VFD wiring

Hi All

I'm trying to install a Mitsubishi E500 inverter with pendant to a Boxford 280 3 phase motor and I know I need to wire the motor to Delta but what wire should I use to connect the Inverter to the motor

Thanks

Theoretically you should use a screened cable connected at Both ends to earth or just the inverter end connected lots of discussion about that practice and which way is correct

But I have found in practice no problem in just using 4 core flex cable...I,m now up to 19 inverter driven machines in my workshops

Three core and earth of about the same rating, per core, as the incoming 230V single phase supply. No need for a screened cable if it is short lead, I believe. Details will be in the manual.... somewhere!

If you don't know, it would perhaps be better to get the job done by an electrician.

The supplier of my VFD recommended 'SY' cable, like this:

**LINK**

https://www.ebay.co.uk/itm/391433134625

[choose an appropriate diameter to suit the current]

... It's screened; and tough; and not expensive.

MichaelG.

'CY' cable would be expected to be superior to 'SY' with regard to minimising radio frequency interference. CY is likely to cost a little more and won't offer quite as much mechanical protection as SY. The differences could be important in a sensitive environment.

Thanks guys for the cable advice

Would any of you also know if I need to add an isolation switch or breaker as it's already on and RCD from my house supply to the garage

Once again thanks

When using screened cable only connect ONE end to earth. That way the screening works and does not lead to the possibility of circulating currents in the screen.

Edited By Martin Cargill on 17/06/2017 09:09:00

Ok Thanks Martin

The 'one end of screen earthed' applies to signal cables - the ones to the pendant. It is generally recommended to earth the screen at both ends for the cable connecting VFD to motor. Best advice is to follow VFD manufacturer's guidance.

VFD to motor you are trying to prevent/reduce generation of RFI (conducted or radiated). Signal cables you are trying to reduce susceptibility to RFI and induced currents form elsewhere.

Despite much contrary advice floating about I believe that's right. The purpose of the screen is to reduce electromagnetic radiation caused by Radio Frequency currents which are a VFD by-product. What's significant at Radio Frequency is negligible at 50Hz and in this context it's unwise to assume AC and RF are similar. In particular a loop can be an efficient radiator. Look at a TV antenna.

I can't find the reference but the most convincing of my books says:

• single shielded cables should be 'earthed' at the equipment end only
• the inner shield of a double shielded cable should be 'earthed' at the equipment end only BUT the outer shield should be 'earthed' at both ends.

I think the reason advice varies is because much depends on individual circumstances. The length of the wires, the frequencies being produced, the quality of the power earth, and the type of filter fitted all have an effect. Compounding the confusion, unless you have a wide spectrum receiver you can't tell if there's an RFI problem or not. Domestic TVs and Radios are far from ideal as test equipment.

Apologies for complicating Lee's question: don't over-think it! If I were Lee I'd use 1.5mm SY cable with the shield earthed at the VFD end only.

Dave

Quick comment as I'm out the door in a minute, but my point is that a power cable with RF on it is best treated as a signal cable. On control wiring shielding is needed to keep muck out, on a power cable the shield is to keep muck in. In both cases a loop in the shield is more likely (at RF) to cause trouble than not.

Good point about following the VFD manufacturers advice though; it might be evidence based whereas I'm armchair bound.

Dave

Haven't we been around this loop before???

(Yawn) - yes, it seems to be an old chestnut here. Some manufacturers recommend connecting both ends of a screened cable, some recommend only the VFD end. But there are lots of experts here who feel there can be only one right answer.

Do whatever is convenient and if you have a problem with interference (on your radio most likely), perhaps there will be an expert who can then help you to resolve it.

Murray

Mitsubishi's EMC guidelines for VFDs may be useful - **LINK**

Thanks everyone for the great replies

.

That's very impressive, David ... Thanks

... and I do like the fact that they offer the option to choose your own name for the download

That's classy

MichaelG.

I'm not going to say anything...

<retires hurt>

Neil

I beg to differ.

If you pick you own name for a downloaded file the odds are that when you come to use it you will never remember its name!

I used to rename downloaded data sheets etc so its name matched say, the IC it referred to because the IC manufacturers often had their own cryptic filenaming system.

Nowadays I put all the PDFs in one download folder using the manufacturers/originators name. I then make shortcuts to them with sensible names.

IanP

Feel free to differ, Ian ... I was only expressing an opinion.

The website offered the rather cryptic name of < 061000.pdf >

I chose < MitsubishiVFD_061000.pdf >

and exported that to 'GoodReader' on the iPad

MichaelG.