View Full Version : 240 volt single phase Phase inverter

08-04-2006, 08:35 AM
Hi Guys,
I apologise I know this subject has been done death, But I can't find out on the other threads where I am going wrong. The diagram I got off one of the single to 3 phase inverter plan sites. My problem is I'm in Australia and we have 240 volt with one active (hot) line, one neutral to the fuse box and one earth. I used a start capacitor wired to a .5 second switch and two 50 uf run capacitors. This worked OK starting the 3ph motor quickly, the trouble is when I go to balance the capacitors.
1. Active leg 9.9 amps at 242 volts
2. Neutral 6.6 amps at about 2volts at
3. manufactured leg 10.22amps at 272 volts
What am I doing wrong? I changed one of the 50 uf run capacitors for 30 uf run capacitor, this brought the manufacture leg down to 7.65 amps at 257 volts. But why is the neutral voltage so low? should I run the manufacture leg off the active line? P.S The repaired bell housing on the rotor motor is running OK now.

J Tiers
08-04-2006, 12:53 PM
The diagram was a little blurred when I went to read the text....

BUT, it sounds like you are trying to read all the voltages to ground

If one of the incoming 240 lines is really a neutral, and therefore is the "grounded conductor" (NOT the equipment grounding conductor), then the line connected to that WILL read low to ground, because it IS grounded (it's the neutral). Then the others will read some much larger voltage.....

What you end up with is 3 phase with one of the phase wires being the grounded conductor. The effective neutral for the three phase is NOT the same as ground, nor is it the incoming neutral. it is a point "floating in space" which would have a voltage to ground also, if you could measure it.

The thing to do is read the 3 phases from one to another.... and see how well they all balance out. It comes to about the same thing, but by realizing that you are measuring the phase voltages, it makes more sense.

In the US the same type thing occurs, except that the 240V is center-tapped for the neutral.... with 120V either side of neutral.

So the incoming 240V neutral in that case is also not the same as the 3 phase neutral, and measurements to ground will look wrong , although with somewhat different voltages to what you have..

Rich Carlstedt
08-04-2006, 09:58 PM
I am not sure how you are measuring amps and Voltages,
I assume amps are read with an ampmeter on the 3 lines to the motor.
That is all that is important.
Voltages mean nothing on a converter because you are not dealing with a true sine wave. it is a spike and a meter will not correctly identify the real voltage.
The only target you should look at is balance under load.
Even then, the world is not perfect. if your readings are within 25 % of each other, you should be OK.

08-05-2006, 04:20 AM
Thanks very much for the advise, I think I have the gist of it now, I was hoping to get 3 phases about the same. So when the clamp metter registered neutral so low, I thought I had it wired wrong.

The other thing was the magnetic switch on the three phase motor side would not work, it would only chatter, it works when pushed down with an insulated screw driver. But I read on another thread, you have to wire it so its not the manufactured leg that is used to start the motor, with magnet switch's?