Protecting motor driven systems with the correct breaker
Motor driven systems are in everything from processes, to commercial heating, ventilating, cooling, and refrigeration. With business productivity so dependent on them, it’s important that they’re protected.
OEM machine builders and electrical contractors need to ensure that there is a circuit breaker used on all motor driven systems. According to Schneider Electric’s Henrique Matheus, selecting the proper type of motor circuit breaker ensures protection of the motor.
This is not the case when using standard electrical distribution circuit breakers: a fact not understood by many. Often selected due to their lower cost, their overload design is for the protection of standard distribution cables, not motor driven systems. By using standard breakers there is a high risk of motor damage, disruption and downtime for the end-customer, or even fire.
The primary reasons for using a dedicated motor circuit breaker as opposed to a distribution circuit breaker are:
1. Avoiding nuisance tripping during motor start-up
When a motor starts, 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 have a magnetic trip threshold set at eight times the rated current, or less. When current exceeds this threshold, the breaker identifies it as a short-circuit event and trips unnecessarily.
In contrast, Schneider Electric’s dedicated GV Series motor circuit breakers have their short-circuit trip threshold set around thirteen times the rated current. This avoids the risk of nuisance tripping of motor driven systems during motor start-up and still protects the motor.
2. Risk of nuisance tripping due to a transient overload
The electrical tripping characteristic designed into distribution circuit breakers is for protecting cables. Their overload tripping times are set according to overcurrent withstand 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. Keep in mind that motor protection relays, and dedicated motor electronic trip units, offer options to configure even slower overload protection for classes 10A, 10, 20, and 30.
3. Risk of nuisance tripping due to high ambient temperature
The design of most distribution circuit breakers is for operation under 30 °C, or in some cases under 40 °C. If enclosure temperature exceeds this rating, the breaker will trip at a lower current than its rating, unnecessarily interrupting a process.
In contrast, motor circuit breakers operate up to 60° C, or 65° C. So, a 10 A rated motor circuit breaker, protecting a 10 A rated motor will not trip when current is 10 A and ambient temperature is 60° C. In this way, machines and operations keep the motor driven systems running whilst still protecting the motor.
4. Risk to motor driven systems by phase fault
A phase fault can happen for many reasons, and distribution circuit breakers are not equipped to trip in these common conditions as they pose no hazard to distribution networks. But a phase fault is a critical event for motors, causing mis-operation, overheating and possible damage to the motor. But motor circuit breakers will trip within seconds when sensing a phase fault. This will protect a running motor, or during start-up.
5. Risk of contactor damage and fire due to short-circuit
EN 60204-1 requires at least Type 1 short circuit coordination device between a motor protection circuit breaker and a contactor. It is unusual for the testing of distribution circuit breakers in coordination with contactors, so there’s no performance guarantee of the combination.
A short circuit may also result in the destruction of the contactor and worse damage. The energy dissipated by the contactor during the interruption process can burn surround materials or start a fire. Motor circuit breakers allow a choice of short-circuit coordination. Type 1 ensures no damage around the contactor, or Type 2 ensures the contactor can still operate after the short-circuit event.
Installing the correct type of circuit breaker ensures the difference between normal motor maintenance or costly disruption to the motor drive system.
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