Another VFD question. Is it worth buying a higher capacity unit?

[Richard]

Is running a 2.2kw VFD with a 2.2kw load asking for trouble, would a 4kw VFD running a 2.2kw motor be under less stress?

Are there any disadvantages to doing that?

Would it just be a waste of money?

[Neale]

Depends on situation but I doubt most home-made machines are capable of working the motor to full rated output anyway. I use 2.2kw spindle and VFD because I get an ER20 collet that way for 1/2" shank cutters rather than its power. I think most VFDs are overload protected; maybe not against dead short but ok against modest overload. My setup is around 5-6 years old now and still going strong.

[m_c]

Running electronics near their limit (especially cheaper designs) is likely to reduce service life, but as Neale says, most things won't run near full load anyway.
I'd very much doubt your typical homemade router would handle sinking 2.2kw of cutting power into the workpiece.

I wouldn't worry about something like running a high speed 2.2kw spindle, however running something like a big old 2.2kw lathe with high starting loads, I would probably consider uprating the VFD, as the extra current would help get things up to speed faster.


However, if you're looking at the generic Chinese VFDs, I'd say it's more luck of the draw anyway. They're more likely to just randomly release their magic smoke, or for something else to cause them to release their magic smoke, so upsizing isn't likely to increase reliability.

[Richard]

The machine is a big combination woodworking machine. It has 3x 2.2kw motors, of which only one is run at a time. So basically it's a saw bench, planer and spindle moulder.

Quite large rotating masses even when not cutting.

One VFD supplier I just contacted recommended a 4kw VFD for a 2.2kw motor... Seems sensible even if it is more £££. It's not much compared to the cost of the machine though.

[Neale]

Ah, that's a slightly different situation. VFDs don't like you switching between VFD and motor while they are live. This applies to the usual HY VFDs, for example. It means that you need a beefier VFD that is designed for online switching. I use a VFD to drive my lathe, but in that case I have permanently wired the VFD to the motor and use the VFD remote control connections for forward/reverse. With your combination machine sooner or later you are going to switch it "live" so you do need to check with the supplier that the VFD is designed for that situation. They are available - people with one VFD and multiple machines use them - but check first.

[mekanik]

Don't think you can switch two motors at the same time, my combi has a selector switch and you have to manually switch to the desired function/saw/planer/spindle

[Neale]

My own combi machine is saw/spindle moulder - planer is separate machine - but all 240V/1P so haven't had this problem. I guess that you normally change over between motors while nothing is running, so I guess the usual kind of VFD would work.However, I don't really like the sound of it, thinking about how you would isolate the motors for blade/cutter changing/adjusting. VFD that is happy to be to have its output switched live seems like the best way to go.

[lp_felix]

take a look
http://www.mycncuk.com/threads/9673-...-flutter/page2

[Richard]

My own combi machine is saw/spindle moulder - planer is separate machine - but all 240V/1P so haven't had this problem. I guess that you normally change over between motors while nothing is running, so I guess the usual kind of VFD would work.However, I don't really like the sound of it, thinking about how you would isolate the motors for blade/cutter changing/adjusting. VFD that is happy to be to have its output switched live seems like the best way to go.

If it was going to be problem to change the motor select switch while the VFD is actually powered up (motor not running), I guess I would have to make an interlock type cover over the motor select switch to enforce complete isolation of the VFD before changing motors.

If it's just a case of waiting for the motor to stop, then I don't think thats going to be a problem.

I doubt I'll be hopping from saw to spindle and back, so the machine will be set up for sawing or planing or routing and be used like that for some time.


I will ask the supplier and see what they say.


I did read somewhere that VFD's don't like contactors etc inline with the motor, but I have also read that people just wire them up to the main input of the machine and they work ok. No bypassing of contactors, etc.

[Neale]

I did read somewhere that VFD's don't like contactors etc inline with the motor, but I have also read that people just wire them up to the main input of the machine and they work ok. No bypassing of contactors, etc.

I think that that's pretty much the point at issue - the common lower-power VFDs don't like live switching - don't put any kind of switch/relay/contactor between VFD and motor - and some (the better ones? Read "cost more...") are OK. Have to check with supplier to find out which is which!