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Using low voltage three phase capacitors in single phase

Using low voltage three phase capacitors in single phase

Using low voltage three phase capacitors in single phase

(OP)
I understand 3 ph PFC capacitors are delta connected (correct me if I am wrong). I plan to apply single phase (415 V) to only 2 of the 3 capacitor terminals for using in parallel with loop test circuit for MV machines.

This is on the idea of reducing the current at the supply point.

Will this work ?


* Basically, I would like a full-time job on part-time basis *

RE: Using low voltage three phase capacitors in single phase

Hello edison123
That will be equivalent to connecting a single phase capacitor with a value equal to 1/2 of the three phase value of the capacitor.
As far as will it work, what are you trying to achieve? What are the KVARs of the single phase load in relation to the capacitor  KVARs?
yours

RE: Using low voltage three phase capacitors in single phase

(OP)
Thx waross.

When we do a flux loop test, especially on large machines, a large inductive current is needed and such a large source is difficult to arrange to sites. My idea was to have caps in parallel to reduce the supply source current.

I will be shortly doing this experiment with 3 ph caps (since I couldn't get my hands on single phase ones immediately) and will be back here with the results.


* Basically, I would like a full-time job on part-time basis *

RE: Using low voltage three phase capacitors in single phase

(OP)

OK. Here are the results

3 ph, Caps 415 V rated

25 KVAR - 3 ph current 34.7 A  - 1 ph current 30.8 A
10 KVAR - 3 ph current 13.10 A - 1 ph current 11.10 A

And the caps didn't blow up!! So, the idea should work, in theory at least.

I will be back with more results when I put my idea into actual use.


* Basically, I would like a full-time job on part-time basis *

RE: Using low voltage three phase capacitors in single phase

I think that your best results will relult when the capacitor current equals (and cancels) the inductive current of your test set. That is the inductive current on the supply side.
What is the rated current and approximate power factor of the test set?
respectfully

RE: Using low voltage three phase capacitors in single phase

I guess I need a little help here...
If 3-phase caps are connected in delta, would not the
current be more in single phase, two-terminal usage?
There should be one cap paralleled with two other caps
in series, adding up to 150% of the value of one three-
phase leg.  No?  And if, for some reason, they are wye
connected, then the terminal value should be 1/2 (assuming
no neutral connection).

<als>

RE: Using low voltage three phase capacitors in single phase

The two caps in series each get 1/2 voltage and the KVARs are 1/4 of rated.
Example Three phase 6 kva capacitor.
One cap across the line = 1/3 of rated kvars. = 2 KVAR
One cap at 1/2 voltage = .5 KVAR
another cap at 1/2 voltage = .5 KVAR
2 KVAR + .5 KVAR + .5 KVAR + 3 KVAR = 505 of 6 KVAR.
There is a root 3 factor between phase current and line current. When this factor is properly applied the current readings supplied by edison123 correlate very closely with the 50% KVARs calculated.
yours

RE: Using low voltage three phase capacitors in single phase

(OP)
Here are the results with with 3 phase capcitors used in single phase in parallel with the loop test circuit.

Loop (ind) Current      42.6    41.7    41.2    41.5    41.1    40.6
Capacitor Current       45.6    39.7    32.7    23.8    18.3    11.6
Supply Current           32.6    30.3    27.9    28.6    30.2    32.6
3 ph Capacitor (Kvar)  35       30      25       20       15       10

Frequency - 50.5 Hz

Strangely, the supply current was not reduced as expected.


* Basically, I would like a full-time job on part-time basis *

RE: Using low voltage three phase capacitors in single phase

A question edison123.
Is the capacitor connected on the input or the output of the test set? I would tend to connect the capacitors in parallel with the input to the test set.
I got the following figures with a rough and ready estimate on the connection with the 25 Kvar cap.
The 25 Kvar looks optimum for the settings you were using.
Reactive current component of the test set; = 30.2 amps
Active current component of the test set; = 27.878 amps
Actual current of the test set; = 41.2 amps.
Uncorrected Power factor; = 67.7%
You mentioned earlier a voltage of 415, so I will use that figure.
Active power; = 11.569 Kw.
Apparent power; = 17.098 Kva
Reactive power; =  12.699 Kvar
Reactive power supplied by the capacitor; = 13.570 Kvar
Over correction of reactive power; =  0.8715 Kvar.

Summary; It looks like you were running at about 67.7% power factor. Your test set-up  must have more real losses than you anticipated.
You have succeeded in eliminating the reactive component of the current with the 25 Kvar Capacitor. Its nominal value on single phase will be 12.5 Kvar. The actual value is well within normal tolerances for power factor capacitors.
Note to the perfectionists. I scaled this out on a piece of paper with a drafting scale. If you apply a rigorous analysis to this data, you will find some small errors. You will also find that the errors are well within the tolerances of the capacitors and probably within the precision of most ammeters. Actually, If any one wishes to apply a rigorous solution and report the percentage errors, I will be delighted.
Congratulations edison123. You have demonstrated a proof of concept in that you were able to eliminate virtually all of the reactive current.
respectfully

RE: Using low voltage three phase capacitors in single phase

(OP)
Thx waross but the result is not what I expected.

Both the inductive and the capacitive circuits are connected in parallel directly from the 415 V supply.

The inductive pf was supposed to be low (at least in theory)since the current was only circulating a working flux around the back iron. Sure, there is a core loss but I thought the watt component of the current will be low and mostly it would be only a lagging current.

Guess, I have to hit the books again.


* Basically, I would like a full-time job on part-time basis *

RE: Using low voltage three phase capacitors in single phase

H edison123
Most of your real losses may be in the test set, rather than the circuit under test.
respectfully

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