What Type Of Switch?

[rdhem2]

No offence to anyone trying to help and reason this out, but this whole thread is pretty much why I have given up on trying to help on this site.
One VFD for three grinders--that way the VFD will not be sitting around unused a third of the time. What about the grinders themselves? Aren't they just sitting around unused? Don't mind the folks trying to tell you to do it correctly. What do they know?

As my Dad used to say, I can bring you 3ph power out with three strands on a barbwire fence. But is the the thing to do?

Do it correctly and safely or not at all.

 

[mksj]

If a contactor is used between the VFD and the motor, it would probably be wise to wire a spare NC terminal on each contactor/overload relay in series between them and also with the control inputs (i.e. in series with the E-Stop). If any one trips, this would disable the drive signals. Seems somewhat complicated to have to add contactors/overload relays for each motor, etc. It probably gets tricky with different loads, acceleration/deceleration, etc. and how this is matched to the VFD drive parameters. You basically give up many of the VFD-motor function/safety features, but no reason it should not work when using a straight volt/Hz control. I have used several of the KB VFD drives (like the KBAC 27D), they have no programming, a few trim pots for minor tweaks, i.e. very simple. It is rated at 6.7 amps. So it may be one reason why it would work, nothing fancy. Just about any 3 pole motor 2 position changeover switch (like a 12-20A on eBay for $15-20) would work for switching between the drives. This could be hard wired from the VFD to a switch enclosure, then 2 output cords to either grinder (I would use female locking plugs on the wires exiting the enclosure that would connect to the grinder male plugs).
http://www.ebay.com/itm/Plastic-Hou...659693?hash=item4acad6e06d:g:m5UAAOSwQYZWznyX

AC Drives FAQ (part 3): Variable-Frequency Drives and Safety-Interlock Circuits
http://www.industrial-electronics.c...epper_motors_servomotors/AC-drives-FAQ-3.html

 

[fixit]

Can't argue with success. If asked, I would have said it wouldn't work well. What was the total HP? Was it possible to start the VFD with all of the motors off, and then switch them in one at a time? This indicates that a VFD could be used in place of a RPC. Now I'm going to have to try this.

The vfd was a 60 hp (over designed) with total connected hp of like 34.5 hp (five 5 hp, 0ne 7.5, & one 2 hp). VFD was on all the time, motors were started one at a time form a PB (start/stop) at operator panel station at machine. The total operation could be ramped up & down to to vary machine output.