Internal motor Earth (Atex).

Tuco88

Hi folks,

Anyone know is it a must to have an internal Earth on a 3ph motor, its in a Ex area driving a pump? So a 4 core cable.

3 core and separate single Earth was marked.

2011

Tuco88 wrote: »

Anyone know is it a must to have an internal Earth on a 3ph motor, its in a Ex area driving a pump? So a 4 core cable.

In my opinion, no.
When you say "internal earth" I take it that you mean an earth within a 4 core cable.
I have installed a lot of Atex motors and have never seen a 4 core as a requirement.
If anything a 4 core is less suited than a 3 core with a separate earth for motors fed from a VFD.
For this reason I often specify motors to be supplied from a 3 core SWA with a separate CPC regardless of whether they are in an Atex or safe area.

It even more important in Atex areas to apply equipotential bonding to the motor frame.

With all things Atex, it is best to read the EC certificate to ensure that you don't do anything that will invalidate the certification. If any doubts remain contact the manufacturer.

3 core and separate single Earth was marked.

What do you mean? "Marked" on the cable block diagram?

Tuco88

2011 wrote: »

In my opinion, no.
When you say "internal earth" I take it that you mean an earth within a 4 core cable.

Correct.

What do you mean? "Marked" on the cable block diagram?

Yes exactly. The norm was always seprate single earth for us attached to a earth fixing on the frame, it also made glanding/terminate much more practical.

It was originally 3core changed to 4core by a new engineer for the above reason apparently. More so we had the cable in already.

We just had doubts on any new changes. E-Mail sent, And a vendor is on site Monday I will interrogate him.

Thanks for the feedback 2011.

2011

As you have been asked to wire the motor in this manner am I correct in assuming that this motor will be fed from a VFD?

I have spent quite a bit of time over the last 6 months retrofitting VFDs to motors in order to reduce energy consumption. During this time we had to work closely with VFD manufactures such as ABB and Schneider to see what modifications we would have to make to the motors, wiring and controls to ensure Atex compliance and that we did not cause premature bearing failure. I assume that your wiring requirement is due to the latter, is that correct?

If a large motor is wired in a 4 core cable and the cable run from the VFD to the motor is long in some cases this can cause bearing failure within a very short time period. This is due the high frequency switching of the VFD (several kHz) inducing a voltage on the motor shaft which finds its way to back to the source (the VFD) tracking across the motor bearings. In some cases this can cause arching across the motor bearings which can dry out the grease, pit the bearing surface and even flat spot or deform the bearings. There are a number of ways to deal with this, one common way is to feed the motor with a screened (or SWA) 3 core cable and separate earth. This has become common practice in many sites in Ireland, less so from what I have seen in other countries I have worked in such as Poland, Czech Republic and Spain. When the motor gets even larger, cable runs are longer or for other reasons additional measures may also be required to ensure a robust solution. This can include installing ferrite rings on the cable, using a concentric type screened cable (this contains 3 earths and 3 phases, also known as "symmetrical cable"), earthing the motor shaft (using an aegis ring), installing insulated bearings on the NDE of the motor shaft, using active filters... the list goes on.

Another consideration for Atex motors fed from VFDs is that minimum rotation speeds (assuming that the cooling fan is mounted on the motor shaft) must be implemented. The reason for this is that if the motor runs slower the fan will rotate slower so the cooling effect is reduced. The consequence of this is that the temperature classification of the motor may be exceeded which may invalidate the Atex certification.

Yet another consideration is that you will also need to ensure that the thermistor will stop the VFD in a compliant manner. ABB and Siemens have a great solution for this.

There is quite a lot to consider when installing motors fed from VFDs in a potentially explosive atmosphere. My preferred solution is to buy the motor and VFD as a certified pair.

Tuco88

Thanks for the info 2011 was not aware of such factors.

Vender in from ABB today stated much the same no internal earth.

You are correct its driven by a VFD. We tried to keep it as close as possible to the motor.

So we have 3 core cable and Separate Earth to the motor frame. Would we still not have interference from the SWA glanded at the motor and at the drive end?

For my own curiosity.

2011

Tuco88 wrote: »

Vender in from ABB today stated much the same no internal earth.

I found ABB to be extremely helpful with VFDs.

We tried to keep it as close as possible to the motor.

This always helps, (obviously the VFD must be installed in a safe area). I was installing an ABB VSD recently, two things you should know:

1) You need to inform ABB that the motor is in an Atex area and that you want to connect a thermistor. They will sell you a different thermistor module for a motor in an Atex area than for a safe area.

2) If the motor is ABB the VFD can detect overheating of windings without a thermistor.

So we have 3 core cable and Separate Earth to the motor frame.

Just to be clear, you need a CPC for the motor in addition to equipotential bonding.

Would we still not have interference from the SWA glanded at the motor and at the drive end?

I won't pretend that I understand this fully, but the recommendation from VFD manufactures is consistent, the insulated bearings are installed at the non-drive end of the motor shaft. ABB emailed me a very good document that explains all of this, I have just found a copy of it here.

I'm not saying that you need to install insulated bearings, the requirement will depend on a number of parameters. It is also crucial to ensure that modifications made to the motor do not invalidate the Atex certification. An easy way to ensure this is to use a company like Campbell Motors who work closely with ABB.

It's a funny thing but on some sites they ignore the recommendations made in the above linked document completely and have no issues. On other sites they implement better installation practices and have serious issues. I think that this is simply because there are so many variables.

tweek84

2011 wrote: »

There is quite a lot to consider when installing motors fed from VFDs in a potentially explosive atmosphere. My preferred solution is to buy the motor and VFD as a certified pair.

Isn't it a requirement that the VFD or VSD has to be match to the motor to be Atex certified? It might just be on a new installation but that was the requirement i have had to work with previously.

2011

tweek84 wrote: »

Isn't it a requirement that the VFD or VSD has to be match to the motor to be Atex certified?

The definitely the simplest way of doing it.
If I buy a VFD and a motor as a matched pair I can have a single certificate for both. It makes less of a difference for Exd (flameproof) motor than Exe (increased safety) motor.

VFD = VSD, just different terminology for the same thing.

It might just be on a new installation but that was the requirement i have had to work with previously.

This is a site standard in many places as it makes life easier. Vendors can be less forthcoming with information when you are connecting a competitors VFD to their motor.

Tuco88

Thanks for the link 2011.

I could not believe the amount of factor's to take when selecting a Motor/Drive/Cable correctly (Must be fun for engineer's). I used to always only have interest in the VFD parameter's more so the 4-20mA AO/AI from and to the drive.

It's a large Brook Crompton motor with ABB drive new ACS model.
So the layout is 3Core SWA, Single PVC Earth for CPC, Frame bonded to local earth bar and a 3Core for the thermistor.

I agree on site standard's are very odd from time to time can be annoying at times to keep a constant practice.
Handy bit of info on this site. Down near the end for cable info.
Here