My angle grinder has a brushed motor, meaning that it can be slowed down
with an SCR-type speed control (e.g., HF's "Router speed control":
http://www.harborfreight.com/router-...rol-43060.html ). I know
that it won't have as much power when slowed down & that won't be a problem.

The question is: is it going to be harder to avoid burning it out? It
can be burned out at full speed, too, and I know how to avoid that. Is
it different at slow speed?

Thanks,
Bob

Never mind: it just doesn't have enough power at reduced speed.

"Bob Engelhardt" wrote in message
...
Never mind: it just doesn't have enough power at reduced speed.

Maybe reduce the wheel diameter instead.

--lowers peripheral speed while still producing full power.

On Tue, 23 Apr 2013 12:26:55 -0400
Bob Engelhardt wrote:

My angle grinder has a brushed motor, meaning that it can be slowed down
with an SCR-type speed control (e.g., HF's "Router speed control":
http://www.harborfreight.com/router-...rol-43060.html ). I know
that it won't have as much power when slowed down & that won't be a problem.

The question is: is it going to be harder to avoid burning it out? It
can be burned out at full speed, too, and I know how to avoid that. Is
it different at slow speed?

Thanks,
Bob

I see you already answered you question, but a few comments anyway...

I had the same problem with speed/torque. I was trying to use a Type 27
flap wheel to remove old paint. If you spin the wheel fast (normal
grinder speed) the paint heats up and sticks to the flap wheel. Slowing
the wheel down helps, but still isn't practical except for small areas.
I would adjust the speed loaded, which is considerably faster unloaded.

The other problem is that by slowing the motor down you reduce the
airflow cooling it. I was using a Harbor Freight Grinder, the paddle
switch model so it wouldn't be a terrible loss. But I didn't have any
problem with that and this was during summerish weather here.
Overheating was in the back of my mind though and I tried not to push
it too hard...

To get this to work you would need a feedback circuit. Something
that measured the output rpm and then controlled the juice to the
motor. But I'm sure you've already figured that out ;-)

--
Leon Fisk
Grand Rapids MI/Zone 5b
Remove no.spam for email

On Tue, 23 Apr 2013 12:26:55 -0400, Bob Engelhardt
wrote:

My angle grinder has a brushed motor, meaning that it can be slowed down
with an SCR-type speed control (e.g., HF's "Router speed control":
http://www.harborfreight.com/router-...rol-43060.html ). I know
that it won't have as much power when slowed down & that won't be a problem.

The question is: is it going to be harder to avoid burning it out? It
can be burned out at full speed, too, and I know how to avoid that. Is
it different at slow speed?


Not sure why you are looking to reduce speed.
In my case I wanted to use Scotchbrite pads for surface prep and these
burned at the full 11,000rpm.
In the end I got a variable speed Makita 5" grinder which works just
fine.
Is that an option for you?

Michael Koblic,
Campbell River, BC

It's likely that the HF router speed control is a very simple diac/triac
circuit. These are very useful for lighting loads, lowering the temperature
of a soldering iron, or other small loads (the label rating suggests 15A
max, IIRC).

The trigger/switch circuit in VSR variable speed drills (for example) is a
better circuit for maintaining motor torque at lower speeds. This is a
characteristic of PWM pulse width modulated circuits.
Many PWM circuits don't require a speed sensor device at the motor, so they
can operate well with only 2 wires for motor power.

--
WB
..........


"Bob Engelhardt" wrote in message
...
My angle grinder has a brushed motor, meaning that it can be slowed down
with an SCR-type speed control (e.g., HF's "Router speed control":
http://www.harborfreight.com/router-...rol-43060.html ). I know
that it won't have as much power when slowed down & that won't be a
problem.

The question is: is it going to be harder to avoid burning it out? It can
be burned out at full speed, too, and I know how to avoid that. Is it
different at slow speed?

Thanks,
Bob

On 04/23/2013 06:46 PM, wrote:

Not sure why you are looking to reduce speed.
In my case I wanted to use Scotchbrite pads for surface prep and these
burned at the full 11,000rpm.
In the end I got a variable speed Makita 5" grinder which works just
fine.
Is that an option for you?

I used a variac recently on my 4.5" grinder to slow it down enough to
use a 9" abrasive wheel. This was done to give me access to the inside
of a welded C channel which was too deep to use with 4.5" wheels.

It took a couple of goes to dial in the right voltage to give me the
proper speed under load, but for my limited need it worked fine.

I don't think my grinder liked the additional mass of the 9" wheel,
though, but for a quick grind it worked fine.

Jon

On Apr 23, 10:26*am, Bob Engelhardt wrote:
My angle grinder has a brushed motor, meaning that it can be slowed down
with an SCR-type speed control (e.g., HF's "Router speed control":http://www.harborfreight.com/router-...ol-43060.html). *I know
that it won't have as much power when slowed down & that won't be a problem.

The question is: is it going to be harder to avoid burning it out? *It
can be burned out at full speed, too, and I know how to avoid that. *Is
it different at slow speed?

Thanks,
Bob

They do have pneumatic versions, should you have shop air. These tend
to be smaller and lighter for the same work capacity and readily
throttled. They had air routers at the motor home plant for doing
cutouts that had throttles on them, seemed to have plenty of power.
Pneumatics don't have the heat buildup problems that electric motors
do, just need good lubrication.

Stan

"Wild_Bill" wrote in message
...
It's likely that the HF router speed control is a very simple diac/triac
circuit. These are very useful for lighting loads, lowering the
temperature of a soldering iron, or other small loads (the label rating
suggests 15A max, IIRC).

The trigger/switch circuit in VSR variable speed drills (for example) is a
better circuit for maintaining motor torque at lower speeds. This is a
characteristic of PWM pulse width modulated circuits.
Many PWM circuits don't require a speed sensor device at the motor, so
they can operate well with only 2 wires for motor power.


You can run series wound universals using DC servo amps too, as long as the
amp has a setting to run in IR mode--I've done this several times and
generally you'll get fairly stablw speed control under widely varying loads.

Specifically, I used the AMC model 25A20--one thing though, it's quite
possible to overspeed the motor with this arrangement in which case the
results could be downright spectacular.