1 VFD--2 Motors can it be done?

Question

I have a 7.5 hp vfd currently used to run a 4500 cfm blower fan. Since the use is intermittent I want to power my drill press as well. The DP is an old delta soon to have a new to me 3ph 2hp 230 1740 rpm motor transplant (more questions on that project later). What I was thinking for the wiring goes like this--- place a 240v dpdt shut off on wall next to DP. wire it to shut off power(240 sp) to vfd. next wire a twist lock to the 3 ph output of the vfd and twist lock plugs to the DP and Blower fan. Plug in what I want to use and get to work. Also keep my eye out for another vfd for the future when the dp is farther away from the fan unit.

About the motor change on the DP, I drill mostly mild steel holes to about

3/4" a few to 1" I'd like to set this up with annular cutters for the larger holes. I'm not sure what my target rpms should be. That is rpm's before I use the vfd to adjust the Hz. The driven pulley is 8" so I figure a 3" (I have a 3" & a 7") drive pulley @ 1740 rpm gives me 652.5 rpm @ the spindle before I lower it with the vfd, that's faster then it runs now but probably a good top end for me. I'm also not sure how the torque drops off when you lower the motor speed or what Hz = what rpm. Anybody do this before? I'd like to size the drive pulley so there's plenty of torque @ the low end.

Thanks

Andrew

Gary Wooding · Accepted Answer

If the voltages of both motors are the same, then the VFD shouldn't care which motor it drives as long as its within its capacity, which yours seem to be.

The motor speed is directly proportional to the frequency displayed by the VFD. If the motor runs at 1750rpm at 60Hz then it will run at

3500rpm at 120Hz and 875rpm at 30Hz. Normally, at speeds below the base frequency (the frequency the motor was designed for) the torque remains constant and, since power is rpm x torque, this means that at 30Hz, for example, the power is half that at 60Hz. Above base frequency the torque drops so that the power remains constant, so you get half the torque at 120Hz. So its constant torque up to and including base frequency, and constant power above base frequency.

I hope this helps.

snarl · Answer

Not affiliated, just saw it in craigs list:

AndrewV · Answer

Thanks for a very informative post , its just what I'm looking for. As far as "sharing" the VFD, given what will happen if something comes loose while the vfd is powered if I can find a 2hp vfd to dedicate to the DP I'll go for it, otherwise what's life without a little risk. On the DP I think I'll go for a lower fixed reduction so it has more power @ low rpms and then just overspeed for higher rpms. I'll look around for a 2" pulley so I'll have 435 rpm's @ 60 Hz & 2 hp and 217.5 rpm's @ 30 Hz & 1 hp. I end up with a top end of about 870 rpm a bit slow for something's but ok for what I do with it. Of course maybe I'll get lucky and find a step pulley with a 7/8's center.

Thanks again

Andrew

1 VFD--2 Motors can it be done?

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