size of flex required

[duncan webster 1Participant @duncanwebster1]

[duncan webster 1Participant @duncanwebster1]

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

[SteviegtrParticipant @steviegtr]

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.

[Dave HalfordParticipant @davehalford22513]

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.

[EmgeeParticipant @emgee]

Duncan

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

Emgee

[SillyOldDufferModerator @sillyoldduffer]

It's not speaker cable is it? 24×0.2 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

[Ian PParticipant @ianp]

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

[duncan webster 1Participant @duncanwebster1]

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!

[Nicholas FarrParticipant @nicholasfarr14254]

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

Regards Nick.

[EmgeeParticipant @emgee]

Duncan

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

Emgee

[duncan webster 1Participant @duncanwebster1]

good point Emgee, I'll have a look tomorrow

[Ian PParticipant @ianp]

Posted by Nicholas Farr on 22/07/2020 22:48:00:

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

Regards Nick.

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

[Ian PParticipant @ianp]

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

[Maurice TaylorParticipant @mauricetaylor82093]

a

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

[brian goldsmithParticipant @briangoldsmith20924]

Posted by Steviegtr on 22/07/2020 20:48:08:

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.

Hi Steve, for 0.75mm flex 6A is the maximum current rating and volt drop has to be allowed for. BS7671 table 4F3A, for 0,75mm flex single phase gives a value for volt drop of 62 mV/A/m. A load of 6amps on a 50mtr circuit will give a volt drop of 18.6 volts which is more than the allowed 5% at 230V.

Brian

[SteviegtrParticipant @steviegtr]

Posted by Ian P on 22/07/2020 23:47:43:

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

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.

[brian goldsmithParticipant @briangoldsmith20924]

Posted by Emgee on 22/07/2020 22:57:24:

Duncan

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

Emgee

Hi Emgee, the emergency stop feature of the vfd is not considered safe enough, the vfd could develop a fault which prevents the e stop function working. The e stop should disconnect all power to the vfd isolating all the equipment from the power supply.

Brian.

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Ian P on 22/07/2020 23:39:35:

Posted by Nicholas Farr on 22/07/2020 22:48:00:

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

Regards Nick.

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

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

[EmgeeParticipant @emgee]

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

[Maurice TaylorParticipant @mauricetaylor82093]

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

[Robert Atkinson 2Participant @robertatkinson2]

Posted by duncan webster on 22/07/2020 22:41:10:

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!

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.

[duncan webster 1Participant @duncanwebster1]

Emgee: yes there are terminals referred to as 'safety stop', but you need a 2 pole switch, and that wouldn't stop the independent table drive anyway

Robert's point about 5A fuse for 0.75 sqmm cable is well made, I suspect a 5A fuse might go on inrush current. I'll get some 1.25 sqmm

Finally I've tried just switching it off at the mains when going full tilt. It stops very quickly, almost as if it uses the last gasp in the capacitors to brake. This made me realise that the switch on the socket it's all fed from is easily within reach, so why am I bothering? I suppose hitting a big red button is more intuitive, and I've got it now.

Thanks for all the input

[Martin KyteParticipant @martinkyte99762]

Hi Duncan

24/0.2 would be 4.5Amp.

**LINK**

I would definitely use the ESTOP fascility on the inverter.

Personally I generally overate cables purely for mechanical robustness especially when they are exposed.

regarding your comment "I've managed for years with just the vfd stop button, but this relies on software and electronics," If the stop button is the normal stop well yes I woud agree, if it's a proper Red Mushroom EStop then no. ESTOP's have rules attached regarding fail safe status. Generally they will be designed to be as separate from software and other electronics as is possible to avoid the very problem you are worried about. If your unit is for example CE marked it will comply with the rules on emergency stop cct's.

If you really want to improve your safety measures you would be better installing a foot kick stop switch. This is particularly usefull when you are using the mill as a drilling machine and have one or both of your hands busy.

Regarding eStops in general, I would be more concerned over the accessibility of the stop button rather than any remote possibility of it's failure assuming it has been designed for purpose.

regards Martin

Edited By Martin Kyte on 23/07/2020 11:43:34

Edited By Martin Kyte on 23/07/2020 11:43:56

[Len Morris 2Participant @lenmorris2]

An Estop is a normally closed button switch that breaks the circuit to the pull in coil on the machines no volt release relay. The current demand of the coil is very small. Once the relay is opened, it doesn't matter where the power is coming from VFD or otherwise. The motor will stop and not restart unless done so manually. I think no volt relays are omitted from no end of home machines. They are mandatory in industry.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Len Morris 2 on 04/08/2020 17:59:39:

An Estop is a normally closed button switch that breaks the circuit to the pull in coil on the machines no volt release relay. The current demand of the coil is very small. Once the relay is opened, it doesn't matter where the power is coming from VFD or otherwise. The motor will stop and not restart unless done so manually. I think no volt relays are omitted from no end of home machines. They are mandatory in industry.

While the current of contactor or no-volt relese coil is relevent to the E-Stop switch contact rating, unless there is a separate (lower rated) fuse for the control circuit, the cable must still be rated at least as high as the fuse in the plug.
If the coil goes short circuit or th cable gets crushed the fuse must blow before the cable overheats.
Indusrial machines often have a control circuit fuse – SlideLoc was a popular type- newer machines have 24V control circuits.
This fuse / cable relationship is pretty fundamental but ofeten mis-understood. Years ago as part of a safety talk at amateur radio club I'd do a demonstration with a car battery, 5A "flex" and a plug with a 13A fuse The cable length was carefully chosen to give 1/2 Ohm cold resistance, Short out the cable and after several of seconds it went up in smoke releasing acrid frumes fro the PVC. Fuse never blew because maximum current of 24A is not enough to blow the fuse befoe the cable smoked (resistance also goes up and current down as cable gets hot.).
I would not do yhis today because of health and safety, even outdoor s the fumes are pretty bad, but it left an impression.

The comment made by Brian about VFD E-stop safety is correct, they cannot be trusted as the sole means to stop a machne that could cause injury if it does not stop quickly. Martins comments on E-stop "rules" are correct, but not applicable to VFDs or similar devices, at least not any I've seen. VFDs shouls not be CE marked as they are components and components are not covered by the relevant directives. Confusingly about the only components that are CE marked are safety interlocks, E-stops and safety relays. I'm not saying the the VFD E-Stop circuit should not be used, it should, but as part of the bigger picture. On most VFDs the E-stop circcuit actually brakes the mootr to stop it as fast as possible (this puts energy into the capacitors in the drive for small drives) One good approch is to have a relay coli across the output side of the E-Stop in the power feed to the VFD with a normally open contact connected to the VFD E-stop circuit. This cuts power even if the VFD has had a sofware failure (or intereference from another machine) but also uses the dynamic braking if the VFD hasn't failed
You might think why not use one contact on the E-Stop for the mains live and one for the VFD circuit but this would be mixing high and low voltage circuits which needs special care.

Only switch the live feed NEVER both the live and neutral (or just neutral) with an E-Stop. If the neutral contact fails the machine will stop but the entire wiring will including the neutrla up to the switch, will still be live, waiting to electrocute someone faultfinding a "dead" machine.

Robert G8RPI.

[Author]

Posts