To explain how rheostats work we need to briefly explain Ohm’s Law. Those with electrical knowledge can skip this article, but for those who find understanding it difficult the following may be useful.

Ohm’s law states that voltage equals current multiplied by resistance and is usually written as

V = IR or I = V/R

Taking the 50 ohm rheostat and a conventional motor with a resistance of 6 ohms the total circuit resistance is 56 ohms. When supplied with 12 volts the current flowing in the circuit is 12/56 = 0.21 amps. This would normally cause the locomotive to remain stationary or to move slowly depending on the tractive effort developed.

Compared with an average of 6 ohms for a conventional motor most coreless ones have a resistance of between 18 and 24 ohms. The ESCAP motor used in the RG7 is typical with an armature resistance of 21 ohms. In this instance the total circuit resistance would be 71 ohms and when supplied with 12 volts the current flowing in the circuit would be 12/71 = 0.17 amps. As these high efficiency motors need a current of only a few milliamps to develop sufficient tractive effort to move a locomotive the resistance of 50 ohms is insufficient to provide acceptable low speed control.

By rearranging the connections to form a potentiometer, as in Figure 1, the voltage applied to the motor terminals when the controller is in the minimum position is zero and hence the locomotive will not move until the controller handle is slightly advanced.


Figure 1. Voltage dropped in rheostat.