# Running 208v motor on 220v service



## toolman_ar (Jun 1, 2017)

I am looking at a grinder and have a question regarding the power requirements.

I have a 2hp Ronk phase converter. It produces 220v, 3 phase power.

The grinder is 3 phase, 208v, 2hp, 5.76 amps.

Do I need to reduce voltage to the grinder to run with my phase converter?

Toolman_ar


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## JimDawson (Jun 2, 2017)

That one is a tough call. Most newer motors will run 208-240 just fine.  If your phase converter is actually putting out 220V you should be fine, but I suspect if your power is anything like mine, you will be closer to 240V.  I would wire it up and put an amp probe on it, if the current draw is within reason, you should be fine.  The motor insulation is good for at least 600V, probably more.


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## toolman_ar (Jun 2, 2017)

Jim, 
After I wrote 220v, I thought I need to go look to confirm. I have not checked, but think you are correct, most likely 240v.

This grinder is most likely bigger than I need. The seller has not confirmed the wheel size. Just trying to do my homework. Before dragging home something I cannot use.

toolman_ar


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## JimDawson (Jun 2, 2017)

I just checked my voltage for today, low volts 241.2 at 10:00 PM, high volts 245.2 at 7:20 AM.  I suspect you will be fine anyway, but it would suck to have to install buck transformers.

You might check with your local motor shop for an opinion.


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## Ulma Doctor (Jun 2, 2017)

generally speaking, if the nameplate voltage is within 10% of supply voltage, you'll be just fine 
for example 240v service - 24v(10%)= 216v
216 is 8v higher than 10%, it should cause no real problems.
the amp draw will be less than nameplate amps @240v


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## JimDawson (Jun 2, 2017)

Mike, I was hoping you would chime in here.   so running a 208 volt motor on 240 volts the amp draw will actually be lower than nameplate?


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## toolman_ar (Jun 2, 2017)

The neet thing about this grinder is that the manufacturer, The United States Electrical Tool Company is still in business.

Tomorrow I will contact the manufacturer and ask what they think. Also it looks like I can provide them the serial number and they can date the machine.

The web site shows the Modle 500, is offered in 240/440.  

toolman_ar


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## Chipper5783 (Jun 2, 2017)

240 volts on a 208V motor?  Not an issue.  As, others have pointed out - check that the current draw does not exceed the name plate value.  Do you have a clip on current meter?  Even a cheap one would be adequate.


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## Ulma Doctor (Jun 2, 2017)

JimDawson said:


> Mike, I was hoping you would chime in here.   so running a 208 volt motor on 240 volts the amp draw will actually be lower than nameplate?


Yes Sir Jim,
we know from Ohm's Law, (if the resistance is kept constant)
the voltage in a circuit increases- the Amperage will decrease


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## markba633csi (Jun 2, 2017)

That's true for transformers but not for resistive loads Mike.  V over R equals I.  Increase V and you will increase I.
Mark S.
ps there may be some goofy inductive effect that makes the current go lower, I don't know


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## toolman_ar (Jun 2, 2017)

Turns out US Electrical Tool is out of business. The plant auction was 4/17/2017. Makes me sad to see an old company like this fold.

Thank you for your advise on the Electrical question!

toolman_ar


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## Rustrp (Jun 2, 2017)

toolman_ar said:


> Turns out US Electrical Tool is out of business. The plant auction was 4/17/2017. Makes me sad to see an old company like this fold.
> 
> Thank you for your advise on the Electrical question!
> 
> toolman_ar


I think you will be good on the voltage difference, and especially when you consider the possibility the motor hasn't seen 208v in it's lifetime. It's close to the allowable guidelines.


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## 4GSR (Jun 2, 2017)

Oh yeah, should work fine.  I ran a 2 HP 208 volt motor on a home made static converter  40 years ago.  Motor never got hot. Go for it!


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## Ulma Doctor (Jun 2, 2017)

markba633csi said:


> That's true for transformers but not for resistive loads Mike.  V over R equals I.  Increase V and you will increase I.
> Mark S.
> ps there may be some goofy inductive effect that makes the current go lower, I don't know


Ohm's law holds for circuits containing only resistive elements (no capacitances or inductances) for all forms of driving voltage or current, regardless of whether the driving voltage or current is constant (DC) or time-varying such as AC. At any instant of time Ohm's law is valid for such circuits.


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## talvare (Jun 2, 2017)

The formula for three phase motor electrical calculations is : I= HP x 746 / 1.73 x E x Eff x Pf. Since the motor data plate doesn't indicate an efficiency rating or power factor we can only calculate a simple theoretical amperage rating. Amperage (I) at 240V = 3.6. Amperage at 208V = 4.14 . This validates Mike's voltage/current relationship.

Ted


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## markba633csi (Jun 3, 2017)

Yep just as I suspected, that darn high-falutin' math strikes again!
MS


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