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Is this leg to high?


Cast Iron
Dec 4, 2009
N.E. Pa.
I have a rotary converter powering my clausing colchester A50 cnc lathe. I measured from leg to leg with the spindle running at 1000 rpm. I got 218, 222, and 247. Is the 247 measurement to high? If it is what can I do with it? If its not do I need to wire it into a certain terminal in the control?
Yes that’s too high, too much of an outlier. No idea how the rotary is constructed so no advice there- what type or brand/model?
Most CNC's use a single phase leg to power the logic side of the control.
That leg is too high, as already stated.( no help there) In addition the logic side of the control should be connected to the non generated output of your converter....
Cheers Ross
The rotary is a homemade using the motor in the picture. So to bring that leg down I need a buck transformer to go from 247 to somewhere between 220 to 230? Is it possible to make a voltage divider to drop it? I have 2 wire wound ceramic resistors about 7 inches long. Not sure of the rating of those.


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Are you just using a 3p idler? Typically there are a number of capacitors between the legs with the sizes selected to best balance the voltages. There are probably 6 or 8 in the control box on my diy rotary converter.
I have a clausing storm 80 using a rotary phase converter .
Close to the same machine you have I also had the same problem you have. My voltage was to high it would throw an alarm .

In your phase converter you have motor start capacitors and motor run capacitors you need to adjust the motor run capacitors to balance the voltage so each leg is close to the same voltage.
To reduce voltage on the output of the RPC, you need to reduce the run capacitance on the leg with 247 volts. To do this use a smaller capacitor on that leg or in my case I took one out.
Also under the machine next to the coolant tank right right side behind the panel (not in the control cabinet) there is a transformer make sure it is tapped for the correct voltage.

My rpc puts out close to 240 volts on each leg .
The Fanuc control is rated +- 10 percent of rated voltage .
So if you have your transformer taps in the machine on
set on 230 volts you should be fine no need for a buck transformer.
I found the transformer it was already set to 225 volt. The schematics in the manual and the nameplate on the door both say 230 volts. Is standard tolerance +/- .010% if it is then the supply voltage could go as high as 253. If this is the case do I need to worry about it at all? On the other hand my phase converter is homemade and has no capacitors at all right now. Can I simply put a few caps between the high leg and one of the others to bring the high one down?


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It was 2001 when I installed the rpc for my clausing lathe .
Haven't touched is sence. But this is what I remember.
The 247 volt leg you can't bring it down with capacitors .
but you can increase the voltage to the other 2 legs with capacitors to come close to the 247 volts its best to have all the voltages close you won't get it perfect If all legs are within 10 volts you're ok .If the voltages are to far apart im told it may be bad for the motor.

You're 10% over the 225 on the voltage tap.

To know if machine doesn't like the voltage. Go in to mdi run your spindle up to 3000 rpm stop the spindle using M5 or reset button.
If the spindle stops with no alarm I think you're ok .
When the spindle stops it turns the motor into a generator
sending the voltage into a resistor if this voltage is to high
you will get an alarm and the spindle will coast to stop instead of braking to a stop then i think you will need a step down transformer.

Also I never hooked up the neutral or the ground wire to the lathe.
There was a reason can't remember why.
My voltages are 241 236 and 234 been running this way for over 22 years .
Can I use this capacitor to help with the balancing? It only has the 3 terminals on the top.


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If your 247V measurement is what your utility supplies, then change the taps on the machine and add some run capacitors to equalize the voltage .
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I have started it yes. I had to switch 2 of the wires going into the machine originally because it started for a few seconds then threw a hydraulic low pressure alarm. I can run it now and at random it will throw that same alarm. It might run for 5 minutes to 15 minutes without throwing that alarm I don't know. And when I say run I mean sitting idle with the hydraulic pump running. I did test spindle direction and coolant flow and they were correct. I even ran a very short facing and turning program. I stopped using it however because after the low pressure alarm about 3 times at random I decided to check the incoming voltage and that's what made me write this post. I saw that high leg and I got nervous I was going to blow up a drive or something. Maybe this whole problem is a problem with the hydraulic unit.
Your line L1 & L2 is in the 240's range. You generated leg L3 is too low causing the low readings of 218 & 222.
How does the idler start? Read your L3 with just idler running. Will probably be a lot higher.
Get some oil filled capacitors and a little religion. Go ask members with your size RPC: What caps do you use?

What typically happens is a RPC with no load has a higher L3. When the load is turned on the L3 drops down to the 240 range.
Is the idler 15 Hp? Does the A50 use a 5 Hp motor? I have a 5 Hp RPC with run caps 50 uF and 90 uF.

I would use a 100 uF and two 100 uF in parallel for the two caps. I scaled my RPC down to 3 Hp once and the run capacitors scaled down as well.
Maybe get a couple of 50 uF and a 25 uF for real fine tuning. You can get withing a few volts phase to phase if you want.

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I think your rpc is ok . Take the front cover off where the pressure gauges are you will see the hydraulic pump motor it will have an arrow for the direction it should turn check it, this is the only way to tell if the phases are correct. The fanuc control doesn't care about the orientation of the phases. Turn the chuck pressure all the way up how much pressure do you have? Check the hydraulic fluid level.
If you still get the low pressure alarm this tells the hydraulic pump
is wore or you chuck actuator is leaking more than it should . On my clausing lathe my pump is worn when i do a tool change I get a low hydraulic alarm pressure goes to zero and goes back up after the tool change. I disconnected the wire to the low pressure switch alarm tool changes work fine. Also to get more pressure out of the pump you can change the hydraulic fluid to a higher viscosity.
Tell us what the pressure is. If you have a tail stock what is the pressure on it.
It might be a few days before I can check the hydraulics. I disconnected my wiring to do the balancing. Hopefully that won't take long.
I finished the balancing tonight. My capacitor values were 10uf between the 2 legs going into the idler motor. 110uf between A and C coming out of the motor and 90uf between C and B coming out of the motor. My voltages between legs was 250,250 and 247. The idler is a 15hp.