Syhcnronous motors 1

[kevindunford]

Can you explain the Tan delta test to me in brief.The results from the Tan delta test a value is given per phase eg. Red 406 at .2pu volt and 449 at .4pu( tan delta times 10000), what is this value? and at the same time you receive a capacitance value on the red phase of 42.19 nf at 02pu volt and 42.66nf at .4pu volt, what is the capacitance value telling you.

 

[Skogsgurra]

The tangens delta is a means of seing if there are any imperfections in the insulation. A perfect capacitor (two plates in a vacuum) has tan(d)=0.000 which is another way of saying that the phase angle between voltage and current is exactly 90 degrees. Which, in turn, is another way of saying that the capacitor has no losses at all.

Delta is the "distance" between phase angle alpha and 90 degrees and also a measure of the losses in the circuit. The reactive power in a circuit with capacitance C, voltage U and frequency f is

Q = 2xPIxfxCxU^2

The losses in same circuit is

Pd = tan(delta)xQ

So, if there are high losses, you will get a high tan(d) number. And low if losses are low.

Losses tell a lot about your insulation. Dirt increases losses. Leakage currents/bad insulation in general also do that. Partial discharge increase losses.

Doing a tan(d) test at different voltages usually says something about partial discharge. If tan(d) increases when you get above a certain voltage, you almost certainly have PD.

The capacitance values are simply that, capacitance values telling you what the capacitance between winding and frame is.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Hi Gunnar;
Thanks for the information.
Is tan-delta similar to what I knew as the power factor of a capacitor?
As I remember, a perfect capacitor had a power factor of 1.00.
Is tan delta basically the same test with the results expressed as a different trigonometric function, or am I completely out to lunch.
Please share some comments with us.
Respectfully

 

[Skogsgurra]

No, Bill. I wouldn't say power factor. "Loss factor" seems to better a better choice of words.

If we are talking power factor (as in cos(phi)), the capacitor has 0.00 PF (no heat in it). The resistor has 1.00. It is possible that there is another definition for capacitors, but I haven't seen that.

By the way: I just had lunch - so I may be completely mistaken. Got no siesta...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Hi Gunnar;
Years ago, I came across some information in an old text book in regards to the power factor of a capacitor.
My initial reaction was similar to yours.
On further reading, I realised that they were expressing the quality of a capacitor by considering the losses and the reactive current and calculating a "Quality" figure that they called the power factor of the capacitor (not the circuit).
I forget the details, but it sounds a lot like the same principal as a tan delta test, but applied to capacitor quality or leakage instead of motor quality.

I just checked some old texts and found a refrence;
Bushings rather than capacitors but the same principal.
Westinghouse Electrical Maintenance Hints, (1974)
Gives a procedure for checking the power factor of bushings over 69KV for capacitor bushings, breaker bushings and transformer bushings. Depending on the temperature and bushing type they show a range of acceptable power factors of the example bushings of from 1.3% to 5.4%
I am sure I have seen similar use of "power factor" to quantify the losses of lower voltage capacitors.
Respectfully
Bill

 

[Skogsgurra]

Yes. That IS the power factor. If you have a 1.3 % loss in the bushing, then your power factor is 0.013. But I am afraid that would cause much unnecessary discussions and misunderstandings. So I prefer using "loss factor" or simply tan(delta), which seems to be the accepted term when looking at capacitance from the insulation point of view.

But, I think you are right physically. It is a power factor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

The same type of testing called tan delta in Europe is called power factor in US.

Mathematically there is a subtle difference. Let's say I have an impedance which can be represented as a series resistance R and capacitive reactance Xc, giving total impedance Z = sqrt(Xc^2+R^2).

pf = R/Z
tan delta = R/X
For typical insulating components, R<<X and X~Z so there is no practical difference.

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[Skogsgurra]

Thanks for that clarification, Pete.

I suspected there was a semantic twist to it.

William A Ross (guessing now) and I usually have the same opinion on things. So I was surprised that we we using different words here. Thanks again. PLS for you.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Thanks for the information Pete.
Right on Gunnar.
Respectfully