Rotor/stator capacitance in induction motors 4

[Skogsgurra]

I am looking for typical values for rotor to stator capacitance for induction motors. Anyone?

If these values are not available I would very much appreciate a list of typical stator bores and rotor diameters and/or air gap widths for standard induction motors all the way from FHP up to 1+ MVA.

 

[GusD]

Hi skogsgurra

Your post mentions capacitance values between Stators and Rotors.Just in case you meant to say inductances I'll give the answer as I know it.Perhaps I am out in the left field,but;it won't be the first or last time.
I normally test our motors with a motor tester that gives me value s of Capacitance as well as Inductance.Your question is "Stator to Rotor Capacitance" so maybe my answer is not what you looking for.
The capacitance values that I normally find on motors from 20 hp to 2500 hp are between 75000 pF to 200,000 pF.
The theory of this Capacitance reading is the following: If you compare the motor to a Cap,then the copper wire in the Stator is one Plate in the Cap.The Iron core is the other Plate and the insulation is the Dialectric (basic a Cap).When the motor is contaminated by "conductive dusts" carbon or other,it increases the size of the plates and consequently the Cap value.The Inductance values are given from a made up formula that takes in consideration the magnectic core area,# of turns ,Freq.etc.TheInductance Value of the Windings are from .010 mH to .90mH or so.
As for the air gaps in nduction motors,it may be 2mm for small motors to 8 mm for larger motors (Total Air Gap).
Dc Motors,Sync motors and others will have greater air gaps.Some large syncs approx.a 1/4" or more.
SKogs,I have a feeling that your question asks for something different than what I provided.
If I didn't help this time,I'll try harder next time.
Good Luck.

GusD

 

[Skogsgurra]

Thanks GusD!

This is the first time I even got close to useful numbers. And there was no mistake, I am looking for nanofarads, not microhenries.

The background is that I am involved in some thinking about EDM in motor bearings. There are - as you probably know - the four failure mechanisms; Capacitive voltage division, Induced shaft voltage, Stator HF voltage and Triboelectric charge of the rotor/shaft. There has been quite a lot of discussions around these things, but I have never seen any hard numbers in these discussions - just estimates or, rather, guesstimates.

So your numbers are really useful and I would like to ask you for some more. If you picture a situation where the bearings were insulated: which capacitance would there be between the stator winding and the rotor? and between the rotor and the frame?

Actually, I am looking for these numbers for the whole range of sizes, from FHP to MVA and if anyone out there knows of any site where they can be found, please post!

Thanks again GusD! And thanks for Eng-Tips it is a great idea and a great site.

Skogs

 

[GusD]

Hi skogs.

If you like, you could send me an E-mail so we can discuss the issue.If we find any info that may be helpfull to others we post it on ENG TIPS.Unless you have a better idea.

My E-mail= dalmeida.gus@syncrude.com

Thanks Skogs

GusD

 

[GusD]

Hi skogs
I just remember we had discussed this issue a previous time
try thread237-30937 . Some info there.
My E-mail is still there.

Good luck

GusD

 

[jbartos]

Suggestion to skogsgurra (Electrical) May 6, 2003 marked ///\\ This is the first time I even got close to useful numbers.
///Some of these figures should be available for the systems grouding calculations since the capacitances to the ground contribute the the ground fault charging currents.
Visit

http://www.pacsci.com/support/documents/APNTE106.pdf

etc for more info
\\

 

[Skogsgurra]

jb,

I am coming back to the question about rotor to stator capacitance because I referred to your link

http://www.pacsci.com/support/documents/APNTE106.pdf

and I am now approached by one of the guys that actually tried to find any useful information in there. He says that he cannot, and I must confess that I cannot either.

Would you please explain where and how I can find any data on rotor to stator capacitance in that paper? And also explain how that value could have any influence on ground fault charging currents?

 

[jbartos]

Suggestion: Visit the link and look for:
Cable and Motor Capacitance on page 9 top.
Motor winding capacitance is often interpreted as stator to ground capacitance, unless there is a wound rotor motor where there would be capacitance between the stator winding and rotor winding.

 

[Skogsgurra]

No jb,

The question was for rotor to stator capacitance. NOT winding capacitance or cable capacitance. Those are values that I can measure or calculate easily. I go to this forum to get answers to things that I do not know myself, not to be misled by irrelevant postings. In this case, I did not do a "sanity check" and just forwarded your link to a collegue, which now came back and asked what I was talking about.

 

[electricpete]

You mentioned that gap.

I think that is the answer. As you know C = epsilon*A/d.
d is gap distance.
A = 2*Pi*r*h.
epsilon = permitivity of free space.

May not be exact but will get you in the ballpark. It seems to me that most induction motors have air gap in the range of 0.1" across a pretty wide range of horsepowers. Maybe someone else can elaborate on typical airgap numbers.

 

[jOmega]

Hi Scogs,

Back in 1995, Messrs. Jay Erdman, Dr. Russ Kerkman, Dave Schlegel, and Dr. Gary Skibinski did some nice research on the subject.

They published a paper on their research that appeared in the IEEE Transactions on Industry Applications, Vol. 32, No. 2, March/April 1996.

The paper is entitled:

Effect of PWM Inverters on AC Bearing Currents and Shaft Voltages

In that paper, they presented a model of the motor showing the various capacitive elements.

The following data for a 15 HP, 460v, 8-pole motor,was presented in the paper.

............. Calc.Value.. Measured Value
C_sr........... 100 pF ..... 100 pF
C_sf........... 10.5 nF ..... 11 nF
C_rf........... 1 nF ....... 1.1 nF
C_b............ 200 pF ...... 200 pF[ul]C_sr = Capacitance Stator to Rotor
C_sf = Capacitance Stator to Frame
C_rf = Capacitance Rotor to Frame
C_b = Capacitance of bearing[/ul]
Skogs, there's also a graph (Fig 16) that plots the C_rf for 4 pole and 6 pole motors against HP.

Also discussed is the methodology used to calculate and measure the values.

Skogs, hope this is helpful to you.

 

[Skogsgurra]

Thanks jomega,

You deserve another red star!

 

[Highspeed]

Another usefull link on that topic:

http://www.ab.com/drives/techpapers/ieee/ieee.html

Good luck
HS

 

[jbartos]

Suggestion: Reference
Doyle Busse, Jay Erdman, Russel J. Kerkman, Dave Schlegel, and Gary Skibinski, System Electrical Parameters and Their Effects on Bearing Currents, IEEE - Applied Power Electronics Conference (APEC), San Jose, CA, March 3-7, 1996, pp. 570-578 and IEEE Transactions on Industry Applications, Vol. 33, No. 2, March/April 1997, pp. 577-584.

available at:

http://www.ab.com/drives/techpapers/ieee/ieee.html

concentrate on:
1.
Fig. 2 Common Mode Equivalent Model
2.
Cb = Bearing capacitance
Csr = Stator to Rotor Capacitance (capacitive coupling mechanism, accomplished by the Stator to Rotor Capacitance)
Csf = Stator winding to Frame Capacitance
Crf = Rotor to Frame Capacitance