VFD for reversing a grinder

[zbang]

I've been doing some work on a friend's Coval surface grinder. It was built with a 3-phase motor to drive the worktable back and forth and does that using contactors (see below) There are no time delays involved, so when the reversing switch changes, one contactor falls out and the other comes on. Makes a racket and must be hard on the motor.... (There's also no speed control because someone so helpfully removed the variable-speed pulley and that's a $200+ part. No limit switches, either; probably removed long ago.)

We're thinking of ripping out the contactors and dropping in a handy 1.5HP Teco VFD to add some sanity to the whole thing. Assuming it's a Good Idea, what sort of accel/decel parameters should we use? Other parameters? I don't recall the ratio, but the existing motor-to-table-drive is maybe 75:1 and that really whips the table back and forth, so speed control is needed.

The existing motor, not original, is a 184 frame Dayton, 1.5HP 1150 rpm SF 1.0 208-220v. The parts list says the original was "Howell motor 3-ph 1/6HP 220/440v 60cy. A-66" which the manual says "is a special reversing motor and is expected to run hot" (I'd think it would, at least that's in the instructions).

 

[retirede]

Those motors are designed for the use. We had those grinders in our shop and I’ve never seen a motor failure on one.
I don’t know the answer to your question, but the ramp can’t be too gradual because you want the workpiece to be moving at the correct, constant feedrate all the while it’s in contact with the wheel.

 

[zbang]

This is not the original motor so that's why I'm more concerned about the slam reversing. Otherwise, the thing is built like a tank.

For ramps, I'm thinking maybe a half second each?

 

[LarryFine]

The existing motor, not original, is a 184 frame Dayton, 1.5HP 1150 rpm SF 1.0 208-220v. The parts list says the original was "Howell motor 3-ph 1/6HP 220/440v 60cy. A-66" . . .

Maybe the problem is that the existing 1.5 hp motor is too powerful for the purpose (9 x the original), and the specified 1/6 hp would be weak enough to be overpowered by the inertia of the work table enough to reverse less abruptly.

 

[ptonsparky]

Maybe the problem is that the existing 1.5 hp motor is too powerful for the purpose (9 x the original), and the specified 1/6 hp would be weak enough to be overpowered by the inertia of the work table enough to reverse less abruptly.

That didn't appear right to me but didn't know why. I think you nailed it.
A half second delay may not be long enough.
IDK what this machine is but ramp down may give grieve with an overvoltage fault as well.

 

[zbang]

Good point Larry, hadn't considered that. Thinking about it, the frame size probably doesn't matter too much as long as long we can bolt it down somehow and we can change out the belt if it needs be. Maybe both a smaller motor and a VFD? (Still want that speed control.) Any suggestions on what to look for in a motor, or just get a general-purpose job with the right mounting and speed? (Regards speed- the motor is already on maybe a 3:1 belt reduction and it'll be hard to fit in a larger pulley on the gearbox.) Hmm... we also have some decent-sized 90v DC motors hanging around the shop....have to check them out, too.

The beast is a Coval No. 10 6"x18" surface grinder; it's used for grinding things flat to a very close tolerance. Here's a pic of a similar model http://docsmachine.com/projects/covel/covel85.jpg.

 

[Jraef]

I would start with a 1/2 second accel ramp, no decel ramp at all, set that to "Coast"

If the existing motor fits and has been working, I would continue using it. An over sized motor is only risky in terms of the torque-shock it can impart on the mechanical components, using the VFD will absolutely take care of that issue. If you ever need to replace it, you will want to get one that is rated as "Inverter Duty". But since you already have this one, I would use it till you lose it.

 

[kwired]

Maybe the problem is that the existing 1.5 hp motor is too powerful for the purpose (9 x the original), and the specified 1/6 hp would be weak enough to be overpowered by the inertia of the work table enough to reverse less abruptly.

Yes, the 1.5 HP has too much starting torque. The OEM motor probably designed to take the heating, 1/6 HP definitely not close to same torque. That said the 1.5 HP on a VFD possibly works as well, it would need de-rated for the frequent starting/stopping. So might a 1 hp or even .75 HP for that matter.