Introduction
Motor protection safeguards the motor, the supply system and personnel from various operating conditions of the driven load, the supply system or the motor itself.
Motor protection categories include
— Overcurrent Protection
–Overload Protection
–Other Types of Protection.
• The National Electrical Code requires that motors and their conductors be protected from both overcurrent and overload conditions.
Overcurrent Protection
Overcurrent protection interrupts the electrical circuit to the motor upon excessive current demand on the supply system from either short circuits or ground faults.
- Overcurrent protection is required to protect personnel, the motor branch circuit conductors, control equipment, and motor from these high currents.
- Overcurrent protection is usually provided in the form of fuses or circuit breakers. These devices operate when a short circuit, ground fault or an extremely heavy overload occurs.
Most overcurrent sources produce extremely large currents very quickly
Overload Protection
Overload protection is installed in the motor circuit and/or motor to protect the motor from damage from mechanical overload conditions when it is operating/running.
- The effect of an overload is an excessive rise in temperature in the motor windings due to current higher than full load current.
- Properly sized overload protection disconnects the motor from the power supply when the heat generated in the
motor circuit or windings approaches a damaging level for any reason.
–The larger the overload, the more quickly the temperature will increase to a point that is damaging to the insulation and lubrication of the motor.
- Unlike common instantaneous type fuses and breakers, overload devices are designed to allow high currents to flow briefly in the motor to allow for:
- Typical motor starting currents of 6 to 8 times normal running current when starting.
- Short duration overloads such as a slug of product going through a system.
- If the motor inlets and outlets are covered by a blanket of lint or if a bearing should begin to lock, excessive heating of the motor windings will “overload” the motors insulation which could damage the motor.
- The overcurrent device will not react to this low level overload. The motor overload device prevents this type of problem from severely damaging the motor and also provide protection for the circuit conductors since it is rated for the same or less current as the conductors.
- Overload protection trips when an overload exists for more than a short time. The time it takes for an overload to trip depends on the type of overload device, length of time the overload exists, and the ambient temperature in which the overloads are located
Other Motor Protection Devices
Low Voltage Protection
Low Voltage Disconnects – Protection device operates to disconnect the motor when the supply voltage drops below a preset value. The motor must be manually restarted upon resumption of normal supply voltage.
Low Voltage Release – Protection device interrupts the circuit when the supply voltage drops below a preset value and re-establishes the circuit when the supply voltage returns to normal.
Phase Failure Protection
Interrupts the power in all phases of a three-phase circuit upon failure of any one phase.
- Normal fusing and overload protection may not adequately protect a polyphase motor from damaging single phase operation. Without this protection, the motor will continue to operate if one phase is lost.
- Large currents can be developed in the remaining stator circuits which eventually burn out.
- Phase failure protection is the only effective way to protect a motor properly from single phasing.
Phase Reversal Protection
Used where running a motor backwards (opposite direction from normal) would cause operational or safety problems.
- Most three phase motors will run the opposite direction by switching the connections of any two of the three phases.
- The device interrupts the power to the motor upon detection of a phase reversal in the three phase supply circuit.
- This type of protection is used in applications like elevators where it would be damaging or dangerous for the motor to inadvertently run in reverse.
Ground Fault Protection
Operates when one phase of a motor shorts to ground preventing high currents from damaging the stator windings and the iron core.
Also read
What is a motor controller?
