ECN Forum Forums General Topics of the Trade General Discussion Area Motor Voltage

A 20 horsepower, 230 volt three phase trash compactor is being relocated from one Public Works Department location to another part of the City. The new location only has 208 volts, three phase available. Would it be acceptable to feed the motor with 208 volts?

I would not do it for long term use. 208V is at the low end of the tolerance range for most motors. At this low voltage you can expect lower starting torque and increased running current, also the life of the motor will be shortened.

This is the perfect application for a buck-boost transformer. Your application will require two single-phase transformers connected in an open-delta configuration. Your 20HP motor will need at least 21.5KVA.
For boosting 208V to 230V (10% change) you will need 1.5KVA transformers.
For 208V to 236V (13.3%) use 2KVA.

You can get an on-line buck-boost calculator at http://www.squared.com. Then go to the Support and Resources section and choose Free Software and On-line tools.

The 208 would be just above a 10% tolerance (207). What was the system voltage that was previously feeding the motor, 240? In my opinion, it would be better to have a properly rated motor. Table 430.150 states that the currents listed shall be permitted for system voltages of 110-120(?), 220-240, 440-480 and 550-600. Conspicuously absent is 208. I think 208 stands alone. I see dual-rated 230/460 motors regularly, but know of no 230 volt systems. These motors may be designed to operate on 240.
BTW, table 430.150 has a column for 3phase 110 volt motors. Interesting!?

Thank you for replies. In the previous location the 20HP 230 volt, three phase motor was fed 230 volts.

What is approx. cost of said boost/buck transformers?

Many new motors that I have been seeing have 3 FLAs listed on the nameplate. One for 208 volts, one for 230 and one for 460.

Has anyone ever worked with a three phase 115 volts system? I've never seen it anywhere, but it must exist or there wouldn't be a column for it in Table 430-150.

Don(resqcapt19)

2002 NEC, See Section 110.4 Voltages.
Throughout this Code, the voltage considered shall be that at which the circuit operates.

The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

Since the motor is rated for a higher voltage than available it would be legal for this app. However, it will not develope it's nominal HP at this voltage and will overwork its self if something is not done to limit the load on the motor. It is probably hydralic so if chain/sprocket coupled, change the sprocket ratio to gear down the motor. There may also be a way to reset the bypass pressure so the motor dosn't work as hard. The other thing to do is resize the overloads to protect the motor. You can make this work but be prepared for the system to be slower or at least to trip out fairly often.
Don

I've got to agree with JBD. A great application for boost xfrmrs.