Member postings for Martin Cargill

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

A couple of points regarding problems that some invertor users have highlighted in this thread:

1. All invertors will produce an amount of earth leakage when used.

2. Earth Leakage is "cumulative" , your household electricity system may already be producing some leakage but below the 30ma trip level, adding an invertor may push it "over the edge" and trip your supply .

3. The most common items in your house that can be producing earth leakage are cookers, kettles and water heaters, devices with older brush equipped motors (washing machines) that have gone through a couple of sets of brushes also tend to have a bit of leakage - if you are having problems with your inverter tripping the supply try switching off these devices - a bit of detective work can sometimes reveal a "leaky" device .

4.When fitting an invertor drive the cable between the invertor and the motor should be a screened type. The screen is there to prevent interference and should be earthed at one point only.

5. A 110 volt transformer is unlikely to produce earth leakage but it can suffer from saturation, depending on a few different factors, when it is switched on it can trip the circuit breaker, this is based on a current overload and not earth leakage.

Hope this helps with a few peoples experiences

Martin

Just joined the forum and I thought I should introduce myself. I'm an engineer during the day and (sadly) I list engineering as one of my hobbies as well. During the day I work on woodworking machines - from simple table top DIY stuff through to quarter million pound CNC controlled machines. When I am not working I have a batch of motorbikes and a 1971 Land Rover to look after. Having recently built a garage/workshop I'm now in the process of equipping it, so far, amongst other things, I have obtained a Clarke CMD mill (with DRO) and two Viceroy lathes (evaluating them to find which of them is the best before I part with one).