Reduced Voltage for Motors 1

[gjenkins]

I am currently experiencing problems with running some motors (25 to 30 hp) at reduced voltage. I currently use 230/460 Volt motors and run them with 208V. I know one way around this would be to get 200V motors, but delivery on 200V motors from our vendors are too long to meet our schedule. Another way is to oversize the motor (what I am now doing), but there are higher costs associated with this i.e. purchase price goes up, motor is less efficient when running below full load.

Any other suggestions would be greatly appreciated.

Gareth

 

[dpc]

Well, it sounds as though you already know the right answer - get the proper motors.

Another possible solution is boost the 208V up to 240V with a small autotransformer. Because the voltage change is small, you don't need a very large transformeer in relation to the kVA throughput.

If your motors are grouped on a single MCC, you can use a single boost transformer. Or use individual transformer for each motors.

Check with GE, Cutler-Hammer, Square D, etc for "buck-boost" dry-type transformers.

 

[electricpete]

If you are forced into using motors below their voltage rating, pay special attention to starting, which may be more severe on the motors. Minimize number of starts. Close discharge valves on centrifugal pumps during start (assuming that can be done without pump damage... perhaps with orificed recirc path) Start at time of day when voltage is highest. etc.

 

[jbartos]

Suggestion: In some cases, it is possible to change taps of the existing transformer (e.g. 5%) that has 208V three phase nominal voltage output. This voltage would increase to 218.4V which could suffice to have your existing 230V motor terminal voltage motors operating properly.

 

[jOmega]

Probably one of the best references around

ACME ELECTRIC CORPORATION

http://www.acmepowerdist.com/ael11.html

Questions & Answers about Buck-Boost Transformers


Here's a sample from the Q & A found on the above website.

[blue]"1) What is a buck-boost transformer?[/blue]

Buck-boost transformers are small single phase transformers designed to reduce (buck) or raise (boost) line voltage from 5-20%. The most common example is [red]boosting 208 volts to 230 volts,[/red] usually to operate a 230 volt motor such as an air-conditioner compressor, from a 208 volt supply line.

Buck-boosts are a standard type of single phase distribution transformer, with primary voltages of 120, 240 or 480 volts and secondaries typically of 12, 16, 24, 32 or 48 volts. They are available in sizes ranging from 50 volt amperes to 10 kilo-volt amperes.

Buck-boost transformers are shipped ready to be connected for a number of possible voltage combinations."

Much helpful information is provided there, including how to calculate the kVA requirement of the Boost transformer.

HTH

 

[Marke]

Hello gjenkins
motor is less efficient when running below full load.
This is not necessarily correct, infact some motors are more efficient at about 75% load than at full load. Additionally, as the motor size increases, the motor efficiency tends to increase also. I have seen installations where a larger motor has been used at around 75% load because a) it was more efficient that an appropriately rated motor at full load, and b) the reduced loading would reduce the winding temperature and prolong the life of the motor. The major dissadvantages at a given voltage are, increased motor cost and higher starting current. Operating the motors on reduced voltage will reduce the start torque, but I don't believe that that will cause any problems for you unless you are requiring a very high start torque.
Best regards, Mark Empson

http://www.lmphotonics.com