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Tuning RPC output, and/or using VFD?

specfab

Titanium
Joined
May 28, 2005
Location
AZ
I have a Tree Journeyman 325, being supplied by a 10HP RPC. As some of you may recall, I had some questions about how to reduce the voltage on the generated leg, in order to meet the factory voltage spec of 230V +/- 5%. The location of my shop receives 240V "single" phase supply, which I then run to my RPC. The outputs are then about 240+ V on two legs, and about 265 V on the generated leg. I want to be much closer to acceptable values for this machine.

I have not delved into swapping caps in the RPC up to now,as this is a mystery project for me. I would prefer not to disable my power source for the rest of the shop for an indeterminate period if not absolutely necessary. I have a couple of thoughts about options:

1. A VFD that will handle the machine's power requirements. Is there a VFD out there that can take 240 VAC single-phase in, and produce 230 VAC 3-phase output? This should be 5HP minimum, as a guess,

2. Can a variable auto-transformer of sufficient capacity be employed on the generated leg (only) to reduce its voltage to match the other two native legs, or is the current in the generated leg so weird that it isn't transformable? If this can be done, would there be any specific gotchas for connecting this setup with respect to grounding (or anything else) within a 3-phase network?

Any input welcome --
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Absolutely a transformer , variable or not, can be used. Preferably not to step down, but to step up instead of "resonating" the system with capacitors, which is relatively unstable.

One thing... many RPCs do not like to start with any load on them, even a transformer. So you might need to have the output contactor before the transformer as well as the rest of the loads.

Another thing... the transformer is best from the generated leg to the idler neutral (star point), but can be used to the input neutral, although that is a different and higher voltage. The input neutral is usually more accessible.
 

13engines

Hot Rolled
Joined
Jun 30, 2015
Location
Saint Paul
Curious what was going on load wise when you measured the voltages. I believe under load the generated leg will get pulled down making the voltages more balanced. Also the more motors running on a phase convertor at the same time, the better the whole system becomes.

If connected properly, the only thing on your Tree that will see that higher voltage is the motors and their drive. Not sure what part(s) of the machine the +-5% call-out is concerned about but...

I think if you were to monitor the voltages under the continuously changing load that you'll find while operating the mill, you'd might be hard pressed to make a decision as to how much regulation is needed, if any. I'm guessing the voltage balance would be all over the map.
 
Joined
Jan 15, 2005
Location
The Netherlands
Most modern VFD s can handle motors with a lower input voltage as the line voltage
Just tell it to the VFD in the parameters
It even holds torque up to a higher frequency matching the linevoltage
For example a 380volts VFD can hold torque on a 220volts motor to about 80Hz
At about 80 Herz the motor is getting the 380 volt

Peter
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
...
I have not delved into swapping caps in the RPC up to now,as this is a mystery project for me. I would prefer not to disable my power source for the rest of the shop for an indeterminate period if not absolutely necessary. ...-

1) home-made, or commercial converter?

2) is your stated manufactured leg voltage with, or without the load machine running?

3) if a commercial unit, is the load more or less in the correct range for the converter's size?

It would be easier to reduce the tuning capacitance than to begin fitting a buck transformer to this, I suspect.
 

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
I had some questions about how to reduce the voltage on the generated leg, in order to meet the factory voltage spec of 230V +/- 5%.

You ever see a chart recorder? The kind with a rotating paper disk and a red or blue ink pen. Measure your power company's single phase power during the day.
At this moment my Fluke 87V reads 249.1 Vac.
 

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
The outputs are then about 240+ V on two legs, and about 265 V on the generated leg. I want to be much closer to acceptable values for this machine.

The generated leg measurement is only going to be meaningful when under the target load.

I think you are not going to gain anything but peace-of-mind if you try for perfection. But you will learn a few things.
 

specfab

Titanium
Joined
May 28, 2005
Location
AZ
Thanks to all for the inputs; much to consider. I haven't been able to get back to this yet, so don't have any additional facts, but a few notes in response to questions:
--The RPC is a commercial unit, rated 10HP. Should be sufficient for the Tree (3HP/5HP) and at least another machine at 2HP simultaneously.
--I haven't measured voltages under load, especially since with the original problem with the Tree mill, I was unable to get the servos/motor turned on. That issue mysteriously "fixed" itself, so more opportunity perhaps.
--The 230V +/- 5% load spec is simply what is in the Tree manual, where an assumption is made that the entire machine sees a single input line voltage. The wiring diagram shows that 3 legs go to motor drive and coolant pump, and 2 legs are split off to the control side where there are several transformers. In the case of this particular machine setup, the 3 leg/2 leg split is actually outside the cabinets, so that I have the native 240V (2 leg) source to the control side, and the full RPC 3-phase to the motor drive side.
 

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
The wiring diagram shows that 3 legs go to motor drive and coolant pump, and 2 legs are split off to the control side where there are several transformers. In the case of this particular machine setup, the 3 leg/2 leg split is actually outside the cabinets, so that I have the native 240V (2 leg) source to the control side, and the full RPC 3-phase to the motor drive side.

I wouldn't want it any other way.
 

13engines

Hot Rolled
Joined
Jun 30, 2015
Location
Saint Paul
I ran a remote controlled 10HP RPC system in my garage for well over a decade without troubles. Including powering up a 7 1/2 HP Shaper. To get rid of the RPC noise, I put it in the well room in the basement. The only thing I brought back up out of the basement was the manufactured leg. The main incoming power was in the garage and a sub panel in the basement. I fed the garage 3ph breaker panel straight off the 240 1ph in the garage and the RPC off the 1ph in the basement, with like I said, the RPC output filled in the 3rd space in the 3ph breaker panel upstairs. So in effect, I was running straight 240 to my machines with the extra RPC output tagging along. If you get what I'm saying. My point is... where all the wires originate from doesn't seem to make any difference.

This idea could save you time and money. All depending on the location(s) of everything.

As an aside, if you think you're going to be adding machines in the future, I'd suggest setting up a 240V 3ph breaker panel right from the start. Then you're all ready for when the next machine comes around.
 
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