Motor Protection: Three Common Mistakes and How to Avoid Them

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When talking about motor protection they are not adequately sized or configured, two possible scenarios can unfold. There are a few cases where they trip continuously and will consume valuable time from your maintenance staff, and in some cases they may not even trip in response to slight Under Voltage or overload, conditions that are not always evident and which reduce the service life of motors.

To avoid some of the common mistakes while configuring motor protections, the following are the steps one should take in mind.

1) Undervoltage Protections Set Too High – Motors that are operating below their rated voltage might suffer from overheating and have a shorter life cycle. The National Electrical Manufacturers Association (NEMA), they do not recommend operating motors below 90% of their rated voltage for extended periods. Make sure that if an under voltage protection is set too high, it can, and probably will, disconnect the motor when not required.

For instance, three-phase motor if it has a rated voltage of 230V, which 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.

2) Thermal Overload Set Incorrectly-The basic requirement for overload protection setting for motors is 125% of their full-load current according to the NEC; however, it makes sure you read the overload relay instructions.

Some manufacturers have the 125% setting built in, which means you must set the overload protection at the motor’s nameplate current.

If the 125% value is not built into the relay, you must set it at the motor’s nameplate current + 25%.

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 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 engine is just lightly loaded which gives 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 adequately from overload. For example, 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.

3) Magnetic Protection Set Incorrectly – In the circumstances like fault conditions, magnetic protections must disconnect the motor immediately but must allow the inrush current without disconnection. Another thing that one should keep in mind is that if the magnetic protection is fixed, make sure its trip curve allows the inrush current, which can be up to 800% of rated current only. Whereas, if the magnetic protection is adjustable, then set a value so that it will not trip with the inrush current. Make sure that the inrush current is lower, if the motor has a reduced-voltage starter, a solid-state starter or a variable frequency drive.


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