RPC and Mill Pictures

[Tom Kitta]

This is new info to me. Normally, the VFD senses the current flow in the motor winding and uses that as feedback to provide better output control. Two motors would confuse the bejezzus out of it.

Was there a VFD setting you used (ie something like ignore feedback and brute force the output) to make that work?

You can get away with small side currents, say your main motor is 5hp, and you also have a light using say 50w, and a control transformer that uses few watts, and coolant pump using 50w motor. I would assume power feed motors should be fine as well - maybe you run into issues with the Z on a mill where power feed motor may be large on old mills.

 

[Susquatch]

You can get away with small side currents, say your main motor is 5hp, and you also have a light using say 50w, and a control transformer that uses few watts, and coolant pump using 50w motor. I would assume power feed motors should be fine as well - maybe you run into issues with the Z on a mill where power feed motor may be large on old mills.

The following is just musing on my part. I'm not pretending to know more than I know....LOL!

I think a light would have zero impact on the performance as long as the light can handle the high frequency characteristics of the VFDs output. In other words, I would not expect all lights to work, but incandescent might be fine. To the VFD, I think they would disappear masquerading as a lower impedance motor with zero shifting around of the feedback signal. Similarly, a very small motor might get masked out by the big motor's dominance of the signals.

It's the two large motors (say 3hp and 1hp) that surprises me. I would not have guessed that would work very well and I would have instinctively stayed away from even trying it.

But obviously, it works for a few others on here so there must be more going on than I would have guessed.

A question for you guys - are you using the variable frequency output of the VFD when you do this or are you just running 60Hz in and out for everything on the output circuit?

 

[Tom Kitta]

The following is just musing on my part. I'm not pretending to know more than I know....LOL!

I think a light would have zero impact on the performance as long as the light can handle the high frequency characteristics of the VFDs output. In other words, I would not expect all lights to work, but incandescent might be fine. To the VFD, I think they would disappear masquerading as a lower impedance motor with zero shifting around of the feedback signal. Similarly, a very small motor might get masked out by the big motor's dominance of the signals.

It's the two large motors (say 3hp and 1hp) that surprises me. I would not have guessed that would work very well and I would have instinctively stayed away from even trying it.

But obviously, it works for a few others on here so there must be more going on than I would have guessed.

A question for you guys - are you using the variable frequency output of the VFD when you do this or are you just running 60Hz in and out for everything on the output circuit?

With 3hp and 1hp you probably can get away with starting them both at once. But if you do not I am like 90% sure most VFDs sized for say 3hp would throw a code if you run 3hp motor and then started 1hp or vice versa. Maybe you could get away with this with a large VFD, say sized for 7.5hp, flipping the 1hp motor on and off.

 

[Former Member]

With 3hp and 1hp you probably can get away with starting them both at once. But if you do not I am like 90% sure most VFDs sized for say 3hp would throw a code if you run 3hp motor and then started 1hp or vice versa. Maybe you could get away with this with a large VFD, say sized for 7.5hp, flipping the 1hp motor on and off.

No this is a bad idea.

Transferring from one machine to another, yes, but not ideal as you can't tweak the VFD to maximum performance to a motor.

 

[Tom Kitta]

No this is a bad idea.

Transferring from one machine to another, yes, but not ideal as you can't tweak the VFD to maximum performance to a motor.

Certainly not a good idea - never tried it. But Susquatch was wondering what one can get away with - or I think he was.

 

[Hacker]

This is new info to me. Normally, the VFD senses the current flow in the motor winding and uses that as feedback to provide better output control. Two motors would confuse the bejezzus out of it.

Was there a VFD setting you used (ie something like ignore feedback and brute force the output) to make that work?

I guess I should have been clearer on this. The tracer is a 2HP motor and the coolant motor is a small fractional HP motor. I start the tracer first and the coolant afterwards so the VFD never really notices that there are two motors there. I checked with the supplier about what I was planning before purchasing the VFD and they said it would be fine. I don't use it a lot but it has worked fine.
I would not do this on a setup with a 3 and 1 HP motor either as it has been mentioned that you have to set the operating parameters to the motor size.

 

[Susquatch]

Certainly not a good idea - never tried it. But Susquatch was wondering what one can get away with - or I think he was.

That's a very good way to put it Peter.

To be clear, it's not something I would advocate as it's clearly outside the envelope of how VFDs are supposed to be used.

But I'm always curious about other ways to do things that are unconventional or outside the box.

So I do wonder if it would work, what the limits are, what the issues are, what trouble you could get into, etc.

Thinking about how a VFD works, it's easy to conclude that it's not a good idea to run two or more large motors simultaneously off the same VFD.

When you move those considerations to a large and small motor, it gets a little harder to conclude anything. It might work quite well or it might not. It's possible that the VFD would accommodate the small motor or that it could trick the VFD into thinking something was happening with the big motor that could lead to very bad control outputs. It might work, but without an analysis and/or full diagnostic testing, I'm instinctively against doing it. Ya just because they are both turning one might think it's all running ok, but then again you might be slowly destroying one or both motors or the VFD. I just don't know. I can only speculate. A VFD is a very sophisticated device - much more so than we tend to think.

On the other hand, I think that a purely resistive load like an incandescent light would probably become a constant factor to the VFD that it can easily handle. Again, just speculation.

The best advice I can give is to use them as they are designed to work - with a single motor. In most cases, that is single phase 220 in and 3 phase 220 out. If the input has a neutral, you can run any peripheral devices on either 120 or 220 single phase power out of the main power box (before the VFD) and still have only one power cord and not have any concerns about the VFD or the primary motor.

I suppose there are peripherals that require 3 phase, but single phase is MUCH more common and many of the 3 phase devices are not really 3 phase because internally they only use one leg of the power input. All I'm saying here is that it's silly to convert 1ph to 3ph and then only use 1 of the 3 phases. Better to just go direct.

My own mill is a good case in point. I am currently running two power lines (115 & 230) because my 230 has no neutral. Basically, the guy who built my shop didn't run a neutral for the 230 circuit. My plan is to rewire that circuit with a neutral, bring one cable into a master on/off switch on the back or side of my mill base, run that to a VFD in a main control box beside it (with speed and direction controls up near the motor), and tap into half the 230 after the main power switch to get 115 for my DRO, power feeds, & light.

Same goes for every other 3phase machine in my shop.

Unlike many others, I don't want 440 or 550 in my shop at all. If I did, I'd probably go with an RPC and transformer. But even then I'd probably still want a VFD to get variable speed, braking, and soft start.