INTRODUCTION
Overload relays are intended to protect motors, controllers, and branch-circuit conductors against
excessive heating due to prolonged motor overcurrents up to and including locked rotor currents.
Protection of the motor and the other branch-circuit components from higher currents, due to short
circuits or grounds, is a function of the branch-circuit fuses, circuit breakers, or motor short-circuit
protectors.
Electrical motors make up a large percentage of power system loads. Market demands for reduced
downtime and increased productivity have compelled the motor control industry to evaluate motor
protection technology continuously. Technology advancements now allow the motor control industry to
offer several options for motor protection.
This briefly reviews traditional motor protection technologies and discusses the new, electronic motor
protection options. After reading this paper, you should be able to understand the available technologies
and how to choose the right solution for a given application. Important factors to consider in determining
the appropriate overload protection include:
• Application requirements
• Cost per feature of a given technology
• Willingness and ability of all parts of the user’s organization to embrace and implement the new
technology.
MOTOR FAILURE AND PROTECTION
Motor failure may be the result of electrical or mechanical factors. A study commissioned by the
Electrical Research Associates (ERA) of the United Kingdom in 1986 indicated the most common
causes of motor failure are:
1. Overcurrent 30%
2. Contamination 18%
3. Single Phasing 15%
4. Bearing Failure 12%
5. Aging (natural wear) 10%
6. Rotor Fault 5%
7. Miscellaneous 7%
Failure modes 1, 3 and 7 are attributable to electrical issues. Modes 2, 4, 5 and 6 are the result of
mechanical (and some manufacturing) issues.
Historically, motor protection provided with the controller was only able to address the electrical causes
of motor failure. These electrical issues account for at least 45% of the most common causes of motor
failure. Motor branch circuits are protected against short circuits (instantaneous overload currents) and
steady state or low level, sustained overload relays. In the U.S., this protection is provided by the short
circuit protective device (SCPD) and the motor overload relay, when they are applied according to the
National Electrical Code (NEC).
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