Inverter questions..

[ColJack]

we use lots of 3 phase inverters at work for energy saving on large fans and such..

due to the "noisy" output of inverters we have to use screened or armoured cables to filter out the EM noise that they create..

some manufacturers aks that both ends of the cable screening is earthed, whereas others ask that both ends of the screening is earthed

if you only earth one end then you can get large voltages at the "open" end, but if you earth both ends you get less effective screening as the EM "caught" by the screening has a circuit to run around then which creates it's own EM field..

which would you use and why?

 

[breezer]

some manufacturers aks that both ends of the cable screening is earthed, whereas others ask that both ends of the screening is earthed

assuming you have my keyboard (what you type is not what the keyboard tells the PC) and you meant to say only connect oone end of cable screening as an option.

If you use armoued cable you have to connect both ends since the cable needs to be secure (unless of coure you fit the gland to thick plastic or paxolin)

If its just screened cable (where you have a choice) i would only earth one end and isolate the other end of the screen.

It is as you said, by connecting both ends of the screening you give all the junk a path to run around also it may "pop up" else where.

it cant if it is only earthed at one end

(I also see i have my keyboard back, one only has one O not two)

 

[ColJack]

and yet i spell checked it before posting.. and again just as i was going to edit it, but it still say's it's ok..

apparently, AKS is a word...?

 

[tim west]

Had advice from an invertor specialist working for Eurotherm some time back, you can remove the noise from the motor windings by adding an inline choke, this doesn't remove the noise altogether but actually shifts it to the choke this could be mounted somewhere it doesn't bother anyone or be enclosed in a sound proof enclosure. Made an otherwise noisy motor dead silent!

 

[ColJack]

by "noise" I meant electro-magnetic interference...

there are suppressors on the inverters but the manufacturers still want screened cables..

I just want advise on whether we need to earth the screen both ends as it's a pain in some motors to earth the screen

 

[bernardgreen]

I just want advise on whether we need to earth the screen both ends as it's a pain in some motors to earth the screen

Having the screen earthed only at the inverter end gives best results in the majority of cases. The connection to earth needs to designed to be a low impedance at very high frequencies so no sharp bends in the wire.

A well earth gland plate with the screen connected to the gland plate gives the lowest impedance.

That said it is a black art and in few cases connecting the screen at both ends or only at the motor end may be the more effective method.

 

[Hallsy]

What sort of distance are you travelling with the cables? Possible that on short runs you won't have as much of an issue anyway.

I'm pretty sure that whenever I have installed invertors I have tied the screen to earth at the cabinet/invertor end, then at the plug/motor end just tie the screen to the casing, which depending on the earthing arrangement of the motor will still be earthed!!

That said, we probably have the wired either way. I've nevre came across an invertor/motor problem that has been caused by noise though.

 

[Hallsy]

Also, where you mention that it's a pain to earth at motor end, are you wiring directly to terminals, or to a plug?

Either way I normally gather up the screen, twist it together and then either solder it to the case of the plug which then earths it if the motor has an earth to casing (normally do) or if going straight to motor, do the same but put a crimp on the end of the gathered screen and fasten to earth terminal.

 

[ColJack]

it's the condenser fan motors..

we use 1.5 4 core ( 3 and earth ) SY cable..

the connection box is plastic and has a small gland..

when we gather the screening and sleeve it it comes out at about 6mm cable size..

the terminals of the motor connections are only made to take about 2.5 at most so we have to put crimp pins on and then get the screen and the earth into it..

I'll get some photo's for tomorrow...

 

[ricicle]

Yes, earth the cable screens at the invertor end only

 

[Hallsy]

I know what you're saying, as has been said if preferable just leave the motor end of screen un-earthed, just tape/heatshrink it off.

As for screen giving to large a cable, I either half the braid and cut the rest off until it is small enough, or I gather it up, tiwst it tight with pliers, solder it as close to insulation as possible, then cut rest off and solder an earth wire of suitable size to go from the screen to the terminals.