How do i make my bench grinder run @ variable speeds?

[gene-pavlovsky]

Old thread, but since it's one of the first results when searching google for "bench grinder speed control", I will post anyway.
These days, there are (relatively) affordable single phase input, single phase output variable frequency drives (VFDs). No-name Chinese units are available on eBay for 60-70 USD including shipping.
Whether or not it's worth it, is up to you.

 

[marinusdees]

How do these cope with the fact that at a low rpm, the centrifugal switch will re-engage the start winding and the motor will overheat and eventually burn up??

 

[seber]

How do these cope with the fact that at a low rpm, the centrifugal switch will re-engage the start winding and the motor will overheat and eventually burn up??

They don't. Cap start motors are not candidates for variable speed.

 

[gene-pavlovsky]

Note that I haven't used such a VFD, nor had I felt a need to slow down my grinders. But since that was the question, and I'm aware such VFDs exist (and are affordable), I decided to post about it.

I wouldn't be so sure about cap start / centrifugal switch. According to a manufacturer of single-phase VFDs,

"Connect VFD's U, V, W phase to induction motor's terminals correspondingly as show in following wiring instruction (The capacitors of single phase motor can be removed if necessary)."
Search for "Wiring a VFD to control single phase motor speeds" to find this page with a connection diagram, I can't post it, since I don't have 5 posts yet.

But maybe you're right about cap starts being out.
Invertek says their single-phase output VFDs are suitable for "PSC (Permanent Split Capacitor) or Shaded-Pole Single Phase induction motors".
Leeson says "Permanent split capacitor (PSC), Shaded pole, and AC synchronous."

So, capacitor start would be not suitable, however capacitor run (=PSC) are suitable. I've seen grinders with motors of both types (e.g. my 1998 Elektra Beckum DS200W is cap start, while all their lower power models were cap-run).

I have heard some people claim they have converted cap-start motors to cap-run (replace centrifugal switch with a jumper, replace the start capacitor with a run capacitor - those are rated for continuous duty and are lower in their capacitance value) and had those motors happily running for years since. Feels more like a hack, though.