Insufficient Electrical Supply? Manual lathe slow to start

[Bill D]

My bad, it's 8AWG in conduit, which is rated to 50A.

It sure is hard to read the black print, on black cable jacket, in the dark, standing on your head. At least that is one possible problem eliminated the easy way.
Bill D

 

[dalmatiangirl61]

I (tentatively) think that if it's not popping the breaker, going to a larger one isn't going to increase the current flow. It's not a hose, it's a switch...

Hopefully one of our electrical guys can help here, but would you confirm that when the machine is in gear (with motor off) the spindle turns relatively easily, i.e. no apparent binding, just normal load from all the rotating elements?

He says it popping the 50A breaker. Code for rpc's allows for over sizing wires for max amp draw.
https://up.codes/s/phase-converters

 

[jim rozen]

SWAG: Your rotary converter is undersized. You can try tuning it with capactitors to bring up the manufactured leg. A better approach woud be to upsize to a 15 hp idler motor for your converter.

 

[candeservices]

This is the major drawback of having a RPC, at startup it is drawing full inrush that can be 3-4 times the running amps.

 

[rons]

California, definitely 60hz.
Just got off the phone with the phase converter company and they recommended boosting the generated leg with extra capacitors. They said 100-200 uF per line. Anyone have an opinion on that? Seems easy enough and inexpensive. The current run capacitors are:
L1 -> L3 (generated): 150 uF
L2 -> L3: 50 uF

Using your reference terminology for my 5 Hp RPC:
L1 -> L3 : 50 uF (C13)
L2 -> L3 : 90 uF (C23)

The ratio of C13 to C23 is 0.55. The same ratio works for me at 2 Hp.
Might be correct for a 10 Hp unit. I worked this out with lathe running at the rpm range most often used.

I would try:
C13 = 180 uF
C23 = 100 uF

The capacitor designations I use have the values reversed from you. That means the direction of rotation is reversed.
I keep everything turning clockwise when looking at the shaft end. I assume your are using oil filled capacitors, the ones
with aluminum cans. I bought my caps at Grainger.

 

[PanarchyMills]

Using your reference terminology for my 5 Hp RPC:
L1 -> L3 : 50 uF (C13)
L2 -> L3 : 90 uF (C23)

The ratio of C13 to C23 is 0.55. The same ratio works for me at 2 Hp.
Might be correct for a 10 Hp unit. I worked this out with lathe running at the rpm range most often used.

I would try:
C13 = 180 uF
C23 = 100 uF

The capacitor designations I use have the values reversed from you. That means the direction of rotation is reversed.
I keep everything turning clockwise when looking at the shaft end. I assume your are using oil filled capacitors, the ones
with aluminum cans. I bought my caps at Grainger.

Thanks this is really helpful. It puts me in the pocket of what the phase converter company suggested. I'll be adding 100uF to each leg so it'll be 150uF and 250uF. I guess I'll just keep an eye on the voltages.

Yesterday I ran an extra 10gauge wire across the shop and spliced it into the breaker and RPC, just to see if larger gauge would help. It would be like having one 6 AWG wire instead of my 8, and there was no change to the voltage drop at startup on that generated leg.

Still curious that the lathe spooled up slowly from the guy I bought it from. He had a 30HP digital converter that was very nice. I wonder if there were 2 issues: #1 a bad motor that was spooling up slowly, and now #2 a phase converter at my place that's not giving enough juice at the generated leg...

 

[rons]

After over 20 years mucking with RPC stuff I would not depend on any run capacitors to fix your issue.
If you cannot get the lathe motor to run freely as a RPC idler then the motor has winding short(s) that weaken the magnetic fields inside.
The only fix is another motor. See post #10.

 

[PanarchyMills]

After over 20 years mucking with RPC stuff I would not depend on any run capacitors to fix your issue.
If you cannot get the lathe motor to run freely as a RPC idler then the motor has winding short(s) that weaken the magnetic fields inside.
The only fix is another motor. See post #10.

Ok, I guess I didn't fully understand that post. Are you saying:
disconnect the 10hp rpc idler motor from the rpc box
connect the 5hp lathe motor (without belts) to the rpc box
run the rpc, and see that the 5hp motor starts quickly like the 10hp does. If it starts slowly then the new 5hp motor is bad
**Do I have to worry about higher line current exceeding the 5HP motor's FLA from the RPC capacitors? Or maybe just a quick on/off isn't much of an issue...
Thanks

 

[rons]

Ok, I guess I didn't fully understand that post. Are you saying:
disconnect the 10hp rpc idler motor from the rpc box
connect the 5hp lathe motor (without belts) to the rpc box
run the rpc, and see that the 5hp motor starts quickly like the 10hp does. If it starts slowly then the new 5hp motor is bad
**Do I have to worry about higher line current exceeding the 5HP motor's FLA from the RPC capacitors? Or maybe just a quick on/off isn't much of an issue...
Thanks

Yes, but you have to determine if the motor swap with no other changes made to the RPC will be ok.
I suspect it will be no problem. Going the other way, 5 Hp to 10 Hp, might make the larger motor stall.

 

[Bill D]

I see no mention of how to calculate the capacitance. Series or parallel makes a difference. Series just add them up. Parallel takes some math to figure it out.
I think some RPC makers do not add and subtract caps they just shift them from series to parallel to adjust the capacitance.
Bill D

 

[jim rozen]

"Series just add them up. Parallel takes some math to figure it out."

Umm, maybe the opposite?

 

[dalmatiangirl61]

Yesterday I ran an extra 10gauge wire across the shop and spliced it into the breaker and RPC, just to see if larger gauge would help. It would be like having one 6 AWG wire instead of my 8, and there was no change to the voltage drop at startup on that generated leg.

Am I understanding this correctly? You ran an extra 10 gauge wire (singular) for the generated leg? Or 2 extra 10 gauge wires for both legs of the single phase power feeding the RPC?

Pulled up the spec sheet for that rpc, its says to use 6 gauge MINIMUM for input power.
https://www.northamericaphaseconverters.com/wp-content/uploads/2019/04/PL-10.pdf

Maybe I missed it, what size wire from main panel to sub panel?

 

[JST]

Adding capacitors might do something, in general it does not fix startup issues. Can fix running voltage issues. Startup is a heavy load, and larger capacitors are also a load..... And, the startup load fouls up the effect that capacitors can provide (the heavy load lowers the "Q" of the circuit)

Since you have only the generated leg dropping very low, your issue is that the RPC cannot hold up the voltage. It's not wiring, or ALL the voltages would be low.

I think you said you replaced the motor with a WEG. What was the original motor? WEG motors are somewhat notorious for being pigs and drawing a lot on startup.

Options: Larger converter, sure. Also running another 3 phase motor unloaded to act as an auxiliary idler. That will be similar to what a larger converter would do. It would be at least a helpful test. Downside is that another motor also draws current as well as adding generated leg current capacity. But so does a larger converter.

 

[PanarchyMills]

Adding capacitors might do something, in general it does not fix startup issues. Can fix running voltage issues. Startup is a heavy load, and larger capacitors are also a load..... And, the startup load fouls up the effect that capacitors can provide (the heavy load lowers the "Q" of the circuit)

Thanks for the reply - It's a good point about the WEG. I replaced it with a TEFC cast aluminum instead of the original ODP (whatever Japanese motor was in there since the 70s). The motor frame size now is a 4 Kw where it used to be 3.7, so a little bigger. Also a good point about running another idler. I was also reading about folks putting a large sheave on their rpc idler to keep the rotational inertia.

The rpc company makes what they call a smart-boost digital rpc. From the website: "The Smart-Boost™ Controller uses solid state switching to re-engage the start capacitors at zero degree crossing. Which in turn, surges 600% more current to the motors during their startup." Do you think that would work?

It'd be the same 10hp motor, and they could upgrade the controls, and I think it'd be only a couple hundo. They recommended trying that if the capacitors don't help. I don't really care at this point about the slow start, but I don't want to be popping breakers when I fire it up at 1000 RPM. I guess the capacitors are easy and I'll see pretty quick once they come if there's an improvement.

 

[JST]

I have no idea if the "boost module" will help.

I'd be questioning the idea if the start capacitors are of the non-polar electrolytic type. If they are using motor-run types as start capacitors, then OK. The usual start caps are not rated for any extended use.

 

[BT Fabrication]

something doesn't seem correct to me,
A - the generated leg is really high, and drops really low under load, that says to me that the RPC is too small for the load.
B just adding caps will raise the voltage up even higher, possibly creating a short as it will go 280V or so. well over 10% rated.
C try to find another motor to run with the RPC to add a little wiggle room

 

[jim rozen]

"RPC is too small for the load."

+1

 

[dalmatiangirl61]

If you go for a larger rpc, you will need heavier gauge wire to feed it, might as well up the wire size first. I'm no EE, but it seems to me you used less than the mfr's recommended minimum wire size, yet you are asking the rpc to do the max it was designed for

Edit: If the problem is not enough amperage supplied to the house, a larger rpc is not going to fix that problem. Eliminate all potential problems with the wiring sizing first.

 

[JST]

As far as what has been written, it appears that the "generated leg" is low, but the others are reasonably OK.

Right there that eliminates all issues from the RPC input out to the pole and transformer etc.

It is THE classic symptom of an RPC that is too small.

What does an RPC do? It generates a correctly phased third hot wire.

How do you know it is big enough? When it holds up voltage on the leg it generates.

What if it does not hold up voltage? Then it is not big enough (or there is some bad screw-up)

 

[rons]

I don't agree with all that. Unless the drive train in that picture is a lot of work to turn.
Remove the belt or put the rig in neutral and see if it turns.
If it turns then connect the belt. If it stalls then I agree about RPC size and a few caps.
But this guy has 10Hp to 5Hp variance and that should be enough.