Insufficient Electrical Supply? Manual lathe slow to start

[dalmatiangirl61]

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.

If I'm reading his first post correctly, it works in every speed except the highest speed of 1800 rpm at the spindle. So the motor is fine, the rpc works, its just the rpc bogs down and trips the 50A breaker when he starts it with 1800 rpm spindle speed. He used #8 wire to feed rpc, when mfr recommends #6 minimum, will upgrading wire size fix that problem, I don't know, but it seems easy enough to check.......

 

[rons]

If I'm reading his first post correctly, it works in every speed except the highest speed of 1800 rpm at the spindle. So the motor is fine, the rpc works, its just the rpc bogs down and trips the 50A breaker when he starts it with 1800 rpm spindle speed. He used #8 wire to feed rpc, when mfr recommends #6 minimum, will upgrading wire size fix that problem, I don't know, but it seems easy enough to check.......

The way I read this is that the lathe is slow to start in general and the 1800 rpm limit is a boundary condition that trips a breaker.
What you say is working is not entirely a happy working state.

The wire size makes a difference if the length is long. You should always follow mfr before asking forum questions.
In post #26 there was a power wire added in parallel to see if that made difference. Just done as a test. No difference.

 

[JST]

If he has good voltage at the supply and bad at the RPC, or if the voltage is good at the RPC and bad at the load, then, yes, wire may be the issue. I have seen that, usually with very long runs.

If the pass-through is good but the generated leg is not, and it is no better at the RPC, then all the wire in the world is not going to help. The problem is between the input and the generated leg, which is the RPC.

Larger wire only helps when wire is the problem.

It may not be that the RPC idler itself is the issue. Could be something in connections etc "around" the RPC. We can't see it, so the OP has to check that.

What the RPC "size" is vs the load is not very important if it does not work. The highest speed setting has the most inertia as seen from the motor, and it can be a very tough load to start. That's not an uncommon problem when the high speed is more RPM than the motor runs at, so there is a step-up.

All the RPC has to do is hold up voltage in the start for that tough load. The powerco can do that, the RPC adds another impedance in series, and there is the basic issue. A 2x ratio from Idler to load is "usually" fine, but that's not a guarantee for every case.

If the voltage drops enough, then there is virtually no current flow into the load from the RPC generated leg, and there goes your "rotating magnetic field", and your torque, down the tubes.

Check the voltages, and if the RPC is not holding up voltage at the output terminals, when it has good input voltage, look in the RPC for the problem, not at the wire.

As a double check, see if the voltage at the RPC output, and the voltage at the load terminals is close. If there is a large voltage drop on the wire, then OK, change wire. But if the voltage is about as low at the load as at the RPC, it ain't wire.

We know the motor works, and high speed is a mechanical drive adjustment, right? If so, and the wire has no special voltage drop, then unless that particular speed has very high friction, the problem is not in the wires or the load.

 

[jim rozen]

Again, another machine, if available, can run on the buss to effectively increase the idler size. If that clears it up, you need to be more than 2X between idler and motor. Things become a *lot* less twitchy when the idler motor is much larger than the load motor.

 

[dalmatiangirl61]

The way I read this is that the lathe is slow to start in general and the 1800 rpm limit is a boundary condition that trips a breaker.
What you say is working is not entirely a happy working state.

The wire size makes a difference if the length is long. You should always follow mfr before asking forum questions.
In post #26 there was a power wire added in parallel to see if that made difference. Just done as a test. No difference.

Quote from post #26 "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."

The way I read that, he ran 1, 10 gauge wire, from the breaker to rpc. I asked for clarification yesterday on if it was 1 wire, or 2, but he did not respond. I also asked about wire size from main panel to sub panel, again no response from OP. He asked about his system, every inch of wire is part of the system. Mfr specifies 30A breaker minimum, 50A breaker Maximum, but only specifies #6 wire as MINIMUM, I would be asking mfr about MAXIMUM wire size. In my mind, minimum means the least you can get by on, but he is asking rpc to do the maximum it was designed for.

Question for OP, what size chuck do you have mounted on the lathe? 1800 rpm at spindle from an 1800 rpm motor means there is no gear reduction. Have you removed the chuck and tried starting? Does it snap up to speed, or still slowly wind up?

 

[JST]

Again, another machine, if available, can run on the buss to effectively increase the idler size. If that clears it up, you need to be more than 2X between idler and motor. Things become a *lot* less twitchy when the idler motor is much larger than the load motor.

Absolutely.

The potential snag is that the extra machine does draw current, so if the supply is limited, there may be a total current issue.

But, if it is simply an issue of not getting current/voltage out on the generated leg, another motor on the system, if it is not loaded (so it should be running without being used), will act as an auxiliary RPC and help supply that current. There is obviously no need for the extra motor to have any start circuits, since the existing RPC will start it.

Yes, a very much over-sized idler will tend to start almost anything without issues. Something like 4 or 5 to 1 is fairly bulletproof (ex. 5HP starting 1 HP), it's impedance is so low that it will act almost like a direct connection to the powerco.

 

[amaranth]

Too small an RPC and/or electrical supply.

I have a 5hp motor on my LeBlond lathe with a 7.5hp phase converter on a 30A circuit. It works fine, EXCEPT if I try to run it at top speed (1800 RPM). It takes a long time to spool up and 9 times out of 10 will trip the breaker. There's just a lot of rotational inertia involved in spinning that up.
My choices are either a larger rpc or not using the lathe at 1800 rpm. I have a 20hp rpc in the queue

 

[604Pook]

Grab the chuck and spin it as fast as you can by hand before turning it on when you want to run at 1800rpm.

And I echo the statement, undersized wire isn't helping at all.

 

[CarbideBob]

Too small an RPC and/or electrical supply.

I have a 5hp motor on my LeBlond lathe with a 7.5hp phase converter on a 30A circuit. It works fine, EXCEPT if I try to run it at top speed (1800 RPM). It takes a long time to spool up and 9 times out of 10 will trip the breaker. There's just a lot of rotational inertia involved in spinning that up.
My choices are either a larger rpc or not using the lathe at 1800 rpm. I have a 20hp rpc in the queue

Ouch..slow starter on the front end for help?

 

[JST]

..................................

And I echo the statement, undersized wire isn't helping at all.

Apparently it is not hurting either, as he put a parallel wire, with no significant difference.

Check post # 13, where the problem is shown clearly..... At the RPC, the output is 220 V on the pass-through lines when starting, but 135 V from either of them to the generated leg. The generated leg is falling a large amount.

Seriously..... When there is a problem, there is a right and a wrong way to solve it.

The wrong way is to guess that a particular issue is the problem, and then just go fix it. That can work, but is generally expensive, since if you do not guess right, you waste time and money on something that was not a problem.

The right way (if you want to actually solve your problem) is to check if that issue is a problem, and not "fix it" unless you can show it is a problem. Any possible "cause" has some effect that will result in what you see. So check if that is present.

For instance.... if the wire is too small, it would have too much resistance, and would cause a voltage drop. So see if the right voltage goes in one end, and the wrong voltage (too low) comes out.

If so, then the wire is at least part of the problem, and you are justified in fixing it. If there is no significant voltage drop in the wire, then the wire is NOT your problem, even though it could have been.

You have to look at the problem you have, check out possible causes, and fix the one(s) that are actually present.

The OP seems to have pretty well proven the wire is not an issue here.

The RPC may be too small for THIS lathe. That has not been tested, but the measurements made suggest the RPC is the problem. We do not know if it is just too small, or if it has some fault.

In this case, one simple way to determine if the RPC is the issue, is by connecting another 3 phase motor, as mentioned, to act as an additional idler. If the RPC is the cause, then the extra motor should make an improvement by supplying additional power on the generated leg, increasing the voltage.

If there is no "extra" motor, then the OP is in a pickle, because the RPC appears insufficient, yet there is no good way to test that.

There is a possible argument that the 240V dropped to 220V on start, so "the problem must be a wiring issue".

Not necessarily, though.... If the RPC were able to hold up the generated leg, then the generated leg might possibly fall to 200V at start. That would be in rough proportion to the drop of input voltage

But it actually falls to 135V. That is way out of proportion, suggesting again that the RPC is not able (for some reason) to hold up the generated leg.

Unless the RPC can be shown to have an actual fault, it appears pretty clear that it is too small to start this lathe at the high speed configuration. That is a known problem with a number of lathes.

 

[PanarchyMills]

Sorry I was out of town for the weekend and away from the computer. Thanks very much for all the thoughtful replies!

I'm with JST on this one, and others that have suggested the rpc is the weak link. To clarify a few themes that I might not have explained very well: Yes the mfg recommends 6awg wire, but I called when wiring the shop and explained my loads, and they said 8awg would be ok. 8 awg in free conduit is sufficient for a 50A breaker, and wire is sized for the running amps, not typically startup. But then thinking perhaps the wire was the bottleneck in startup, as some of you were suggesting, I ran an extra 10awg wire from a different breaker to the 1~ input of the rpc, which would be equivalent to running one 6awg wire to the input. since the generated leg (the weak point) still was dropping low at startup, I reasoned that the wire gauge isn't my bottleneck.

I have this rpc wired to a bridgeport mill and I actually build 3-phase granite flour mills for bakeries and farmers. So I'll fire up a flour mill later and then try and start the lathe. The flour mills use a 1HP motor, so it should be ok for the total load, and the rotational inertia of a 110-lb piece of granite spinning at 200 rpm should be a good stabilizer. Using another motor as an additional idler has been a great suggestion.

The response on this thread has far exceeded my expectations, and I've learned ton about rpcs through this. Thanks for all the feedback. I don't think I will be upsizing my rpc at this point. I will be turning mostly steel around 500 rpm and that spools up ok. For the occasional faster aluminum at 1000 and 1800rpm, I'll hopefully be able to tune the rpc (or start a flour mill as an idler) so that the breaker doesn't trip. Or giving it a little spin by hand is a good suggestion too. I'll update how the capacitors change things when they arrive this week. Thanks all

 

[rons]

If that's the case, putting a 3Hp or 5Hp in parallel to the existing idler. If the starting improves then you know.
A cheap test if the second idler is laying around.

 

[PanarchyMills]

Ok so a happy ending to this saga:

I tried adding 100uF capacitors to each capacitor bank (bringing it to 150 uF L1 & L3) and 250 uF (L2 & L3). This boosted the voltage to my generated legs quite high at idle (275V ish). Fired it up anyway and it helped a little, but not a huge change, and I was uncomfortable hitting my motor with that high voltage at startup, so I removed the capacitors.

Got back on the phone with North America Phase converters and we talked about their smart boost unit (SB-10), which monitors the voltage drop at startup and delivers a boost if it drops too low with hard-start loads. The company was very helpful and reasonable and sent me a smart boost panel, which I attached to the same 10HP rpc motor I had before, and I returned the original standard panel (PL-10). This solved the problem immediately and now the lathe snaps right into action on startup. I'd imagine a larger 15HP rpc would've solved the issue, but the 10HP is appropriately sized for the FLA and now the smart boost unit delivers all the juice it needs at startup. I couldn't have a higher recommendation for North American Phase Converters - their customer service was terrific, and the final product is great.

Thanks for all the help and feedback to everyone who contributed.

 

[jim rozen]

Thanks for the update, and also the 'attaboy' for the company.

 

[metalmagpie]

My Nardini is a pig to start. 6.5hp two speed WEG motor. Ten horse RPC wouldn't touch it. So I had to go to a 15hp RPC. I had to change the RPC output fuses from 30A to 40A. Now it starts my Nardini fine.

Just redo your phase converter with a bigger idler.

 

[rons]

Ok so a happy ending to this saga:

I tried adding 100uF capacitors to each capacitor bank (bringing it to 150 uF L1 & L3) and 250 uF (L2 & L3). This boosted the voltage to my generated legs quite high at idle (275V ish). Fired it up anyway and it helped a little, but not a huge change, and I was uncomfortable hitting my motor with that high voltage at startup, so I removed the capacitors.

When you have a correctly balanced configuration, idler and target running closely running voltage, the off-line volts are a little higher.
I get 275 and higher. But start the idler and target and I get roughly 2 Vac difference between legs. Like 246 - 248.
When you understand that once current starts moving is when volts count, then there is comfort.