VFD for horizontal/vertical mill

Hi All,

First, I want to say thanks for all the advice being shared here. The information I gathered from this forum helped me greatly in getting a VFD successfully wired on my first 3-phase machine.

Well, I've been lucky enough to bring in a horzontal/vertical mill into my little shop and I am looking forward to getting this machine running. The spindles run on separate 3-phase motors, 1.5hp for the horizontal and 0.75hp for the vertical. I've been reading the archives on how to power two motors with a single VFD and have decided this is the route that I would like to pursue.

I've determined the VFD model that I am going to get and confirmed that it will support 2 motor settings. However, rather than placing a twist lock plug/receptacle downstream from the VFD, I read that some folks were using a drum switch (reversing?) to switch the power between the motors. I'd be interested to see a wiring diagram of how this was done. Are any modifications required to be made to the drum switch since I think these usually only switch 2 poles to achieve the reversing function?

I've also looked for a selector switch that might be able to do this, but was unable to find anything from the available resources (MSC, McMaster). I may not be looking for the right type of switch though. Any pointer to the right type of selector switch would be appreciated.

Thanks again and I look forward to receiving any responses.

I've got 2 machines wired to my hitachi SJ200.

There are a few different styles of drum switch I've seen. The type I'm using has 2 multipole switches ganged together that close or open depending on the position of the knob. You'll have to make jumper wires to connect the VFD to both sides of the drum. Another type has a common set of terminals, and you connect the VFD to those. Another type is just for reversing, has 3 input, and 3 output terminals, this type won't work for this. When the "set" terminals are closed, it tells the VFD to use motor 2. You use the control terminals on the VFD for this. I drew a cheesy schematic for you, to give you an idea how to wire your VFD to both motors, with both kinds of drum switches. Any 3 or 4 pole double throw switch will work if it can handle the current. If you use a 3 pole, you'll have to wire the set function to a separate toggle, which tells the VFD which motor is connected. The only problem is that you can forget to hit the set switch, and if the two motors have differences like the number of poles, or RPM, or set the current limit for the smaller motor, it can cause problems. I'd use a 4 pole so that you can't forget to hit the set switch. Again, DO NOT switch between motors with a motor spinning! Your VFD will be instant JUNK!

Thank JunkyardJ, the wire diagrams make sense of it.

I'm realatively new to this, so I didn't realize there were different flavors of drum switches. For the first two drum switches you mentioned that will work for this application, are these typically used for 2 speed motor machines? I guess I am wondering how I go about looking for one of those since all I see are reversing drum switches.

I guess if it comes down to it I could just put multiple toggle switches in line to switch between the 2 motors and make sure they are all switches in the same way. On the other hand that may be a disaster waiting to happen.

Some fwd/rev drums will work.

See, I recently ran into a purpose built fwd/rev switch on my buddy's hendey lathe. It's designed specifically for reversing 3 phase motors. You can tell because it only has 6 screw terminals, where the regular ones have 9, 12, or 16. Now MOST drum switches, including those used for fwd/rev operation, are the type that will work. You just have to make sure that it's not one of those purpose built switches. Most of the time they take a regular drum switch and simply wire it for fwd/rev on most machines. I've never seen one of the purpose built buggers until I saw it on that hendey lathe. It does simplify wiring for fwd/rev of a 3 phase motor, so if that's all it's used for, it makes sense. All it does is switches two of the power wires around, and sends the third straight through. So, just because the switch is used for fwd/rev, doesn't mean it won't work. The switches that will work are called (3 pole double throw) 3PDT, or (preferably) 4PDT (top schematic), or are dual ganged 3 or 4 pole, single throw switches, which you can wire to be a 3 or 4PDT switch (bottom schematic). Both types can also be used for fwd/rev functions, and are pretty common for use that way. I dunno how common the purpose built fwd/rev switches are, I've only seen 1! Now the switches for dual speeds will work if they're wired like the one I saw. There's more than one way to wire a dual speed motor, so a drum switch used for that MIGHT NOT be what you want. You just have to verify what type of switch it is by either looking, or it's description. I took the purpose built switch off my buddy's hendey lathe to replace the fwd/rev switch on my B&S mill, and gave him the switch off the mill, to select which machine he wants to run off his VFD. That way I can still re-wire my mill so all the original controls work if I ever get rid of it. I left all the original wiring in tact on my machines, and simply wired the motors to the selector switch off the vfd. Just goes to show you have to figure out for yourself what type of switch it is, rather than choose by how it's used. Confused yet???

You will likely find it cheaper to buy another VFD for the smaller motor and not do any switching. I will definitely work better.

How cheap is a small VFD for a 3/4 HP motor???

It's GOT to cost more than a used drum switch off fleabay, some wire, and an enclosure. Plus you'd have to wire up both VFDs to power, or use the same wire to hook up the drum switch. It would have to be a hella cheapo VFD to cost less than a drum switch, a box, and some wire. Plus you can spend a little more on a nicer VFD if you don't have to buy two of them. As for it working better, the ONLY advantages would be no risk of switching the output while it's running, and being able to operate both motors at the same time. There would be less resistance from the VFD to the motor, but that wouldn't make much of a difference unless the resistance was high enough to cause trouble. Cheaper and work better???

If you compare the price of a new quality drum switch to a scalar type VFD you may be surprised that the VFD is not a huge amount more. If you compare used prices then compare a used VFD to a used Drum switch. I bought a used 7.5 hp VFD for $29.95 on ebay and it is a current model. This can not be done every day but it happens. By the way, the 7.5 hp was a closed loop vector type VFD. New cost was over $2500. Two weeks later I found the matching Vector motor for $250 and only 200 miles from my shop. The combined new cost would have exceeded $5000. Now I am just waiting for a Monarch 10EE to come by in search of a new home.

I have been using VFDs for the past 20 years and can assure you that trying to run two motors from one VFD is a PIA unless you want to start and run both at the same time as in a slaved drive. I have used as many as 80 motors running from one VFD in a slaved application but I do not even consider trying to switch motors on one drive.

The bottom line is simple--it is your application so do it the way you want it.

Perhaps I look at it differently.

I have things like drum switches laying around, or know whrere to get them cheap. The one I use to select between my machines came off my mill drill. The enclosure was something out of my junk collection. There are TONS of them on stuff @ the local scrapyard, and I see surplus ones cheap from different places. I dunno, $5 from the scrap yard, VS trying to either score a super cheap VFD that may or may not be a scam or defective. With my luck, I get hosed when a deal looks too good to be true. Some people just get lucky on deals like that, NOT ME!!!