Insufficient Electrical Supply? Manual lathe slow to start

[PanarchyMills]

I'm having continued issues getting my manual Mori Seiki lathe (MS-850) to come to full speed quickly. This was a machine-specific question but I'm moving it to the general forum because I've narrowed it down to a more general electrical issue.

The problem is: at startup, the motor starts slowly and takes a long time to come to speed. At standard 500 RPM it's not bad (3-5 seconds) but at max RPM (1800RPM), it will trip the 50A house breaker after about 5 seconds, and never reach full speed.

It has a brand new 4kW (5.5HP) 3 phase motor, wired for 240V. The old motor seemed shot, so I replaced it, and the problem persisted. When I bought the lathe, the issue was there at the guy's shop, so I figured it was internal to the lathe (like the motor). But after replacing the motor and having the problem continue, I chased a bunch of threads and ended up simplifying the problem by just wiring up a brand new contactor and 'on-off' switch (with all new wires) directly to the lathe motor (bypassing all of the lathe wiring and just starting directly). You can see that in the photo. the problem was still there. So now it seems to be either my rotary phase converter (10-HP unit, rated for 5HP load), somewhere in my supply (200A house main supply @ 240V), or the new motor I got has something wrong with it... I also switched breakers at the panel to rule out a bad breaker...

But it would be curious if the problem is on the supply side because the issue happened at the shop where I picked it up.

The gears in the head are clean, I drained and replaced the oil, and the spindle spins smoothly in neutral. There's a brake caliper on the motor that I've removed so there shouldn't be any physical resistance, unless there's something I'm missing that I can't see.

I'm really stumped. Can someone help out with how they might go about this? I don't have a clamp on current meter (The current would exceed my multimeter's 10A max current reading), but could buy one if it would help diagnose the problem. Thanks for the help, I'll be thrilled if this can be resolved

EDIT: sorry maybe this should be in the 'Transformers, Phase converters and VFD' Channel. Not sure how to move it over there...

 

[dalmatiangirl61]

Is rpc running off main panel, or a sub panel? Wire sizes to sub panel (if applicable), rpc and machine? Length of runs?

If wires are all properly sized, maybe try a larger breaker? My 20hp rpc runs on a 125A breaker as per mfr's recommendation.

 

[Milland]

If wires are all properly sized, maybe try a larger breaker? My 20hp rpc runs on a 125A breaker as per mfr's recommendation.

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?

 

[john.k]

Accelerating a heavy 4 jaw chuck takes a lot of energy.

 

[Robert R]

The motor slow start can be caused by a supply transformer that is undersized for the peak start up load. A 5 HP 3 phase motor at full load at its rated speed draws 14 amps on each phase. At start up it can draw up to four times that current. The panel circuit breaker will be a time delay type. If the motor has a slow start due to low voltage from a undersized transformer the circuit breaker will time out.

The 200 amp house panel does not guarantee that the pole mounted transformer supplying your house is rated for 200 amps.

This article will provide a more detailed explanation of motor starting currents:

Understanding Motor Starting (Inrush) Currents, & NEC Article 430.52 | JADE Learning

Inrush current, also referred to as “locked rotor current,” is the excessive current flow experienced within a motor and its conductors during the first few moments following the energizing (switching on) of the motor. This current draw is sometimes referred to as “locked rotor current” because...

www.jadelearning.com

 

[EPAIII]

Bad wiring would likely add resistance to the circuit and that would draw less current. That is the opposite of tripping a 50 Amp breaker so I doubt it is the wiring.

Bad contactor? That could be a more complicated question, but you have replaced it. So it can probably be eliminated.

The rotary phase converter? You do not say if it came with the lathe or not. If it came with the lathe, then it could be suspect particularly because you say the problem existed at the previous shop. But if that was a separate purchase then it may or may not be the problem.

And then there is mechanical drag.

I would take the belts off and see if everything spins freely. Motor, pulleys, idler, anything else the belt comes in contact with.

You don't need a clamp on meter for this. If you are drawing more current than the circuit can supply, there will be a Voltage drop. A simple VOM will measure the single phase Voltage to the RPC. Take a reference reading before starting the lathe and then read the meter while it is starting. An analog meter would be best because you can see the needle dip. Digital meters, particularly inexpensive ones, can show confusing readings when the Voltage is changing rapidly. But even an inexpensive analog meter will show those momentary dips.

If there is a significant dip (<20%) in the Voltage when the motor is starting then you may have a wiring problem in the shop/building wiring. It could be that the 230 V circuit that powers the RPC has conductors that are too small for this load. That would add resistance and a Voltage drop so the RPC would not get the full Voltage. And that would mean a slow start.

Or there could be a loose connection in the shop/building wiring. Again that adds resistance and produces a Voltage drop. Etc.

If you don't feel comfortable working with the building wiring, call an electrician.

 

[TheOldCar]

My MH1500 did the same thing. I imagine no clutch on these Mori Seiki manual lathes means correct RPC and correct beaker are a must. Not much wiggle room, just has to be right. I've used a bit undersized RPCs in the past, and scraped by. On this harder-starting lathe I simply (duh) had to use the proper converter and breaker.

 

[GregSY]

I have the same lathe (Whacheon) and it starts with no issue off a 30 amp breaker. It takes exactly 6 seconds to reach full speed no matter what gear I'm in.

First...the lathe (I believe) should have a 7.5HP motor. Some of the older versions did have a 5HP but I wonder if you are simply short on starting torque.

Second....the reason my lathe takes 6 seconds to start is I have in a VFD and that is the accel time I programmed into it.

I am a fan of RPCs and am not a 'use a VFD on everything' believer, but in your case....it sounds a lot like you need a VFD.

 

[Philabuster]

VFD or a soft starter to reduce inrush current.

I put a soft starter on my 15hp turret lathe so I did not brown out the CNCs running at the same time. Works great.

 

[rons]

Check your lathe motor by using it as a idler on your RPC. Disconnect the belt and see if it starts up immediately as a RPC idler should.
If it starts up fast on it's own then the next step is reconnect the 10 Hp motor as the idler and connect the 5 Hp as a idler with no belt.
If that works good then you might have to consider a soft start with the belt connected.

Was the original lathe motor a 5 Hp, or was it 7.5 or 10?

 

[deltap]

Check voltage on third leg of rpc motor. You may have to adjust capacitance.

 

[DDoug]

No worries, the saturation bombing OP got his answer, just didn't keep all informed:

New Motor Issues on Mori Seiki Lathe

I replaced the motor on my Mori Seiki MS-850 lathe that I'm trying to get fired up. I replaced it because it was spooling up very slowly, and I took off the old motor and it was full of what looked like drywall dust and in bad shape. So I replaced it with a brand new WEG that fits the same frame...

www.practicalmachinist.com

 

[PanarchyMills]

Wow, incredible, thank you for all the great and prompt responses. I will try to give some more info related to the above answers:

To the last post: that was my old thread. I reposted here because it's become a more generic electrical issue and the problem isn't solved

Keep in mind that this issue presented itself at the shop when I bought the machine. The guy had a pretty trick setup and I don't think he skimped on anything, which is why I thought it was something in the lathe. The motor was full of dust and looked hammered so I thought the new motor would fix it, but obviously no-go.

Also my friend who lives nearby has the exact same lathe on a digital phase converter and it fires right up, no problem.

TLDR: see the photo of the voltage readings. The generated leg on the RPC drops to 135V when the lathe is starting, even with the lathe in neutral. Does this mean the phase converter is undersized or that something is wrong? Thanks for the recommendations.

--- more detailed responses to above ---

My RPC is off a 125A subpanel, located 15ft away from the main. The rest of the house is under light load while I'm in my shop, and nothing else is on in the shop while I'm doing this. From the sub, a 50A breaker with 8 AWG wire in conduit leads to the RPC 25 feet away. The RPC is brand new, North American Phase Converter PL-10, rated for hard startups at 5HP and requires 30A min 50A max breaker. I tried it on a 30A breaker too. I've used this company with other things and no issue. When I tested another breaker it was a 60A. I know the wires aren't properly sized, but I did it quickly to see if it started quicker and switched back when there was no change.

With the lathe in neutral, motor and belts turn easily and smoothly by hand. Spindle is the same. Put into gear, same thing, just mechanical drag depending on the gear selection of course. No drag with belts off. I replaced the three belts.

It's a good point the supply transformer could be undersized... The Locked Rotor Amps on the motor are 116A @ 230V. I'd imagine the 125A subpanel can handle that spike. I'll see if I can get info from the electric company. House is 1975, main panel is original, transformer is 60 feet away, so I'd imagine they sized the panel for the transformer.

Soft starter isn't a bad idea. I've wired up VFDs in the past, but since I got the rpc for this, I'd like to keep that as the primary. But it seems there should be a solution that's with the DOL starter.

I've started the lathe motor without belts and it starts fine. It also starts fine in low gear. Its 500 RPM and above that it's noticeable and an issue. I'm not sure that hooking it up as an idler in place of the 10HP rpc motor would tell me much...

The old motor I replaced was 3.7kW (4.96HP), and I replaced it with a 4kW (5.36 HP). The frame size/mounting holes/brake rotor on the shaft didn't accommodate a 7.5HP motor.

Voltage readings in the photo attached. All measured inside the RPC with a fluke multimeter using voltage min/max to catch the lows. Seems the generated leg is dropping way too low. But also not sure why this issue would be at the shop where I bought it. Maybe his phase converter was undersized...

thanks for the help

 

[rons]

I think the bottom line is:

If you witnessed that lathe run slowly with utility line three phase power, the problem is in the machine.
Either the lathe motor is too weak in horse power or there is something wrong in front of the motor.
After the motor is the transmission system. Belts?

 

[Bill D]

LOCATION? Maybe you are on 50Hz not 60? Mains voltage in your country.
Bill D

 

[PanarchyMills]

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

 

[Bill D]

10AWG copper wire size off the 50 amp breaker is very undersized leading to voltage drop. For 50 amps it should be #8 or #6 depending on insulation.
Bill D

 

[Fish On]

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

My mechanical sheet metal shear is one hard starting SOB, and probably a similar type load to your lathe, and is also a 5 hp motor.

Hooked up to an American Rotary ADX-20 (twice the RPC you have, and they're ADX series is their high end digital version with the extra capacitors and circuitry to kick them in during startup), and it still took forever for it to spool up. The entire time of startup, the light on the RPC indicating it was providing 'boost' current would be illuminated. For comparison, that RPC never had to provide the boost to my 7.5 hp hydraulic press brake or ironworker during startup.

Now, in a different shop with 208v 3 phase coming in from the street, that shear spins up in half the time.

Point being, the inrush current on starting up a flywheel brings even way oversized RPCs to their knees. I would say yes, add the extra capacitors, but also expect that slow startup is just a part of RPC life on that kind of load.

 

[PanarchyMills]

10AWG copper wire size off the 50 amp breaker is very undersized leading to voltage drop. For 50 amps it should be #8 or #6 depending on insulation.
Bill D

My bad, it's 8AWG in conduit, which is rated to 50A. Edited my post above to correct, thanks for catching it

 

[reggie_obe]

10AWG copper wire size off the 50 amp breaker is very undersized leading to voltage drop. For 50 amps it should be #8 or #6 depending on insulation.
Bill D

Yes, 50A circuit, 6ga copper, none of that Aluminum crap.