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

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.

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.

Mike

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

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.

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

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!

Dave

Thanks SOD, a concise explanation !

FWIW, My VFD came already in a small enclosure, and wired to the lathe. (So no idea what shielding or connectioins are used )

Not wishing to feed any noise back into the single phase mains, it is fed through a suppressed metal clad double socket.

In the sixteen years since first use, have never heard of anyone complaining of strange noises or effects on Radio, TV or computers. If there had been, am sure that at least one of my next door but two neighbours, (being on the same phase ) would have moaned, even if not realising who the culprit was!

Howard