Rockwell Lathe motor humming when plugged in but not running

[Dan Hahr]

I have an old Rockwell variable speed lathe from I believe the 70s. It has the Reeves Drive and a one horse power motor. I’ve never noticed it before but while cleaning around it today, I heard a faint humming noise coming from either the motor or the electronics. The switch is mounted near the top of the lathe and is original with the two push buttons. The motor starter is attached to the cabinet under the lathe outside the cabinet and I can’t tell where the humming is coming from. I did notice while turning some pens the other night that the motor was overheating and I’m guessing some sort of thermal overload protection was turning the lathe off. After cooling down for a few minutes it would restart. I put a fan in the cabinet, which stays open, and while running the fan did not have anymore problems.

I haven’t done much research into what is causing the motor to overheat although I could tell it was quite warm to the touch. Is there something in the starter box that would be humming when the motor is not running? Do you have any suggestions on what could be causing the motor to overheat?

Thanks, Dan399850

[Brice Rogers]

If this is a standard lathe with the Reeves Drive and it has not been modified to have an electronic motor speed control, there basically aren't any electronics AFAIK. Some people have put in an electronic motor speed control when their Reeves Drive stops working and they cannot find replacement parts. But without info from you, I'll assume that this is basically a "stock" unit.

So, it just has an on-off switch. The motor speed is controlled by the Reeves drive. In the off position, there shouldn't be any current flow. But your comment that the motor overheated has me perplexed.

Please come back and tell us whether this is a standard/stock lathe or has been modified.

BTW, in your text you referred to the lathe as a "variable speed ladies". Don't you hate auto spell-correct?

[Bill Orbine]

You might have a transformer in there for the motor controls. You might have an LCV (Low Voltage Control) and these have a transformer that makes a humming noise. I can't begin to tell you what is causing your motor to over-heat as there can be numerous issues. Check to see if the lathe turns freely and make sure all the wiring connections are clean and tight. Make sure the motor is not clogged up with dust.

As you have stated, you have a 1 hp motor.... running on what? Single phase or three phase? What volts?

[Bohdan Drozdowskyj]

Your motor may have a mechanical centrifical start switch that is stuck 'on'. This would cause the motor to hum when not running and overheat when it is.

[Dan Hahr]

It’s a single phase 120 volt motor. I hadn’t thought of the centrifugal switch sticking. I’ll check that before I take the wooden box off to get to the starter. Will a little oil on the moving parts free it up or do they generally wear out?

And yes, I can’t hardly type anything on my phone...

Thanks
Dan

[Bill Bukovec]

Do you have a volt meter? If so, check to see if there is any voltage on the motor side of the switch. I'm wondering if your "on" switch is stuck closed. Also, most centrifigal switches make a click type noise when the motor has been turned off and is slowing down.

[Dan Hahr]

Okay. I checked the switch. The meter shows .3 volts on the black line in and 2.1v on the red line in. The other side of the switch is showing . 3 v as well. I pulled an access panel and can see and hear the centrifugal switch opening or closing, whichever it’s doing ... there is a spark when it starts inside the housing. It appears to be very clean inside and the bearings seem smooth enough. The humming is coming from the starter box which I haven’t had the time to open yet. The capacitor on the motor looks okay.

Thanks, Dan

[david privett]

check the grounds and neutral and if the start switch is kicking in and out normally chances are the cap is ok if it is a little sluggish it might be breaking down. usually heat is a resistive problem , as motor is pulling to many amps compared to normal running.

[Dan Hahr]

All the wiring seems fine. It starts and runs fine except getting warm. Of course I have other motors that get pretty warm but they don’t shut off.

BTW, I’m not sure why this was moved to the Turner’s forum. Is there anyway a moderator could move it back to the general forum? It’s more of a motor question than turning.

Thanks, Dan

[Brice Rogers]

Dan,

I'm an electrical engineer and am having trouble following or understanding what is going on. What I am understanding is that
1. Your motor hums when the on-off switch is in the off position.
2. You measured the voltage at the switch and "The meter shows .3 volts on the black line in and 2.1v on the red line in. The other side of the switch is showing . 3 v as well. "

hmmm... (no I'm not trying to simulate the humming that you're hearing). If you have a red line in and a black line in, that almost sounds like a 220 volt source. But I think that your lathe is probably a 120 volts. So perhaps you are referring to the internal wiring that may be using a somewhat non-standard color scheme. With a 120 circuit, black is usually "hot", white is "neutral" and green is ground. Most people measure voltage from ground or neutral to both sides of the switch. It wouldn't hurt to measure the voltage from ground to neutral. It should be close to zero. If you were measuring the voltage from ground to "hot" and it was close to zero, then it sounds like your receptacle may have some of the wires swapped. Or the internal wiring was swapped at some point. Or if you have an extension cord it was miswired.

Also, you mention that the voltage at the input side (red or black) is close to zero. That doesn't make sense unless the lathe wasn't plugged in - - or the receptacle is mis-wired.

You may want to take the voltmeter and measure the receptacle from the round ground pin to the "hot" (of the two blades, it is the narrower one). It should measure around 120 volts. Then measure from the round ground pin to the neutral (it is the fatter of the two blades). It should measure close to zero but perhaps a few tenths away. Then measure from the round ground pin to each side of the on-off switch. One side (the line side) should read 120 volts and the switched side should read around 0 (switch in off position). Also, make sure that you are using the voltmeter on the AC position (I'm not trying to be abrasive or demeaning but I don't know your background or familiarity with this stuff). I'm actually trying to be helpful and am very curious with what is going on.

BTW, be careful with your measurements. You would be working with live circuits and there is some danger of either getting a potentially lethal shock or throwing scary sparks. I think that is why they refer to electrical engineers as "sparkies". ha ha.

With most appliances, if the switch is off, there shouldn't be any way that the motor should hum. It doesn't matter if there is an issue with the centrifugal switch or capacitors - - if the switch is off, there shouldn't be any voltage to cause a hum. The centrifugal switch and AC capacitors are "down stream" of the switch. I suppose that if the switch is defective and is arcing internally you could hear some "hum". If someone added an RFI filter on the lathe (unlikely) and put it on the wrong side of the switch so that it is always in the circuit, then I suppose that you could hear a hum.

Let us know what you find.

[Dan Hahr]

Gonna check all the wiring and I’m going to take the starter apart today. I’ll report back with any new details.

Thanks for all the help so far.

Dan

[Dan Hahr]

Well, I took the starter cover off and I see that it does have the low voltage control and a transformer.

I have a basic understanding of electricity. I did wire my own house last year but that never involved much more than four wires.

The outlet is properly wired and showing about 122v and the neutral is less than .1v to ground.

I am more concerned with the overheating than the humming. I’m just surprised I never heard it before. It is very faint and I had to get my head level with it to hear it but I’ve had the lathe for 7 or 8 years.

I’ll post photos of everything. If I need to test any of the components, let me m know. Thanks again!

Dan
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[John K Jordan]

Is it a 60hz hum? Transformers can get a bit loose inside and hum. Make sure the screws/bolts are tight. You can try whacking, er, tapping it on the top or side with a screwdriver handle or something - I do that with a fluorescent work light that hums loudly and that stops it for a bit. If coils or plates are loose you can usually fix them with lacquer if you can get into the transformer.

But typically, a humming transformer causes no problems other than annoyance. If it doesn't get hot now in operation it probably won't.

If it's not the transformer or if you are not sure, there are simple ways to locate the source of the hum, just ask.

Voltage readings in the 1/10s of a volt are normal, wires can pick up a bit from induction.

JKJ


Well, I took the starter cover off and I see that it does have the low voltage control and a transformer.

I have a basic understanding of electricity. I did wire my own house last year but that never involved much more than four wires.

The outlet is properly wired and showing about 122v and the neutral is less than .1v to ground.

I am more concerned with the overheating than the humming. I’m just surprised I never heard it before. It is very faint and I had to get my head level with it to hear it but I’ve had the lathe for 7 or 8 years.

I’ll post photos of everything. If I need to test any of the components, let me m know. Thanks again!

Dan

[Ken Fitzgerald]

It sounds like transformer hum which isn't uncommon.

[Dan Hahr]

The vibration seems to be coming from the vertical device on the left side of the box with the spring in it. If I push it sideways a bit the noise changes. I tightened everything I could and closed it up. Which part is the transformer?

thanks , Dan

[Bill Boehme]

If the centrifugal switch is working OK then it is time to look at other things (It is generally easy to tell ... just listen of a click about a half second or so after the motor is turned on and then after the motor is turned off and the motor is winding down you should hear another click when the motor drops below about 25% of full speed). Just to be sure you could remove the rear cover of the motor and examine the centrifugal mechanism ... check the condition of the contacts to see if they are badly pitted, check the weights and springs to make sure that nothing is broken.

I suspect that the humming might be a malfunctioning relay in the LVC with the contacts chattering. My opinion is that you might be better off just getting rid of the whole LVC and start/stop switches and just replace it with a toggle or push-button switch. The main purpose of the LVC is for industrial safety. It serves a couple functions:

The start stop switches are low voltage momentary contact types. It provides operator safety by not having high voltage at the switch.
The other function is that the machine won't start up after a power outage until the operator pushes the start button.


Finally, the motor is pretty old and it might be slowly dying. The insulation on the field windings weren't as good as it is now and the insulation might be breaking down where the wires are making sharp bends. One last thought is to make sure that the armature spins freely. Give the shaft a spin and verify that it spins for a few seconds. If you can feel any dragging or if you hear any noise like clicking or scraping then maybe the bearings are worn out. Also check to see if there is any lateral play in the shaft which is an indication of bearing failure. If there is a lot of play you probably ought to pull the armature out of the motor and see if there has been any rubbing between the armature and field. That could be a bad sign because it might result in eddy currents in the iron which just result in wasted energy heating up the motor.

[Dan Hahr]

Bearings seem to be in good shape, but it is quite old. I think the centrifugal switch is working right, as well. I am going to watch it and see if I can determine if the motor is actually overheating or not. I would also like to find a matching 3 phase motor for this machine as the Reeves drive is getting noisy in its old age.

Thanks again, Dan

[Bill Boehme]

Are you sure that you want a three phase motor? Your current motor is single phase and three phase power is not normally available for residential service. If you're thinking of getting a VFD, I would not recommend it in your situation because you already have variable speed with the Reeves drive. The mechanical Reeves drive has one major advantage over electronic variable speed ... there is no significant power loss in the drive train. Full motor power is available at all spindle speeds because the motor is always running at full speed. On the other hand, with VFD speed control power is directly proportional to the motor speed and that's something worth considering if you will be roughing large diameter pieces at slow speeds.

[Bill Orbine]

In looking at the electrical diagram of your motor control box, I see no mentioning of a thermal overload. However, the pictures you provided of you motor plate, I see a time rating of 1 hr. I'm not sure what that means but I'm under the assumption the motor has a duty cycle of 1 hr before it overheats. The motor probably has a thermal overload fuse. If you were running that motor a long time without ample time to cool off, that could well be your issue. If you were running a very short time, then you likely have a problem.

Otherwise, the next best thing to do is measure the amp draw. If you are drawing to much, then you need to find out why. One way is to take the belt off and see what is the amp draw. As I said before, make sure all the connections are clean and tight and everything turns freely.

[Bill Orbine]

I'm copying and pasting the some information about time rating I found on the internet:

Time rating — Standard motors are rated for continuous duty (24/7) at their rated load and maximum ambient temperature. Specialized motors can be designed for “short-time” requirements where intermittent duty is all that's needed. These motors can carry a short-time rating from 5 minutes to 60 minutes. The NEMA definition for short-time motors is as follows: “All short-time ratings are based upon corresponding short-time load tests, which shall commence only when the windings and other parts of the motor are within 5°C of the ambient temperature at the time of the test.” By using short-time ratings, it's possible to reduce the size, weight, and cost of the motor required for certain applications. For example, you may choose to install an induction motor with a 15-minute rating to power a pre-operation oil pump used to pre-lube a gas turbine unit because it would be unusual for this type of motor to be operated for more than 15 minutes at a time.

[Bill Dufour]

Sounds like the contactor armature is humming. Probably needs to be removed and cleaned. likely there is slight rust and dust binding motion so the contacts are not closing all the way leading to arcing under load. I would pull the plug and put a strip of business card between the contact faces and saw the card back a forth while holding the contacts closed. This may be enough to clean the faces. If not you may have to get more like sandpaper.
Bill D.

[Dan Hahr]

Well, I googled the contactor part number and found a manual for the LVC. According to it, the overload block is the item that is humming. I will check out the contactor, but everything in the box looks brand new- it was sealed very tightly. Is there something in the overload block that could be cleaned to stop the humming? Or could it be malfunctioning, causing the motor to shut down?

Thanks again to everyone.

Dan