Milling Machine Motor Madness - need advice

[HoosierBuddy]

Hey guys,

My old U.S. Burke Millrite milling machine is starting to act more and more "funky" on me. For some time, when started in the forward direction, the OEM GE 3/4 single phase motor has "balked" at starting in the forward direction. By "balked" I mean it will do one of the following:

1. Hum and not move.
2. Run forward, but at a much slower speed than it should.
3. Run forward normally.
4. Start and run BACKWARDS at a slow rate.

Being a non-electrician I did the obvious and changed the starting capacitor. Didn't help. Didn't make it worse.

Typically what I end up doing is just starting it over and over until 3. above occurs...but I feel like it's getting harder and harder to get it to work right. Sometimes I might have to start it 4 or 5 times to get it to spin up and run right. Once it's running it functions normally. While I almost never run the machine in reverse, I think it always starts and runs properly in reverse.

An electrician friend diagnosed the issue over the phone for me as "worn bearings".

Question 1. Do you think that makes sense?

Question 2. Is this user serviceable?

Question 3. There is a motor repair place about an hour from me. Is it normally feasible from a cost perspective to take a 50 year old motor in and have it rebuilt...or is that going to be as costly as buying a new motor?

Thanks guys for any help you can provide. If it matters, this motor is a 120V/240V motor that the electrician (same guy I talked to about the bearings) wired up for 120V. It ran fine after he got done with it, but has progressively become "balkier" as I've used it over the past 6 years or so since he wired it up.

Phil

 

[jav]

Answers: (IMHO)

1- yes- it could make sense.
2- it might be but without direct knowledge of the motor- it's tough to say. Generally- the older the motor, the more "serviceable" it was designed to be. Parts availability is another issue.

Another thought is that a wire/tap has become loose. Some of these motors require moving tap wiring to change voltage/direction. Have you unplugged it and confirmed the wiring looks tight and well isolated from ground and/or other conductors?

 

[Whitworth]

Centrifugal switch. Contacts dirty. Yes, self serviceable. But if you've never had a motor apart and feel intimidated, might want to bring it to a repair place.

 

[Wes J]

It sounds more like the switch than the motor.

 

[matt_i]

Hum and not move on single phase is classic sign of start capacitor being bad or not engaged by centrifugal switch mentioned above. Changing out the bearings is relatively simple, the new versions are typically -2RS (double sealed) and not open or single shield like the older ones. Some careful work with arbor press and dial caliper should yield the replacement measurements.

 

[LS6 Tommy]

The fact that it has a reversing switch leads me towards it being a switch problem. If it didn't have a reverse switch, I'd say it's classic symptoms of a bad run cap, depending on motor type. A bad start switch can cause reverse rotation, but it's REALLY rare. A bad start cap does not reverse rotation, only causes a no start or a slow start with hum. A start switch stuck "closed" causes slow run, followed by overloads, internal thermal safeties or fuses/breakers popping. A start switch stuck "open" also causes no start/slow start and hum. I have seen bad run caps cause a motor to run backwards many times, although I have never really given any thought to why it happens. I think it would have to do with the phase being reversed on the start winding by the bad run cap.

Tommy

 

[catalytic]

Before you touch the motor, check EVERY SINGLE CONNECTION between the power source (wall plug) and the motor leads.

I have worked on a lot of motors. Most often when something hums and only turns a little bit as you describe, there is a loose connection in the switch or between the motor leads and the cord going to the switch.

As others have said, if it's not this, then test the capacitor and make sure that the centrifugal switch contacts are OK.

But my money is on loose connection. I have often thought I had a dead capacitor until I did a really, really thorough check of the connections.

And by the way, the problem is not bad bearings... bad bearings don't make your motor hum and do nothing or turn backwards slowly (unless they are completely seized? But you can turn the shaft by hand...right? so that's not the problem).

 

[JoeFin]

Hum and not move on single phase is classic sign of start capacitor being bad .

Ding Ding Ding - we have a winner

 

[Kevin54]

My mills run on three phase through a phase converter, but on my CNC, the servo was acting up terribly. We thought it was in the mill wiring, or possibly the encoder. When we tore the servo motor apart, the magnetics had broke and let loose. So when I would go to rapid feed the table, it might move, it might move slow, or it would jump terribly. I don't know how the motor is on your mill, but could that be a possibility. I'm not an electrician, but maybe some others can answer that.

 

[JoeFin]

My mills run on three phase through a phase converter, but on my CNC, the servo was acting up terribly. We thought it was in the mill wiring, or possibly the encoder. When we tore the servo motor apart, the magnetics had broke and let loose. So when I would go to rapid feed the table, it might move, it might move slow, or it would jump terribly. I don't know how the motor is on your mill, but could that be a possibility. I'm not an electrician, but maybe some others can answer that.

You running AC stepper motors on a Gecko set up ??

Most CNCs come with DC Servo Motors. Now days "Brushless DC Servo Motors". But ya - I'm still running DC Brushed Motors on mine too. Had problems with the Servos until I took the single phase transformer feeding the DC rectifier off the "Wild-Leg" of my 3 ph RPC

 

[HoosierBuddy]

Hey guys,

I just wanted to close up this tread as the issue is now fixed.

I took the entire motor/cord/switch assembly into an electrician friend and he diagnosed it as contacts in the switch for FWD are shot.

To test and as a bandaid fix, he rewired FWD and Reverse so that the Reverse contacts are now the Forward contacts and vise versa (How many times do you really need to run a mill backwards?).

I put the motor back on the mill and have about 20 hours of run time on it since and it has not missed a beat. The motor starts and immediately spins up to the proper speed every time. I just have to remember to move the FWD/OFF/REVERSE switch the opposite direction from how it used to be.

SO...bad contacts.

In case anyone does a search.

Phil

 

[rsanter]

Sounds to me like the motor is old, dirty, and in need of service.
Wiring could also be old and causing resistance, dirty or burnt contacts, bad connections...etc

Blow it out, service it up and go from there. Ask the local shop what they would charge for a clean, lube, and service

 

[BillK]

Sounds to me like the motor is old, dirty, and in need of service.
Wiring could also be old and causing resistance, dirty or burnt contacts, bad connections...etc

Blow it out, service it up and go from there. Ask the local shop what they would charge for a clean, lube, and service

If you read his last post he told you the answer