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Joined: Aug 2001
Posts: 335
One of my guys ask me this and I have no (legitimate) answer therefore I need this answered by someone more knowledgable than I on motors and buck boost xfmrs. We have a 240v (single phase) motor being run on 208v with a B/B xfmr ahead of it. It's been there for quite a while but has been a prob due to burned motors (2 in 3 yrs). It is wired exactly as shown in the Acme Xfmr diagram. This puts the H1-4 in series; the X1-4 in series and the H in series with the X, basically one long series hook up. Input is the 2 ends of H and output is the extreme ends. In this configuration, one input is tied directly to one output. That means that one output reading to ground is prob. The other output has the boost added and reads about 140. The reading between the two outputs is about 238... no prob. Now the question: Is there any short or long term effect on the 240v (single phase) motor if the voltage on one leg is higher than the other? My gut feeling was that it's not a prob and we need to look elsewhere for the cause of the prob but I couldn't justify my answer other than to say "It's always worked before".
Any help would be appreciated

Last edited by Steve Miller; 06/12/09 04:23 PM.
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Joined: Jul 2004
Posts: 787
Some questions:
1) How often does the motor start?
2) Does the motor start under a load?
3) What is the HP rating of the motor?
4) What is the B/B transformer rating in KVA?
5) What is the voltage and current at the motor leads when it is starting?

What I am thinking of, is that the motor might be overheating due to under voltage when it starts.

Larry C

Joined: Aug 2001
Posts: 335
1) Runs 24/7.
2) NA
3) 1.5
4) it's correct
5) they're ok

Hence the prob. Everything reads correct (when we're there). We don't have recording eqpt. We're thinking the input voltage spikes and drops and takes it's toll over time.
We're ok figuring the cause (we'll get it eventually). My real question is the motor theory question and the uneven 2 legs.

Joined: Jul 2001
Posts: 599
JBD Offline
The motor is a 2-wire device. All it cares about is the voltage across its line leads.

Joined: Mar 2005
Posts: 1,213
Originally Posted by JBD
The motor is a 2-wire device. All it cares about is the voltage across its line leads.
I agree, there's no problem here. Having one input and one output connected is normal (and OK) for an autotransformer, too. I'd look somewhere else for the issue.

Q. Is the transformer sized properly for the motor load, and cabling sized properly for the voltage drop? If either is too small, it would have the same effect as running a long cheap extension cord: low voltage, high current & burnt-out motors.

Joined: Jun 2007
Posts: 301
Some motor manufacturers list 120/208/240 on the nameplate. Some manufacturers will not honor a warranty on a motor running on 208 if it's not listed for 208. That tells me and you that it does matter. If it's not 120/208/240 get one that is. Of course wait until this one fries. Have the correct one ready and on hand.

Joined: Mar 2004
Posts: 251
I assume this motor does not have a VFD on it. If it does DO NOT exceed the manufactures input voltage, if you do it will damage the VFD.

Joined: Jun 2004
Posts: 1,273
The motor may be undersized for the load...

Field wiring may be undersized...

Was the thermal protection constantly popping in the old motors?

Does this drive a load that is slow to come up to speed?

Do you see a lot of cycling? ( on-off-on-off-on-off )

Is the environment clogging up your bearings?

For a single phase motor the winding that burns out is going to be the start winding -- almost every time.

With 208V power available why isn't a TEFC 3-phase motor being used? Three-phase motors run cooler than single-phase. They also cost less and last longer.

I find it very hard to believe that your motor truly runs 24/7. I've never seen one do that outside of heavy industry -- and there only rarely. What is it driving?

There is no impact on the windings from boosting one leg to 140 over ground. Because the phase angles are shifted the 120V + 140V = 240V at the leads.

I'm not going to work through the phasor mathematics...

If you're interested pick up a book on physics or electrical engineering or math for the sciences.

If you want to dig around, Scott has posted essentially ALL of the wiring diagrams used for transformers.

BTW, all things being equal, it's better to have the buck/boost transformer back near the breaker. The higher voltage will reduce the amperes flowing in your field wiring.

If it were me I'd be looking at a 2 hp 3-phase motor and toss the buck/boost transformer onto my special projects pile.

Remember that a buck/boost that lifts 208 to 240 will also lift 240 up to 277. Sometimes it can be helpful to bump commercial light circuits before permanent power is available. Just remember that the neutral in this situation cannot be bonded with the ground -- anywhere. Both legs coming off the transformer will be either 120V or 140V above ground. OCPD is necessary, of course.


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