VFD for machine with 3 motors

[HMF]

Any idea what the answer to this question is? It was asked on another forum and got no response:


"I'm picking up a automotive surface grinder, its an older machine and I'd like it to give me a surface finish better then what it was originally designed to do.

It has a 5 hp spindle motor, two speeds, it has a 3 hp table motor, two speed with hi/low gear box, also a small coolant pump motor. All motors are three phase.

I want to be able to slow down the table motor to a crawl to get a better finish, much lower RA numbers than about 30 it will do now. I'd like to see 15 or better.

I don't have three phase here, so a VFD would give me my three phase needed from my single phase power. How can I use the VFD to slow down the table motor and not the other motors.

Are there VFD's for multiple motors, is there a way to wire this so I have three phase to all three motors but can only slow down the table motor?

For some reason this just isn't clicking for me now. Probably easy. I appreciate any input you might have about running this the way I need."

 

[Tony Wells]

Simple answer:No.

VFD changes the frequency that drives the motors. If you feed then 30Hz instead of 60, they run about half speed. I don't like the idea of running three motors on one VFD anyway. I don't believe you can do it, as recommended my manufacturers. It's not something I've read specifically, but seems reasonable.

 

[brucer]

I think you would have to run 3 separate vfd's (one to each motor), the coolant pump would be easy enough to swap out for a 110v pump..

you can use a vfd to power 2 different motors, but pretty sure you have to have a switch to choose which motor you want to use.. such as 1 vfd to power a mill and a lathe but not both machines at the same time..

I'm not an industrial electrician and I didnt sleep at a holiday in express last night..

 

[arvidj]

I agree with the previous comments. I only have experience with two VFD installs and know enough about electronics to be in the "dangerious" stage -- yet another hobby -- but (a) I do not believe one VFD would not work for three motors -- at least not for very long -- and (b) it is impossible to get the multiple frequencies you need to run the motors at different speeds.

A rotary converter would solve (a) but it will not solve (b).

You say that the 5hp spindle motor is two speed but do not say how -- an electrical switch, a gear box or belts. If it is switched then when installing a VFD you would want to eliminate the switch and control it with the VFD. The output of the VFD should be connected to the motor such that the only time the switch can be moved is when the VFD is not providing any power to the motor. If the switch is moved while it is driving the motor the voltage spikes may very well do in the VFD.

 

[Starlight Tools]

Nelson

There are a few important details that were left out in your inquiry.

Main one, what is the voltage that these motors run at?

Also how is the 5 hp 2 speed obtained?

Like Turbo said, if you have two motors and they will both run at the same time and the same speed, then you can, although not totally recommended gang them to a single VFD. If however you want to vary each motor individually, then each motor needs a separate VFD. Generall you do not want to ever slow down the coolant pump.

If you operate more that one VFD on a machine, IE it has more than one 3 phase motor, then there should be line chokes installed as well.

Now it is very possible to feed this machine with a RPC, that is going to give it a good clean three phase power and run it like it was supposed to. Now to slow down one of the motors, add a VFD to that motor only. This way you get the best of both worlds.

Now if the motors are 208/230V 3 PH then this is relatively easy to accomplish, with either the VFD's, the RPC or combination of both.

If the motors are 440V or 600V then it becomes a bit trickier. Now we need very special VFD's (pricey) or we need a RPC feeding a transformer. If you look in the electrical section for my shop wiring set up, you will see that i have a very similar situation with my B&S #13

240V RPC, to 10KVA 240 - 600V transformer feeding 3 separate motors on the grinder. Wired it up, tested for proper rotatation and done. Machine runs just like it was supposed to do.

Walter