Why Protect Your Three Phase Motors?
Historical data shows that 44% of motor failures are due to overloads and single-phasing. (Cooper/Bussman’s Protection Handbook Bulletin http://www.bussman.com/library/docs/spd02/SPDSection11.pdf)
This can be serious, especially since a problem isn’t detected until the motor fails. When that happens, which unfortunately may be on the weekend or night when it is difficult to get a repairman, the consequences may include inconvenience, loss of revenue, extensive repair costs, or irate customers.
What Causes Overloads
Overloads are caused by excessive load on a motor. This results in current that exceeds the FLA
What Causes Single Phasing
Voltage imbalance means that the voltage between the phases isn’t equal. Single phasing is the worst condition of voltage imbalance because one phase is lost (open) and the current in the other two phases increase to keep the motor running. The result is overheating and a burnt out motor.
Some causes are damaged motor starter contact, damaged switch or circuit breaker, open fuse in the circuit breaker, cable failure on the transformer primary or secondary side, or poor connections in terminals (Bussman, p5).
What Can You Do
HECO’s Rule 2C on “Motor and Equipment Protection” says that the customer is responsible for providing protection devices for motors. This includes damages from excessive overheating resulting from open phase or low voltage.
The National Electric Code Article 430 describes the protective devices required by code. Keep in mind that each device has specific applications. In order to assist our customers on the various devices on the market, we have enclosed HECO PQ Tips (HECO Powerlines No. 2 Summer 2007).
It is very important to consult with a reputable vendor to ask the right questions and select the proper devices so that your business won’t suffer from unexpected and expensive motor damages.
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