When motor protections are not properly sized or configured, two possible scenarios can unfold. In some cases they trip continuously and consume valuable time from your maintenance staff, and in other cases they may not trip in response to slight undervoltage or overload, conditions that are not always evident and which reduce the service life of motors.
The following are some common mistakes when configuring motor protections, and how to avoid them.
Motors operating below their rated voltage suffer from overheating and a shorter service life. The National Electrical Manufacturers Association (NEMA) does not recommend operating motors below 90% of their rated voltage for extended periods. However, if an undervoltage protection is set too high, it can disconnect the motor when not required.
Assume a three-phase motor has a rated voltage of 230V. This means the lowest operating voltage acceptable according to NEMA is 207V (230V x 90%). However, if an adjustable undervoltage relay is set at 220V, a 5% reduction in voltage will be enough to disconnect the motor.
If you have a motor that is constantly being disconnected by the undervoltage relay, check the operating voltage and the relay settings. Do not assume that voltage is the issue right away; maybe the relay is set too high, and the solution is as simple as reducing the set point.
The required overload protection setting for motors is 125% of their full-load current according to the NEC, but make sure you read the overload relay instructions as well.
For example, assume you want to protect a motor with 60A of full-load current, and you have an overload relay that can be set from 50A to 100A. If the device already factors in the 125%, you must set it at 60A. If not, the correct setting is 75A (60A + 25%).
If an overload protection is set too low, the motor can be disconnected even when operating normally. For example, if the protection device described above came with the dial set at 50A and it was left that way for a 60A motor, it may not trip immediately if the motor is lightly loaded, giving the impression that it is working correctly. However, higher motor loads that bring current above 50A will trip the device.
Of course, overload protection shouldn’t be set too high either, since the motor will not be protected effectively from overload. For instance, if you add 25% when setting an overload relay that already has the 125% value built in, the actual overload protection value will be 156%, which does not meet the NEC.
Magnetic protections must disconnect the motor immediately under fault conditions, but must allow the inrush current without disconnection. If the magnetic protection is fixed, make sure its trip curve allows the inrush current, which can be up to 800% of rated current. On the other hand, if the magnetic protection is adjustable, set is value so that it will not trip with the inrush current. Keep in mind that the inrush current may be lower if the motor has a reduced-voltage starter, a solid-state starter or a variable frequency drive.
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