Motor Protection

Motors are a significant investment and often run critical processes. Motor protection relays are used to protect the windings in the stator from damage due to electrical faults and thermal overloads. Adequate motor protection not only prevents motor damage, but also ensures optimal process efficiency and minimal interruption. Cost recovery for protection is achieved by extending the life of the motor, preventing motor rewinds and reducing downtime.

Motor protection and the NEC®

The NEC® requires the motor be protected by overload devices against excessive heating due to overload and failure to start (NFPA70 430 Section III). NFPA 70 430, Part IV also specifies the use of devices to protect against overcurrents such as short circuits and grounds. Both of these NEC® requirements and many additional functions can be met with the use of a multifunction motor-protection relay.

NFPA 70 430.32 (A)(4) requires the use of a protection device having embedded temperature detectors that cause current to the motor to be interrupted when the motor attains a temperature rise greater than marked on the nameplate in an ambient temperature of 40°C for motors larger than 1500 hp.

The NEC® defines minimum requirements and is intended to provide protection from fire. Protection relays can provide many enhancements to a facility above simple fire protection.


Network communications can be added to a motor protection relay to allow remote metering of currents, voltages and temperatures. Datalogging is a useful feature for troubleshooting and comparing event sequences with process stages. Analysis of information can often show operational issues.