GFCI tripping immediately with 500W brushless DC motor - Page 1
Forum section: electronics / general

author: skinnerfam95
author_rank: Newbie
timestamp: January 06, 2017, 04:38:21 am
content: I have a mini lathe with 500W brushless DC motor that trips the GCFI immediately- works fine on a non-GFCI circuit.https://littlemachineshop.com/products/product_view.php?ProductID=5100Seems that most people with the issue are just removing the GFCI- my outlet also services exterior outlets. Some are replacing with a hospital grade GFCI and are having success blaming EMF/higher harmonics generated by the motor controller. But nothing that really elaborates with really is the problem. The found solutions are not specific to my lathe and some are VFD related-- on larger, 3 phase equipment.The littlemachineshop.com says that the lathe cannot be used on a GFCI protected circuit.Seems like there should be solution to manage EMF/noise and use the GFCI. I have no data to support the hospital grade GFCI will work with my machine, so I'm not ready to buy and install the new outlet.Any ideas to make it work are greatly appreciated. I have access to a 100mHz oscope if you could suggest troubleshooting.I considered putting this under the RF topic, but I don't know enough as to what is causing my issue.I hope someone finds this interesting. I'm having trouble finding a credible answer to this problem.The mini-machining industry is almost exclusively Chinese imports. The lathe I have is somewhat unique coming with a brushless DC motor. I am aware of only one other US supplier called Micromark- I understand they are produced in the same factory and as most import lathes.Thank you.

author: Red Squirrel
author_rank: Super Contributor
timestamp: January 06, 2017, 04:51:10 am
content: I tried to put my bathroom fan on a GFCI as I could save on some wire nuts/room in box by feeding off the plug, but it would trip as well. I guess they don't like the characteristics of motor loads. GFCIs are constantly comparing current on neutral, hot and ground so I imagine a very noisy load like a motor is going to cause issues there.

author: station240
author_rank: Unknown
timestamp: January 06, 2017, 05:32:21 am
content: The back EMF has to go somewhere, possible they are shunting it to ground or neutral.I've got Thyristor equipment that has resistor between active and Neutral to stop leakage currents backfeeding, those resistors are also the correct value to trip a GFCI.

author: mij59
author_rank: Frequent Contributor
timestamp: January 06, 2017, 06:17:12 am
content: If there's a connection between the ground and the neutral of the lathe the GFCI could trip.A mains filter with capacitors to ground could pose a problem.Try a less sensitive GFCI e.g. 30 mA is considered save.

author: skinnerfam95
author_rank: Newbie
timestamp: January 06, 2017, 04:00:40 pm
content: Hi thanks for the recommendation. It gave me more terms to search on that I didn't know was an option. But I have two concerns about the 30ma RCD.1) Per a NEMA slide show on fault indication: 30mA -- IEC RCD Standards:: Breathing Difficult, Fibrillation in children2) I've only found panel mount for several hundred dollars (also not sure if it would be compatible with my panel) or the cord end (that plugs into an outlet).Thanks to all your inputs I think I have a better short term solution to mitigate risk. Instead of removing the GFCI, I'm going to add one, on GFCI outlet to the line side of the existing outlet. I am going to use a non-fuseable disconnect (like on the condenser of a whole house AC unti) on that non-GFCI circuit so it is only live when I'm operating the lathe.Any other comments are appreciated as I would eventually like to eliminate the non-GFCI circuit from my garage.Thanks!

author: rstofer
author_rank: Super Contributor
timestamp: January 06, 2017, 04:21:59 pm
content: Quote from: Red Squirrel on January 06, 2017, 04:51:10 amFunny thing is, it isn't all motors. I have one DC motor that will never work on a GFCI but I can run any number of drill motors, routers, Sawzall, etc and none of them ever trip the GFCI. It's just this one particular motor. I don't use it often but when I do, I have to run an extension cord from an unprotected outlet.

author: skinnerfam95
author_rank: Newbie
timestamp: January 06, 2017, 04:55:03 pm
content: My first intuition was bad wiring in the equipment, but the GFCI issue is a known issue with no workaround from the supplier-- very credible supplier in the hobby machining space.I have a "direct from china" electric jackhammer that I've seen posts indicating the the wiring is incorrect, dangerous, and can be fixed. On my list of things to do... I used it for a week on a project and had not fatal injuries.. found the post after.Maybe your tool that shorts is not wired correctly. In my case I am suspicious the brushless controller or power supply is low budget causing circumstances to trip the GFCI. Just a project I was planning when I bought the lathe.I think with the world market, liability issues blur the manufacturers investment in standards and safety.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 07, 2017, 12:52:27 am
content: OP, welcome to the club.Yes this is a known problem with VFD. HV capacitive coupling to frame. I won't ask why you are trying to use it with a GFI. There was a company that was working on a new motor design to add a barrier. It never happened. It's a real problem with some places with kitchen appliances now.Most GFI does not work by detecting leakage to ground. Its a differential sensor across the two AC lines. If this difference gets too large, it faults. So one way to solve this is with a transformer. No need to float the ground. The two AC lines to the transformers primary will match and the GFI will be happy. Not sure how many HP your motor is.Personally, I would handle it differently....

author: skinnerfam95
author_rank: Newbie
timestamp: January 07, 2017, 04:19:45 am
content: Hi joeqsmith. Thanks for the feedback- it was very thought provoking. Two questions if you are able-- or anybody willing to chime in.1) "Personally, you would handle it differently." What would you do?2) Its interesting to me you that you say, "I won't ask why you are trying to use it with a GFI"... because I seen many similar statements on forums where people are using somewhat heavy duty equipment for wood or metal working. My understanding of electrical code (googling) is that ground fault protection is required by code for all garage outlets. I've never seen a youtube video of someone turning a gokart hub in the living room wiping machine oil from the couch cushions (challenge accepted). What are legitimate alternatives in the garage?Am I totally missing the code requirement or is it just common to disregard? Granted, in 45 years I've never been "shocked" with exception of the occasional poor judgement.. But I have kids who could get shocked and heaven forbid I start a fire or something... I risk liability if I've disregarded the electrical code.The isolation transformed idea was interesting until I read some more... The link below best describes K-rating as it pertains to harmonics and faults (I'm out of my league in the details.) I found mostly $300+ for a 1000VA isolation transformer... which I would have to further research K-ratings not advertised.... and it is a large, heavy shop anchor. But a very interesting rabbit hole to go down.http://www.protekpower.com/isolation_transformer.html

author: Ian.M
author_rank: Super Contributor
timestamp: January 07, 2017, 04:49:04 am
content: You are still allowed dedicated (on a separate circuit) or inaccessible non-GFCI protected outlets in your garage - seehttp://ecmweb.com/code-basics/nec-requirements-ground-fault-circuit-interrupters-gfciThe 'kosher' way of doing this would be to get a licensed electrician to run you a new circuit to the garage, to a suitable outlet that doesn't accept domestic plugs, mounted high enough to be inaccessible to children and labelled 'Machine Tools Only'

author: joeqsmith
author_rank: Super Contributor
timestamp: January 08, 2017, 12:43:52 am
content: Sorry, I am not who you would need to ask about what you need to do to be in compliance. I have heard that its a big problem with some of the refrigerators. Of course, eating spoiled food may pose more risk. The transformer would prevent the GFI from tripping if this is your only concern. I could believe $300 and up. You may want to check what having a certified electrician has to say.There are smarter GFIs available. Some get fairly pricey but if you wanted to solve it this way it would be possible. Expect it may cost more than the lathe and isolation transformer combined.If you have the ability to program the VFD, you could try and reduce the operating frequency. I am not referring to the low frequency for the speed control but the high frequency. Some are programmable.This is about all I can offer. Good luck with it.