3 Phase Motor Voltage

Leo G

I have ordered a 3HP 3 Phase motor. I had a choice between 2 motors, a TECF and a TECF Haz Location. I chose the latter and didn't notice the voltage requirement was different from the TEFC . The standard TEFC had a 208-230/460 volt range and I made an assumption that the other motor was the same. I just noticed that it is a 230/460. My power system is a 208 3 PH. Am I going to be able to run the motor off this lower voltage without doing harm? Or am I going to have to get a boosting transformer? The voltage is about 9.5% low, usually a 10% variance is considered OK for motors. There is a thermal shutoff control within the motor. Will this protect it if it gets to hot from the low voltage? Thanks for your help.

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mdshunk

A 230 motor run at 208 will run a slight fraction slower, a bit hotter, and have slightly shorter service life. In industrial establishments, if a 230 motor must be run, a buck/boost transformer is normally utilized to mitigate these issues. I suspect that the hazardous location version of this motor is actually the same motor inside. Because of the extra sealing involved with giving it the hazardous duty rating, they likely don't want to list it as being suitable for 208 since it will run hotter at the voltage, and the extra sealing keeps the heat in even more. Luckily for you, you say this motor has a thermal overload. Many or most 3 phase motors do not. I'm not sure what application you're using this for, but if it was for myself personally, I'd wire it up and examine the need for a buck boost transformer later. This motor would only take a very tiny buck/boost transformer anyhow, which is not very expensive.

Be advised that the thermal overload in this motor may not be what you think it is. Some 3 phase motors have an extra set of leads marked "T" and "T". These are a set of simple make and break type wires hooked to the thermal unit in side the motor. They are intended to be run out to connect to the stop circuit of the motor starter. Not sure if that's what you're getting or not... just a heads up.

Leo G

Yes, the thermal protection is just that - a thermostat that gets connected to the starter control stop connection. If it gets too hot it turns the contactor unit off, good enough.

If and when I get a buck/boost transformer, is there a 3 phase buck/boost transformer or do I need to get 3 separate transformers and connect them to each phase? I already have installed a boosting transformer to my tablesaw (boosted 208 to 220 vac 1PH), piece of cake, about $125 (Acme). Could you tell me the size of the transformer I would need? Or just a link to a site that sells them would be good.

The motor is going to be used to drive a 36" Tubeaxial fan at 950 RPM, so it's a good sized load.

A question about your comment that the motor will run slightly slower. Aren't 3 PH motors actually frequency driven for speed? If I am using it on a 60Hz circuit shouldn't it drive the motor at the nameplate revolutions. Unless you are talking about less voltage means less current, which will mean that motor slippage, will be more likely. I was going to give you a link to the motor that I am getting at Graingers, but their site is having maintenance at this time, I'll try later for this info. Thanks MD:Thumbs:

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mdshunk

The ACME buck/boost transformer would be the one I'd recommend. That's the brand I use. They have a fantastic website, that will provide you with all the information you need to select your buck/boost arrangement. For 3 phase motors, sometimes you need two transformers, and sometimes you need three. Get me the name plate amp draw of the motor, and I can spec the buck boost arrangement out for you.

mdshunk

True, in part. Two identical UNLOADED 230 volt motors, one connected at 230 volts and the other connected at 208 will run at nearly the same speed. Load both of those motors near their name plate horsepower, and the one connected at the lower voltage will drop off in measured RPM's and torque.

Leo G

I would agree. Torque is directly related to ability to draw current from the source. 230v will give more ablilty to draw more current according to Ohm's law. Thanks for the clarification. Not bad for a woodworker eh? I took a bunch of classes in electronics and electrical during high school and college. Then I ended up working in wood, who would of known?

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mdshunk

I took wood shop and ended up twisting wires together for a living. I guess you and I balance each other out. All is right with the universe now.

Leo G

Here is the link to the motor. This has the specs to the motor including the amperage draw. It is a link to the Grainger Catalog.

http://www.grainger.com/Grainger/pro...3135936&ccitem=

If the link doesn't get you there type in grainger.com and in the search put in 6JR76 use the Grainger item # for the search. Thanks again.:Thumbs:

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mdshunk

Okay, for your application, you'll need two Acme T-1-81050 transformers, connected like the drawing below. These cost about 70 bucks each, and you'll need two.

Attached Images

 

Leo G

I was looking a the schematic and have a question for you. I notice that the two transformers are connected in a "Y" configuration (delta wye?). I can see how that #1 and #3 phase are boosted by the transformer. But the #2 phase seems to be connected straight through to itself form input to output. Does that mean that phases #1 & 2 are at 230v but the #2 phase remains at 208v? I looked and looked but couldn't figure out how the 2nd phase gets boosted. I can wire it like this no problem, but would really like to understand how it works too. Thanks.

I looked up the transformer and also found a different schematic which was a wye configuration. This I can understand how it boosts the 3 phases incorperating the neutral. With your diagram I assume that the neutral goes straight through and the transformers ride on the phase to phase voltage. Does this put the system in an unbalanced condition? Is this the price you pay to use two transformers instead of three? Is the wye configeration (my schematic) a better way to do it or are they both equivalent? Sorry for so many questions, but as I said, I'd like to understand how these things work so in the future I won't have a need to ask questions because I can figure things out for myself. BTW, do you still dabble in the woodwork? Thanks agoin MD.

Attached Images

 

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mdshunk

Umm... no, it's not hooked in a Y configuration at all. It's hooked in an "open delta" configuration, which is why you only need two transformers. It may help to look over the two catalog pages that I used to select the transformer and get the proper connection diagram:

Selection chart
Connection diagram page

Each phase is not boosted individually per se. When you measure the output side, you will measure 230 volts between any two phases, I assure you. If you were to measure any one phase on the output side to ground, you'd get wacky measurements. For your motor, you don't need a neutral on the output side or we'd need to take another boosting approach.

You will note when you study the pages in the links that we could have used three T-1-81048 transformers connected like diagram AA to serve your motor too. I elected not to spec it that way for you, since the you'd need three transformers instead of 2. The cost of the ...48 transformer and the ...50 transformer is about the same. This also eliminates the extra expense running a neutral to the transformers and possible associated conduit upsizing.

Leo G

Hey Md,
Where are you getting them for $70? The place I get mine from want $98. Do you have a link you could share? Or are you getting them from a local distributor? Thanks.

Leo

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mdshunk

From my local distributor. You wouldn't want to buy them online, since the shipping would kill you. These things are like bricks. I suspect I get a better pricing schedule than the man on the street.

Leo G

Shipping isn't horrible, $20 for both. I guess i'm stuck with the $98 bucks apiece.

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