Chinese AT vfd heatsink and grounding

I guess more than a few of you have purchased cheap imported VFD units. I bought one last week but now I'm looking at wiring it up I have had a closer look. No protective screen behind the PCB and heatsink, and the heatsink is not connected to ground. If there is a short to the heatsink it could be lethal.

I need to buy a couple of inverters soon, one for a qualters & Smith Qdm750 drill I just took delivery of yesterday, and my new lathe, I've decided to go with invertek as someone on this forum mentioned they are made in this country and I feel it's important given current world trade to support companies in this country, and didn't think the prices were really that high, £95 plus vat for a 1/2 hp inverter, out of interest, how much was your 'cheap' inverter?

Well that's a coincidence! I'm wiring up a qdm 750 right now.

£50 for the VFD

Since i cannot see it Be cautious. It could be that the heat sink may run live ! Earth it and you may need new mosfets or what ever. Stick to what the instructions say, if there are any that make sense. Good luck. Noel

Did you buy the one on ebay down on the south coast somewhere? There was two, that one and one from a dealer in Birmingham, I bought the one from a dealer as the other didn't have a post option when I bought it, first impressions are really good, there's no noticeable movement in the quill when extended to about 130mm, which is pretty impressive.

Well it's good there's another qdm750 owner on here, I hope you get a safer vfd, your life is worth more than an extra 50 quid.

Just tested the heatsink. 52VAC with respect to mains PE (ground). Not happy with this.

No, I didn't buy the one you are thinking of.

Yes, I agree with your comment about safety. I will be reporting this one.

Both heatsinks on my easily accessible VFDs are grounded. However, a VFD is classed as a component, intended to be used in a larger assembly. So there's no absolute requirement for the heatsink to be grounded. It could be floating due to the design, or could just be a case of you get what you pay for.

Andrew

Good choice on the drill, i've been happy with my QDM750.

Shouldn't the VFD be mounted in it's own enclosure? When you go buying commercial components rather than retail finished products makers expect a different level of competence from the installer.

Typing too slow for Andrew 

Edited By Dave Halford on 17/02/2021 12:01:39

Yes, I agree it should be mounted in an enclosure. But what happens when the VFD is mounted in an earthed metal enclosure? The heatsink currently floating above 50VAC would then be shorted to ground. Or how about an internal fault which brings the (unearthed) heatsink to mains potential? Seems dodgy to me.

The seller offered an 80% refund. I replied and said I'm expecting a full refund.

A high impedance DVM can show some surprising readings from floating electronics.

For example I have just checked a mobile phone charger, 50 vac to earth and an ipad charger, 80 vac to earth. Both OEM chargers supplied with the devices.

Michael

I would stand the VFD off, mounting it on plastic posts within the enclosure using plastic screws if you have to.

As to a return, 'A bird in the hand' etc.

The 52v could just be capacitively coupled to a floating heatsink. If it was really unsafe it would be much higher I suspect. The key question is whether you can safely earth reference the control input. Quite a few DC motor speed controllers are designed just for control potentiometers and need an isolator to use with a CNC system for example.

Understood, but that doesn't resolve the problem of an exposed metal heatsink that is in close proximity to mains voltages with no apparent regards to safety. Clearance between the incoming live and heatsink is minimal with no attempt at insulation.

See what Trading Standards have to say

The Schneider ATV12 that I bought has the heatsink connected to ground, and as the metal box bought for it to fit in had a steel plate in the back, the VFD is bolted to it using thermal grease. The box has ventilation top and bottom which is ideot proof. The plate is included in the earthing system, for the box and its door. As this was for use in a museum, the close inspection by a qualified electrician was mandatory for insurance purposes.

As VFDs are almost always components rather than free-standing consumer items they should be mounted in a protective box. If the box is metal it should be earthed, not the heat sink. Never assume a heat sink should be earthed. Electronics and power distribution are only similar, not identical.

VFDs are in the same class as contactors and other "don't touch" electrical internals. Remove the cover from almost any electrical appliance and there be dragons.

Always possible Ajax's VFD is a genuinely negligent death trap, but I don't find the evidence convincing (yet!). But it's not safe unless properly installed. My lathe's VFD is mounted on a rack inside the headstock behind a screw-secured panel. Even with the panel off, it's difficult to touch the heat-sink because VFD's rear is protected by the enclosure. I'd have to deliberately dismantle the setup to touch the heat-sink.

Dave

I fully agree with S-O-D's post regarding the component aspect of a VFD. The controls are there solely to be used for setting up the parameters and there should be no need to ever touch one when in use. A problem exists for many users where the instructions are so difficult to understand, that any thoughts of proper remote controls are set aside. Full size controls in secure cases and in easy to reach places on the machinery can reduce the possible danger of touching any hazardous part of the VFD.

It would help to know the make and model of the vfd in question in case others have the same model and want to compare theirs to see if the same ‘fault’ is present. Having just spent a long time sorting out a small control problem with my own Huanyang vfd, I can confirm that there is a variety of voltages present between various terminals when measured with a DMM. And there is a difference between ground, common and earth terminals.