Under-Voltage Motor Control Remedy

[1madison]

Given: An across the line connected 30 hp 208v 3ph motor will drop out the contactor due to a thermal overload condition because the poco is generating power at a reduced voltage (approx.190v) because of network feeders burning out due to high demand.
Will installing a VFD in place of the across the line controller enable this fan motor to continue to operate even if at a lower speed.
Also, can a drive start a motor during an under-voltage condition.

Any drive experts out there.
Thanks in advance

 

[kingpb]

This is a multi-issue question. You did not state what the motor is running, so that could be a big factor in all this.

A 30Hp motor, rated 200V (nameplate) should be able to start and run continuously at 190V, fully loaded. IF, the motor is not overloaded, or being continuously operated in it's service factor, and the O/L are sized, or set properly, and the ambient temperature in the installed location is within the rating, they should not be tripping the motor at this voltage.

Things to check:

Where is the 190V available, i.e. service entrance, low side of utility transformer, on the motor control center (MCC) or at the motor. This is important, because if the voltage is at the source you most likely do not have 190V at the motor terminals and therefore it is operating at a voltage below it's rating. If there is a transformer between the motor and source, then consider using the trnasformer taps to improve the voltage to the motor.

Is the motor operating within it's thermal damage curve? The motor may be in need of repair and therefore drawing more current then it's nameplate, or the ambient temperature is greater than 40 deg C.

Are the O/L set, or selected properly. This can be common and easily rectified. Also, I assume the starter is a size NEMA 4 for this size and voltage, not a NEMA 3.

Yes, a VFD should be able to take a 190V input, and be able to maintain the motor at rated load. But, if all other things are sized and operating correctly, the VFD probably is not the answer, or required.

 

[boater bill]

Most of the drives will trip on an undervoltage fault if your feed is that low. The VFD will regulate the output so you wont have the big inrush current that happens when starting across the line. The VFD is not the solution for your problem. See if the 208 V transformer has multitaps and step it down one or 2 windings to step up the output voltage to something the motor will use without tripping the overload.

 

[plate]

I agree with the previous post in that it may be cutting out on undervoltage. If you have a 3 phase monitor such as a Time Mark Corp monitor, try adjusting the undervoltage value lower. Just a thought.