size of flex required

I'm adding an emergency stop button and trying to tidy up the wiring around my milling machine, at present it looks like an explosion in a spaghetti factory. I have a 0.75 kW three phase motor driven off a vfd, so I'm guessing about 4.5 amps. (750/230/0.75 efficiency)

I have some flex which is 24 strands 0.2mm diameter, which I make to be 0.75 sq mm. It's in free air clipped to the wall, not buried in insulation

According to the interweb this is good for 6 amps. Does this sound sensible? Bigger means larger bend radii, which is what I'm trying to avoid

Are you talking about driving the motor with the flex, or the e-stop circuit. Either way it is big enough.

If it gives 6amps in the regs, then that is for 50mtr run. Taking it you have a short run then it will be considerably higher.

Steve.

The cable should have the cable sq.mm and voltage moulded on the sheath.

Your 6A rating is for 60degC sheath in other words it runs hot and the volt drop might be noticable.

Duncan

What is stated on the motor plate for FLC ?
I think possibly your estimate is an amp high.

Emgee

It's not speaker cable is it? 24x0.2 speaker wire is OK for 6A, but the insulation may not be up to mains volts. ( 6A into an 8 ohm speaker is less than 50V.)

Dave

Is the flex you mention actually flex, or is it cable? I have always understood flex to be the twisted sort of stuff used on things like table lamps and antique telephones, cable is mostly multiple insulated conductors in an overall jacket.

I presume the wiring is between the VFD and the motor rather than the mains input lead? The mains lead will be carrying the total power whereas the motor wires only carry about a third of that.

Ian P

The motor plate says 1.8/3.3 As I'm connected in delta I assume the 3.3 is appropriate, but as it's a 3 phase motor on a single phase VFD the single phase current must be higher? Sqrt 3 comes into it I think

It's not speaker cable, definitely mains. Whatever is moulded into it is illegible. I've just checked my kettle lead, that is 1.25 sqmm and 10 A

I'm putting the estop between the plug and the vfd. It is rated AC15 230V 6A, and is intended to be very much emergency only, ie not regularly switching. I've managed for years with just the vfd stop button, but this relies on software and electronics, it just seems better to have a mechanical switch, am I being overcautious? If so it will be a lot easier to just leave well alone. I suppose in extremis I can pull the plug out!

Hi Ian P, flex or cable, that is the question, maybe this will help. **LINK**

Regards Nick.

Duncan

It is likely there is an output pair on the VFD for a remote emergency stop, check your manual.

Emgee

good point Emgee, I'll have a look tomorrow

It does help, but for another reason. It confirms something I mentioned in a recent thread on this forum recently when I advised a poster not to believe everything he read on the internet!

There are several facts in the definitions given on that website that are patently wrong but one obvious example is that it states, 'cable' has solid and not stranded conductors. The company claim to be certified professionals so how they could come up with that trinket I have no idea.

Ian P

As emgee says, the VFD's own E-Stop feature is the one to use because it will stop the motor quickest.

The disadvantage with cutting the incoming mains is because of the stored power in the capacitors of the VFD which continue to run the motor until discharged. If the drive is only lightly loaded the longer stopping time could be significant from a safety point of view.

Ian P

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Edited By Maurice Taylor on 22/07/2020 23:52:25

Correct. i have one fitted to the inverter. If hit it stops instantly. With a power cut probably would be slower due to the above quotes by others. If you wire the circuit using this method the flex will only carry a few milliamperes.

Steve.

Hi Ian P, the point I was trying to make is that many of us, myself included, don't use the word properly, but most of the time we understand what has been said. Flex seems to have many meanings, but ultimately it means to bend something pliant or elastic, normally repeatably without fracture or deformation to any significant degree. When talking about cable or wires, we should therefore describe them as either flexible or solid or maybe even semi-flexible. Cable as I understand it, has two or more wires or strands, that are insulated from each other, inside one or more sheaths whether they are flexible or solid.

Regards Nick.

P.S. Of course in this context, cable and wire should also be proceeded with the word electric.

Edited By Nicholas Farr on 23/07/2020 08:27:06

Brian,

Does that apply to all VFD units or just the make Duncan has ?
I'm rather surprised nobody has suggested replacing the motor cable with a screened type, SY for example.

Emgee

added SY

Edited By Emgee on 23/07/2020 08:33:14

I would fit a mains rated latching emergency stop button between the mains plug and the VFD ,using same size cable/flex that is between the plug and VFD.

I can’t understand why these threads get so complicated ,when all it needs is a simple answer.No doubt there will something wrong with my answer.

Edited By Maurice Taylor on 23/07/2020 09:13:59

The cable rating must not be less than the rating of the fuse in the plug. If using 0.75mm2 cable you must have a 5A fuse in the plug. For a 13A fuse you must use 1.25mm2 or larger cable.
The load does not matter. The cable must be protected against overload and that falls to the fuse in the plug. In the UK at lest.

Robert G8RPI.