The over-current devices (fuses and circuit breakers) in the service entrance panel of a building are designed to protect the circuits from shorts and excessive current. The starting current of electric motors is greater than the operating current. In addition, when an electric motor is overloaded it will attempt to rotate until the electricity is disconnected or the lock-up torque is reached. Both of these situations can cause an excessive current in the motor circuit. The over current devices must be designed to withstand temporary overloads. Common breakers and fuses do not meet these requirements.
When a motor is overloaded for an extended period of time or on a frequent basis, it will overheat. Some motors include an overload protection device to protect the motor from overheating. These devices work on the principle of resistive heating. Because the current demand of a motor increases as the load increases, and because the greater the current flow through a resistance the greater the heat, engineers can predict when certain temperatures will be reached and design controlling devices using temperature as the trigger.
Three types of motor overload protection devices are used:
1. Built-in thermal overload protection: Many motors have this device as an integral part of the motor. It may have either an automatic reset or a manual reset. If the automatic reset type is used, the device will disconnect the motor from the circuit at the designed temperature, and when it has cooled down, will automatically reconnect the electricity. This type of device should be used where the motor is supplying power for a critical purpose, as in livestock ventilation vans and sump pumps. Automatic reset devices can be more dangerous than manual because they will attempt to restart the motor as soon as they cool down. If the power is not disconnected before anyone works on the machine, it can restart during repairs and cause an injury. When a manual reset device is used, it must be manually reset before the motor will start. Even with this device, the power should be disconnected before anyone works on the motor or the machine it is powering.
2. Manual starting switch with overload protection: A manual starting switch overload protection device can be added to motors without built-in thermal protection. This device is an integrated switch and thermal overload device that opens the circuit when the motor overheats. It must be reset by hand before the motor can be restarted.
3. Magnetic starting switch with overload protection: Magnetic starting switches are fast-acting switches for large motors and are equipped with a thermal overload (heater) device that will open the circuit if the current draw is excessive. These devices are manually reset.
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