DC motor control

Rekarp · Post by **Rekarp** » Thu Jun 19, 2008 3:06 am

Well I picked up a drum sander for my workshop the other day for $350 which was a steal as the usually go for $900 (Delta 18x36!). So I was using it today when the drive motor stopped working and the smell of burning electronics filled the garage. Doh!

Took apart the motor to find that the windings where all melted. So I ordered some magnet wire to rewind the motor (done it before, not hard)

So I was like I should check on the the voltage regulator so I stuck my multimeter on it and nothing. Took it apart and it was fried... no fuse protection at all.

I can buy a new one for $50+shipping but I think I can build one for cheap.

The motor runs off of 90VDC and draws .45A. It has to run off of 120VAC so I was thinking of using a dimmer switch for a fan and run it through a bridge rectifier. Do yall think that will work? I will install a 1A fuse infront of the whole thing to protect the circuit encase the motor decides to melt down again.

edit//

Ok I think the plan is to get a 1A fuse to protect the whole thing. Then a dimmer followed by a bridge rectifier. Then a large cap to smooth the DC current. Should work.

timmeh87 · Post by **timmeh87** » Thu Jun 19, 2008 11:29 am

Ok probably a silly question, but what about returning it?

I can see a few problems with a dimmer switch, the first one being that if you just drop in a standard light dimmer, you would probably fry your motor. I mean, eventually someones going to turn it up to 170V (or at least past 90) by accident or something. Yes, rectified 120v is 170v.

The second problem is that only about 30% of the range will be a safe voltage with the other 70% being about 170v. Half of the range will be 170v, as it will include the peak. Check out the diagrams on this page:

http://home.howstuffworks.com/dimmer-switch2.htm

You would probably be best off using a transformer. Isnt there already one in there?

Rekarp · Post by **Rekarp** » Thu Jun 19, 2008 11:47 am

timmeh87 wrote:Ok probably a silly question, but what about returning it?

I can see a few problems with a dimmer switch, the first one being that if you just drop in a standard light dimmer, you would probably fry your motor. I mean, eventually someones going to turn it up to 170V (or at least past 90) by accident or something. Yes, rectified 120v is 170v.

The second problem is that only about 30% of the range will be a safe voltage with the other 70% being about 170v. Half of the range will be 170v, as it will include the peak. Check out the diagrams on this page:

http://home.howstuffworks.com/dimmer-switch2.htm

You would probably be best off using a transformer. Isnt there already one in there?

Can't return it. Bought it used at a garage sale. Plus it will cost less then $100 to buy OEM parts so a drum sander for $450 is still a steal.

The old voltage regulator turned up to 124V. Industrial 90VDC motors can run straight of rectified 120V. It doesn't have a transformer. Has a weird part labeled A70084 but I can't find any info on it. The AC voltage passes through a makeshift rectifier (4 diodes) on its way and an inductor to limit the current change.

Ive heard of people using dimmers for this but you get a crazy DC signal.

edit// Ok this is what I decided to do.

Harbor Freight sells a router speed control for AC motors for $14. It uses a rheostat to cut the voltage and can handle up to 15Amps which is way more then I need. I hack it and take the guts and install it into the Drumsander. Pass it through an Inductor and a filter cap taken from the old regulator. Then into a bridge rectifier and into another filter cap taken from a busted PSU.

Oh and the speed control already has a fuse holder.

Should work great and only $17 after taxes.

timmeh87 · Post by **timmeh87** » Fri Jun 20, 2008 10:29 am

Im liking the simplicity. I also like large rheostats.

For some reason it makes more sense in my head to put the resistor after the rectifier. Im trying to think why. Heres what i came up with: If you are going to put decoupling caps on the rectifier, then if you put the control resistance on the other side, wouldn't you get some lag, as the capacitors are trying to resist that change in input voltage? The motor is decoupled from the AC side, so in theory a change out there should not affect it, and in reality its going to affect it on the scale of the 'time constant' of the RC circuit made by the rheostat and (both)? decoupling caps. Which might only be a few ms.

I mean theres lots of other sources of lag in here, the inductor, the momentum of the rotor. So you might not even notice it. I guess i just wanted to work it out on paper.

Anyways good luck.

Rekarp · Post by **Rekarp** » Fri Jun 20, 2008 10:50 am

Well I wired it up last night. We will know soon when the winding wire gets here if it works or not. Interesting thing is the Router speed control device uses a Triac (dimmer switch doh) but its rated well above and is built for this application so I should be safe.

Oh I placed an inline 1Amp fuse to. The winding wire is rated at 1.5Amps so this should protect the windings from burning up again.

Rekarp · Post by **Rekarp** » Mon Jun 23, 2008 2:42 pm

Wire came in today. Will be rewinding it when I get off work. During my lunch break I stuck the voltage regulator on an oscilloscope. Looked fairly decent but its not a perfect linear DC line but it will do. Interesting enough I looked at the older voltage regulator and it is built similar to the Triac that was in the router speed control device meaning it was a glorified dimmer switch as well.

I tore apart a 19" CRT that I got for free and took enough capacitors to get 1000uF of capacitance which looks to be enough. We shall see how the motor affects it because the motor "pulses" onhy the motor the current draw and the Triac sends a semi pulsing DC current. How the caps will fair will be interesting.

I think I am going to rewind the motor to get more torque out of the motor by adding windings. Before the burn up you had to crank the voltage to get it moving (wasn't a gear or load problem). This probably is the reason why the motor burned up when it jammed. More torque should allow it to run at a slower speed with less voltage atleast in theory.