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Home > Archive > Electrical Engineering > August 2005 > Calculating Balanced Voltage
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Calculating Balanced Voltage
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| gtslabs 2005-08-24, 12:21 am |
| In my Phase converter I took measurements of unloaded and loaded volts
and amps. Knowing that my loaded Amps on L3 is 1/4th of L1 and L2 can I
calculate the balanced capacitance needed from L1-L3 and L2-L3 ??
I know it can be done using trial and error using different capacitance
values. But is there some vector mechanics that would get theoretical
values?
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| Don Kelly 2005-08-24, 12:21 am |
| "gtslabs" <gts@nb.net> wrote in message
news:1124850660.229583.281740@z14g2000cwz.googlegroups.com...
> In my Phase converter I took measurements of unloaded and loaded volts
> and amps. Knowing that my loaded Amps on L3 is 1/4th of L1 and L2 can I
> calculate the balanced capacitance needed from L1-L3 and L2-L3 ??
> I know it can be done using trial and error using different capacitance
> values. But is there some vector mechanics that would get theoretical
> values?
>----------
Probably yes, IF you have more information. You do not mention whether or
not the voltage (unloaded) is balanced or not, nor do you have any phase
information for the currents -i.e. load power and power factor on each
phase. Also, is the output Y, Y grounded at neutral, or delta. Ifg grounded,
what is the neutral current?
--
Don Kelly @shawcross.ca
remove the X to answer
----------------------------
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| gtslabs 2005-08-24, 12:21 am |
| Don, this is the information I collected:
Volts
L1&L3 L2&L3 L1&L2
Unloaded 225.4 222.4 238.5
Loaded 205.2 194 226.7
Volts
L1&G L2&G L3&G
Unloaded 121 119.3 189.5
Loaded 110 108.6 156
Amps
L1 L2 L3
Unloaded 19.1 19.1 <2
Loaded ~60 ~60 ~15
Idler: 15hp
Load: 10Hp
I do not have a neutral so I used Ground for my measurments.
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| gtslabs 2005-08-24, 10:21 am |
| This morning I put 60 uF between L1 and L3. It brought down my unloaded
idler Amps from 19 to 14 amps on L1 and L2.
It however made no difference when I was running my 10hp compressor. I
have problems starting my compressor under load and thought balancing
the amps would help. My compressor does unload correctly so it has to
be an electrical problem. If I put the run capacitors across the
compressor motor will that help ? - But the I dont see how that would
be isolated from the idler circuit.
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| Don Kelly 2005-08-29, 12:21 am |
| ----------------------------
"gtslabs" <gts@nb.net> wrote in message
news:1124852190.580344.184510@o13g2000cwo.googlegroups.com...
> Don, this is the information I collected:
>
> Volts
> L1&L3 L2&L3 L1&L2
> Unloaded 225.4 222.4 238.5
> Loaded 205.2 194 226.7
>
> Volts
> L1&G L2&G L3&G
> Unloaded 121 119.3 189.5
> Loaded 110 108.6 156
>
> Amps
> L1 L2 L3
> Unloaded 19.1 19.1 <2
> Loaded ~60 ~60 ~15
>
> Idler: 15hp
> Load: 10Hp
>
> I do not have a neutral so I used Ground for my measurments.
Off hand this looks as if you have a high impedance on line 3. This is
likely the reason that the voltage on 3 is high. Have you accounted for all
loads? The balance is poor. Without ground, there is no reason for the
voltages to ground to be equal but they shouldn't be this far out.
Again, is it Y or delta ? If Y, where is the neutral connected? Are the
load motors Y or delta and, if Y , are the neutrals tied to the motor frames
and hence to ground?
Your line to line voltages aren't balanced but are not bad enough to cause
the current variation shown. Your loaded line to line voltages follow
along.
The unloaded currents seem to indicate the presence of a damn near single
phase load between line 1 and 2 and the loaded currents are similar. You do
have a large neutral shift. in both cases.
Line 3 appears to have a high impedance either in the converter or in the
motor.
What load is actually present in the unloaded case? You indicate idler 15Hp
and load 10HP which makes little sense without more detail .
Have you any measurements of the motor impedance (motor disconnected from
supply) between phases and phase to frame as well as frame to ground?
In any case it looks as if you are almost single phasing which does present
problems.
--
Don Kelly @shawcross.ca
remove the X to answer
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