Motor & Generator Winding Capacitance

[VLFit]

Hello All,

Does anyone know of a table that shows common motor sizes and the average capacitance from the terminals to ground, particularly for stator windings?

I am aware of the many influences on this and the impracticality of my question, but I just thought I would ask if there is some chart showing the typical motor sizes, in terms of kw, V, A, pf, etc. that also shows the typical winding capacitances. I can calculate it a number of ways if I know the necessary data, and usually have to, but I was hoping there is some table or rule of thumb based on kw and voltage that has estimates of the capacitance.

Thank you

 

[Gr8blu]

VLFit Total (AC stator, OR wound rotor induction rotor) winding capacitance to ground is found using the following formula, which requires knowledge of both winding conductor arrangement and stator (core and slot) geometry.
C = 9.15 * S * (L-n*b) * 2 * (hs-hw) * 0.0000001 / [0.5 * (W - cu)]
where
C = capacitance in microfarad,
S = number of stator slots
L = gross stator core length in inch
n = number of stator vent ducts
b = axial dimension of stator vent duct in inch
hs = radial depth of stator slot in inch
hw = radial depth from stator ID to bottom of stator wedge in inch (or from rotor OD to bottom of rotor wedge for wound rotor induction)
W = width of stator slot in inch
cu = total width of BARE copper in stator slot in inch

Since most "standard" (e.g., sub-375 kW) ratings use random wound stator designs, accurately calculating the winding capacitance to ground can be tougher than one might think, as the last variable becomes moderately randomized due to the nature of the winding and the multiple options for winding a given rating.

Converting energy to motion for more than half a century

 

[petronila]

VLFit,

The capacitance value is calculated mainly for MV and HV stator windings and you will need some core dimensions(core length, slot depth) and the number of slots.

For stator core sizes in inches, you can use the formula C= K Q L (2H+W)/d (see Benefits of the AC hi-pot for new form coil stator windings, EASA CURRENTS, August 2013)

where C is the capacitance in microfarads, k is the insulation permittivity= 0.9*10-6 microfarads/in, Q is the number of slots, L is the gross stator core length, H is the slot width and d is the single insulation thickness (in) that relies on the rated voltage of the machine ( 0.035 for 2.3 kV,0.060 for 4 kV, 0.076 for 6.6 kV and 0.135 for 13.8 kV).

Best Regards

Petronila

 

[VLFit]

Thank you for your messages. Yes, I can calculate the uF if I have all the info as to the design of the motor, but that is usually not the case. I was hoping that there was maybe a rule-of-thumb or some such shortcut to approximate the capacitance, particularly for a stator winding, based on the input voltage, kW rating. full load current, etc. Even something with a 25% accuracy rate would help in determining what kVA AC hipot a motor shop might need: 1kVA, 10 kVA, 100 kVA...For now we just have to continue to dig for info, like the charging current from a previous test, the current for maybe only a 1 kVac test and then scale it up to the higher test voltage, or just go through the exercise of calculating it from the construction data.

Thanks,
VLFit

 

[Gr8blu]

VLFitCheck the following link. On the right side of the white paper are some fairly generic current limits based on the type of electrical equipment undergoing the AC hipot test. If you know your test voltage, you can quickly figure a reasonable test set kVA rating.

https://hvinc.com/wp-content/uploads/2019/04/AC-Dialectric-Test-Set_Update_Feb25_2019.pdf

Converting energy to motion for more than half a century

 

[petronila]

VLFit,

Based on the information provided by Gr8blu and the formula extracted from EASA´s current article seems the capacitance should be calculated and you will need to know more detailes about the stator winding and its sizes thus I am afraid that such rule of thumb might not be available and I will say that will need to build your own table based on your own experience. However, if you don't want to make calculations theere are some Meggers that can provide you capacitance readings.

Best Regards

Petronila

 

[edison123]

In our repair shop, we test individual coil/bar and calculate the KVA needed for the final AC test of completed winding. We have multiple AC HV kits ranging from 100 KV/150 mA for coils/bars and 36 KV/6 A for completed windings.

For overhauled machines where rewinding is not done, we use AC HV test or tan delta kit at 1 KV to measure current / capacitance and calculate from thereon.

Muthu

http://www.edison.co.in

 

[VLFit]

Thanks to all. I guess that's a no to my question - which is about what I was expecting. I appreciate the various recommendations.

Thanks,
VLFit! - on motors and cables.

 

[electricpete]

For resistance grounded systems, design of ground resistor requires estimate of the system capacitance in the design stage. Accordingly Industrial Power Systems by Kahn (CRC Press 2008 ISBN 978‑0‑8247‑2443‑6) gives the following gross estimates for 4kv motors of various ratings / speeds

Table 4.8 Zero-Sequence Capacitance Data for Induction Motors (μf/phase)

Rating in kW, kV, 1800 rpm, 1200 rpm, 900 rpm, 600 rpm
225 4.0 0.009 0.012 0.014 0.016
300 4.0 0.01 0.013 0.015 0.017
350 4.0 0.012 0.014 0.016 0.018
400 4.0 0.014 0.016 0.017 0.021
Source: Westinghouse, “System Neutral Grounding and Ground Fault Protection,” publication PRSC-4B-1979, Westinghouse, 1979.

I cut/pasted and the columns don't line up but I think you can figure them out.

There might be a factor of 3 required to obtain total capacitance to ground since it's zero sequence capacitance stated as "per phase" (I'd have to think about that a bit).

I agree with the caveats provided by the others, these are just rough numbers for particular machines, and old ones at that.

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(2B)+(2B)' ?

 

[electricpete]

Here is another reference from which you could estimate motor capacitance to ground

https://megaresistors.com/products/neutral-grounding-resistor/capacitance-to-ground/

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(2B)+(2B)' ?