Winding Resistance

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Ken9876

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I have a 120 HP 460 VAC Inverter duty motor driven by a VFD, yesterday the drive tripped on output overload. While checking resistance between the load size terminals to the motor (which were isolated from the drive and only connected to the motor) I measured a very low resistance of .1 ohm between windings. I then checked the leads to ground and found that the circuit is grounded, which I still need to troubleshoot. I thought the reading between windings was low so I checked another similar drive and motor and found the resistance between windings low as well. So what would be a normal reading between windings? I thought a reading of a few ohms was the norm.
 
Ken9876 said:
I have a 120 HP 460 VAC Inverter duty motor driven by a VFD, yesterday the drive tripped on output overload. While checking resistance between the load size terminals to the motor (which were isolated from the drive and only connected to the motor) I measured a very low resistance of .1 ohm between windings. I then checked the leads to ground and found that the circuit is grounded, which I still need to troubleshoot. I thought the reading between windings was low so I checked another similar drive and motor and found the resistance between windings low as well. So what would be a normal reading between windings? I thought a reading of a few ohms was the norm.
Click to expand...

I guess my question to you would be what resistance did you anticipate?
This is a very simple explaination.
When you measure winding resistance you are measuring a simple length of wire of that winding. Would not one expect that the resistance be very low then since you are measing resistance with a DC source? If you measure the resistance of each individual winding one would anticipate that each one would result in a very similar resistance reading since all windings would be made up of the same length and gauge of wire. As such measuring winding resistance of each winding in and of by itself really is of not relevance unless you compare the reading of each winding and find that there is a relevant difference which may indicate some of the windings may have shorted to themselves resulting in a lower resistance or open which would be strange should the motor still be operational.
When AC voltage is applied to the motor it is the interaction between the stator and the rotating rotor that provides the needed stator resistance which is not evedent when measurng the simple winding resistance with a ohm meter when the motor is de-energized.
Then there is an insulation test that would also assure that the winding to ground resistance is within acceptable values and that there isn't insulaion failure in progress.
Personally, I would target an insuation test by having the motor meggered to varify the insultion integrety. You may not find a thing an would have to consider another issue.
 
Ken9876 said:
So what would be a normal reading between windings? I thought a reading of a few ohms was the norm.
Click to expand...
OK. In round figures a 120HP motor would draw around 140A at 460V.
Assume that passes through a winding of a few ohms - say, 2 ohms
The dissipated power, being i2R, would thus be in the region of 40kW.

Does that help explain why it cannot be a few ohms?
 
Besoeker said:
OK. In round figures a 120HP motor would draw around 140A at 460V.
Assume that passes through a winding of a few ohms - say, 2 ohms
The dissipated power, being i2R, would thus be in the region of 40kW.

Does that help explain why it cannot be a few ohms?
Click to expand...
Your example concerns load impedance. Though impedance is also in units of ohms, DC resistance of [uncompromised] windings is always lower than the nominal operating impedance.
 
Well, considering I was asking this question with the knowledge that small single phase motors, such as a compressor motor would have 3-4 ohms resistance between windings or are the books I?ve read deceiving me? I was expecting something more than I had measured, but thanks again for the perfectly clear answers.
 
The reactance part of the impedance doesn't doesn't draw any real power. The windings are heated by only the resistance (xI^2). My translation of what Bes said is the winding resistance can't be 2 ohms. If it was, at FLA, the i^2R losses would 40kw - and that just isn't possible.

ice
 
Smart $ said:
Don't believe me. See Table 14.4 at the following...

http://books.google.com/books?id=4-...a=X&ei=g9D4T5CLA-Kf6wGLjtnXBg&ved=0CFMQ6AEwAw
Click to expand...
Thank you for the information.:thumbsup:
Somewhere between 0.04 and 0.09 ohms according to that table.
Which rather nicely confirms the point I was making.

As does the following data from an actual motor in service on a variable speed drive in a pumping station.

625kW02.jpg
 
iceworm said:
The reactance part of the impedance doesn't doesn't draw any real power. The windings are heated by only the resistance (xI^2). My translation of what Bes said is the winding resistance can't be 2 ohms. If it was, at FLA, the i^2R losses would 40kw - and that just isn't possible.

ice
Click to expand...

Thanks ice for helping to clarify my point.
 
Besoeker said:
Thank you for the information.:thumbsup:
Somewhere between 0.04 and 0.09 ohms according to that table.
Which rather nicely confirms the point I was making.

As does the following data from an actual motor in service on a variable speed drive in a pumping station.

625kW02.jpg
Click to expand...
I'm reading what you are saying?can't be 2 ohms and 0.04 and 0.09 ohms according to that table... rather nicely confirms the point I was making?so I must be misinterpretting (obviously from your perspective :p). In saying the winding resistance can't be 2 ohms, there is an infinite amount of values it can be not taking stated conditions into account. Taking conditions into account, are you saying it is some value less than 2 ohms... because most certainly somewhere between 0.04 and 0.09 ohms according to that table (which I agree with) isn't 2 ohms, and definitely impossible to be 2 ohms???
 
Besoeker said:
All I'm saying is that the winding resistance can't be several ohms.
Click to expand...
But what I'm saying is saying it can't be several ohms or 2 ohms isn't saying what it can or should be. Could be more. Could be less. Please state in terms of what it can or should be!!!
 
The straight answer and exquisitely simple example calculation are in post #3. I clearly understood the language that Besoeker used to teach myself and the OP.

As a field electrician, I'm asked by my supervisors to troubleshoot things like multiple ballasts parking lot lighting circuits and manufacturing machine tools with nothing more than an ohm meter. Besoeker's explanation will help me better interpret motor winding resistance measurements.
 
With all due respect often time there are those who want to be given a blank or white answer, that is that they don't have a clue as to what the purpose of what they are doing is or why but only want a go or no go value. As such they don't have to think as it's either good or bad.
What is being asked here is what the ohmic value should be which there is none unless the motor manufacturer may have a published value available but probably not. There may be someone who has an idea about what the typical value one should expect from previous experience.
But, never the less, a winding of a motor consists of a length of wire. If you remove that winding and un-wide it and lay it out, then take an ohm meter and measure it's resistance would that value be any different that if you left it as been wound in the motor?
As I started in a previous post the dc resistance of a length of wire will be dependent basically on the gauge of the wire and its length.

Doing a resistance check doesn't accomplish much except to check for an open winding or comparing the measured resistance value of one winding to the others. Should there be enough difference to be of a concern to you then further investigation as to the cause would be warranted.

When doing electrical maintenance work it is important to have an understanding of the process and not be dependent on absolute values.
 
Besoeker said:
Winding resistance for the 140HP motor can not be several ohms.
How more straight and direct do you want it???
Click to expand...
First, OP stated 120HP motor. You had it right in earlier post. Perhaps a typo here???

As for straight and direct, perhaps something like this:

Winding resistance for a 120HP 460V 3? motor should be about 0.08 ohms.
 
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