Potential electrical problem with DC Lathe Motor

[GeoBruin]

I picked up a lathe the previous owner had converted to run with a DC motor. He actually had the lathe and a drill press all wired to the same controller setup so I've spent some time separating out the two and I ordered a KB Controls KBMD-240d controller to drive the 1.5 HP Baldor Shunt motor on the lathe. I was able to get it wired up and working, but today as I was messing with things, I noticed a little "tingle" when I was adjusting the speed on the controller and I still had my hand on the lathe. My first thought was "wow, I'm glad I got a "tingle" as a reminder to run a ground wire and not something much worse". I decided to test just how much voltage there was so I touched a volt meter to the lathe and to the case of the controller and I show about 40 volts. It turns out, that number drops progressively as I increase the speed of the lather using the controller (in case that information is valuable).

Anyhow, the situation is this: I have a 240v plug plugged into the wall with a 3 conductor cable (Hot/Hot/Ground) connected to the AC input on the controller with the ground wire clamped down to the equipment ground lug on the case of the controller. Then, I have a 4 conductor cable leaving the controller to the motor (2 armature wires, 2 field wires). I'm thinking I should also have a ground wire bonded to the case of the motor, and traveling back to the ground lug in the controller. If it's that simple, it's an easy fix.

However, I got to wondering about that 40ish volts I'm measuring and I'm thinking that may be indicative of another problem. Should I be seeing that much voltage "leaking" from the motor or do I have a short in the motor?

Thanks in advance.

 

[sdelivery]

First I suggest a ground from motor to drive and to machine as you stated.
I think what you have here is because you have no neutral.
Neutral is the wire that returns a hot leg to the system. Your ground is not supposed to be a current carrying conductor.
Not seeing the machine or the schematics I could be in left field but.....

 

[markba633csi]

Run the wire from the motor case to the controller case and also check to see if you are actually using the neutral or the ground from the utility box.
In your case you need to be using the ground not the neutral.
Many homes don't have an actual earthing ground rod driven into the soil, but there should be a couple of ground wires tied to your cold water pipes with at least one coming back to the breaker box.
The power company always ties the neutral to ground at the power pole but it has been known to come loose. Never hurts to have extra grounds

 

[GeoBruin]

Thanks for the responses so far. I'm quite certain it's an actual ground wire because I pulled it myself. It comes from the sub panel I installed in the shop and I drove the ground rod into the ground myself.

But none of that matters because there's currently no ground between the motor and the controller. If there was, I wouldn't have been the path taken!

As discussed, I think that ground wire is a no brainer. I'm more concerned that I've got a fault inside the motor or something. Otherwise, why would I be measuring 40ish volts from the motor to ground?

 

[Nigel123]

Thanks for the responses so far. I'm quite certain it's an actual ground wire because I pulled it myself. It comes from the sub panel I installed in the shop and I drove the ground rod into the ground myself.

But none of that matters because there's currently no ground between the motor and the controller. If there was, I wouldn't have been the path taken!

As discussed, I think that ground wire is a no brainer. I'm more concerned that I've got a fault inside the motor or something. Otherwise, why would I be measuring 40ish volts from the motor to ground?

With DC brush motors sometimes carbon dust builds up and tracks to ground
Might be a good Idea to remove the brush covers if possible and take a peek
If you pull the brushes out make sure they go back in the same orientation

 

[markba633csi]

It's not unusual to have some leakage voltage- the current is likely pretty low but it still could be dangerous if you don't tie it down
The controller output is DC with an AC component which shows up between the two chassis because the motor isn't perfectly isolated-
There's also capacitive effects

 

[GeoBruin]

Great thoughts. I can certainly check for dust in the motor case.

Mark, that is definitely over my head but in any case it seems like the voltage I'm detecting may not necessarily be from a fault.

I think the first order of business is to ground the motor. Of course that means ordering some 5 conductor wire.

Thanks again for the input.

 

[markba633csi]

Even with perfect insulation, line operated appliances can have leakage due to tiny parasitic capacitances which you can't eliminate totally
60 cycle ac can pass from wires or windings to the chassis at low current thru these "phantom" capacitances

 

[rabler]

Great thoughts. I can certainly check for dust in the motor case.

Mark, that is definitely over my head but in any case it seems like the voltage I'm detecting may not necessarily be from a fault.

I would start by unplugging the machine and then checking resistance to ground between non-ground leads at the plug and at the motor. DC resistance to ground should be infinite on all including the neutral wire. Anything else implies either mis-wired or a short. Ground/frame should be grounded at <1 ohm. If those tests don’t pass they need to be resolved before plugging the machine back into power.

 

[Jeff.64]

If you're using a digital multimeter, the input impedance will be 10 MΩ or more. This will sometimes show a high voltage from a source only capable of supplying low µA of current. If you use analog multimeter, which has a much lower input impedance, the voltage reading would be close to zero in that case. However, if the source can supply many mA of current, then both meters will measure a similar voltage. If that is the case, you have an unsafe situation.

In addition to the great previous replies, check the following.

The potentiometer connected to some DC motor speed controllers can have a high voltage on the terminals. With power cord to the lathe unplugged, check the resistance between the pot terminals and the shaft of the pot. Or check the connections to see if the terminals of the pot are bent in such a way as to make contact with the panel. If you have a high resistance (in MΩ's) reading, the pot is OK for shorts to ground.

Check all current carrying wires for a connection to the lathe. Look for chaffing at power cord entry to the enclosure for the speed controller, the motor and control panel on the lathe.

Check other devices on the lathe like a work light and etc.