Inverter with motor needed

[Martin CargillParticipant @martincargill50290]

Regarding my comments about screened cables.

The screening of the cable between the invertor and the motor is required to reduce radio frequency interference. An SY type cable with braid is probably sufficient for most invertor installations. The reason for earthing screening at one end only is to prevent the chance of a circulating current being set up in the screening, probably not a problem in an invertor circuit but it can be a problem in cables being used with encoders etc because they can pick up pulses that have been induced into the cable (if its not screened or if both ends of the screening are earthed). If an controller starts to count extra pulses, and not just the ones from an encoder then it will lose its position. On a CNC router or similar machine this can be serious. Hence the convention for earthing screening at one end only.

Its probably worth mentioning that, in my opinion, swa cable armouring and SY cable braid (where SY is used as an incoming power cable) should, obviously, be earthed but additional earthing should also be provided. In the course of my work I regularly find machines that are not earthed because the wire armour or the braid has been pulled out of the cable gland.

Martin

[MuzzerParticipant @muzzer]

Posted by Steve Sharman on 21/10/2016 23:50:22:

Well done, Andy Ash, some sensible rmarks at last! Many of the comments regarding RCD's show a complete misunderstanding of their function and, indeed, where they are required under the current wiring regs.

Nicely put. Clearly not one to make undue friends or think before opening your mouth. Great to have such a modest expert in our midst!

[John RuddParticipant @johnrudd16576]

Posted by duncan webster on 21/10/2016 23:09:06:

Never had any trouble with RCDs and inverters, and I've had 4 of them, but the first one used to blow the MCB on startup because of inrush current to the capacitors.. You can get 'slow blow' MCBs I've since found, must get one for the welder circuit.

Teco inverters that I have serviced have a thermistor preceding the caps to prevent inrush current surges. It is not until the under voltage detection circuitry has cleared then the main power relay is energised bypassing the thermistor….

As for mcb rating, there are different trip curves, A, B, C and D…..these are what you are referring to…..

[Steve SharmanParticipant @stevesharman33815]

Posted by Muzzer on 22/10/2016 14:45:15:

Posted by Steve Sharman on 21/10/2016 23:50:22:

Well done, Andy Ash, some sensible rmarks at last! Many of the comments regarding RCD's show a complete misunderstanding of their function and, indeed, where they are required under the current wiring regs.

Nicely put. Clearly not one to make undue friends or think before opening your mouth. Great to have such a modest expert in our midst!

PMSL

We armchair experts are renowned for our thoughtful, informative and sensitive replies when responding to some of the drivel misconceptions written (about) on this august website.

However, it is difficult to endear oneself to all and sundry whilst retaining one's position in the saddle of such a lofty mount.

Regards, Steve

[not done it yetParticipant @notdoneityet]

Not followed this thread, but have now looked back at replies, and editing times in particular. It seems that (perhaps) we can guess that some information or other has been deleted/changed since some original post(s)? If that is the case, it would be better all round if mistakes were highlighted with an edit to make the correction clear.

[Neil WyattModerator @neilwyatt]

Posted by Steve Sharman on 21/10/2016 23:50:22:

If contemplating setting up a new workshop, it is well worth talking to your local spark. It could actually save you money, and it may save your life if things go pear-shaped. But, hey-ho, we're engineers and know it all, don't we?

We had an electrical check recently as our wiring is a few decades old. Found a few things and the comment was 'the best bit is the wiring in the workshop' as its the only bit completely up to modern standards. Guess who wired up the workshop

N.

[Roderick JenkinsParticipant @roderickjenkins93242]

We had to get a new washing machine a couple of weeks ago. Today was the first time I've used the mill, which has a Newton Tesla 3ph package on it, since the arrival of the Bosch washing machine. Lights out in the workshop again. The washing machine needs to be switched off at the wall if I'm to use the mill, so no milling on Sundays 'cos that's the day we get our free electricity from British Gas and the washing gets done.

Sheesh

Rod

[Mark CParticipant @markc]

Martin, the screen on the motor/inverter cable definitely should be connected to ground at both ends. You will find that this is the instruction from all the manufacturers.

Mark

[HevansccParticipant @hevanscc]

post deleted

Edited By Hywel Evans on 17/01/2017 08:24:52

[Martin KyteParticipant @martinkyte99762]

Well put Andy Ash.

I would add the observation that whilst the topology of mains filters is generally symetrical Live to Earth and Neutral to Earth, the single phase supply is assymetric in as far as the neutral and earth potentials are usually within a volt or two. The 50Hz leakage component in the filter is predominantly on the Live side which is what causes the imbalance and as Andy has pointed out is additive when more filters (devices) are connected. Hook up enough filters and sooner or later you will exceed the trip current (30mA).

In our old building (Lab) we had no end of issue with earth current running through both the pipework and the reinforcing to the extent of many amps. I spent a week selectively earth strapping a water pipe in one of our electron microscope rooms in order to reduce the field generated by it. I built an active field cancelling unit in the end. We were aware that the main culprit were the many hundreds of computer mains filters each dumping current into the safety earth.

regards Martin

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Mark C on 17/01/2017 00:06:38:

Martin, the screen on the motor/inverter cable definitely should be connected to ground at both ends. You will find that this is the instruction from all the manufacturers.

Mark

That is only the case when the screen is being used to provide the safety earth bonding. For reducing radio frequency interference it should be connected at one end only and the safety earth provided by a fourth conductor. Otherwise high frequency currents in the screen will give rise to radiation in accordance with Maxwell's laws.

Russell.

[Mark CParticipant @markc]

Russell, you might want to take that up with the inverter manufacturers.

Mark

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Mark C on 17/01/2017 09:49:08:

Russell, you might want to take that up with the inverter manufacturers.

Mark

No need Mark. I have Mitsubishi and ABB inverter drives and neither manufacturer makes the statement you claim that all manufacturers make.

Russell.

[David JuppParticipant @davidjupp51506]

What confuses many, and isn't always clearly set out, is that screen should be grounded at both ends for the power cable between inverter and motor but at one end only for control signal cables (e.g. pendant to inverter or PLC to inverter).

[Mark CParticipant @markc]

Russell,

No, you are creating a Faraday cage arrangement. Here is the guidance note produced by ABB. You might want to read your installation guides again…

**LINK**

Mark

[John HaineParticipant @johnhaine32865]

We can learn a little from RF engineering here. If you had two screened boxes connected by a coax cable carrying RF energy from one to the other, but only connected the outer screen at one end, then interrupting the coax screen means that currents that otherwise would flow on the inside of the screen will have to flow on the outside when they reach the break. As long as they flow on the inside they can't radiate RFI, but once they start flowing on the outside of the cable it starts to radiate. So to stop RFI generated by the switching current waveforms carried on the inner conductors being radiated you have to ensure that the outer screen is connected right through.

[Martin KyteParticipant @martinkyte99762]

Generally I would ideally screen a motor cable at both ends so that the cable screen extends around the teminal housing on the motor and the motor casing. The object being to limit emissions from high current conductors.

For signal cables I generally want to screen one end only to obtain some shielding of my signals from external noise and to avoid ground loops and multiple earth points.

Generally for control gear using low impedance sensors (such as switches and pots) the main thing you are concerned about is EM radiation rather than noise immunity so it doesn't really matter too much unless the neighbors complain.

You have to remember that currents case magnetic fields and voltages cause electric fields so how you ground bond, ground reference and earth protect depends on what you are required to do and what you wish to achieve.

Have a go at building a PA sound system and earth everything at both ends and have a listen to the hum.

Connect your system according to the manufacturers instructions. If it works leave it alone. If you have problems like you cannot hear the test match on long wave or your tacho doesn't work properly then think about changing your arrangement.

regards Martin

[MuzzerParticipant @muzzer]

I've seen both methods recommended by motor drive manufacturers and ABB certainly count as credible authorities. That looks like a pretty sensible guide from them.

Bottom line is you need to test the final installation for compliance to the required EMC standards (EN55011 for industrial, scientific and medical appliances in the EU). As the manufacturer, you can recommend and sell filters etc that will typically enable the system to meet the requirements but each implementation is different so it's not possible to provide a definitive solution.

You can certainly find problems if you leave one end of the motor screen floating. The screen forms a rather nice undamped half wave stub that will resonate if you provide it with the correct stimulus eg a PWM voltage – I can tell you myself from experience that in that situation, grounding both ends can indeed be very helpful. There should be a separate protective earth anyway (for electrical safety) and usually any sizeable motor will be firmly bolted to a large conductive (and thus earthed) assembly, so the motor isn't going to start flapping up and down (electrically) either way. However, a resonant stub aerial isn't going to do anyone any EMC favours.

Murray

[Author]

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