Eureka grinder RPM controller

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#1: Post by CowboyCoffee »

I am looking for anyone with knowledge on wiring in a motor rpm controller. I have a silenzio an want to install one.



#2: Post by crwper »

Do you have a particular controller in mind? Generally, it's a lot more difficult to add speed control to these things than you might think.

The easiest way to control speed on an AC motor is to change the peak voltage, but this reduces the torque of the motor, and in grinding applications is likely to lead to a stall.

A much better way is to keep the full voltage and change the frequency instead. A variable frequency drive is a pretty serious piece of equipment, and will probably set you back about $200.

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#3: Post by NicoNYC »

You could use something like a router speed controller for very basic slowing of the motor:

This reduces the power to the motor by clipping the AC wave (it's the first type of controller crwper explained). I took a quick look at the silenzio's circuit board and at first glance, it doesn't look like it would do well with reduced voltage (looks to be a transformer to 9v, then I can't read the regulator or timer chips), which means you should wire the controller in right before the motor (and before the motor starter capacitor).

At home I have a speed controller that isn't enclosed. Just some screw terminals for wires in/out. I'll take a photo of it this evening so you can see, maybe there is space for it inside the case.

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#4: Post by Jeff »

If it's a synchronous motor, a VFD is pretty much the only way to go.

Those router-style speed controls are, AKAIK, for devices with DC motors. "Motor starter capacitor" tells me that it is an AC motor in there.

The fan-speed controllers "work" as the goal is less airflow, where torque is not so much of an issue. As noted by previous posters, those controllers typically reduce the torque, which can lead to stalling in a grinder.


#5: Post by NicoNYC »

If I'm not mistaken, it's more to do with whether it's a brushed motor or not. But to be fair, my knowledge on this stuff is pretty confined to the woodworking & metalworking machines I usually work on - the induction motors all require VFDs. For low-powered stuff like drill presses, we've had good luck with the cheap (aka. drop-shipped from China) eBay ones.

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#6: Post by Bluenoser »

I'm also interested in RPM control, but of an Urbanic grinder. I think most of these motors are single phase AC, brush type.. with a starting capacitor. (So you'd need to find the exact type/model of motor in the Eureka) At least on the cheaper grinders.. pretty sure they aren't synchronous motors.

You can use a router controller, but I think they are just rheostats (basically a resistor) that just decreases the voltage to the grinder...

But if you lower the voltage on a basic AC motor, from what I've read, I think the motor tries to keep the power constant and pulls more current. P=VI. This will heat up the windings.. now maybe for grinding a short time this is not an issue.. But if you draw twice as much current to slow the motor 50%, the heat might cause long term failure.

How would an SCR speed controller work on these? Do these chop the wave.. so you get the same voltage but for a shorter time? Would these be safer?

Motors were never something I understood well my electrical courses :)