# Question about using dimmer switch as a motor control

Joined Jul 27, 2017
8
Hola everyone,
Just a basic 'safety' question. I just watched a video on YouTube where a gentleman used a 600w standard dimmer switch as a "Variable Current Limiter" motor control. I was wondering:
A) How safe is this?
B) To determine power tools/motor sizes that are ok to do this, just use Ohm's law to figure the watts & keep it under the dimmer switches wattage rating?
EG: if I have a bench Grinder that is 300watts, would it work to use a 600watt dimmer?
C) If this isn't a wise thing to do, what would you guys recommend as a quick-n-easy speed controller for things like bench grinders, drill press, etc?

Thanks for any input,
Rod

#### AlbertHall

Joined Jun 4, 2014
10,917
This may depend on the dimmer - some of them may not like the inductive load of a motor.

#### Dodgydave

Joined Jun 22, 2012
9,912
The dimmer can be uprated by changing the Triac and filter inductor, your problem will be as soon as you put a load on the motor the speed will drop slightly.

Joined Jul 18, 2013
22,390
For tool usage it is only good for Universal motors, not induction , unless a shaded pole fan etc.
For large wattage power tools with a Universal motor you could check the triac used and up the rating with a suitable replacement.
They also generally have a small ferrite wound choke in the circuit, as opposed to the lamp type for suppression.
Max.

#### DickCappels

Joined Aug 21, 2008
6,974
I built my own motor speed control. It was a classic triac lamp dimmer circuit but I found a diac that had very symmetric breakover characteristics so that the firing angles on both polarities of the line were nearly the same. The reason for this is to minimise the DC component of the current through the motor. The triac I chose is a "snubberless" type from ST. The reason for this is that the motor is a highly inductive loads and can cause high changes in voltage with time.

Dimmers made for incandescent lamps don't worry much about this and some would work well and some won't. Also, lamp dimmers rarely bother with snubbers.

Joined Jul 18, 2013
22,390
The reason for this is to minimise the DC component of the current through the motor. .
Would this really matter with a Universal motor?
Or was yours shaded pole induction?
Max.

#### DickCappels

Joined Aug 21, 2008
6,974
I don't know -there was no documentation. Would the average current through the triac be higher with a universal motor is there was a significant DC component?

Joined Jul 27, 2017
8
I appreciate all of the replies, truth is, most of it was greek to me, I know just enough about electronics to get myself in trouble. More or less I am looking for a solution to control the rotational speed of a bench grinder that I want to dedicate ( it is one of those ~$20 ones from Harbor Freight) to sharpening shop tools / knifes. I thought about purchasing a 'router speed controller' https://www.harborfreight.com/router-speed-control-43060.html but ran across the video of the dimmer switch. So I guess the next question would be, can anyone point me to a schematic of a simple motor controller. just keep in mind that my experience level is very low, (but I can follow instructions and solder on breadboard) again, Thanks a ton everyone, I appreciate it immensely, Rod (just FYI, I would just purchase speed controller but being 100% disabled gulf war vet living on disability income, I simply cannot afford a$200-$300 unit, so any ideas are appreciated) #### MaxHeadRoom Joined Jul 18, 2013 22,390 WHAT is the nature of the motor? A clue is the rpm rating on the plate, it will only work with that controller if it is a Universal (AC/DC) motor. Why do you want to slow a bench grinder down? Max. #### mcgyvr Joined Oct 15, 2009 5,394 So I guess the next question would be, can anyone point me to a schematic of a simple motor controller. just keep in mind that my experience level is very low, (but I can follow instructions and solder on breadboard) The next step is to know the type of motor used in that grinder.. You need to give info on that.. Its likely an induction motor and as stated cannot be speed controlled easily/cheaply.. You are likely SOL as far as slowing down the grinder.. "simple" likely won't cut it Thread Starter #### Frask Joined Jul 27, 2017 8 WHAT is the nature of the motor? A clue is the rpm rating on the plate, it will only work with that controller if it is a Universal (AC/DC) motor. Why do you want to slow a bench grinder down? Max. Does this help? https://drive.google.com/open?id=0B_EUm6EjhPYLbVBRU0FGUHptM000OGpHbTlPU2hmeTBmZWVj I want to slow it down for sharpening/honing knifes, lathe tools, scissors, chisels, etc. I bought it years ago from Harbor Freight for under$20 & recently upgraded to a nice Dewalt one & had the brilliant idea to try & speed control this made in china one, & so far it is being more of a PITA than what I had imagined lol.
Thanks again for all you guys input
Rod

Joined Jul 18, 2013
22,390
The fact it is 3450rpm indicates a 2 pole induction motor so not suitable for Triac 'dimmer'
control.
All those same task I use mine for I would not want to slow mine down.
Max.

Joined Jul 27, 2017
8
The fact it is 3450rpm indicates a 2 pole induction motor so not suitable for Triac 'dimmer'
control.
All those same task I use mine for I would not want to slow mine down.
Max.
hmmm okay, well it was a thought lol
Thanks again to all who replied

#### Dodgydave

Joined Jun 22, 2012
9,912
Here is a circuit i use for dimming and motor control for universal motors, D1,4 are 1N4007 the Triac is a BTa08-600, it will work on 230Vac,,,you could try an ordinary lamp dimmer and modify it to your needs.