Running two motors on one VFD

I am in the process of putting VFDs on the spindle and work head of a Heald cylindrical grinder. The primary reason is that the work head has one fixed speed that is too fast for heavy unbalanced parts. The feed is from a separate motor and gearbox, not geared like a lathe. We were considering putting a VFD on the feed motor, also, when the owner asked if we could run both motors off one VFD, keeping the ratio between them. The work head has a 5 hp motor and the feed 3/4 hp, both three phase 9 wire configuration. The VFD is a 10 hp one that we happened to have. I currently have it haywired in a test setup on the work head and is working fine. I don't see a reason we can't do that. The only objection might be that the feed motor will come up to speed quickly and the work head with a large 4 jaw chuck takes longer. Since I have it set to ramp up, that shouldn't be a consideration, but I thought I would ask. Would braking be a problem?

Bill

Most VFDs specifically allow you to do this.

You need individual overload protection on each motor, though. The VFD can't detect over-current because it can only measure total current.

Check the manual for your VFD. If it doesn't have specifics on programming it, set the motor output power and current to the sum of the two motors.

9100 said:

The work head has a 5 hp motor and the feed 3/4 hp, both three phase 9 wire configuration.

Click to expand...

I regular type mill with a 1hp spindle motor and 1/3hp x traverse feed motor would be a scaled down example. Can see that the feed motor is turned on after the
main motor is running, and turned off before the main motor turns off.

Some good points. The manual does say that two motors are acceptable. I was mostly concerned with the disparity between them.

Re overloads, this machine is so old that overload heaters are not available. It is set up for 480 V and has 480 V coils on the contactors and start-stop buttons, the sort of thing that makes a safety inspector's blood run cold, especially on a machine with coolant splashing around. The shop has mostly 240 and 480 only in a few places, so part of the mission is to switch it to 240. I could wind lower voltage coils but there is a transformer already there, so we will operate relays with lower voltage coils to control the contactors and get the 480 off the front panel.

We bought separate overload units and I can connect one for the feed motor to the contactor supplying power to the VFD.

Amazingly, the machine is still mechanically very accurate so the investment is worthwhile.

Bill

I would use 2 vfd.s and sink them with the controls...Phil

I tried it today and nothing smoked. The usual operator is on another job right now so it may be a couple of days before I get any feedback.

Bill

I operated 4 individual gear-motors off a single VFD for 27 years with no runs, drips or errors. None were individually over current protected..just a single VFD program setting for all 4, and they were vastly different HP's as well. I didn't know any better in those days.

Stuart

9100 said:

Some good points. The manual does say that two motors are acceptable. I was mostly concerned with the disparity between them.

Re overloads, this machine is so old that overload heaters are not available. It is set up for 480 V and has 480 V coils on the contactors and start-stop buttons, the sort of thing that makes a safety inspector's blood run cold, especially on a machine with coolant splashing around. The shop has mostly 240 and 480 only in a few places, so part of the mission is to switch it to 240. I could wind lower voltage coils but there is a transformer already there, so we will operate relays with lower voltage coils to control the contactors and get the 480 off the front panel.

We bought separate overload units and I can connect one for the feed motor to the contactor supplying power to the VFD.

Amazingly, the machine is still mechanically very accurate so the investment is worthwhile.

Bill

Click to expand...

If you're installing a VFD, you need to use the start/stop buttons to switch signals to the VFD. Do not switch (either through contactors or manually, except for isolation purposes) the feed into or out of the VFD.

SomeoneSomewhere said:

If you're installing a VFD, you need to use the start/stop buttons to switch signals to the VFD. Do not switch (either through contactors or manually, except for isolation purposes) the feed into or out of the VFD.

Click to expand...

I have done various VFD installations, just never with two motors that different. The final setup will allow the operator to light up the VFD with the spindle off and have a meter calibrated to read spindle RPM, which can be as high as 11,000 RPM after two pulley stages, before actually starting the spindle. One of the reasons for the installation is that they want to use larger diameter stones that cannot handle that speed and be able to see where it is set before starting. The grinder is a great machine but it has single fixed speeds, not even stepped pulleys.

Next is a Cincinnati cylindrical grinder to get the same treatment.

Bill