A motor protection circuit breaker, or MPCB, is an electromechanical device that performs three important functions upstream of an electric motor: isolation, motor protection against overload and short circuit, control of the motor (on/off).
When a motor starts up, it draws current up to 10 times its rated value. This can last up to 30 seconds until it reaches its steady speed. Distribution circuit breakers typically have a magnetic threshold set at 8 times the rated current, or less. When current exceeds this threshold, the breaker identifies it as a short-circuit event on the distribution and trips immediately. For a motor, this type of overcurrent can simply represent regular starting up current. So you can see that a distribution circuit breaker used to protect a motor will very likely, and undesirably, trip when the motor starts up.
In contrast, dedicated motor circuit breakers are designed with their short-circuit trip threshold set much higher, usually 13 times the rated current. This avoids the risk of tripping during the motor startup phase.
Electrical distribution circuit breakers are designed to protect cables. Their overload tripping times are set according to overcurrent with stand of cables, which is usually shorter than that of motors. So distribution breakers are likely to trip before the situation becomes dangerous for a motor.
A phase fault can happen for many reasons: wiring errors after maintenance, loose connections, a utility phase loss, or even motor aging. Distribution circuit breakers are not equipped to trip in case of phase unbalance or phase loss, as these common conditions pose no risk to distribution networks. But a phase fault is a critical event for motors, causing mis-operation (e.g. speed deviations) or the motor to overheat and eventually be damaged.
Motor circuit breakers are designed to trip when sensing a phase fault. This will occur after a couple of seconds when the motor is running at regular speed, or after only a few tenths of seconds when it’s starting up.
What a motor protection circuit breaker is
A motor protection circuit breaker, or MPCB, is an electromechanical device that performs three important functions upstream of an electric motor:
isolation,
motor protection against overload and short circuit,
Control of the motor (on/off).
In the event of overload or short circuits, the MPCB automatically disconnects the circuit. It has internal contacts that connect the power supply line to the load. An internal coil detects the short-circuit currents (high or very high current values), while a heat-sensitive element, welded on the bimetal, detects overcurrents (current values slightly higher than the nominal value) which may damage the motor. In the event of overcurrents, the coil or the bimetal determine the opening of the contacts.
Protection against overload
Overloading is an unprogrammed operating condition which overheats the motor, causing it to draw a higher than nominal current, which may occur when:
The resistant torque is too high for the motor in question
the rotor jams
The motor is starting too frequently
One of the three power phases is lacking
The starting time is too long
The overload protection device must be sized based on the motor rated current, and in choosing it, pay attention to the minimum motor start-up times against which it is necessary to identify one of the intervention classes identified by the IEC standard: 2 – 3 – 5 – 10 – 10A – 20 – 30 – 40. The most common intervention classes are 10 – 10A, which corresponds to an intervention within 10 s for an overcurrent equal to 7.2 In
Protection against short circuit
Short circuit is the most serious and dangerous fault. It can be caused by the contact between two phases or between a phase and the neutral/earth conductor. This is due to an accidental contact or an insulation loss by natural aging or overcurrents damages.
Choosing the protection device, it is necessary to check that its breaking capacity is greater than the maximum short-circuit current calculated at that point of the system (it does not depend on the motor nominal current, but on the characteristics of the upstream network) .
Protection against phase loss
To increase the level of their reliability, motor protection circuit breakers also often ensure protection against phase loss, which puts the motor safe from dangerous overheating of the windings or even the burning of them. The SM… series implements this important feature.
A motor protection circuit breaker, or MPCB, is an electromechanical device that performs three important functions upstream of an electric motor: isolation, motor protection against overload and short circuit, control of the motor (on/off).
When a motor starts up, it draws current up to 10 times its rated value. This can last up to 30 seconds until it reaches its steady speed. Distribution circuit breakers typically have a magnetic threshold set at 8 times the rated current, or less. When current exceeds this threshold, the breaker identifies it as a short-circuit event on the distribution and trips immediately. For a motor, this type of overcurrent can simply represent regular starting up current. So you can see that a distribution circuit breaker used to protect a motor will very likely, and undesirably, trip when the motor starts up.
In contrast, dedicated motor circuit breakers are designed with their short-circuit trip threshold set much higher, usually 13 times the rated current. This avoids the risk of tripping during the motor startup phase.
Electrical distribution circuit breakers are designed to protect cables. Their overload tripping times are set according to overcurrent with stand of cables, which is usually shorter than that of motors. So distribution breakers are likely to trip before the situation becomes dangerous for a motor.
A phase fault can happen for many reasons: wiring errors after maintenance, loose connections, a utility phase loss, or even motor aging. Distribution circuit breakers are not equipped to trip in case of phase unbalance or phase loss, as these common conditions pose no risk to distribution networks. But a phase fault is a critical event for motors, causing mis-operation (e.g. speed deviations) or the motor to overheat and eventually be damaged.
Motor circuit breakers are designed to trip when sensing a phase fault. This will occur after a couple of seconds when the motor is running at regular speed, or after only a few tenths of seconds when it’s starting up.
What a motor protection circuit breaker is
A motor protection circuit breaker, or MPCB, is an electromechanical device that performs three important functions upstream of an electric motor:
isolation,
motor protection against overload and short circuit,
Control of the motor (on/off).
In the event of overload or short circuits, the MPCB automatically disconnects the circuit. It has internal contacts that connect the power supply line to the load. An internal coil detects the short-circuit currents (high or very high current values), while a heat-sensitive element, welded on the bimetal, detects overcurrents (current values slightly higher than the nominal value) which may damage the motor. In the event of overcurrents, the coil or the bimetal determine the opening of the contacts.
Protection against overload
Overloading is an unprogrammed operating condition which overheats the motor, causing it to draw a higher than nominal current, which may occur when:
The resistant torque is too high for the motor in question
the rotor jams
The motor is starting too frequently
One of the three power phases is lacking
The starting time is too long
The overload protection device must be sized based on the motor rated current, and in choosing it, pay attention to the minimum motor start-up times against which it is necessary to identify one of the intervention classes identified by the IEC standard: 2 – 3 – 5 – 10 – 10A – 20 – 30 – 40. The most common intervention classes are 10 – 10A, which corresponds to an intervention within 10 s for an overcurrent equal to 7.2 In
Protection against short circuit
Short circuit is the most serious and dangerous fault. It can be caused by the contact between two phases or between a phase and the neutral/earth conductor. This is due to an accidental contact or an insulation loss by natural aging or overcurrents damages.
Choosing the protection device, it is necessary to check that its breaking capacity is greater than the maximum short-circuit current calculated at that point of the system (it does not depend on the motor nominal current, but on the characteristics of the upstream network) .
Protection against phase loss
To increase the level of their reliability, motor protection circuit breakers also often ensure protection against phase loss, which puts the motor safe from dangerous overheating of the windings or even the burning of them. The SM… series implements this important feature.
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