![]() While overloads are allowed for a short time (usually minutes), prolonged overloads will use thermal action to cause a protective device to trip. ![]() Systems are protected by overload protection relays. This is caused by too much load on a motor. It is caused by excessive amounts of current drawn by a motor, which may be as high as six times the rated current. The term “ overload” describes a moderate and gradual rise in the value of current over a relatively long period of time. The initial high value of current is called inrush and can cause severe line disturbances and nuisance tripping if fuses and circuit breakers are not sized accordingly. This current creates a magnetic field that causes the motor shaft to spin, and that spinning action creates a counter-EMF (CEMF), which limits the current to its normal running value. As such the motor starts to draw very high values of current. When a motor is first started, before the shaft has a chance to pick up speed and begin to rotate, the characteristics of the stator coil are that of a short circuit. ![]() Terms and Definitions 3 Overload and Overcurrent ProtectionĬlick play on the following audio player to listen along as you read this section.
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