DC Motor Control

[GeeksGoneBad]

Not sure if this is the right forum, and I'm not sure if this is "news" to anyone but I've worked out a cheap solution to DC Motor control and wanted to share it

Basically it's a "Fan Control" Light Dimmer from Home Depot and a Bridge Rectifier - that's pretty much it! Check out my video below

YouTube- Cheap and Easy DC Motor Controller

YouTube- Cheap and Easy DC Motor Controller

[Jesse B]

Pretty cool. Quite ingenious.

You should see if you can't find a dimmer switch that has the "Off" position before the lowest setting; the sliding type are all like this.

[GeeksGoneBad]

Thanks At least no one has told me I'm crazy yet! hehe

I'm thinking to gain more control (meaning slower speed) I'm going to try and wire two dimmers in a row and see if that lets me slow it down - at this point I'd need a 1" to 9" pulley system to get my speed down to around 600 and I don't have room in my little set up for 9" pulley

It's worth a try anyway

[Pandinus]

I'm going to try and wire two dimmers in a row

Now we get to call you crazy lol

Now if I remember this all correctly...

The way the controller work is that the pot on the control sets the voltage that the Triac turns on at and then the Triac turns off when it crosses "Zero" volts on the Sine curve.

So depending in the way the triac is set up you are probably getting a minimum of 90degrees on time and a power reduction of 50%??? I do not see putting two triacs in series like you are suggesting working at all...

[GeeksGoneBad]

I did read up on the dimmers and saw a chart where they cut off the juice in cycles like you explain and I realized that it wouldn't work

I was at the store ready to buy another fan dimmer and thought I'd try the regular light dimmer - just to see

because with the fan dimmer I could not get to 0 volts (I guess that's why they call it a fan dimmer because it doesn't go all the way down?)

anyway the light dimmer worked too and let me lower the voltage all the way down, but at some point the motor just kinda sputtered and eventually would not turn

maybe I need a big capacitor too? smooth out the DC coming out...

[Jesse B]

Originally Posted by GeeksGoneBad I did read up on the dimmers and saw a chart where they cut off the juice in cycles like you explain and I realized that it wouldn't work I was at the store ready to buy another fan dimmer and thought I'd try the regular light dimmer - just to see because with the fan dimmer I could not get to 0 volts (I guess that's why they call it a fan dimmer because it doesn't go all the way down?) anyway the light dimmer worked too and let me lower the voltage all the way down, but at some point the motor just kinda sputtered and eventually would not turn maybe I need a big capacitor too? smooth out the DC coming out...

A capacitor may help. I assume that the motors will only run off of so little voltage, and once it drops below that level, they "sputter" and won't turn.

[Andre' B]

The main difference between a light dimmer and a fan speed control is the fan speed control has more electronics in it to make sure the switching events on the positive and negative parts of the sine wave are more balanced.

With a light dimmer you can be getting more on time in one or the other sides of the wave resulting in a DC offset in the current. For AC motors and transformers this DC current makes for extra heating, noise, and or rough running.

Also the speed control does some things at slow speeds to try and smooth things out. One of which is to put a limit on how low you can set it.

[Fastest1]

So much easier to watch than read about it. Excellent and I was just wondering if supply voltage on a dc motor was responsible for speed or if a signal voltage was sent to a driver. It seems like both ways work.

[GeeksGoneBad]

Thanks for the info on the fan control!

I tried a capacitor (3 different ones actually) and they didn't seem to do anything at all

So to summarize - the fan control works the best so far

[vger]

Another difference on the fan control circuitry is that it is designed to run an inductive load, IE, a fan motor. I will have some filtering to keep electrical noise out of the supply line, and most probably some protection to keep from destroying the triac if the supply current to the control is interupted mid cycle. An resistive load, like a light bulb, will just go out when the incoming power is removed mid cycle. An inductive load, as in a motor, on the other hand will have a magnetic field built up in the windings when mid cycle. Removing the power at this point causes the field to collapse (very quickly) and there will be a voltage spike coming back from the winding that is opposite in polarity of the originally applied current. The voltage of that spike can be much higher than the originally applied voltage.

I have used a simple speed control for a DC servo motor for some time that is made with a "Variac" (adjustable autotransformer) and a bridge rectifier. Basicly preforms the same as your control control but is a little smoother down at low speed. I use it to spin a 6" diameter plastic disk that has 4 Neo magnets spaced equally around the edge. I put the edge of the disk near a metal object and adjust the speed untill the object vibrates in resonance. I then check the motor RPM and multiply by 4 to get the natural resonant frequency of the object. Anyway... using the variac and the servo motor, I can get all the way down to just a couple of RPM very smoothly.

Steve

[GeeksGoneBad]

Sounds like a cool machine!

Thanks for the info - I'll have to get some Variacs and have a play with them!