Why Is Motor Protection Required?

A motor protection device is a device that monitors and controls one or more parameters (such as current, voltage, temperature, speed or torque) of a motor or its circuits. The purpose of a motor protection device is to prevent or minimize damage to the motor and its circuits in the event of a fault or abnormal condition. Fuses: For low voltage motors, over-current and short circuit protection can be performed by fuses associated with switch disconnections or instantaneous trip circuit breakers that respond to instantaneous (nearly instantaneous) current values from short circuits, ground faults, or locked rotor currents. 

Fuse: Fuses are generally not suitable for overload protection because if sized to provide overload protection, they will blow when the motor starts due to the high motor inrush current, although they can be used as overload backup protection.

Circuit Breaker: A circuit breaker is an overcurrent protection device. It automatically opens and closes the circuit with a predetermined overcurrent. When a circuit breaker is properly applied within its ratings, turning the circuit breaker on and off will not damage it. A circuit breaker interrupts a circuit when high current flows due to a short circuit or overload. The circuit breaker simply resets after the fault is corrected. Circuit breakers can be divided into: thermal circuit breakers and magnetic circuit breakers. A circuit breaker consists of a pair of contacts that are opened or closed by an electromechanical mechanism triggered by a sensing element. Circuit breakers are more advanced than fuses because they provide the following features: reusability and reset ability after each operation. Protect against overload and under voltage by adjusting trip settings. Indication and isolation of fault location by manual or automatic operation.

Overload Relays: These motor protection devices interrupt a circuit when a high current flows due to an overload, and are often used on large low-voltage and medium-voltage motors. They consist of a sensing element that measures current and contacts that are opened or closed by electromechanical or electronic mechanisms. Overload relays are designed to protect motors from overheating and insulation damage due to prolonged overload or voltage imbalance. 

Overload protection is usually ensured by thermal overload relays. There are two main types of overload relays: thermal overload relays and electronic or digital overload relays. Thermal overload relays use bimetallic strips or heating elements to detect temperature rise in motor current. When the current exceeds a preset value, the thermal element bends or melts, causing the contacts to open or close. Electronic or digital overload relays use a current transformer or shunt resistor to measure motor current and a microprocessor or solid-state circuit to control the contacts. When the current exceeds a preset value, the electronics send a signal to open or close the contacts. Electronic or digital overload relays are more advanced than thermal overload relays.