I Should Probably Quit Cussing the Innocent Now

Man, this is embarrassing.

For days, I have been trying to get my 3HP 3-phase motor to work with a 3HP TECO variable frequency drive (VFD). I have had no end of problems.

The drive kept pooping out at high RPM’s. I could not figure it out. I disabled DC braking and did all sorts of other things. I got really intimate with the Chinglish owner’s manual. I changed this setting. I changed that setting. I even made a new drive pulley for my 2HP motor, in case I had to go back to it.

Tonight someone suggested there might be a “wiring fault.” I could not figure out what he meant, but I went back to the motor itself, to check what I could.

It was wired for the wrong voltage.

Many 3-phase motors come ready to accept either 230V or 460V. Or 220V or 440V. For some reason, we can’t seem to settle on a number for voltage which is double the usual American household voltage. Sometimes we call it 220; sometimes we call it 250. Whatever. A lot of motors come ready to handle twice OR four times the standard wall socket voltage.

When you use a motor, you have to fiddle with the wires coming out of it in order to make sure it knows which voltage is coming. I didn’t do that this time. I’ve dealt with a number of 3-phase motors, and none showed up wired for 440. Or 460. Whatever. It was always 240.

The one I just bought was wired for 460.

The motor wanted twice the voltage I was giving it, so naturally, it kept running out of joie de vivre.

I feel so stupid. I should have checked this carefully.

Now I have to add to my Google legacy, for other belt grinder owners. Yes, you CAN run a 4-pole 3-phase 3HP motor at 120 Hz on a VFD. You can probably go somewhat higher.

I rewired it. It runs great. It has a ton of power. I’m happy.

With that behind me, here is an obvious question: do you really need a 3HP motor on a 2×72 grinder? My feeling is that you don’t. I fired up a 2HP motor tonight, and I had a hard time bogging it down. The 3HP motor is significantly stronger, but I can’t say I felt like I needed the added grunt.

I think you want 3HP if you plan to go above 5000 FPM, for sure. To do that, I would want a pulley over five inches in diameter, in order to avoid revving the life out of the motor. The added torque of a 3HP motor would allow you to work very aggressively at high belt speed, in spite of the tension you would lose to the big pulley.

The thing is, if you shop for motors on Ebay, you’re likely to find a 3HP motor for the same money you’d pay for 2HP, so why not go for it? The only real down side is the weight. Moving a motor that weighs over eighty pounds gets old.

This grinder is unstoppable now. It is a seriously impressive machine, by garage-doofus standards.

Guess I should make something with it.