can not be several ohms is pretty direct.
Winding Resistance
- Thread starter Ken9876
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You're entitled to your opinion, as am I... and given the context, I disagree.
You have likely gleaned what you wanted to know by now, but to answer your query directly...
A few ohms is the norm for small motors. Winding resistance varies somewhat inversely with HP rating, decreasing as HP increases.
As such there are no specific values but ball park values of what should be expected. Such values have no relavence other than as an indicator that further testing is required. Since a 3ph motor has 3 separate windings one would look for an ohmic value to be the same or very close to the same for all three windings. The exact value is not important. If one winding had an issue you should see an obviously different measurement. But it would be highly unlikely that all 3 windings had the exact some issue. No magic answers.
As such, what is the goal of measuring winding resistance other than making it easy for an inexperienced technician to conclude that it doesn't measure what the specifications say it should.
As such, what is the goal of measuring winding resistance other than making it easy for an inexperienced technician to conclude that it doesn't measure what the specifications say it should.
The context is a120 HP 460V motor.
And you disagree with my comment that the winding resistance can not be several ohms.
Yet your proffered table gives a fraction of a ohm.
My submitted calculation provides clarity on why it can not be several ohms.
My example of a of a real motor does the same.
So what exactly are you disagreeing with?
Your own data?
My calcs?
My real life example?
Alternatively, what silly game are you trying to play?
kwired
Electron manager
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Forget reactance and other AC characteristics and think of what you are measuring for a moment. Think of the motor winding as nothing more than a reel of insulated conductor when measuring DC resistance and you have access to each end of the conductor to connect your meter leads to. I don't know what size or how long for this 120hp motor, maybe somewhere around 4 AWG. If you were measuring a reel of copper conductor that size and same length of the winding what kind of resistance would you expect?
You later mention maybe 3-4 ohms for a smaller motor. The motor in that example may only be wound with 16 or 18 AWG conductor, wouldn't you expect it to have a higher resistance? measuring it would be like meassuring a reel of 16 AWG vs measuring reel of maybe 4 AWG for the larger motor - and somewhat similar length of conductor is probably being measured.
Larger conductor of same material will have less resistance than a smaller conductor of same material.
If you are getting continuity to ground you likely have windings that are no good, double check to make sure you don't have unintended path you are reading through. Motors on VFD's are usually well protected by the VFD from having violent fault current pass through them, but if this were to be connected across the line and there is a ground fault it will likely let out a lot of smoke and molten metal and will be even more obvious there is a problem, where when connected to the VFD it trips the drive much sooner to protect the drive from the resulting fault current that would happen if it did not trip as quickly.
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I see the context as...
No. But 1) it doesn't answer the question, and 2) it could be less ambiguous. Perhaps, must be less than 2 rather than not 2...??? I liken your comment to you asking me what I'm doing, and I tell you one thing I am not doing.
Correct.
An opnion which I do not share. You imply 2Ω is not possible because 40kW of I?R loss is not possible. Consider for a moment that a reader, especially the OP'er, cannot make the connection between winding resistance and heat dissipation.
It's an example, nothing more... It does not "provide clarity" from my perspective. It's not even a good example IMO: different supply voltage, output rating in[IEC?] kW, and has six values in units of ohms, not one of which is clearly labeled as winding resistance, nor did you indicate.
".can not be several ohms is pretty direct."
A truly silly question.
No.
No.
None. But I believe many individuals reveal their disposition when questioning mine.
mike_kilroy
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measure inductance instead of ohms
measure inductance instead of ohms
an idea perhaps worth sharing is to measure L instead of R. As stated, ohms is typically too low for anything other than showing it is missing or significantly different than the other windings. Even small hp motors can have R too low to measure accurately (we just finished a 1/3hp motor with 0.007 resistance). Inductance in most motors large or small is most of the time high enough to get out-of-the-mud readings. these compared phase to phase quickly identify a shorted or partially shorted winding.
And as found in OP, any R from phase to ground less than a couple hundred Kohms usually means a problem.
measure inductance instead of ohms
an idea perhaps worth sharing is to measure L instead of R. As stated, ohms is typically too low for anything other than showing it is missing or significantly different than the other windings. Even small hp motors can have R too low to measure accurately (we just finished a 1/3hp motor with 0.007 resistance). Inductance in most motors large or small is most of the time high enough to get out-of-the-mud readings. these compared phase to phase quickly identify a shorted or partially shorted winding.
And as found in OP, any R from phase to ground less than a couple hundred Kohms usually means a problem.
I've been watching these posts and wonder what the point is and I agree with you. There will be no DC winding resistance value which will have any relavent meaning other that it may lead to warrant further inspection using a relavent method. Measuring inductance is the only way that will provide a measurable value.
But the fact remains what is the objective even if you are able to measure what I think the term should be impedance.? Now what? So, you now have an impedance value. Where is that going to get you?
As I said in my previous post people want a number where they can turn their brain off and simply use the resulting value to compare it to a chart with specific values. It makes the job simple because there is no thinking involved. They want to measure the resistance, if it doesn't fall within a given value then the motor is judged to be defective. As such it no longer requires on skill and knowledge. If replacing the motor doesn't solve the problem then it's on to the next device.
I believe you and I in agreement that this is not going to happen. Winding resistance taken with a common ohm meter may cause you to suspect that there may be an issue. If there is an open winding or if you compare winding resistances you would anticipate that the would be reasonably close to the same value.
IMO, what is usually missed is the method of measuring. If you use a regular ohmmeter, maybe you can get a reliable value, maybe not. I cannot rely on ohmmeters to decide whether windings are good or not. I'd use a DC source and a lab grade ammeter if I wanted to have it accurate. But that just me.
Electric-Light
Senior Member
Let's simplify this.
If you have a 100' spool of 22/2 awg control wires, the resistance is sufficiently large that you can use a common DC resistance meter to know if the two wire are jumpered at the end or there's a short somewhere in between, because the wire resistance is large enough to provide accurate reading that makes the contact resistance at connections negligible.
If the 100' spool was 4/2, the per/foot resistance is too low to resolve length with a common meter and connection resistance would add excessive error.
Lacking specialized equipment, you could push a few amps with a power supply through a resistor while measuring the current. For example, if you hook up 2 ohm resistor and push 5 amps with a 10v power supply and you see a 11mV drop across the winding, you know that you've got 0.011/5 = 2.2mohms.
R = V/I
To read change in number of turns due to a short, you'll have to read the inductance. Measure the inductance on all 3 windings. There should be no continuity between coils and reactance should be the same between the three.
If you have a 100' spool of 22/2 awg control wires, the resistance is sufficiently large that you can use a common DC resistance meter to know if the two wire are jumpered at the end or there's a short somewhere in between, because the wire resistance is large enough to provide accurate reading that makes the contact resistance at connections negligible.
If the 100' spool was 4/2, the per/foot resistance is too low to resolve length with a common meter and connection resistance would add excessive error.
Lacking specialized equipment, you could push a few amps with a power supply through a resistor while measuring the current. For example, if you hook up 2 ohm resistor and push 5 amps with a 10v power supply and you see a 11mV drop across the winding, you know that you've got 0.011/5 = 2.2mohms.
R = V/I
To read change in number of turns due to a short, you'll have to read the inductance. Measure the inductance on all 3 windings. There should be no continuity between coils and reactance should be the same between the three.
The very opening words of the thread are:
You might not consider that context. I do
I didn't post that.
"cannot be a few ohms" were my exact words.
From the OP:
I used 2 ohms as the smallest number you might interpret to be a few ohms to demonstrate why the winding resistance couldn't be a few ohms in the very specific example given.
It isn't an implication. It's a simple fact clearly demonstrated.
Consider for a moment that the original poster made this comment:
It is very much more. It is a real motor in service and I don't see you posting real data from real machines so it's a bit rich to criticise mine.
I'm sure you can convert kW to HP, can't you?
As I think you have more than adequately demonstrated.
I agree with your way of thinking if the results would even still be meaningful. The may be more accurate but then what?
You have to wonder if it is a challenge or if you are able to get some sort of more accurate value what is the objective? It appears as though people are going around in circles with this issue. If the method being discussed was a relevant way to test the windings of motors I would believe a standard for this type of testing procedure would have been established years ago.
Well, thank you very much!!!
As for the remainder of your response, you appear to be adamantly close-minded... so I'll leave you to your opinions and move on to more productive issues.
Have a good day...
Good points! As a maintenance man in my other life, I kept a database of every motor that walked in my workshop; HP, voltage, amperage, winding connection, frame size, etc. including testing procedures to cater to the different motor troubles. But life changed much since I last went under that roof! The required response for troubled equipment has forced us to source-out repairs and all we do was replace every motor that failed and left the repair works to dedicated outsourcing shops! Most of us got rusty but we still remember the important points if ever some newbies ask.
The statement that the winding resistance cannot be a few ohms is demonstrably correct.
It is thus fact, not an opinion.
The OP has accepted my explanation.
And, whilst your disparaging personal remarks about me cause me no offence, they have no place on this forum. Do it in a PM if you want.
For winding resistance measurements I would expect all three or however many coils are brought to terminals to read the same for each phase or section of a phase to be the same for all three phases. My experience is that the resistance is generally too low to yield anything other than winding continuity or not and that's probably the most you can get out of such a test.. Difficult to determine, for example, shorted turns.
I've moved on. I suggest you do the same...
Very well stated to say the least. Spot on. At the very best you may get a result that causes you to be suspicious enough which would lead you to do more relavent testing.
Winding Resistance
I just checked the winding resistance on some of our 600 VAC, 100Hp motors. The Line to Line winding resistance is between 0.1 and 0.2 ohms. So, your 0.1 ohm reading on a 460VAC 150Hp motor sounds very reasonable. If your winding resistance is 0.1 ohms, then your losses per phase (assuming 200 Amps FLA) would be 2.3kW (or 6.9kW for all 3 phases). In any case, your winding resistance reads normal assuming all three phases are similar.
I just checked the winding resistance on some of our 600 VAC, 100Hp motors. The Line to Line winding resistance is between 0.1 and 0.2 ohms. So, your 0.1 ohm reading on a 460VAC 150Hp motor sounds very reasonable. If your winding resistance is 0.1 ohms, then your losses per phase (assuming 200 Amps FLA) would be 2.3kW (or 6.9kW for all 3 phases). In any case, your winding resistance reads normal assuming all three phases are similar.
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