Phase converter/electrical question

[slowtwitch73]

I have a 5hp 20 amp RPC that has happily run my mill, bandsaw and previous lathe for years. They are all 1hp motors.

The 3 phase comes from RPC to a 3 phase safety switch where each leg goes through a 5mp fuse then on to whatever machines are wired in.

I just wired up a new to me lathe with a 5hp motor. When I try to start it, one of the leg blows a 5 amp fuse.

Is this indicative of the RPC being too small? General rune seems to be double the machines hp, but I've also read 1.5x will do.

From what I can gather getting a 5hp motor to start off a 5 hp RPC isn't going to happen... but I'm not clear on that, and wondering if larger fuses would help.

 

[Bert_]

5hp 3ph motor on 230v pulls about 12A. I'd be looking for 20A fuses at minimum.

As to weather it will start it, you're not out anything to try. Depends on how much load is on the motor when starting.

 

[slowtwitch73]

Ok, sounds good. Would it be detrimental to run the 1 hp motors with 20A fuses?

I'm thinking not a good idea. Looks like 1 hp 3 phase is recommend to be just under 5 amp.

I'll try the 20a for the lathe and see if it starts up and figure out the disparate fusing after that.

 

[Bert_]

Ok, sounds good. Would it be detrimental to run the 1 hp motors with 20A fuses?

I'm thinking not a good idea. Looks like 1 hp 3 phase is recommend to be just under 5 amp.

I'll try the 20a for the lathe and see if it starts up and figure out the disparate fusing after that.

Each piece of equipment should have it's own overload protection. It's usually part of the starter on this type equipment.

 

[Steve from Socal]

A rotary phase converter can use smaller motors running as idlers to improve the manufactured leg. try starting a couple of small motors before the 5HP. You need enough current to supply the load on the inlet side 1.73 times the single phase current.

The motors should be protected by overload heaters/relays AT the motor.

Bert's mention of 12 amps is for a three phase motor, the current to the idler would be 12X1.73 or 20.76 amps.

Steve

 

[slowtwitch73]

Each piece of equipment should have it's own overload protection. It's usually part of the starter on this type equipment.

Starter? I don't follow. I only run one machine at a time.. until this lathe, they were all 1 hp so required identical fusing. It's the same 3 fuses for all the machines, but like I said, I only run one at a time. Flip a switch or push a button at machine and they start.

 

[Lonnies Performance]

You have a 5HP RPC & want to operate a 5HP lathe with it?
That could be a problem... your RPC should be at least 1.5x preferably 2x the operating load.

 

[Bert_]

Starter? I don't follow. I only run one machine at a time.. until this lathe, they were all 1 hp so required identical fusing. It's the same 3 fuses for all the machines, but like I said, I only run one at a time. Flip a switch or push a button at machine and they start.

Example of a starter.

 

[Bert_]

You have a 5HP RPC & want to operate a 5HP lathe with it?
That could be a problem... your RPC should be at least 1.5x preferably 2x the operating load.

I've heard people say this before. Commercial phase converters are usually rated to start a motor as big as the idler though.

It is very dependant on how hard the load motor starts. A motor with lots of load and inertia to get spinning is much harder to start and will require a bigger phase converter to be reliable.

 

[slowtwitch73]

You have a 5HP RPC & want to operate a 5HP lathe with it?
That could be a problem... your RPC should be at least 1.5x preferably 2x the operating load.

Yes, and that's what I've gathered as well, but there are those out there who say it may work... so ill try with some 20 amps and go from there.

 

[slowtwitch73]

Example of a starter.

Thanks.. I did some searching last night and figured that was it. So there's fuses in there? Bussman type? From what I could find, not cheap.

My stuff is all old enough, it's just drums switches etc, bur pending outcome of this little electrical journey, I may need to regroup.

 

[Bert_]

Thanks.. I did some searching last night and figured that was it. So there's fuses in there? Bussman type? From what I could find, not cheap.

My stuff is all old enough, it's just drums switches etc, bur pending outcome of this little electrical journey, I may need to regroup.

I would honestly be surprised if your equipment didn't have it's own overload protection. When that stuff was made it would have been required to have some form of overload protection

 

[slowtwitch73]

Well good and bad news.

The lathe started up with the 20a fuses, but it wasn't pretty. The garage lights were flickering and the lathe seemed to be running erratically

Turned it off, checked that the fuses were ok, tried to start it again and nada..

Opened the door to the electricals on lathe and.... the smell was not good.

I think poop show project 2021 just showed up.

on edit, I got into the lathe cabinet with a light and didn't see anything amiss. There was some recent dust down low, I figured it came from this component, I reset it and the lathe ran again, then eventually tripped it again.

 

[Bert_]

I'm assuming you made sure the controls aren't running off the generated phase. That's a big no no. The voltage isn't very well regulated, you can fry a coil.

Have you checked the current on all three lines with the lathe running? I'm assuming the generated leg isn't doing much.

Often when you have a motor as big as the idler you have to add extra capacitance. Best to connect it at the lathe

 

[slowtwitch73]

I'm assuming you made sure the controls aren't running off the generated phase. That's a big no no. The voltage isn't very well regulated, you can fry a coil.

Have you checked the current on all three lines with the lathe running? I'm assuming the generated leg isn't doing much.

Assume nothing lol. I connected L1,L2,L3 of lathe to corresponding legs coming out of converter. Not sure how it could be any other way. Something could be mislabeled at lathe or converter but that's low odds.

The lathe electricals are clean, well labeled and from what I can gather stock/unmolested.

No I have not checked current.

I appreciate the tips.

 

[Bert_]

The lathe sounds like it has contactors. They have single phase coils. You need to make sure they are not connected to the generated phase.

With utility 3 phase this isn't an issue since they hold a tight tolerance on voltage. But the generated leg out of the converter has pretty big voltage swings depending on the load. On the generated leg it's not uncommon to see 270v unloaded or under 180v when starting a motor. A 230v coil doesn't like that.

Normally I keep track of the generated phase with orange tape on the wire.

 

[slowtwitch73]

I dug up the paperwork on the rpc. They show the 3 legs coming out, and one (red) is labeled at 'hot'. I assume this is the same as 'high'. But is it also the manufactured/generated leg? Is there a convention for which legs get labeled 1,2, or 3?

RPC running no load is 252, 248 and 248

With lathe running (spindle not turning) they read 226, 247, and 242

I measured L1 to L2, L1 to L3, L2 to L3

Given 10 seconds or so, the racket from lathe dies down and it will run smooth with spindle not engaged. Engage the spindle and it's very herkey jerkey, then it either settles in and is constant and quite, or it trips that deal I posted a pic of.

Sounds like it's trying.... like contactors clanking around.

I'm feeling optimistic this a rpc/wiring issue instead of a lathe problem.

 

[txvwnut]

You need to up the size of your RPC. With that 5hp lathe motor the in rush current needed for start up is greater than the RPC can overcome. When dealing with RPC's and even SPC's you need to have a convertor that's twice the size of largest motor your trying to start.

Also have you checked what the voltage of the lathe motor is?

 

[Bert_]

Post up some pictures of the main connections in the lathe. And if you have paperwork for your converter post it up too.

I would assume what they label "hot" is the generated leg.

If the contactors are chattering I would say you have the control circuit powered from the generated phase.

 

[Strouty]

At this point if you think it is the contactors, try swapping the wires from one leg to the next. As for running larger motors, you should be able to do what others have said and run one or more of your original machines then try starting the 5 hp motor. You may need to get a more powerful capacitor (IIRC). There are also ways to balance out the legs, but I agree it is best just to not run the controls on the generated leg. Also to qualify these statements, I only have one rotary phase converter and I did not stay at a Holiday Inn Express last night.

 

[slowtwitch73]

Pics...

 

[Bert_]

That's all that came with the converter?

Can't really tell anything from the pictures. If you can't trace the wires it might just be easier to swap the line wires around until the contactors don't chatter.

 

[slowtwitch73]

Yup, that's all.

I swapped some wires and it's much better and it spins the correct way but it's still a bit wonky.

RPC sounds like it digs pretty deep on startup. Not a big deal in and of itself. Lathe motor a bit slow to spin up. For a while it would go forward and reverse, but then it would only go reverse and give the 'nnnnnn' sound when I tried get it to go forward.

I'm chalking it up to the small rpc.

 

[Bert_]

Those issues sounds like not a big enough idler. You can try running some other motors that have no load on them. Doing that is like having a bigger idler motor.

Still might have issues with unbalanced current unless you add capacitors.

 

[slowtwitch73]

Thanks for the help. I'm off this project for a week or so. I'll report back when I a bigger rpc. I've had the current one quite a while and basically got it for free. I'll sell it to offset a new one.

 

[slowtwitch73]

Alright, the saga continues.


Biggger RPC has no substantive effect other than being dead quiet and providing a nice startup.

Lathe will run reverse (lever up) but not forward (lever down). I get that same 'nnnn' sound and lights flickering.

If I swap wires, it runs forward with lever up (which should make it go reverse) and still get the 'nnnn' sound with lever down.

At this point I can limp along and do some work with wires swapped and the spindle rotating forward.

I'm thinking one of the contactors is loopy.. further advise appreciated!

 

[gorilla]

When a 3 phase motor makes the nnn sound it means that it's not getting all 3 phase's connected. You probably have a bad contactor. Current draw is real high when a motor tries to star on only 2 phases.

 

[Bert_]

Yup^

Probably single phasing. Probably a contactor since it works in one direction. Pretty easy to check with a meter

 

[slowtwitch73]

I took out the fwd/rev contactor unit.. looks like two identical units ganged together.. one of them had that electrical dust smoke residue on it.. I tried to get it apart to see if a cleaning up would do it, but at a certain point I couldn't get it apart any further. They are readily available on Ebay at a decent price, so I'll try that next.

 

[slowtwitch73]

Alright, got the sob going as it should after replacing the contactors.

Had a scare along the way. I took lots of pics of the wiring before removing the old one, and while deleting some junk pics my son took the whole 'camera roll' just went **** into thin air. 300+ pics just gone.. not in the trash, not in the cloud, nowhere. I was pretty torqued for about an hour. Downloaded some app called Disc Digger and my god it worked.. found old stuff from years ago.

THANKS ALL!

 

[slowtwitch73]

Ugh... that was short lived.

Went out today to make some chips. Lathe starts, sounds great, start making some cuts, and then the power cuts out. I try reverse and that works. Look in the cabinet and there is whispy smoke coming from cantactors and stink. All the fuses are fine. Try again in fwd, no dice. So I swap L1 and L2 thinking I can limp along in 'reversed reverse' and after about 5 seconds same thing happens. Little smoke n stink, no busted fuses. Now it wont go at all. So wtf........

The contactors were new. I didn't replace the contact blocks that slide onto the front of the contactors (LA1 DN22). Could those be the culprit? The contactors aren't even making the 'noisy contactor' sound now. Frustrated.

 

[slowtwitch73]

Alright, I went out and started removing components. Noticed right off one of the contactors wouldn't move. I took it apart and the insulation around the coil was crispy and the blue plastic had actually deformed so the center of the large metal piece was hitting against it.. you see can in the center just to left of serial number. So that one is toast.

Then I grabbbed the half of the old one I had left over and noticed it says 120v and the new (toasted) ones say 24v. Curious considering they were the same part#.

 

[Bert_]

Old ones are 120v coil. New ones 24v coil. 24v coil doesn't last very long when you feed it 120v

 

[slowtwitch73]

Smells bad.. burning pastic with a slight hint of charred money.

 

[slowtwitch73]

Alright, got the thing going and ran it for a good long while. Forward, reverse, no smoke, no smell. Thanks for the help.

 

[cnc-me]

Just get a 5HP single phase motor and keep your existing setup.

 

[kwb]

VFD is how I would move forward.

 

[u3b3rg33k]

VFD is how I would move forward.

Same. a 5HP single phase input VFD is very affordable.

 

[slowtwitch73]

This is a done deal. I wanted to run all my machines of varying hp off one RPC and somewhat quickly without going down the VFD rabbit hole. Someday down the road when I have time to get up to speed on them I can see it happening.