I got shocked - Page 1
Forum section: electronics / projects

author: Halvmand
author_rank: Regular Contributor
timestamp: May 24, 2017, 05:53:04 pm
content: So I'm mounting an new motor and a variable frequency drive on my cheap drill press.It was assembled and working until today where i got a shock and the RCD tripped. It was not a huge shock.I know VFD's has high leakage currents, and I measured mine to 15 mA AC. No DC leakage.Now the whole machine is disassembled to check for wiring errors. I found none.The motor appears fine to. isolation resistance is about 500M ohm.VFD is a Danfoss VLT2811Motor is an AEG 0.55 kWThe question is now how such a high potential could develop and trip the breaker? In the instructions for the VFD it says that an RCD must be installed, and rightly so since it saved me.But would there in general not be a high risk of the RCD's tripping when using VFD's?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 24, 2017, 06:08:33 pm
content: Where did you get the shock from? (I.e., where did you touch?)Your error is exactly there.Sure, once you fix it (by proper grounding), you might have a problem of RCD tripping all the time, but at least nothing is shocking you.Maybe you should consult a professional electrician.

author: Niklas
author_rank: Frequent Contributor
timestamp: May 24, 2017, 06:57:29 pm
content: VFDs can damage metal ball bearings over time by electrical discharge from the capacitive coupling between the windings and rotor. One common workaround is to use ceramic ball bearings. When driven with 50 Hz sine wave, the current through the capacitance path is low. When the VFD is added, then the switching frequency is much higher and so is also the leakage current. The leakage current can change with the speed and load on the motor when the voltage and duty cycle is changed.

author: Kalin
author_rank: Regular Contributor
timestamp: May 24, 2017, 09:30:54 pm
content: I'm an electrician who work with VFD's alot. Could you post or DM a picture of the connections on the VFD? Also, is your bonding conductor connected to the line side bonding screw on the VFD and then the bonding conductor for the motor connected to a separate bonding screw on the VFD?Edit: After glancing at the manual for that VFD I would make very sure that you bonding conductor coming in to supply the VFD is attached to the PE screw on the line side (labelled L1+L2) and connected to the equipment through a jumper conductor and then make sure that you bond the PE on the load side as well. I have seen dangerous voltage and blown up VFD's from missing or high impedance path on the line side bonding conductor. I think that is probably your issue.

author: Halvmand
author_rank: Regular Contributor
timestamp: May 25, 2017, 05:13:17 pm
content: I was holding the material that I was drilling and the handle when i got the shock.It's completely apart now, so I can't really take any pictures of the whole installation.The earth wire was directly connected to the VFD, and from there to the motor and chassis.I live on the first floor in an appartment building that probably was built between the two world wars.I have a bachelor in marine management and technical engeneering, wich includes all the theori a licensed electrical installer would have.That made me think about equipotential bonding, and I think there is none of it, taken the age of the building in account. It is after all an appartment and not a workshop.Thanks for the answers guys. I'll seek out an actual electrician and take a look at the problem.

author: Halvmand
author_rank: Regular Contributor
timestamp: May 25, 2017, 05:24:46 pm
content: Kalin:In industriel situations where VFD's is used have you experienced the drives tripping the GFI's? I think that must be a major problem unless the GFI's is the 300mA types.

author: Kalin
author_rank: Regular Contributor
timestamp: May 25, 2017, 09:42:14 pm
content: Quote from: Halvmand on May 25, 2017, 05:24:46 pmWe don't have separate? ground fault protection for motor branch circuits as you would see in a house. The drives I have worked with have had it integrated within the device and the only time I've seen ground fault current faults on the drives have been in cases where there is moisture within the motor terminal box, a burnt winding or failing disconnect switch.Sent from my SM-G930W8 using Tapatalk

author: Kalin
author_rank: Regular Contributor
timestamp: May 25, 2017, 09:51:21 pm
content: Quote from: Halvmand on May 25, 2017, 05:13:17 pm