Overload Protection of D-C Motors

Author: E.E. Kimberly

Overload protection of a motor may be obtained by the use of simple fuses in series with the power line. However, fuses are generally unsatisfactory because they may open the circuit on a short-time overload which would not injure the motor. For the specific duty of motor protection there have been developed thermal overload devices with heating characteristics as nearly as possible like those of the motor to be protected. Such a device will disconnect the motor only after an overload has been sustained for a dangerous period of time. It is relatively difficult to make a heater-type relay with the same heating characteristics as the motor which it is intended to protect. In some cases, and particularly in protecting very small and totally enclosed motors, the relay is therefore placed on or in the motor, so that it may also be operated by heat received by convection or by radiation directly from the hottest part of the motor.