Yes you should use the manufacturer’s recommended filter. There is no doubt these devices do generate RFI in varying amounts. I have filters and screened multiway cables with one end grounded as per instructions on all my installed Inverters.  Manuals usually show which cables may/should be grouped. Type SY-Multi-Flex cables are excellent for this use. Very flexible and transparent smooth outer sheath.

Edited By Steve Garnett on 20/10/2010 12:39:22

Screening with 'SY' cable is the right way to go, as described above. If this is a 230V inverter, take care if you fit an EMC filter and have an RCD protected supply. The Filters have capacitors connected to earth and the small amount of current passing through them may trip your RCD.

If RFI is a significant issue, CY cable is preferred over SY for connecting the motor. Also, contrary to what has been said above, don't worry about ground loops (that is an issue for the control connections, not really for the cable to motor). For the power cables better to have earths everywhere you can get them.

Most inverters (not all) are designed to be mounted in a control panel (i.e. a metal box). Mount inverter securely on metal backplane, ground the screen of motor cables to the backplane close to the inverter (or use. EMC bracket if supplied), a saddle clamp over the screen is one method.

If an RFI filter is used in the supply, mount it on the backplane, as close as possible to the inverter.

Ensure door of control cabinet is earthed – the simple mains earth cable won't do much at RF frequencies, add either a braided copper strap or use semi-conductive foam or mesh door seal.

Keep motor cables as short as possible – if motor cables become too long, an output filter may be needed, these are bulky and expensive.

Use metallic cable glands (preferably proper EMC rated ones) to attach the motor cable to motor terminal box and to the control cabinet.

Edited By David Jupp on 27/04/2016 09:58:58

John, thanks for the explanation. As The Inverterdrive Supermarket we supply large numbers of Inverter Drives, mostly to Industry, but a great many go to home users. When we get the chance, we advise Customers to use the types we know have low leakage or a means of removing the filter. One make I can think of has high leakage for its domestic level built in filters at 70mA (Manufacturers own figure). Many of the other makes are between 10mA and 20mA for their built in Industrial and one at leat for their domestic level. ABB, Bosch, Invertek, Schneider, Teco and Siemens all have links or screws to remove the filters.

Generally if the motor cable is screened 'SY' cable, is less than 10m or so, is correctly installed and permantantly connected to earth without loops and with a good cross section of cable, the leakage is minimised whatever level of filter is resident in the Inverter Drive. This is always necessary to comply with the Manufacturers installation instructions.

'Fraid not, John. The neutral connection is at ground potential, so no current is generated by the Y cap on that node. However, the Y cap on the live node generates a small AC current. They don't balance out, otherwise life would be a lot easier for the engineer at EMC time. Ask me how I know.

Y caps are fitted to reduce common mode noise between the AC inputs and ground (X caps are for differential mode ie between L and N) and generally the more Y caps you fit, the lower the CM RFI. Unfortunately, there are low limits on allowable leakage current (typically 3.5mA) otherwise you'd simply fit masses of Y caps.

That looks like an industrial quality installation, with screened and terminated cables, metallic enclosure and grounded inverter. If only this were the norm!

Inverters are designed to be capable of meeting most "normal" installations by following good practice like this – and have filtering incorporated to get them in the right ball park but every installation is different. However, if you assemble a complete cabinet as a standard product (for sale etc), you would be expected to make it meet the EMC requirements at the cabinet level.