controlling grinder speed-->variac?

A variac reduces voltage, which means a loss of torque. You need to use SCRs to reduce speed and these units are available. The professional way is to use frequency inverters. They work better, but are expensive. Steve

Stick on a 3ph motor and use a cheap VFD as your speed controller

Gunner

Political Correctness

A doctrine fostered by a delusional, illogical liberal minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.

You need to be slightly more specific, here. What, exactly, type of grinder do you intend to use? When I read your article, I was thinking of something like an angle grinder which uses a universal (brush-type) motor. A variac or an SCR-type controller will work fine, although you will lose torque as the motor slows. If you are talking an industrial grinder with a 3-phase motor, a VFD is the way to go. On the other hand, a common single phase tool grinder is going to run at the designated speed. Putting a variac on it will just make it run poorly and get hot...

Have you considered using a belt-drive? If you add a jackshaft, you can choose pulley sizes to get you any speed you want.

Jerry

I'm thinking the OP is talking about a grinder based on an induction motor, which I don't think the HF item will work on.

Steve

Ignoramus32311 wrote:

This unit is for universal(AC/DC) brush-type motors ONLY. Yes, it doesn't really say so, but routers have brush-type motors. No good for induction motors like most bench grinders have.

As far as slow grinders are concerned, the O.P. should look at the woodworker's supply sites, they've licked this years ago. Not variable speed, but do you really NEED this? As long as the speed is slow enough to control the heat, you should be good. Or go with coolant like most industrial grinding setups do.

Stan

Motors with speeds somewhere near 3450 RPM and those somewhere near

1750 RPM are induction motors that cannot be speed-controlled with a variac or SCR device. Those with higher speeds have brush universal motors that will respond to speed control with a variac or SCR device. Brush motors with gear reduction (e.g., drills, some angle grinders) are readily available but the gears are noisy.

The best approach would probably be the slowest brush motor motor you can find, with its max speed reduced to your desired max speed by belt reduction. A DC PM (permanent magnet) servo motor would work very well here if you can contrive a suitable DC supply and make or buy a speed controller. These motors are pricey new, but are available surplus.

I stand corrected.

igor

Take a look at a Bader grinder (

) They have variable speed machines. They also have thought out and engineriered grinding jigs and systems for every application imaginable. After scrounging parts and planning to build a belt grinder for 2yrs (or more) I broke down and bought one. It stung till I got it set up in the shop and fed a 1 1/2" square bar into it, I've been smiling since. Also its a family owned biz and they are nice to deal with.

Enjoy

AndrewV