Motor/Generator Winding Capacitance's for AC Hipoting

[VLFit]

To calculate the power required from an AC Dielectric Test Set one must know the capacitance of the load. Simply listing the electrical specs of the machine does not reveal the capacitance to permit the use of A=(2pif)CV to calculate the current required at the test voltage required. Is there any table, chart, calculation, etc. to help in figuring the uF of the windings to get an idea of the kVA needed for the test? Perhaps each vendor has a chart revealing this info or some rules of thumb for calculating the capacitance based on the HP, pf, V, etc. of the motor.

Thanks for the help.

 

[edison123]

Each winding has its own capacitance. You need an AC Schering bridge to measure its capacitance. A tan delta kit has this bridge and will measure the capacitance and the tan delta.

Motor HP, pf and V do not enter into this calculation.

Muthu

http://www.edison.co.in

 

[VLFit]

Thank you for your reply. Yes, we can always measure the capacitance, but often the people involved do not have that equipment. We produce high voltage testers for this and other purposed and I was really hoping for some rules of thumb or charts with approximate uF ratings based on the m/g size and other parameters were available. I realize the accuracy will not be precise but that is ok. I want to know if the uF rating is 0.01 uF or 0.2 uF , 0.8 uF, etc

 

[edison123]

As I said, it varies from winding to winding. There is no thumb rule on this, only actually measured data on the records. The AC HV kits for these machines vary from tens of KVA to hundreds of KVA. As a rewinder, I have multiple AC HV kits to cater for machines from a few KW to hundreds of MW.

Muthu

http://www.edison.co.in

 

[electricpete]

There was a lot of discussion of a similar subject in thread237-278636

As mentioned there, you could get a ballpark estimate for a form wound machine if you knew the number of slots, slot dimensions (Area = slots * 2*L*depth+L*width), ground insulation thickness (d) and ground insulation dielectric constant (epsilon, somewhere in the range 2-4 times epsilon0) simply using C = epsilon * A / d.

Also one reference tabulated in that thread tabulated zero sequence capacitance (which I’d interpret as the same capacitance you’re interested in for testing… capacitance of entire winding to ground) for a handful of motors of various ratings.

If I wanted to make some guesses about how winding capacitance to ground generally varies with voltage, power and speed, I’d say A is proportional to the stator volume (which itself is roughly proportional to hp and inversely proportional to speed) and the d is proportional to voltage among form wound machines. But as edison mentioned there is expected variation among designs (even with identical ratings). You could attempt to make a statistical prediction if you had a large number of measurements of motors (along with their ratings). Doble probably has that. Obviously nothing beats a measurement of a machine with identical design to the one you're interested in.


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(2B)+(2B)' ?

 

[VLFit]

Thanks for your post electricpete. I knew i was asking for too much due to the complexities and the multitude of winding designs. It makes it tough to properly size an AC test set for withstand and PD and TD testing of generators when the user doesn't know enough about AC versus DC testing and has little idea of the capacitance of the load. Thanks