Not sure what model or Kw rating your VFD is, but based on your current consumption figures I would go with a 16 or 20 amp Type C MCB or RCCBO.

Andy

With a VFD the worst case demand is the switch on surge to charge the capacitor bank providing the DC supply that the VFD chops up to generate 3 phase AC. If you happen to switch on at just the right point in the incoming AC cycle you get a very large inrush current for, relatively speaking, quite a long time. Enough to take out a quick blow fuse unless its very overrated relative to the nominal steady state draw.

Ages since I had owt to do with this sort of thing but I think worst case switch on point is around 150 volts.

Ideally the switch on should be remote via a relay and zero crossing detector so the actual switch is at zero volts letting the capacitor voltage rise alongside the AC input rather than trying to jump from 0 to a high voltage. Fuse suggestions imply that your inverter is direct switch on not zero voltage controlled.

Clive.

One that I sold recently needed a 32A type D mcb to hold on when switching the unit on but that was supplying more current to the bigger motor.

Emgee

Have checked the mcb and it is 32A. It does supply other things as well as the workshop.

Martin C

That's only a 0.75kW / 1hp drive, so I can't imagine what business you'd have fitting anything like the ratings you are talking about.

VFDs have a soft start circuit to limit inrush current (there are regulations that require it to be limited as you might expect), so talk of silly big currents at turn on is plain unhelpful.

Perhaps you could link to the relevant page in the spec or paste a screenshot.

Murray

PS I'll be installing a 3.3kW Yaskawa VFD later this weekend, so may have some more insight to offer then.

I see from the installation manual that the fuse ratings you mention are for compliance with UL / cUL ie for industrial installations in America and Canada. The fuse ratings are somehting like 3 times the rated input current. You should ignore them. These drives are something like 97-98% efficient, so the input power isn't going to significantly differ from the output power. Don't overthink this!

Edited By Muzzer on 14/04/2017 14:46:05

I have a Yaskawa VS mini J7 series inverter, some kind person "here" helped me out when I was unable to get a slow speed, they pointed me in the right direction enabling me to alter an item in the program. Now I would like to increase the top speed from 2000rpm to 3000rpm.which item is it please. John

Bit of a guess, but I think what's going on is the manufacturer is recommending the best possible fuse for a particular set of circumstances:

1. On power up the VFD capacitor bank might draw a short burst of very heavy current if the switch happens to be operated close to the top of the AC cycle. To the supply it will look like a short except it only lasts a few milliseconds. This is worst case, if by luck the switch is turned on 10mS earlier, the voltage will be zero, and current starts to flow gently. 5mS after the peak and the voltage is well on the way down. I feel this is less likely to happen with modern VFD because it's not difficult to design in zero point starting. BUT
2. The motor pulls a lot of extra current on start-up, especially if it's already loaded and the lubrication is stiff on a cold day. Again the supply 'sees' something suspiciously like a short circuit except it doesn't last long.

Given there are two false short-circuits how best to defend against a dangerous real short? One way is to fit a slow blow fuse sized to the expected 'normal' current; these take a heavy current for long enough not to be fooled by brief overloads. But each time it happens the fuse is stressed and it will give up the ghost eventually: as a solution it's safe but causes interruptions. Alternatively, fit a fuse large enough to take the full short-term overload current plus a little extra, but make sure it has a fast blow characteristic. A big quick blow fuse will be resilient to brief overloads but can be sized to pop quickly in the event of an actual fault.

In amateur use an ordinary 13A fuse should be fine. The VFD is unlikely to be powered up and down as much as a professional set-up, and the ordinary fuse may last decades. Eventually it might get tired and die, but that's hardly a crisis – just put a new one in. This approach wouldn't do in a professional shop and the VFD supplier would get complaints. To avoid them, I suggest he recommends an expensive fuse that balances safety and reliability. All we need to is keep it safe.

I'm reasonably comfortable using a small fuse in place of a big one. What would be a gigantic mistake would be to replace the makers recommended expensive fast blow fuse with an nice cheap slow blow of the same amperage. A lot more damage will be done in the time it takes a slow-blow to eventually break.

Ignore any and all of the above if a professional electrician disagrees!

Dave

Thank you Mike, I will check things on Thursday. The motor is a 4 pole type and is fitted to my bench top Naerok mill.. I've had the mill almost 30 years and don't remember any critical comments about Korean products like I frequently hear regarding model engineering items from China.,strange old World. At the time I was mighty glad to get what I have, the casting finish ought to have been better, but it given me a lot of pleasure. John