Given that electric motors are used in virtually every industry today, it only makes sense to ensure that these critical machinery are equipped with sufficient protection. Having the right protections not only avoids unwanted failures and unplanned maintenance like electric motor rewinds but also helps maintain or even increase efficiency throughout the entire plant. And while there are no clear-cut rules when it comes to motor engine protection, there are several key elements to consider to mitigate avoidable issues.
Below, we first go into more detail about the importance of motor protection and then outline some of the most common mistakes associated with it.
Why is motor protection required?
Motor protection is indispensable to any electrical motor system, and it mainly involves safety devices installed in the cabling that power the motor. These protection devices do more than just safeguard the motor itself from various faults and damages; they also protect the load and any other devices connected to the motor system. In general, motor protection devices are categorised into three levels:
- External short-circuit protection for the entire motor
- External protective devices usually consist of various types of fuses or short-circuit relays and are mandatory to have as per countless safety regulations.
- External protection against overloading of specific equipment
In other words, this type of protection is designed to handle electrical currents and issues like excessive overloading to prevent motor damage and failure.
Built-in motor protection
These devices include components such as thermal overload protection and almost always require an external circuit breaker, while others also need an overload relay.
3 Common errors in motor protection
Incorrect selection of low voltage protection values
Setting a motor to run below its rated voltage may subject the system to overheating and shorten its service life. The National Electrical Manufacturers Association (NEMA) states that it is highly inadvisable to set electrical motors to operate anywhere below 90% of their rated voltage for a very long period of time. Also, keep in mind that setting under voltage protection too high may unintentionally disconnect the motor when needed.
Let us imagine that we are working with a three-phase motor, for instance. It has a rated voltage of 240V, with 216V being its lowest acceptable operating voltage. However, if this motor comes equipped with an adjustable low-voltage relay that is set to 230V, a 5% drop in voltage can cause the undesirable effect of disconnecting the motor despite not going below the 90% threshold.
Thermal overload setting
Moving on to motor overload protection, the NEC has established that 125% of full-load currents is the basic requirement that organisations should abide by. That said, one should also consider your motor OEM’s overload relay instructions. This means that if the relay does not come integrated with the recommended 125% option, the next best alternative is to set it to +25% of the current found on the motor nameplate.
Otherwise, setting the overload protection too low may disconnect the motor even during normal operation, potentially causing unplanned downtimes and not having sufficient protection for the motor. On the flip side, setting the overload protection too high will cause the motor to be inadequately protected against possible overload scenarios.
Magnetic protection setting
A motor’s magnetic protection needs to work as intended and disconnect the motor immediately in situations like fault conditions while still allowing inrush current without disconnection. The most important thing to keep in mind here is that if the motor’s magnetic protection is fixed, the inrush current and trip curve values should be allowed to work together in coordination. Lastly, it is helpful to ensure that the inrush current is lower, particularly if the motor has a soft starter, low-voltage starter, or variable frequency drive.
Apart from sticking to a regular maintenance schedule, periodically checking on motor protections and verifying their function is key to steering clear of avoidable issues. Ensure that you avoid motor breakdowns by addressing common pitfalls in motor protection. Should you need professional help in configuring your motor’s protection devices or resolving any of its other issues, consider reaching out to firms that specialise in mechanical and electrical engineering in Singapore to get your machinery serviced and running optimally once more. These experts can help ensure the efficient operation of your machinery by providing specialised services, including the management and maintenance of electric transformer oil.