460v motor winding resistance

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dogleg

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Good morning, sorry for any rule breaks its been years since posting here. I have a 200 hp compressor motor fed from a 200hp soft starter cabinet. There is 1 disconnect between soft starter and motor . Disconnect is a few feet from each compressor. The picture is the name plate for the 200 hp motor. It's a 12 tap single Voltage motor. It came with the taps set up in delta run configuration. So that is how I connected my 350 kcmil via crimp lug and nut and bolt. My concern is that when I measured resistance between legs on load side of disco with it open the resistance is very lol. When testing my meter leads I get about 0.2 ohms. Also I get the same between all phases on load side of disco which leads me to believe resistance is basically 0 ohms. I have 2 of these compressors and both read the same. I megged between motor casing/egc and each phase and get 2.2 gig which is max for my fluke. So if the resistance between windings is indeed zero ohms does this indicate a problem? I was expecting to see slightly higher resistance. There are no anomalies between different phases they all read same resistance. These mtrs are brand new. I checked taps before connecting they all matched name plate diagram all the way to the right for delta-run. Any thoughts from the large brains in here would be appreciated. Sorry for slang and punctuation if any. Entering this via mobile phone. Thanks
0d8634c397c61b196da262c08394ea9f.jpg


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Welcome to the forum, or welcome back, as appropriate.

Could this simply be the difference between resistance and impedance?

Most motors and other inductive loads, such as solenoids, have a much lower DC resistance, especially while at rest, than an AC impedance, especially while operating.

You can use Ohm's law to calculate the operating impedance with the voltage and current numbers.
 
You're measuring the DC resistance of the motor windings. As long as there's no continuity from the phases to ground, you should be OK.


SceneryDriver
 
If you want to test motor windings assembled I believe the tool you need is a "impulse winding tester".
 
When running at a 252A line current, a delta winding would be conducting about 145A (assuming PF=1.0 just for a very rough calculation). If the series resistance of the winding was 0.2 ohms, then there would be a 145 x 0.2 = 29V drop across this resistance which is 6% of 480V. And so 0.2 ohms measured at DC does not raise any red flags.
And as ptonsparky has noted, a general purpose handheld meter lacks sufficient accuracy at such low resistance levels.
 
Thank you all for your comments. I just put a fluke 87 on resistance. I understand that I don't have the right equipment. I basically just was looking for differences of resistance between the legs. Ans so there weren't any that I could detect. I expected maybe something like 0.2 or maybe 0.5 somewhere in that range. Zero just concerned me a bit ide like to keep the magic smoke in. I have more studying to do on the matter.

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There are lots of different tests that can be run on motors, that all look at different characteristics.

For measuring the DC resistance of the windings you will need a 'milliohm meter'. I've used this one and it does a fine job:

I agree with the others above, 0.2 ohms measured using an ordinary meter wouldn't raise any red flags for me. The number is quite high for a motor of that size, but with an ordinary meter the error in the measurement is greater than the value you need to look for. I'd expect a number in the 0.01 ohm or below range if measured with a proper low resistance tester.

-Jon
 
When I was young I remember seeing the guy in the motor shop using a really old telephone receiver to check motor windings. I don't remember how that worked, but the old man would listen to the receiver and if it made a sound the winding was shorted.
 
So like I said it's been like 10 years since last post lol. I thought I may get disciplined for asking in wrong section of forum. Or for maybe incomplete info or something. But that didn't happen which will encourage me to post more. I respect the opinion of the long term moderators and members. Appreciate it.

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Just get yourself a Megger and test for insulation leakage between windings, and for leaks to ground. If there is any kind of fault, in the stator most likely you'll find a weird reading.


megger.jpg

They can be a lot of fun around the shop too 🤣
 
My fluke 87 has a nS scale (nano-Siemens).

In my experience, if there is a shorted coil (where you would expect a slightly lower resistance common to one motor lead) the motor will trip on overload or burn up quickly. If there is an open coil, it will make a lot of noise as it single phases. An open coil on a wye connected motor is easy to find. Any reading using that lead will read open. A delta connected motor is not as obvious, but can still be determined. The resistance measurement across the open coil will read an amount twice the resistance of the other two combinations of leads. If this happens, suspect a poor connection in the pec... um, the connection box.
 
TwoBlocked said:
My fluke 87 has a nS scale (nano-Siemens).
Click to expand...
That is 1/gigohm, correct? A very high resistance reading, not a suitable for measuring winding resistance.

TwoBlocked said:
In my experience, if there is a shorted coil (where you would expect a slightly lower resistance common to one motor lead) the motor will trip on overload or burn up quickly.
Click to expand...
Yup, but the short of a single coil might be a 1 milliohm change on a 10 milliohm reading, not something you will see with a multimeter :)

TwoBlocked said:
If there is an open coil, it will make a lot of noise as it single phases. An open coil on a wye connected motor is easy to find. Any reading using that lead will read open. A delta connected motor is not as obvious, but can still be determined. The resistance measurement across the open coil will read an amount twice the resistance of the other two combinations of leads. If this happens, suspect a poor connection in the pec... um, the connection box.
Click to expand...

Strongly agree on the above.

-Jon
 
ptonsparky said:
I had to go look at my 87. Never, ever did I use the nS scale. I don't believe I even knew it was there.
Click to expand...
I haven't either and kinda forgot what it was for. I seem to recall that scale being recommended in checking wiring on a ship's ullage system. Changed and/or swapped all components and still had intermittent problems with just that one tank. Wonder if they ever solved it.

What I was really getting at, but not saying directly in my last post, is you can detect opens on a motor, but don't expect to detect shorts. The magic smoke will beat you to it. The way it was explained to me is a shorted winding acts like an autotransformer with a shorted coil. Very high current which get things very hot and quickly destroys the insulation.
 
Out aboard ship in north Pacific. Needed to check a 3 MW rotating rectifier generator rotor winding resistance as output was not regulating.

Soldered 6 D cells in parallel (enough to drive near 10 Amps) and hooked up 2 fluke 77 meters, one for current, the other voltage. Wired as 4 terminal resistance tester.
The D cells drove 9.xx amps, voltage read on the mv Fluke scale. All OK. You could use the same setup for your motor dc resistance test. There are commercial 4 termial low resistance testers as already discussed.

Found that a 'fast-onl' spade connector on Basler regulator had gotten partially loose from diesel vibration which is what caused the problem.
 
LarryFine said:
Can't you use a low-voltage or a variable transformer for motor-winding testing?
Click to expand...
We had a mix-up on the leads of a compressor motor one time. Trying to remember just what we did. Took 120VAC and put a heat gun in series to reduce the amps. We then put two motor windings in series and measured the voltage across them. Then switched the polarity of one of the coils and measured the voltage again. The polarity with the least voltage was the correct polarity because of the impedance. ... or something ... About that time we got better info from the manufacturer which proved we chose correctly.

But that was to check polarity. You can determine the reactance of a coil or capacitor with a variable resistor and an AC source. Put the resistor in series and adjust it until the voltages are equal. The resulting resistance of the variable resistor then equals the reactance of the coil or capacitor at that frequency. A calculation can then be made to determine the Henries or Farads. (If you don't have a variable resistor, there's additional math you can use.)

To compare one motor winding to another, seems you could just use a filament light bulb in series with each coil and measure the voltage across the coil. A lower voltage than the rest would indicate less reactance, suggesting an internal short. The bulb would also burn brighter.

I think you could use just the primary or just the secondary of a small transformer the same way as a choke coil. This would give you two choke coils in series, the motor winding being the second one.

Oh, probably be best to remove the rotor when testing. The relative angle between the rotor bars and the motor coils would affect the impedance.
 
junkhound said:
Out aboard ship in north Pacific. Needed to check a 3 MW rotating rectifier generator rotor winding resistance as output was not regulating.

Soldered 6 D cells in parallel (enough to drive near 10 Amps) and hooked up 2 fluke 77 meters, one for current, the other voltage. Wired as 4 terminal resistance tester.
The D cells drove 9.xx amps, voltage read on the mv Fluke scale. All OK. You could use the same setup for your motor dc resistance test. There are commercial 4 termial low resistance testers as already discussed.

Found that a 'fast-onl' spade connector on Basler regulator had gotten partially loose from diesel vibration which is what caused the problem.
Click to expand...
Got it! Put a known DC amperage through the coil and measure the voltage across it. The higher the voltage , the higher the resistance.
 
Oooo, better add some caveats before I get jumped on:

Everything that conducts electricity has some capacitance, inductance, and resistance. There will be some error in determining the reactance of coils and capacitance in the way I mentioned. One cool thing that can be observed is the voltage drops across the resistor and the reactor, when added together, will be more than the source AC voltage. This is because the currents will be up to 90 degrees apart.
 
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