Variable speed for angle grinder

[Janger]

I'd like a variable speed angle grinder for grinding stainless and also using 3M type pads at slower speeds for finishing. They are expensive. $250 for this one with a rat tail handle which I want. It's about the only one I can find with a rat tail handle.

https://www.amazon.ca/Metabo-601243...r_1_30?keywords=metabo&qid=1576274094&sr=8-30

There are these router speed controllers which I gather work with brushed AC motors. like this thing for $55.

https://www.busybeetools.com/products/router-speed-control-115v-15a.html

Apparently it also reduces torque. I'd like it to go down to 2500 or 3000 rpm. Would the tool still be useful (enough torque?) at those speeds?

@JohnW @Johnwa You guys seem to know all about stuff like this... ?

Comments?

 

[PeterT]

FWIW I have a Metabo variable speed compact die grinder 13-34Krpm. First time I ventured into that product line. Its well made, kind of reminds me of Bosch tools. The variable speed is great. I'm not actually sure if its brushless or not, it doesn't say specifically. They mention Vario-Constamatic (VC)- Electronics for work with materials requiring customised speeds, which remain almost constant under load. ... which sounds a lot like brushless or ??. The parts don't show classic pairs of brushes, but there is a 'brush' part, part of the overload sensing/cutoff? I also dont see 3 wires to the motor but I dont really know what I'm looking at in this schematic. Maybe someone can decipher. I haven't put a lot of run time on it but it didn't seem to bog on low rpms, mind you I wasn't doing high load or anything. Check for prices & deals they seem to vary & list price can be spendy. Cant recall where I ended up getting mine in the end but it was about $100 lower than the norm.

I'm not too keen on those speed controllers but its kind of application specific. If you need low rpms but work duty is light it might be OK. But yes, the common complaint is low rpm torque goes for poop.

 

[YYCHM]

https://www.princessauto.com/en/detail/variable-speed-fan-control-switch/A-p8534232e ????

 

[PeterT]

YYC, that princess unit is only rated to 2.5A. For example the angle grinder Janger referenced was 1500 watts so ~12.5amps.

 

[Dabbler]

Princess auto has a variable speed angle grinder - that last one I bought was on sale for 29.99 or 39.99 - I can't remember which. I see it is 74.99 now:

https://www.princessauto.com/en/detail/5-in-variable-speed-angle-grinder/A-p8549727e

 

[Johnwa]

PeterT
From the schematic it looks to be a standard brushed motor. I don’t know why the part list calls it a cut out though.

 

[Tom O]

Is there any chance that you could use a dimmer switch for it? I use one mounted with a outlet to control the speed of my blower when sandcasting.

 

[JohnW]

Regular dimmers are not designed to switch inductive loads. Significant voltage spikes can be generated when they switch the power on and off to an inductive load. That will work to some extent but usually not for their full rating. i.e. running a 1 or 2 amp motor on a 10A rated dimmer might be OK. The dimmers are cheap though, the worst thing you would do is burn out a cheap dimmer. When it burns out, it will either fail open in which case your motor stops, or it will fail shorted, in which case your motor will go to full speed. Either way nothing really spectacular happens.

I ordered a "router speed control" for use with my Makita die grinder to be able to reduce the speed when using long grinding burrs (reaching into cylinder head ports). Torque was not an issue since I am spinning only a half inch diameter cutter. My worry was that spinning a long (6") grinding bit at too high a speed can vibrate a lot, or even catastrophically fail once the shaft bends a little bit, then very quickly bends more and more and starts swinging the bent cutter around violently. I did that once using an air die grinder. It could have been a bad thing - beyond just destroying an expensive long carbide burr.

The unit I ordered for the die grinder is: https://www.aliexpress.com/item/32899895408.html

It seems to work quite well. You would definitely loose torque as you lower the speed, since you are lowering the speed by effectively reducing the voltage, so the power to the motor would drop along with the voltage. That is different than using a VFD on a 3-phase motor where the frequency is reduced to reduce the motor speed. The voltage is reduced as well in that case, but voltage is only reduced to the point where the same strength of magnetic field is generated, so the torque (which is related to the strength of the magnetic field) remains quite constant. When using a "dimmer" you are reducing the voltage far more, which lowers the torque, which makes it spin slower.

The speed of a brushed motor is limited by how quickly the magnetic fields in the armature can build, drop, and reverse as the brushes switch the coils. How fast a magnetic field changes in a coil is proportional to the voltage driving it. A lower voltage takes longer to rise and fall and does not build as strong a field. the motor will only spin as fast as the magnetic fields can change at that voltage.

Hope that makes sense.