Motor Overload Protection

Large alternating current motors are vital to modern life and industry. While motors are relatively simple devices, they can be installed or operated incorrectly or develop operational problems. For protection of the motor and for safety reasons, electrical motors require protective devices on their power supply systems. Circuit breakers and fuses are used to provide protection from very high-current short circuits and ground faults. Overload devices are used to protect from longer period, moderate overcurrent events. This module will focus primarily on the role and function of the overload protection devices.

• Describe the purpose of fuses and circuit breakers • Differentiate between a short circuit fault and an overload • Explain the principles of operation behind thermal overload devices • List some causes for motor overload • Explain the principle of operation behind magnetic overload devices • Explain the principle of operation behind electronic overload devices • Describe the NEMA trip class numbering system