Induction motor shaft voltage and electrical safety for the operator

Thread Starter

amph

Joined Nov 13, 2018
6
Hi everyone!

I'm planning to build a small machine tool using a three-phase AC induction motor (squirrel-cage type) driven by a VFD.
The motor is to drive a metal platter directly, wihout a belt. In other words, the platter will be mounted directly onto the motor shaft.
The operator will have his hands above the platter.
My question is: is there a risk of electric shock should his hands touch the platter for an instant? (for the sake of the discussion, let's ignore mechanical injuries and focus on electrical safety only)

What is more, the machine may be operated in an old building with an old electrical installation, which may have a bad earth connection or even no earth connection at all. So maybe my question should be split into two cases:
(a) Assuming the whole system is perfectly grounded, is there a risk of electric shock should the operator touch the platter?
(b) Assuming the system is not grounded, is there a risk of electric shock should the operator touch the platter?

I've heard of phenomenons such as shaft voltage (which are, if I'm not mistaken, in the order of millivolts) and bearing currents, but I can't really wrap my head around these and don't know whether they are of relevance to this question.

Thank you very much for your help
 

Thread Starter

amph

Joined Nov 13, 2018
6
P.S.: I just realized this thread might have nothing to do in the Automotive section... My apologies. Maybe a moderator could move it in the General electronics section, if they find it more fit? Sorry about that.
 

shortbus

Joined Sep 30, 2009
6,979
If your that worried, why not run a wire from your motor to a known ground? Wouldn't need to be rewiring the whole building, which would be better, but just a single wire for that motor. But to answer what I think is your real question, most induction motors if they had an internal short, which is what would be needed to cause the situation you talk about, they wouldn't even rotate. The wire coils in an induction motor are insulated from the housing and from each layer in the coil. and all metal nonelectrical parts of the motor are connected by them being metal.

Sounds like your making a disc sander. You may still get a small shock from the friction of sanding some materials, but other than scaring you, they can't hurt you in of themselves.
 

Thread Starter

amph

Joined Nov 13, 2018
6
Thanks for your answer!

You guessed almost right! I'm making a gem faceting machine. But granted, the construction is very similar to that of a disk sander.

Thank you for you clarifying things about the insulation of the different motor parts.
So the shaft is insulated from the coils, right? That's what I suspected, but I needed to hear it from a more knowledgeable person. :)

As to your suggestion of wiring the motor to a known ground, well, the thing is... there's no known ground.
Let me explain a little bit: this machine is a relatively small table-top one which will probably be operated in an apartment.
So if the earthing of the building (or, at least, the apartment) is bad or nonexistent, well that's it, I don't see any other connection point where I could get a ground from. But this is another question altogether...

Thanks again for your help
 

MaxHeadRoom

Joined Jul 18, 2013
19,033
How does it get certification operating a Apartment building with no electrical safety in place?
Sounds like a law suit in the offing?
Max.
 

Thread Starter

amph

Joined Nov 13, 2018
6
Oh, sorry, my bad, I've used the wrong word.
It's not, like, a multi-storied building with lots of apartments and co-owners.
It's an old two-storied European-style house. The electrical installation probably dates back to the 50's or 60's, but that's just my guess.
I don't know what are the exact regulations in my country, but the house has always belonged to my family since generations, and nobody ever bothered giving the whole electrical wiring a revamp. Mind you, most AC outlets at the ground floor are of the old 2-pin type without earth.
 

MaxHeadRoom

Joined Jul 18, 2013
19,033
Original domestic circuits in Europe were two conductor with no Earth.
Mainly used for two conductor lighting and outlets did not exist in the beginning.
There was in some cases a earth GND conductor set up by using inground water supply pipes etc. In some cases a ground rod was set up.
Max.
 

recklessrog

Joined May 23, 2013
988
Get yourself a metre of copper pipe and drive it into the soil outside leaving 6 inches (150mm) sticking out, get enough earthing cable to reach your machine, securely attach the wire at each end, wrap the connection to the pipe with self amalgamating tape to protect is from the weather, and at least you have a basic earth that should provide a good degree of protection.
Try and site the earth rod into an area of moist ground, and not into rocks or stones. Water it from time to time if needed.
 

MaxHeadRoom

Joined Jul 18, 2013
19,033
You can obtain ground rods from most hardware stores and electrical suppliers, IMO 1m is far from sufficient depth.
The regulation rod is a minimum of 2 metres.
In N.A. the earth is connected to the Neutral at the service panel Only!.
Max.
 

Thread Starter

amph

Joined Nov 13, 2018
6
Thank you all for your valuable answers.
I'll assess the quality of the house's earthing next time I'm there and see what I can do about it.
All the best
 

recklessrog

Joined May 23, 2013
988
You can obtain ground rods from most hardware stores and electrical suppliers, IMO 1m is far from sufficient depth.
The regulation rod is a minimum of 2 metres.
In N.A. the earth is connected to the Neutral at the service panel Only!.
Max.
Hi Max, this year I went with a another "Ham" to our local big electrical stockist and bought 10 copper earthing rods to make up an improved ground plane for his antenna tower. They were all just over a metre long, and unless they were old unsalable stock, (we had told them what they were for) I assumed that one metre would be sufficient, although obviously longer would be better. We are in the U.K so regs may be different.
 

MaxHeadRoom

Joined Jul 18, 2013
19,033
It has been several decades since I practiced in the UK, but here they are 2m rods.
With a recommended driven depth of 2.4m (8ft).
Max.
 

recklessrog

Joined May 23, 2013
988
It has been several decades since I practiced in the UK, but here they are 2m rods.
With a recommended driven depth of 2.4m (8ft).
Max.
Just found this for the U.K

5.2.1 The earth rods shall have a nominal outside diameter of 13 mm and a nominallength of 1 50 mm. The rod shall consist of a steel core of minimum grade AS 3679.1-250. 5.2.2 The copper sheath shall be either molecularly bonded or extruded so as to maintain intimate contact with the steel core of the earth rod.
 

MaxHeadRoom

Joined Jul 18, 2013
19,033
Hi Max, this year I went with a another "Ham" to our local big electrical stockist and bought 10 copper earthing rods to make up an improved ground plane for his antenna tower.
Similar here to transmission towers and sub station transformers etc, There is a Ground plane made up of net of copper mesh laid into the ground under the apparatus and ground rods driven at intervals around it.
I recall when I started in the UK there was a prevalence of using the main water pipe as a ground, then with the advent of PVC etc, it was mandatory to place a GND rod and Earth Leakage Breaker.
Max.
 
Last edited:

shortbus

Joined Sep 30, 2009
6,979
Thank you all for your valuable answers.
I'll assess the quality of the house's earthing next time I'm there and see what I can do about it.
All the best
I have a dumb question about your faceter. How fast does the motor spin, RPM? Most motors unless they have a gearbox built in will be too fast for gem cutting.
 

Thread Starter

amph

Joined Nov 13, 2018
6
I've settled for 1500 RPM, which seems to be the most common speed among commercial machines.
The VFD is there to allow the user to lower the speed using a knob.
 

shortbus

Joined Sep 30, 2009
6,979
My oldman did some of this stuff back when I was a teenager, now I'm 70. The 1500RPM still seems like it is way faster than his disc was turning. I never payed that much attention to his hobby, so I could be wrong, but it seemed like his disc was only turning around 500 or so RPM. I do remember it was made of cast iron though, so the abrasive paste would embed into the disc.
 
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