Yet another VFD question - Neutral?

[Richard]

So now I have got hold of a big combination woodworking machine, it has 3.8kw motors in so will be looking at 5.5kw VFDs.

One thing I have spotted in the wiring and installation diagrams is that it uses a Neutral to run the contactors (via an isolation transformer I think)

I am assuming this will be an issue with a VFD.

I have a 2.2kw rotary converter on my lathe and this has a neutral provided for exactly this reason.

[Neale]

I'm assuming that the machine will run off something like a dedicated 16A 240V single-phase supply? You probably won't be using the "variable frequency" feature anyway and would leave the VFD set to 50Hz. One option is to leave the VFD running continuously, see if the device has a neutral connection on the output, and just pretend that you have a three-phase supply and everything works normally. However, you could consider something similar to the approach I took with my lathe with its 3HP/2.2KW motor. My problem was that the switchgear, contactors, etc, ran off 110V via a 415-110 internal transformer but I also wanted to use the VFD in soft-start mode, so there was no 415V available to actually start the thing. All a bit Catch-22.

My solution was to separate out the supply to the switchgear from the motor supply. I disconnected the transformer and supplied the switchgear with 110V from an external source (actually a repurposed site transformer) and used a spare contact on the main motor contactors to switch the VFD which was permanently connected to the motor. Could you look at something similar by supplying your switchgear isolating transformer direct with 240V mains rather than from one 415V phase and neutral, and using the existing contactors (via a spare or additional contact?) to switch the VFD? This would give soft-start capability, so reducing start-up surge, as well. Even if it needs an extra 240V relay in parallel with the main motor contactors, this might be a way to do it. Mine was a bit more complicated as I needed to also run the motor in reverse, but especially because you already have a single source of power to the switchgear, it might be an easy thing to do in your machine.

[Richard]

I'm assuming that the machine will run off something like a dedicated 16A 240V single-phase supply? You probably won't be using the "variable frequency" feature anyway and would leave the VFD set to 50Hz. One option is to leave the VFD running continuously, see if the device has a neutral connection on the output, and just pretend that you have a three-phase supply and everything works normally. However, you could consider something similar to the approach I took with my lathe with its 3HP/2.2KW motor. My problem was that the switchgear, contactors, etc, ran off 110V via a 415-110 internal transformer but I also wanted to use the VFD in soft-start mode, so there was no 415V available to actually start the thing. All a bit Catch-22.

My solution was to separate out the supply to the switchgear from the motor supply. I disconnected the transformer and supplied the switchgear with 110V from an external source (actually a repurposed site transformer) and used a spare contact on the main motor contactors to switch the VFD which was permanently connected to the motor. Could you look at something similar by supplying your switchgear isolating transformer direct with 240V mains rather than from one 415V phase and neutral, and using the existing contactors (via a spare or additional contact?) to switch the VFD? This would give soft-start capability, so reducing start-up surge, as well. Even if it needs an extra 240V relay in parallel with the main motor contactors, this might be a way to do it. Mine was a bit more complicated as I needed to also run the motor in reverse, but especially because you already have a single source of power to the switchgear, it might be an easy thing to do in your machine.

Hi Neale,

Thanks for the reply.

Yes I have a dedicated feed from the consumer unit.

My lathe (M300) had the same 110v switchgear with a transformer. One of the reasons why I got a used rotatry converter was to use all the switchgear as is and suds pump, light etc. Alas the combi machine turns out to be 3.8kw and not 2.2kw as advertised, so I can't just bung it on the rotary.

The VFD softstart would be nice to have I guess. I think wiring in a direct 240 single phase for the switchgear is the best solution. As long as the machine isolator cuts the 240 into the VFD and the switchgear, it should be fine.

One more thing I have yet to work out - mainly due to the machine being outside under a tarp, is if it's direct start or Star-Delta start and if it has a brake circuit. All are mentioned in the schematics.

Another option in the manual is a wiring option for 220v 3 phase instead of 380v. (delta wired motors). Which means being able to use a 220v out VFD.

Schematic showing most complex options:

[Richard]

Just had a look in the electrical cabinet. Inside there was a full schematic on a bit of paper folded up in the bottom! (its basically the same as what I posted above)

Looks like is has the Star-Delta start and braking (which is DC across the motor right?)

I doubt a VFD will like any of that, so will need to bypass it and wire the VFD direct to the main and Delta outputs of the start contactors (assuming I go for a 220v - 220v VFD)

[magicniner]

Ideally you will not use the Start Contactors, any decent VFD will have inputs for all the required functions, completely re-wire for the VFD manufacturer's Best Practice installation as shown in their documentation.

[Richard]

Just bought a 5.5kw VFD from ECOGOO. 220 to 380v version. 5.5kw giving me a bit of headroom for 3.9kw motors.

Will see what I get charged for vat and duty etc. £185 so far.

I'll bypass all the brake and startup contactors, mount the VFD on the machine with a remote start stop switch on magnets so I can put it where ever I am working.