Electric motors drive everything from processes, to commercial heating, ventilating, cooling, and refrigeration. With business productivity so dependent on them, it’s important that they’re properly protected. Article by Henrique Matheus, Offer Manager for Switching and Controls for Schneider Electric.
It’s estimated that there are more than 300 million electric motors in the world being used in industry, infrastructure, and large buildings, with electric motor-driven systems (EMDS) accounting for between 43 to 46 percent of all global electricity consumption. Motors drive everything from processes, to commercial heating, ventilating, cooling, and refrigeration. With business productivity so dependent on them, it’s important that they’re properly protected.
In most jurisdictions, motor protection is mandatory, so OEM machine builders and electrical contractors will ensure that there is a circuit breaker used on each motor circuit. If the proper type of motor circuit breaker is selected, the motor will be adequately protected.
Unfortunately, there are many examples in which a standard electrical distribution circuit breaker was chosen for this purpose. These breakers are typically selected because they are lower cost. But they are designed to protect standard types of circuits and loads, not motors. If such a breaker is used, there is a very high risk of motor damage, disruption and downtime for the end-customer, and fire.
Here are five critical reasons behind why a dedicated motor circuit breaker should be used to protect a motor, not a distribution circuit breaker. Please note that, in some cases, I am referring to IEC standards and application, which may not be appropriate for use within NEC, CEC, or other standards jurisdictions.
Risk Of Nuisance Tripping During Motor Startup
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 eight 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, 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.
To save money while avoiding the risk of tripping on motor startup, some contractors may select an overrated distribution circuit breaker, i.e. one with a higher threshold. Let’s use the example of a 7.5 kW motor with 16 A rated current, and startup current likely to reach 160 A. In this case, the contractor may choose a distribution circuit breaker rated at 25 A, with a magnetic threshold of around 200 A, so that motor startup will not immediately trip the circuit breaker. However, there is the serious risk if the motor becomes overloaded during regular operation, reaching a current of up to 24 A, the 25 A rated breaker will not trip and the motor will be in serious risk of being destroyed after only a couple of minutes.
Note that, in the next few years, IE3/IE4 high-efficiency motors will become mandatory in many regions. When using direct starters, these motors will have higher starting current that today’s motor designs. This will make it even more important to choose the correct type of circuit breaker to protect these motors.
Risk Of Nuisance Tripping Due To A Transient Overload
Electrical distribution circuit breakers are designed to protect 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.
Risk Of Nuisance Tripping Due To High Ambient Temperature
Most distribution circuit breakers are designed to operate under 30 deg C, or in some cases under 40 deg C. If ambient temperature exceeds this rating, the breaker will trip at a lower current than its rating, unnecessary interrupting a process.
In contrast, motor circuit breakers are set for operation under 60 deg C, or optionally 65 deg 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 deg C. In this way, machines and operations keep running while motors are still properly protected.
Risk Of Motor Damage Due To A Phase Fault
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 hazard 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.
Risk Of Contactor Damage And Fire Due To Short-Circuit
The international standard for safety of machinery, EN 60204-1, requires at least Type 1 coordination between a circuit breaker and a contactor, in the case of a short circuit. Distribution circuit breakers are usually not tested in coordination with contactors; therefore, there’s no performance guarantee of the combination. This means that if a short circuit occurs, the contactor can be destroyed and, worse, the amount of energy dissipated by the contactor during the interruption process can burn surround materials or ignite a fire.
With motor circuit breakers, short-circuit coordination can easily be selected: Type 1 ensuring no damage around the contactor, or Type 2 ensuring the contactor can still operate after the short-circuit event.
Hence, choosing a motor circuit breaker is critical to properly protecting motors and the installation. If you’re an OEM, you may not always know in what kind of environment your machines will be installed. To maintain your customer’s perception of quality, you need to ensure that your motors and machines are operating reliably under all conditions. During warranty periods and beyond, you need to make sure motors are protected while delivering the performance you promised. Having the right type of circuit breaker can be the difference between normal motor maintenance or costly substitution, and between continuous operation or costly disruption to your customer’s productivity, or even a catastrophic fire.
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