Motor Protection Circuit Breaker (MPCB) - How It Works

Motor Protection Circuit Breaker (MPCB) is a type of circuit breaker used for providing protection to electric motors. MPCBs are specially designed for protecting electric motors only.

Read this article to learn the functions, working, and applications of motor protection circuit breaker (MPCB).

What is an MPCB?

Motor Protection Circuit Breaker is a specially designed circuit breaker used for protecting electric motors from faults or abnormal conditions.

In any industry, electric motors are very important and significantly expensive devices. Therefore, they have to be protected from faults and abnormal conditions that occur in power system. For this purpose, MPCBs are used. MPCBs increase the overall life span of electric motors and reduce the maintenance cost significantly.

MPCBs are primarily used in industrial installations for electric motor protection and are used in domestic applications.

Functions of MPCB

A typical MPCB performs the following three major functions in an electric motor circuit −

Protection Against Overloading

Overload protection is the primary function of the motor protection circuit breaker. MPCB continuously monitors the current flowing through the motor circuit, if there is any abnormal increase in the circuit current, in other words, if the motor circuit current exceeds the specified limit for a certain period of time, then, the MPCB operates and trips to interrupt the flow of current and prevent the damages to the motor.

Protection Against Short-Circuits

MPCB also protects the electric motors from short-circuits. As we know, short-circuit is a dangerous condition in which sudden and high-intensity currents flow through the circuit. These high currents can burn the windings of the motor. Therefore, it is necessary to interrupt the short-circuit currents within a specified safe time limit. This function can be accurately performed by an MPCB.

Protection Against Phase Loss or Phase Imbalance

Phase loss or phase imbalance is an abnormal condition that occurs in a three-phase motor system when any of the three phases is disconnected from the motor circuit. MPCB is designed to continuously monitor the phase voltages and currents of the three-phase motor systems. If there is any abnormality is detected in the system, then MPCB operates and protects the electric motor from damages.

Types of MPCBs

Motor Protection Circuit Breakers (MPCBs) can be of different types depending on the operating mechanism used for providing protection. These different types of MPCBs are described below −

Thermal MPCB

Thermal MPCBs are those motor protection circuit breakers that operate on the principle of heating effect of electric current. A thermal MPCB consists of a bimetallic strip or a thermal overload relay to detect the fault in the motor circuit.

Thermal MPCBs continuously monitors the temperature of the electric motor. If the motor’s temperature exceeds the specified limit, the MPCB operates and protects the motor from overload condition.

Magnetic MPCB

Magnetic MPCB is another type of MPCB which provides both overload and short-circuit protection. The operation of magnetic MPCBs is based on the principle of electromagnetic effect. In other words, magnetic MPCBs detect the faults in the motor circuit using electromagnetic effect and interrupt the motor circuit if any abnormal condition is detected. This type of MPCB is relative faster and provides quick protection to electric motors.

Electronic MPCB

Electronic MPCB is one that utilizes microprocessor-based technology to monitor the circuit and protect the motor against faults. These MPCBs are known for their better fault detection capabilities, rapid operation, and precise motor protection.

Electronic MPCBs are capable in providing all kinds of protections such as short-circuit, overloading, phase loss, phase imbalance, etc. Also, these MPCBs have an advanced feature of communication interfacing that allows them to integrate with smart grid and SCADA systems.

How to Select a MPCB?

It is an important process of selecting an MPCB of correct size. The size of an MPCB used for motor protection depends on the following two major parameters −

• Rated voltage (nameplate voltage) of the motor

• Rated current (nameplate current) of the motor

Therefore, before selecting an MPCB, we first confirm the rated voltage and current values of the motor to be protected.

Advantages of MPCB

The following are some key advantages of MPCB (Motor Protection Circuit Breaker) −

• MPCBs have an adjustable current setting that can be changed as per the needs of application.

• A single MPCB can provide protection against different types of faults like short-circuit, overload, phase loss, and phase imbalance. Hence, it is a versatile device.

• Currents settings of the MPCB are easy to adjust.

• MPCBs have a compact size.

• MPCBs require very less maintenance, hence their maintenance cost and down time is low

• MPCBs do not require any external thermal overload relay.

Disadvantages of MPCB

The major disadvantages of MPCBs are as follows −

• The initial cost of MPCB is high.

• MPCBs are use and throw device, as they cannot be repaired if burnt.

• We cannot adjust the short-circuit tripping limit of an MPCB.

Applications of MPCB

MPCBs are widely used in commercial and industrial applications for motor protection. The following are some examples of applications of MPCBs −

• In industrial applications, MPCBs are used to protect electric motors equipped in manufacturing plants, pumps, conveyors, compressors, HVAC systems, cranes, etc.

• In commercial applications, MPCBs are used for protecting motors used in lifts, elevators, water pumps, etc.

• In residential applications, MPCBs are also used in some special residential motor operated devices like washing machine, air conditioners, refrigerators, lifts, etc.

Conclusion

In this article, we elaborated all the major concepts related to MPCBs (Motor Protection Circuit Breakers). An MPCB is a protection device or a circuit breaker, specially designed for protecting electric motors against short-circuit, overloads, phase loss, and phase imbalance conditions. MPCBs are preferred in motor protection applications because of their easy installation, high reliability, and versatility.

FAQs Related to MPCBs

This section of the article answers some of the important and frequently asked questions related to Motor Protection Circuit Breakers (MPCBs).

Q 1. What is the function of an MPCB?

MPCB is a switchgear device. Its role in a circuit is to protect the electric motor connected in the circuit against short-circuit, overload, phase loss, and phase imbalance. MPCBs are used for protecting electric motors from short-circuit and overload currents.

Q 2. Where can I use MPCB?

MPCBs can be used in any kind of electrical installation in commercial, industrial, or even in residential applications to trip the circuit during short-circuit or overload condition of the motor.

Q 3. How to size an MPCB?

An MPCB is sized according to the nameplate ratings of voltage and current of the electric motor.

Q 4. What is the range of MPCB?

MPCBs are available in a voltage range of 230 V to 660 V. We have to select an appropriate voltage rating depending on the nameplate ratings of the motor.