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Slow Startup on Mori Seiki MS-850


Oct 27, 2022
I'm setting up a nice clean Mori MS850, but there is a problem where it takes longer than it should to spool up to speed. For example 500 rpms takes about 5 seconds to get to speed, 1000 rpms takes a good 12 seconds to ramp up. My friend has the exact same machine and his is very snappy. Not a huge deal, but I'd like to get it resolved.

The problem existed when I bought it, so I can rule out my phase converter or supply side or any changes during transport.

Things I've done:
Complete drain and refill of the oil in the headstock (Mobil DTE Light)
3 new drive belts
Spun the motor without the belts to make sure the brake wasn't binding it, nope...
In-line current of each phase at idle is roughly the same amps
Resistance between each phase is the same (~3.3 ohms)
No shorts to ground
Spindle is nice and tight and spins freely in neutral. Still not super snappy starting in neutral. Gears in headstock all looked great when I had it opened up.

I've sourced a replacement motor for about $500, but if the current motor specs out with resistance, amperage, and no shorts, I can't figure out why it's weak and would need to be replaced. But there isn't anywhere else I can point to that would make it spin up slowly other than the motor.
The only thing I haven't done is pulled the motor and looked at the windings or cleaned it out.


You already checked what my issue with my Whacheon copy of your lathe and that was the hydraulic brake was hanging up.
haha thanks. I'm stoked
I pulled the brake off completely to make sure it really wasn't binding and it still has a slow ramp...
I pulled the motor apart and it was filled with dust and looked terrible. It was hard to tell what was going on, but the insulation on the windings looked black in places and it was quite brittle, showing signs of overheating. Even though there weren't any shorts phase to phase or to ground, perhaps all the added resistance from the dust caused it to run sluggish. A replacement was relatively affordable so I sprung for that and it should fix the issue. For anyone interested its a 112M frame 4 pole IEC motor, 4kW (5.5HP), slightly more powerful than the original 3.7kW. A NEMA 184T 5HP was very close and might work, but the shaft was 0.023" bigger so you'd probably have to overbore the rotor/pulley assembly. mine was on there VERY tight!

Sounds almost like a voltage mismatch... I.E. wired for 480 internally but plugged into 240...

And congrats on a very nice lathe, that was one of my ideal make/models when I was looking for my first lathe. Around here they rarely seem to come up and tend to be clapped out or stupid priced!
That's a really good point. I hadn't considered that. It has 220/440 on the original 1973 motor plate. But it only has the 3 main wires, so maybe it was somehow wired internally for the higher voltage. Either way, a 2023 motor will get it ripping
Thanks I feel like I lucked out. If you're NorCal, I'm NE Cal. Scored it from a friend of a friend and very happy to get it running.
If there are only three leads coming out to the box I think something is up... If you haven't already pull the box off the side and see if there are some wires behind it... Hopefully the motor has a diagram on it to reference.

Definitely the best way to get a lathe! That way you get some of the real history and condition when you are buying it...
Ok so this is curious. I bought a new WEG 00418ET3EAL112M-W22 motor and got it all hooked back up. This motor is 4kw and the old one was 3.7, so I figure no biggie, same frame size and a little extra power never hurt.

But strangely, when I turn the machine on, it's actually the same issue - the lathe doesn't snap right to speed like it should. And even this time if I try to spool it at 1800 RPM, it takes so long it trips my 50A breaker at the house. FWD and REV have the same issue.

The lathe is powered by a North American Phase Converter Model PL-10, which is rated for hard startups to 5HP 3 phase, which this is.

The lathe transformer is powered directly from the house mains (not the generated leg). Photo below, but I think it's wired correctly

Strangely, the thermal relay (set to FLA around 15A) never trips, so I wonder if it is bad, or if it's just not engaging because it can ignore startup current. I suppose I could jump it to see if it improves.....

I've removed the brake caliper for now to remove any rotor drag from the equation.

But I'm completely stumped. Mechanically this thing is smooth and sound. Anyone have thoughts on where the added drag or resistance could be coming from?

Thanks for the help, at least it's operational for the range of steel, but I'd like to figure this out....

I'm not a motor guy at all but have you checked the resistance on each leg from the top of the starter to the motor. Hold the starter in manually. Have you checked the starter contacts?

Have you checked the amp draw on each leg during startup?

From the 'Line in' connections to the end of the thermal relay are all 0.5-0.7 Ohms with the contactors pushed in. The motor connections are buried now so I measured just upstream of the motor terminals. I guess that rules out the contactors and the relay...

Line to line (measured at the thermal relay) are all 0.7 ohms

When I got it, the switching was wrong and the brake disconnect didn't work so I actually went through the entire wiring, mapped it, and compared it to my friend's functional and identical machine and fixed the wiring. So all the circuits have been through and seem as correct as they can be