the torque. Instead a reduction in the speed of the motor, speed of the differential compound motor will be higher than the shunt motor. In fact, the speed will rise as load is applied to the motors. Fig.4. graphically illustrates the torque and speed characteristics of the motors.

2. Comparing torque of cumulative compound 3. Compare speed of cumulative compound

and shunt motors and shunt motors

Care should be exercised when starting a differentially compound motor. The large inrush current in the series field during the starting period may produce flux greater than that produced by the shunt field This will cause the motor to start up in the wrong direction. It is best to short the series field when starting to prevent such events.

Series motor. In a series motor the flux depends upon the current in the series field, which is the same current that flows through the armature. At light loads the armature current is small; therefore the flux and torque are small. An increase in the load increases both the armature current and the flux, pro­ducing a much greater increase in the torque.

As long as the magnetic material is not saturated, the torque is proportion­al to the square of the armature current. Thus, if the armature current is dou­bled, the torque will increase four times. Fig. 5 shows the torque of the vari­ous types of motors.