High Voltage on Drill Press Motor Shaft - Page 1
Forum section: electronics / repair

author: rstor22
author_rank: Regular Contributor
timestamp: June 26, 2021, 12:01:27 pm
content: I was noticing the the other day that with my mini drill press (see photo) plugged in and turned off, I could feel a slight vibration when touching the base. I decided to use my contactless mains voltage detector and it was showing high voltage when I brought it close to the unit (such as the base).The mini drill press does not appear to have a polarized plug and it is not grounded. I found that if you plug it one way, and then measure the voltage between the metal base and ground, it was about 60V. If you plug it the other way around it is 0V (between the base and ground) however once turned on it is about 40V.I decided to look to see if there was any exposed wire however I could find none, the mains goes through a switch and then directly to the motor (see attached). With the device plugged in (one of the two ways), still switched off, I measured the voltage between the shaft of the motor and ground and it was about 90V.Question:1) The drill press was plugged into a GFCI outlet (passed internal test feature). How come when I touched the base it didn't trip the GFCI? I tried shorting the base to ground with it plugged in (where the base was showing about 60V earlier) and the GFCI nor the main breaker tripped. Why?2) I am assuming this is a fault with the motor? Any idea what has happened and the next steps to repair?Thank you!

author: sleemanj
author_rank: Super Contributor
timestamp: June 26, 2021, 12:19:44 pm
content: Quote from: rstor22 on June 26, 2021, 12:01:27 pmMains voltage motor, metal casing, metal drill press, metal everywhere, and not grounded? Sounds a bit dodgy.Anyway, I'm gonna totally and uneducatedly guess capacitive coupling through the case and you to earth. Plugged in one way the Active (Live, Phase) is connected all the way through the motor to the switch - all that forms one side of the capacitor, the casing, you and earth forms the other.Plugged in the other way it is stopped right at the switch and it's Neutral which is running all the way through the motor, no capacitance since it's (roughly) same potential both sides.Turn on the switch and you wind up somewhere in the middle.There is no power behind the coupling, enough to feel that hum/vibration/tingle, but not enough to actually pass current.

author: Manul
author_rank: Super Contributor
timestamp: June 26, 2021, 12:22:44 pm
content: Voltage you see is most likely because of capacitive coupling or some small leakage through isolation. It is not at all uncommon and it does not necessarily mean that there is a fault. If you would measure current, most likely you would find out that the current is extremely small. However, any such equipment should be grounded, no question about that.1.GFCI outlet measures current going into its ground, but the drill press is not connected to the outlets ground.As mentioned above, the current is likely so small, that it would not be enough to trip GFCI.2. I would assume there is no fault. Just ground your drill press properly.

author: themadhippy
author_rank: Super Contributor
timestamp: June 26, 2021, 12:41:09 pm
content: Quotethey measure the current imbalance between live and neutral,they dont care were the imbalance current goes to ,it could be ground or it could be a point before the GFCI they'll still see an imbalance and trip.

author: Manul
author_rank: Super Contributor
timestamp: June 26, 2021, 12:55:46 pm
content: Quote from: themadhippy on June 26, 2021, 12:41:09 pmThanks for correction. I kinda knew it, I don't really understand now why I wrote it like this, brain fart moment.

author: rstor22
author_rank: Regular Contributor
timestamp: June 26, 2021, 01:02:14 pm
content: Thank you all for the replies. From this I am understanding that:- It seems like the current leakage is so small that it doesn't even trip the GFCI.- The high voltage measured on the motor shaft could be from capacitive coupling / leakage and can be considered normal so no need to replace the motor or open the motor to fix anything- The drill press should be groundedRegarding the grounding of the drill press: The mains wire goes directly to the inside of the motor. In the past where I had to ground some equipment like an old Kikusui power supply (https://www.eevblog.com/forum/repair/kikusui-pab-18-5-5-grounding/msg1283502/#msg1283502) I had cut the end and attached a three prong grounded plug. I then ran a separate ground wire to the chassis. This probably is not per electrical code but I found it easier to do instead of removing the existing wiring from inside.I am thinking I will do the same for the drill press, in this case, I will have a grounded plug and run a separate ground wire and connect it to some metal part of the frame.Are there any other measures I should take or would this be good enough?

author: TimFox
author_rank: Super Contributor
timestamp: June 26, 2021, 02:28:22 pm
content: In most sets of regulations, power tools that do not have a ground/PE connection must be "double insulated", so that two layers of insulation have to fail before there is conduction from the line to any exposed metal component. Still, I would be happier if a metal drill press had a ground connection to the frame, chuck, etc. Can you change the line cord?

author: rstor22
author_rank: Regular Contributor
timestamp: June 26, 2021, 02:39:16 pm
content: Quote from: TimFox on June 26, 2021, 02:28:22 pmThe line cord is going into the motor. I suppose I could try opening the motor to change the entire line cord...not sure how difficult that would be.I am leaning towards adding a grounded plug and adding a separate grounding wire from the plug to the frame. I am thinking this will hopefully achieve the similar results as if I were to change the entire line wiring.I have attached pictures on how I have done it on my Kikusui power supply that was originally ungrounded. This may not be following electrical code.

author: mathsquid
author_rank: Frequent Contributor
timestamp: June 26, 2021, 04:57:30 pm
content: This sounds like exactly the same problem that I had a few years ago.https://www.eevblog.com/forum/reviews/my-power-supply-has-80v-on-chassis/That thread contains basically the same advice and information as others have given here.

author: amyk
author_rank: Super Contributor
timestamp: June 26, 2021, 10:08:14 pm
content: How much current can you draw out of that voltage?If it's in the microamps range, there is no real problem besides it being capacitive coupling; it is effectively like a capacitive dropper supply, but with a very tiny capacitance. If it is significantly more (I believe 300uA is the safety limit), then there is a big problem and it could be something like insulation failure.Either way, as others have mentioned, grounding will remove this stray voltage.

author: james_s
author_rank: Super Contributor
timestamp: June 26, 2021, 10:30:04 pm
content: Does it have a brushed motor? If so, it may have RF suppression capacitors wired between each terminal and the motor case. If there is exposed metal, it should be earth grounded. I would advise either replacing the cord, or running a separate equipment ground attached to the metal housing.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 27, 2021, 02:51:00 am
content: Try measuring the voltage with a low impedance meter. I'm guessing it will be near zero.

author: TimFox
author_rank: Super Contributor
timestamp: June 27, 2021, 03:06:57 am
content: If you don't have an old-school Simpson 260 VOM, you can put 10 k or so across your DMM in AC volts mode.

author: rstor22
author_rank: Regular Contributor
timestamp: June 27, 2021, 05:15:38 pm
content: Quote from: TimFox on June 27, 2021, 03:06:57 amWithout the resistor it is about 68V from the case to ground. With a 10k resistor across the DMM probes it is 0.27V

author: james_s
author_rank: Super Contributor
timestamp: June 27, 2021, 06:09:24 pm
content: Yeah that's got to be capacitive coupling, could just be the capacitance between the windings and the rotor and stator pieces. Ignoring the input impedance of the DMM I get 30 microamps, that's orders of magnitude less than is required to trip a GFCI or give you a dangerous shock. The metal still should be earth grounded though, you could easily get a real fault condition where you have a low impedance connection to the live wire through the same metal parts.

author: TimFox
author_rank: Super Contributor
timestamp: June 27, 2021, 06:26:01 pm
content: That's about 36 nF by direct calculation, or possibly two 100 nF capacitors across the 120 V single-phase mains to the chassis.I have seen similar voltages with computers or test equipment that have conventional three-prong line filters at the input, but where the ground connection at the mains socket was defective. In 220 V countries, the unexpected small shock from the chassis to a real ground can be startling enough to cause injury (like falling off the ladder), although not directly lethal.