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Excessive motor current draw question

I have a Tiawanese manual lathe with a 3 phase 3hp motor. I run my shop on a phase converter. I have a CNC lathe with a 15hp spindle and a CNC mill with a 10hp spindle. Other machines too but it's these 3 that make me wonder. When the CNC lathe spindle is starting it starts fast. Bang! The mill also starts fast, but not quite as fast. Both of these machines can start at the same time with no problems. Even when the chuck is on the lathe. The chuck wieghs about 50 lbs. so it is a lot of inertia to spin up. But if I start the 3hp lathe at the same time as the CNC lathe it sometimes faults the spindle drive on the CNC and it shuts off. The crappy little motor never runs hot enough to smell, and I've had the machine for about 20 years. It does run rough though and always strains to start. When starting the manual lathe in the highest gear it takes longer to spin up to 1150 rpm than the CNC lathe takes to spin up to 5000 rpm with the 50 lb. chuck. So what could make this motor draw so much current? Thanks, Eric

Reply to
etpm
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I had a big blower motor that would spin as easy and free as a child's pinw heel in a gentle breeze, but virtually lock up when AC was applied. It wou ld howl and struggle to build RPMs. Even after AC was cut, it was still sti ff until I turned it over several times when it relaxed. And it wasn't hea t either - I could switch on the motor for a split second, and it stiffened right up. There was no end play, but it had bad bearings..

Reply to
John-Del

S.W.A.G: if this motor has DC-injection braking, one of the relays (or capacitors) may be failed or intermittent, which would explain both the slow start and/or sufficient noise on the line to interfere with the CNC machine.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

Nah, it's just a crummy 3 phase motor connected to the line through standard contactors. The motor has always, from day one, had electrically caused vibration. It's pretty easy to tell as motor vibrates when powered up but if power is removed the vibration stops instantly. I would expect this with a single phase motor but not 3 phase. Eric

Reply to
etpm

Have you tried measuring the current on each leg to see if they are ballanced ?

Along the same line, measure the voltage to see if all 3 legs are being fed with the same (with in a couple of volts) voltage.

Reply to
Ralph Mowery

change the motor and also use a inverter drve to each machine for a softstart. The old moter most likely has a centrafugel switch and the bearings are most likely sloppy causing drag...

Also, I don't know what region you are in but if you have 50 Hz motors on 60 Hz supply things don't run as well. etc..

Reply to
M Philbrook

I guess you didn't read what I wrote. The motor is a 3 phase, so no starting switch.

Reply to
etpm

ok, I missed that one sorry but still, you could have a bad 3 phase contactor with a weak leg on it. That causes poor startups too, long spin cycles. Actually, you may want to do a phase to phase voltage test after the contactor or switch that engages these legs to ensure all three legs are getting equal voltage.

You may have a case where you could only be getting enought to set the propper direction, this causes lots of drag and over currents on the legs that are working..

Many phase inverters shut down when seeing unbalnaced loads like this..

Reply to
M Philbrook

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