Inverter de-rating

supply

Theory: Input is passing through two legs of input rectifier bridge and not three, so if they are being used flat out you need to de-rate to 67% of original load. There is a theoretical consideration of the main reservoir capacitor(s) being too small as now charging is happening less frequently.

Practice: in the two occassions I've set machines up this way I've not had to de-rate at all and there's been no hot smells from within. I assume that the input three phase bridge is in most cases pretty well over specified to 'the next size up' as are the reservoir capacitors.

There MAY be an issue regarding noise pushed backwards into the mains due to the input current now being more 'pulsey' but I doubt it will trouble you or youe neighbous unduely.

Suck it and see!

AWEM

Hi, There are usually specs published for using Inverters from what is effectively single phase mains. What will usually happen is that you would get a 'phase lost' trip as normally the less clever inverters will tend to have their phase trips based on the DC link ripple. You can get away with using an inverter but there are serious de-rates involved. Usually you end up only having about third of the rating. The guidance is that the more current you are using then the more severe the de-rate. Also bear in mind that if the motor is running at something less than base speed then this will favour you. Don't for get that motor power is speed times torque, P = radians/second times by the Nm of torque.

I used to design inverters, and the criteria for our designs was to preserve the lifetime of the DC link capacitors, which are the biggest risk in an inverter.

Hope this helps!

Rob.

A while ago I was going to experiment with different sizes of additional DC buss caps but didn't get time to finish playing around. It should be possible to reduce the ripple without introducing any problems (the bridge, etc should cope with the additional in-rush current).

Dave

Thanks The reason for asking is that frequently the manuals don't say what the effect is. I have a 3 phase 240 input inverter that does document the down-rating but I've not seen it on 415 volt ones.

Also your final comment is interesting. Are you suggesting that a 2 pole 3000rpm motor run at 25hz will produce the same power as a 4 pole

Regards

Charles

Hi Charles, What I am saying about running at a lower speed is that the motor will put out less power at the shaft, because the speed component (?) of the P = T * ? is less, so that the net output power is less. The torque stays the same (if the inverter has any slip compensation) at the lower speed.

You probably need to think of the drive/inverter as an energy pump. If you are running the motor at lower than base speed then you give the DC link cans a much easier time. The other guy who suggested putting extra C in the DC link is right, but I think that the Rectifier might take a pounding. You have the option of putting in a more manly front end rectifier and supply the drive via its DC terminals, if it has any. Careful not to wire it to the brake transistor if it has one!

Try talking to the manufacturer, as they probably get this question asked of them repeatedly. I know I did when I was in the industry.

I should say that if you don't know what your diong, do not attempt any of what I have said. Just to be clear, if you blow yourself up, don't blame me! :o)

Cheers,

Rob.