# Determine voltage of single voltage motor?



## Barncat (Jun 30, 2022)

Let’s say you have a 3 phase motor with only 3 leads, and no motor nameplate, so it is a single voltage motor.  Based on size you guess it is 5-7.5hp. You have no other info to go on but just have the motor sitting on your bench.  Does anyone know of a definitive way to determine through some sort of electrical test if it is a high or low voltage motor?


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## cathead (Jun 30, 2022)

An ohm meter probably wouldn't help much.  I would likely apply 220 volts to two of the wires (any two) and spin up the motor with 
a piece of rope and see what happens.  If it runs, I would let it run and watch for any signs of overheating.  If it was a higher voltage
motor, it would probably not spool up but basically this type of thing would have to be classified as an experiment.  Do so at your
own risk.


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## rabler (Jun 30, 2022)

I'm assuming you want not only a definitive test but a non-destructive test!  
And that your limiting the options to 220 vs 440.  There are some other voltages 208, 380, 400 ...

Two options, neither one is perfect :
1) You could measure the full impedance of the motor and make a reasonable guess based on similar motor locked rotor behavior.  This would take more than your basic ohmmeter as you need the inductance as well as resistance.

2) Hook it up to 220 for a second or two, and see if it spins up normally, if not try 440.  220 briefly won't damage a 440 motor.  440 applied _briefly_ to a 220 motor probably won't instantly damage the motor, but is more risky, and it would spin up.

edited to add: @cathead was a bit faster


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## WCraig (Jun 30, 2022)

rabler said:


> And that your limiting the options to 220 vs 440. There are some other voltages 208, 380, 400 ...


And if it came from Canada, could be 575 volts!

Craig


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## cathead (Jun 30, 2022)

I am referring to what I would do in the shop using what I had at hand, that being 220V single phase.  A three phase source would
make it somewhat easier.  As I said, it is an experimental approach and quite safe if executed safely.  (No pun intended)


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## tq60 (Jun 30, 2022)

If no data plate then guessing at hp.

Somewhat simple approach.

Get a line cord to 120 volts first.

Clamp on Amp meter.

Connect across any 2 wires, place Amp meter across one and plug in to measure current then unplug.

Do NOT touch the motor and have it resting on wood.

Repeat across a possibilities. 

Lower voltage goes same place as high voltage but less risk of damage .

If no shorts then try 240 volts.

You can compare the Amp readings to online charts for motor hp.

If the motor looks to be 2 hp but draws about 1/2 current it may be wired as high voltage, current close then low voltage.

Sent from my SM-G781V using Tapatalk


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## Barncat (Jun 30, 2022)

I am reasonably sure it is either 220 or 440, it came from a local defunct business, and those are the options in the area.


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## Barncat (Jun 30, 2022)

rabler said:


> I'm assuming you want not only a definitive test but a non-destructive test!


Certainly my preference, yes.


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## markba633csi (Jun 30, 2022)

I'd do what Cat suggested, try it on 220 with a spin.
No info at all? No brand? No idea where it came from or what it was used on?
-Mark


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## ddickey (Jun 30, 2022)

How do you know it's three phase if you have not seen the nameplate?
So there are only three wires in the peckerhead?
How about a pic.


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## graham-xrf (Jul 1, 2022)

If you are going to take the route of "trying out voltages, starting from the lowest first", at least do a visual check of the connections to discover if it was wired in "star" connection", or "delta" connection. Three wires and no sign of capacitor makes it three phase.
DO check continuity of any of those wires to the ground frame connection. They should be insulated, with no leaks to ground.

Then - start with the "star" connection. This is safest, it being the higher voltage type series connection.
This forum, and Google, has the most extensive descriptions of these, but I am assuming that all be stuff you already know.

If you can temporarily hijack a VFD, or a 3-phase variac, you can test it to your hearts content.


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## ddickey (Jul 1, 2022)

Not all single phase motors require start capacitors. If it truly is only three wire it will be connected internally. You more than likely won't be able to tell if it's connected delta or wye.


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## Barncat (Jul 1, 2022)

Only three wires, no capacitors. None of the three go to ground. Similar resistance between any two of them. I have actually asked a similar question already about the motor in question in this thread
https://www.hobby-machinist.com/threads/help-with-a-440-volt-motor.95693/
I am revisiting this because the person I got it from informed me he may have changed motor starter/coil or some other things when he couldn’t get it to run, but couldn’t remember because it was so long ago. So my assumption of it being 440 based on coil in the starter may be wrong. Hence wondering if there was any concrete way to electrically test if it is 220 or 440. My local motor shop quoted me $1500 to rewind to 230.


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## rabler (Jul 1, 2022)

Barncat said:


> Only three wires, no capacitors. None of the three go to ground. Similar resistance between any two of them. I have actually asked a similar question already about the motor in question in this thread
> https://www.hobby-machinist.com/threads/help-with-a-440-volt-motor.95693/
> I am revisiting this because the person I got it from informed me he may have changed motor starter/coil or some other things when he couldn’t get it to run, but couldn’t remember because it was so long ago. So my assumption of it being 440 based on coil in the starter may be wrong. Hence wondering if there was any concrete way to electrically test if it is 220 or 440. My local motor shop quoted me $1500 to rewind to 230.


Based on the machine, do you have any idea of what RPM it ran at?   Typically a bit less than 1200, 1800, or 3600?  Spinning at that RPM with another motor, and seeing what voltage it generated with some reasonable load (such as another 3 phase motor) would be another test of interest.


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## BGHansen (Jul 1, 2022)

+1 on Randal's comment, beat me to the post.  Couldn't you run the motor with a second motor and measure the voltage output?  You'd have to make some assumptions on the RPM's.

Bruce


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## Barncat (Jul 1, 2022)

Will running off another motor make the  nameplate voltage? With no input voltage?


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## Barncat (Jul 1, 2022)

I believe the rpms are around 3600


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## rabler (Jul 1, 2022)

Barncat said:


> Will running off another motor make the  nameplate voltage? With no input voltage?


Possibly, it depends a bit on how long since the motor has run, and if it has any residual magnetism.  It will either ramp up to generating some voltage, or nothing (zero).  If it ramps up then measure the voltage.  Ideally you would put some nominal load on the wires.  If it was me, I'd wire two  100W incandescent bulbs in series between a pair of wires and measure the voltage there.  If the bulbs quickly burn out you can assume it tried to generate more that 240 volts.


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## Barncat (Jul 1, 2022)

rabler said:


> Possibly, it depends a bit on how long since the motor has run, and if it has any residual magnetism.  It will either ramp up to generating some voltage, or nothing (zero).  If it ramps up then measure the voltage.  Ideally you would put some nominal load on the wires.  If it was me, I'd wire two  100W incandescent bulbs in series between a pair of wires and measure the voltage there.  If the bulbs quickly burn out you can assume it tried to generate more that 240 volts.


This seems like a non-destructive test at least. I will try this next time I have some free time. I have a small motor I could spin it with, but it is currently hooked to a rpc as a pony motor. I need to see what rpm it is. Thanks, some good things to think about here.


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## Barncat (Jul 9, 2022)

Well I spun it up to about 3750 rpm (based on the pulley sizes on it and my drive motor) and got 7 volts. I guess there isn’t enough magnetism left to generate 230 or 460?


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## ddickey (Jul 9, 2022)

How did you measure? was it 7v everywhere? line to line, phase to ground?


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## Barncat (Jul 9, 2022)

Line to line, all three combinations were close to that.


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## ddickey (Jul 9, 2022)

If you could check phase to ground and it was the same as line to line then you'd know you had a delta wound motor and it would be 240 or 480. More than likely it is.


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## rabler (Jul 10, 2022)

Barncat said:


> Well I spun it up to about 3750 rpm (based on the pulley sizes on it and my drive motor) and got 7 volts. I guess there isn’t enough magnetism left to generate 230 or 460?


Sorry, should have made this clear earlier. Did you run it with something loaded, or nothing connected? It needs some current path to build up beyond self-excitation, electro-magnets work on current, not voltage.  Without a path to build some current flow (through some load), the rotor doesn’t get an induced current, so it never gets started building a real voltage.    Long shot that there is enough residual magnetism to start that process, but without a load to act as a current path it is doubly handicapped.


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## Barncat (Jul 11, 2022)

What kind of current path? Like a light bulb that is just wired between 2 wires? Or? And then how to test, remove the load and test voltage between 2 wires?


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## rabler (Jul 11, 2022)

Barncat said:


> What kind of current path? Like a light bulb that is just wired between 2 wires? Or? And then how to test, remove the load and test voltage between 2 wires?


Yes, like the incadescent light bulb.  Ideally 3 bulbs wired to a common center, but you could try one bulb between two wires.  Leave them in and measure the voltage across 2 wires.  You are basically building a three phase induction generator.


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