Rockwell Lathe motor humming when plugged in but not running

[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