Motor Protection Circuit Breaker or MPCB

Motor protection circuit breakers are a specialized type of electrical protection device that is designed specifically for electric motors, like their name implies. Electric motors have plenty of applications and are used to drive mechanical devices of all types, so it is very important to protect them adequately with MPCBs. The following are just a few examples of devices driven by electric motors in commercial and industrial buildings:

  • Rooftop air conditioners, chillers, compressors, heat pumps and cooling towers.
  • Extraction and injection fans, as well as air handling units.
  • Water pumping systems.
  • Elevators and other hoisting devices.
  • Industrial conveyor belts and other machinery used in manufacturing processes.

In all of these industrial and commercial applications of electric motors, the MPCB has the key role of providing electrical protection.

What is a Motor Protection Circuit Breaker and what are its Functions?

A motor protection circuit breaker, or MPCB, is a specialized electromechanical device that can be used with motor circuits of both 60 Hz and 50 Hz. It has several functions that allow it to provide a safe electrical supply for motors:

  • Protection against electrical faults such as short circuits, line-to-ground faults and line-to-line faults. The MPCB can interrupt any electrical fault that is below its breaking capacity.
  • Motor overload protection, when a motor draws electric current above its nameplate value for an extended period of time. Overload protection is normally adjustable in MPCBs.
  • Protection against phase unbalances and phase loss. Both conditions can severely damage a three-phase motor, so the MPCB will disconnect the motor in either case as soon as the fault is detected.
  • Thermal delay to prevent the motor from being turned back on immediately after an overload, giving the motor time to cool down. An overheated motor can be permanently damaged if it is turned back on.
  • Motor Circuit Switching – MPCBs are normally equipped with buttons or dials for this purpose.
  • Fault Signaling – Most models of motor protection circuit breakers have a LED display that is turned on whenever the MPCB has tripped. This is a visual indication for nearby personnel that a fault has occurred and the electric motor must not be connected again until the fault is addressed.
  • Automatic Reconnection – Some MPCB models allow a cool down time to be input in case there is an overload, after which the motor will restart automatically.
    Electric motors are expensive equipment, so the role of the motor protection circuit breaker is very important. If a motor is not protected correctly, it may be necessary to carry out costly repair works or even replace the equipment completely. An electric motor that is adequately protected with an MPCB will have a much longer service life.

Motor Protection Circuit Breaker Working Principle

The motor protection circuit breaker can be considered a subtype of thermal magnetic circuit breaker, but with additional functions that are specially designed to protect electric motors. The basic working principle is similar to all other circuit breakers.

  • Thermal protection is used to guard the electric motor against overload. It is based on an expanding and contracting contact that disconnects the motor if excessive current is detected. It is very important to know that thermal protection has a delayed response, to allow the high inrush currents when a motor starts. However, if the motor is unable to start for some reason, thermal protection will trip in response to the extended inrush current.
  • Magnetic protection is used when there is a short circuit, line fault, or other high current electric fault. Unlike thermal protection, magnetic protection is instantaneous; to immediately disconnect the dangerous fault currents.
  • The main difference between the MPCB and other circuit breakers is that the MPCB can provide protection against phase unbalance and phase loss. Three-phase circuit motors require three live conductors with balanced voltages in order to operate effectively. An unbalance of more than 2% will be detrimental to the motor’s service life. If one of the phase voltages is suddenly lost, the effect is even more damaging because the motor will keep on running with only two phases. The motor protection circuit breaker is capable of detecting these conditions by measuring the differences among phase voltages, and disconnects the motor immediately when they occur. It is important to note that phase current unbalance is normal in three-phase systems that power separate single-phase loads, but is unacceptable when the three-phase circuit powers an electric motor.
  • MPCBs are also equipped with a manual interruption mechanism, allowing disconnection of electric motors for replacement or maintenance.
  • Motor protection circuit breakers are available in a wide variety of current ratings, and one of their best features is that many models allow the current rating to be adjusted. This means that the same MCPB can be configured to protect motors of different capacities.

Asynchronous Motor Protection

Most motors used in the industry are an asynchronous motors, also known as squirrel-cage induction motors. These motors use three-phase power to create a rotating magnetic field, which in turn magnetizes the rotor an creates rotational movement. When designing the electrical protection for an asynchronous motor and selecting motor protection circuit breakers, there are some very important factors to consider that aren’t present when protecting other types of electrical circuits.

  • Asynchronous motors draw a very high inrush current during startup, because they must establish a rotating magnetic field. This current can reach values of 500% to 800% of the rated value for a few fractions of a second. For this reason, the MPCB magnetic protection trips at values greater than 10 times the rated current, unlike some types of miniature circuit breakers which trip at values as low as 3 times rated current. In these cases, using a circuit breaker other than an MPCB will not even allow starting the motor before the magnetic protection trips. In order to reduce the inrush current, a very common practice is to complement the motor protection circuit breaker with a reduced voltage motor starter.
  • Asynchronous motors require the three phase conductors to have a balanced voltage in order to operate properly. If the phase conductors have an unbalance greater than 2%, the motor will suffer damage over time and will have a reduced service life. The electric motor will also tend to overheat, causing additional energy expenses as waste heat. For this reason, a motor circuit breaker must be able to detect phase imbalance and disconnect the motor accordingly.
  • If one of the phases is disconnected completely, the motor will keep operating but the current in the remaining two phases will rise above the rated value due to the electrical unbalance, and will probably burn the motor’s windings. For this reason, motor protectors must trip immediately as soon as phase unbalance or phase loss is detected. This is normally achieved by measuring the differences in current among the phase conductors. If one of the phase currents rises or drops considerably compared with the other two, it is indicative of unbalance. Likewise, if one of the phase currents drops to zero while the other two remain, a phase loss has occurred.

Then, what kinds of breakers can be used for the protection of asynchronous motors? Manufacturers generally offer three different motor protection circuit breakers, available for a wide range of voltages and currents, in order to meet most asynchronous motor protection needs.

It is very common to complement motor protection circuit breakers with a contactor to allow automatic control of motor startup and disconnection. The system might also include and under-voltage protection device, which disconnects the motor in case the system voltage drops considerably below the rated value.

Motor Protection Circuit Breaker Sizing (Selection Guide)

The two main factors that determine the adequate motor protection circuit breaker size are the nameplate voltage and nameplate current, of the motor itself.

  • The MPCB voltage rating must match the nameplate voltage of the motor. Normally, motor protection circuit breakers can be used in a wide variety of voltage ratings such as 230 V, 380 V, 415 V, 440 V, 500 V, and 660 V AC.
  • Once the voltage is known, it is necessary to check the nameplate current of the electric motor. It is important to note that the actual operating current may be lower than nameplate current, especially if the motor isn’t fully loaded. However, the MPCB must always be selected according to nameplate current value in order to allow the inrush current when a motor starts. For example, a motor with a nameplate current of 20 amperes might draw a much lower current during part-load operation, but the MPCB must be selected according to the rated value of 20 amperes, or it might trip if the motor is used at full load.
  • Motor protection circuit breakers can then be calibrated to the exact current value that is adequate for the electric motor being protected. They typically have an adjustment range. For example, a MPCB rated at 32 amperes might be usable for motors with rated currents as low as 22 amperes. This is very useful if an electric motor is replaced with a more efficient model that requires a lower current, since it will not be necessary to replace the motor breaker.
  • Even if a motor protection circuit breaker is sized correctly according to the electric motor being protected, it is also important to use adequate wiring. In order to provide adequate protection, the wire must be able to conduct the rated current safely. An undersized wire will overheat, the insulation will melt, and electric faults may occur even with a breaker installed.

Conclusions of Motor Protection Circuit Breaker

Motor protection circuit breakers have a very important role in electrical safety, since the motors they protect have a wide variety of applications in commercial buildings and industry.
Asynchronous motors, the most common type of electric motor in industrial and commercial settings, has special protection requirements that can only be met by a motor protection circuit breaker. It is also possible to complement MPCB with other protection or automation devices such as under-voltage protection, timers, and reduced voltage motor starters.
Adequate selection of the MPCB is key in order to provide reliable motor protection. An undersized MPCB will not even allow the motor to start, while an oversized MPCB might be unable to detect over-current conditions for the electric motor being protected.

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