Motor Protection Application Guide


When applying protective relays to motors or any other equipment, you must decide how much protection is sufficient. The answer depends on motor rewind cost, loss of production, effect of downtime, new versus old installation, metering, control, and the consequences of a motor failure on the electrical system and process.

This guide presents an overview of motor hazards and a discussion of detection and protection options. Basler Electric BE1-11m relay typical setting value ranges are given along with considerations to help designers and users select Basler for motor protection.

Most of the protection functions apply to squirrel cage, wound, induction motors. Additional protection elements needed for synchronous motors are mentioned in this guide.

 ES Series Protection Relays
ES-27 Undervoltage Relay on DIN Rail
ES-55 Power Factor Relay


Motor protection is a challenge because there are so many different incidents that can affect a motor and the associated load. Motor hazards include the following:

  • Motor induced
    • High internal temperature
    • Insulation failure (faults within the motor)
    • Bearing failure
    • Mechanical failure
    • Failure to start
    • Synchronous motors - loss of field
  • Load induced
    • Overload and underload
    • Jamming
    • High inertia, especially at starting
  • Environment induced
    • High ambient temperature
    • High contaminant level or blocked ventilation
    • Extreme cold or wet conditions
  • Source induced
    • Loss of phase or phases
    • Voltage unbalance
    • Overvoltage
    • Undervoltage
    • Phase reversal
    • Out-of-step condition resulting from system disturbance
  • Operation induced
    • Synchronizing or closing out of phase
    • High duty cycle
    • Jogging
    • Rapid reversing

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