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3 phase cable for a VFD

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Andrew Tinsley17/05/2020 18:15:31
1449 forum posts

I am just about to order some 4 core cable to go between VFD and 3 phase motors. In the past I have never bothered to use screen cable. Lots of argument here about which end of the screening to earth or both! So far I have not been bothered with air or mains born interference.

However I have run out of the old unscreened cable so should my new purchase be screened cable, just in case I ever run into RFI or mains born interference?

If so any recommendations for a supplier. My local and usual go to firms, either supply unscreened cable or armoured cable (which seems to be a bit OTT for the application.



AdrianR17/05/2020 18:32:46
538 forum posts
36 photos

My go to company for everything electrical is TLC Direct, The prices seem reasonable for the cable **LINK**

They have 4 and 5 core so you can wire star or delta.

Re which end of the screen to connect, I would say use brass glands both ends which will earth both ends, but also connect the earth wire so you don't just rely on the screening. To ensure the screening is earthed you can also use earth tabs on the glands.


Andrew Firman17/05/2020 18:55:07
37 forum posts
18 photos

I’ve decided to use SY cable. Search the well known auction site where you can buy various lengths

Andrew Johnston17/05/2020 18:59:31
6137 forum posts
675 photos

When I fitted a VFD for the high speed spindle on my CNC mill I used 300/500V rated 3-core plus screen cable from Farnell. It makes no difference if the motor is star or delta, you only need the 3 phases connected. I connected the screen to earth at the VFD but left it unconnected at the motor. The motor casing is earthed via the CNC mill.

A sweep with a spectrum analyser didn't show any extraneous RF, just the WiFi at 2.4GHz.


John Baron17/05/2020 20:00:19
471 forum posts
186 photos

Hi Guys,

The only reason for connecting a screen at one end is so that it does not form an electrical loop which could allow a feedback voltage between points of unequal potential. Something usually found in audio equipment to avoid creating hum loops.

As far as VFD's and three phase motors are concerned almost invariably the motor will be earthed for safety reasons and so would the rest of the machine. So leaving the VFD end open shouldn't matter. Some VFD's have an earth pass through connection, if so I would use it.

The danger is that some of the older machines used the neutral (Common of the star) as an earth/ground connection which could leave the machine frame at some above ground potential. Particularly if there was a phase to frame leakage when using a VFD. With a normal 3 phase supply, this condition would trip the breakers.

not done it yet17/05/2020 20:02:04
6099 forum posts
20 photos

I daresay Medlock, off Ryhall Road, would be able to supply?

Mike Poole17/05/2020 20:07:22
3015 forum posts
71 photos

SY is a cable with a galvanised steel braid primarily for mechanical protection. CY has a copper screen which is intended for use in applications where EMI needs controlling.


Andrew Tinsley17/05/2020 21:22:03
1449 forum posts

Thanks everyone for your comments and help. i have the cable on order.


Andrew Tinsley17/05/2020 21:22:04
1449 forum posts

Thanks everyone for your comments and help. I have the cable on order.


Edited By Andrew Tinsley on 17/05/2020 21:22:50

J BENNETT 117/05/2020 21:31:47
55 forum posts

You should not use the screen on this type of cable as a protective earth. If the far end requires earthing it should be done with a cable of the correct size in accordance with the wiring regs. The shielding on armoured cable can in some circumstances be used as the PE but it depends on the size of the cable. Tables are available.

Bob Brown 117/05/2020 21:49:11
1021 forum posts
127 photos

I used SY cable and also have a RF filter on mine all fitted in a cabinet


Steviegtr17/05/2020 23:02:59
2150 forum posts
303 photos
Posted by J BENNETT 1 on 17/05/2020 21:31:47:

You should not use the screen on this type of cable as a protective earth. If the far end requires earthing it should be done with a cable of the correct size in accordance with the wiring regs. The shielding on armoured cable can in some circumstances be used as the PE but it depends on the size of the cable. Tables are available.

J Bennet you are not getting the correct picture. The screen is only a screen, just that. You still connect the earth core at both ends. It is just the screen you do not connect at one end, for shielding interference.


mgnbuk18/05/2020 08:11:30
1004 forum posts
69 photos

What does the manual for your particular drive recommend ?

Generic "advice" about connecting just one end of the screen, or not using the screen as a PE is not always helpful or correct where inverter drives are concerned - the ABB ACS 150 drive that is today's job at work is quite specific about the cable requirements for the motor and how to connect it. Both ends of the motor cable screen are to be connected & the screen can be used as PE if the total CSA of the screen is equal or greater than the conductor CSA, otherwise use an extra core or a separate external earth cable of appropriate size.

When all else fails - RTFM !

Nigel B.

AdrianR18/05/2020 08:57:30
538 forum posts
36 photos

The problem with using the screen as the primary PE is that it may be steel not copper. You need a considerably larger CSA for steel.

For safety both ends of the shield should be connected to PE to ensure it is connected incase of mechanical damage to the cable.


martin perman18/05/2020 09:12:38
1994 forum posts
83 photos

When I was working I worked for a company who made large industrial washing machines, side the control cabinet there are two VFD's driving large water pumps that needed to have speed control, because these machines sometimes had to go into already constructed building they came in parts big enough to go through standard doors, when I was let loose on building one of these machines I had problems with the VFD's and other control systems so I contacted our Italian masters and explained the problems, there response was have you connected both ends of the screening on the motor and VFD, answer yes, disconnect the motor end, all the issues disapeared.

Martin P

Andrew Tinsley18/05/2020 10:33:07
1449 forum posts

OH dear, I was hoping NOT to have all the arguments about earthing and screens etc. I said quite clearly in my post that I didn't have any screening and up to now, I had ZERO problems with mains born and RFI.


SillyOldDuffer18/05/2020 10:37:24
7364 forum posts
1620 photos

Confusion galore about earthing cable shields, but I think I can offer a rational explanation.

First, remember earthing is done for two different reasons:

  1. For electrical safety, mostly in power circuits.
  2. To ground unwanted electrical currents, mostly in signal circuits, but also to prevent interference.

As earthing is done for different reasons the engineering solution can be expected to vary. There is no one size fits all.

Wiring up a motor, reason 1 is top-priority and SY cable with its steel mesh shield is a reasonable choice. It's purpose is to add strength if the cable is suspended, to protect against casual physical damage, to inhibit radiation, and it can be earthed as part of a safety system. I wouldn't rely on the shield as a safety earth on it's own because it's prone to rust and has low current capacity. But because SY cable is primarily part of a power system, the shield should normally be earthed at both ends. The main concern is electrical safety.

At this point Reason 2 intrudes. A VFD works by generating and manipulating high-frequency pulses. Although the output waveform is good enough to fool the motor that it's being fed ordinary low frequency 3-phase AC, zooming in reveals the waveform is actually built from high frequency AC. Unfortunately the high-frequency component behaves as a radio wave that can cause chaos to nearby radio services, maybe up to a few kilometres away. It's well worth suppressing, and to do this signal rather than power engineering techniques are needed.

An AC current has magnetic and electrical components. As the frequency rises, AC behaves more and more like a radio wave. It's totally unlike well-behaved 50Hz power which stays on the wire. As high-frequency alternating power can leak energy capacitively and inductively the best shielding prevents both:

  • A cable shield earthed at one end prevents capacitive coupling but not magnetic. Certainly not a waste of time and useful in some circumstances, but probably not when shielding a VFD to motor cable. However often done in electronics to prevent earth loops (see below)
  • A cable shield earthed at both ends will prevent magnetic coupling and capacitive coupling to the outside world because the loop so formed shorts out inductive currents. It blocks both capacitive and magnetic coupling. Usually but not always the best way to stop energy escaping from a cable.

For shielding high frequency AC, shields should be as well-made as possible; ideally a tight copper mesh woven over a copper or aluminium foil wrapper. As this is expensive, SY is a useful compromise for signal shielding despite being steel rather than copper and full of holes. Not perfect, but far better than not bothering! But in a severe case of interference, there is better shielding than SY in the armoury!

Earth loops cause exceptions. They're a serious problem in signal engineering because sensitive electronic amplifiers easily react to tiny voltage differences across a ground-plane that's theoretically all at the same potential. Actually, the ground-plane can be full of unwanted circulating currents. Poor layout causes otherwise correct circuits to malfunction. One way of disrupting earth loops is to only connect signal cable shields at one end. This tactic probably doesn't apply to a VFD though. However, earth loops can also cause difficulty in power distribution systems. It's possible for the ground potential between two buildings to differ enough to cause significant current to flow. In this case a power engineer might also choose to earth shields at one end only.

As a general rule with VFDs and motors, it's probably best to earth both ends of a shielded cable. But read the manual! It's possible that the general rule doesn't apply to a particular machine or a particular VFD, in which case follow the manufacturers instructions. Their recommendation depends on an analysis of radio effects and circulating currents as they apply to their design, and we can only guess if these matter or not.

An uncontroversial way of reducing radiation from a cable is simply to keep it short. A long dangling wire works as an antenna invisibly spraying electronic excreta all over the neighbours! Be aware that not detecting interference in your home absolutely doesn't mean all is well. Maybe Mr Angry in the next street would give you a jolly good kicking if he found out your incompetent wiring was to blame for his years of misery!

For what it's worth, the advice for double shielded cable in signal applications is usually that the inner shield should be earthed at one end only and the outer should be earthed at both. Everybody is right!


blowlamp18/05/2020 10:43:18
1493 forum posts
97 photos
Posted by Andrew Tinsley on 18/05/2020 10:33:07:

OH dear, I was hoping NOT to have all the arguments about earthing and screens etc. I said quite clearly in my post that I didn't have any screening and up to now, I had ZERO problems with mains born and RFI.




Point taken, Andrew.

Nevertheless, here's a link to a .pdf from an inverter manufacturer that may help someone looking for guidance in this matter. **LINK**


Edited By blowlamp on 18/05/2020 10:44:32

John Baron18/05/2020 10:54:51
471 forum posts
186 photos

Thanks SOD, a concise explanation !

Andrew Johnston18/05/2020 11:18:56
6137 forum posts
675 photos

Some of the statements by SoD need elaboration. The PWM frequency used in VFDs is usually a few kilohertz. The reason that the signals radiate at much higher frequencies is because the edges of the waveform are fast, probably a microsecond or faster, fall and rise times, not because of the fundamental frequency.

All AC currents will radiate an electromagnetic wave, even at 50Hz. However, there are two things we can say about radiation at 50Hz. First the wavelength is long (6000km) so even the power distribution network is electrically short, and electrically short antennas are very inefficient. Second, in the near field (below about half a wavelength) the magnetic field dominates. So in practice interference from 50Hz is by magnetic induction.

Copper or aluminium shielding will not stop a magnetic field. Instead you need a high permeability material such as mu metal.

Ground planes are a whole different ball game! When laying out PCBs, especially a mix of low noise analogue and switchmode power supplies one needs to 'visualise' how and where the currents are going to flow to prevent interference. Way back multiple ground planes for different areas where in vogue. But the modern view is to use one solid ground plane with possibly small local planes for noisy circuits like switchmode supplies and be careful how high current tracks are run. The return current in the ground plane will try and follow the track as closely as possible.

Just because the OP says he doesn't have a problem doesn't mean it isn't there, and is possibly affecting somebody else. It isn't clear if he's actually measured the radiation?


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