Conversion of Compound Interpole Type Machine

A compound interpole machine used as a motor with line polarity as shown in Fig. 14-2 will have currents in its armature and field windings as indicated. If this motor be disconnected from the power supply, but its speed be maintained by an external driver, the motor will become a self-excited generator in the same manner as that of Fig. 14-1. If then the generator be connected to a load, the load current Ig (together with the shunt-field current) will flow through the armature in a direction opposite to the direction when used as a motor.

If the motor in Fig. 14-2 was connected cumulative compound (in which the series field aided the shunt field), the reversed direction of current in the series field of the generator would weaken the total field and so cause the generator to be differential compound. The current through the interpole winding would also be reversed. However (page 131), when the machine is used in the same direction of rotation and with the same main-pole polarity, the interpole polarity for motor use must be opposite to that

for generator use, and hence no change is necessary in the interpole connections. In the case of Fig. 14-2 the series-field connections must be reversed if the generator is to be cumulative compound. Current is said to enter a motor at the positive terminal and to leave a generator at the positive terminal.

The maximum voltage obtainable by self-excitation of a motor used as a generator is seldom more than 5 per cent greater than the rated voltage of the machine as a motor.

Example 14-1. - A cumulative compound interpole motor is to be operated as a cumulative compound generator in the same direction of rotation, but the terminal which was positive as a motor is to be negative as a generator. Give instructions for accomplishing the change by reversal of remanent magnetism.

Solution. - Let the two diagrams of Fig. 14-3 show the magnetic polarities,current directions, and direction of rotation in the machine used as a motor.

Fig. 14-3. Machine of Example 14-1 Used as a Motor

Let the two diagrams of Fig. 14-4 show the magnetic polarities, current directions, and direction of rotation of the machine after all the changes have been made. The conditions of Fig. 14-4 will be determined as the solution progresses.

The armature-generated emf in the motor is toward the positive terminal (always opposite to the direction of the armature current). With the same direction of rotation as before, the reversal of remanent magnetism as specified in the problem produces the required terminal polarity by reversal of the generated emf. The shunt-field current will hence be reversed, as in Fig. 14-4, and the generator will proceed to build up its voltage from the reversed remanent magnetism. Current in the series field must flow in the direction opposite to that it had for the original magnetic polarity. This it does not do; and so the series field must be reversed, as in Fig. 14-4. Each interpole in a generator must be of the same polarity as the main pole which it precedes in the direction of rotation. Since the current through the interpoles has not been reversed, their polarities are correct as shown, and their coil connections need not be changed. The instructions for converting the motor to a generator are as follows:

1. Reverse the remanent magnetism of the field by separate excitation of the shunt winding only.

Fig. 14-4. Machine of Example 14-1 Used as a Generator

2. Reverse the series-field coil connections.

A self-excited generator cannot reverse its own remanent magnetism because its ability to produce demagnetizing current in its own shunt-field winding is dependent on the presence of some flux. After the interpoles of a direct-current machine have been properly connected in series with the armature, no subsequent conversion will require changing that connection.