Finley did it, home made speed control

JTknives

JTknives

Joined
Jun 11, 2006
Messages
8,650
Well after much searching and testing i made a dc speed controler that keeps a constent speed nomater what the load is, which is nice because when i am grinding real hard the speed stayes the same. because i had such a hard time doing this i will shair with you the plans

motor.gif


this works very very good for being so simple, the nice thing is that it works on all brush type motors even ac if its a brush. you plug your motor into where it says ac socket. where it says 10K that is the pot which is the speed control. what happens is this. ac comes in and goes through the bridge rectfyer so that we get a pulsing dc out put whech is feed to the scr. now and scr is a switch that turnes on when some voltage is put to the gate which it the wire coming off at an angele. but with an scr once it turnes on it stayes on till the input power is cut, that means even if your remove the gaste voltage it will stay on. but here is the trick, because the voltage is a pulsing dc voltage that is suplayed it turnes on and of real fast. but then you adjuse the pot it charges up the caps to keep voltage to the gate, the lower the pot is the slower it takes the caps to charge to switch the gate. so what happesn s that the scr mecomes a pwm which stands for pulse with modjulation. it sends pulses of power to the moter at difrent speeds depending where the pot is set at. the nice thing is that this does not reduce tork as the pulses will incresse in speed to keep the rpm the same. im shuer there is some one that can explane it bettere than me but that is a jist of it. i found i get a better speed control if i turn the pot all the way "max speed" then slow it down to the speed i want. this is baucse of how the caps charge thay will just turn on and the motor will be around 1/2 speed, and if you turn it down then the motor stopes. but if you turn it up all the way the down to your desired speed it works perfect.

warning that anyone following the instructions are doing so at their own risk
 
Do you have a list of the componet values? I can fiqure out a couple of them but the rest I have no idea. I am glad it is working for you. It is alot less expensive to build circuitry even compared to buying imported products. I would put a fuse or breaker inline in case the SCR blows, you woul have a dead short comming off of the bridge rec.
Thanks for posting the schematic.
 
not a problem when i get home i will put up a list of the parts i used. i still cant beleve how good it works, i mean a $10 speed control should not workl this good.
 
The circuit comprised of the 10k pot, the two 100 ohm resistors and the 50uf capacitors delivers gate drive of the SCR. The diode D1 protects the circuit from reverse voltage spikes. The ratings of the bridge rectifier and the SCR should be 25 amps and PIV 600 volts. The diode D1 should be rated for 2 amps with PIV of 600 volts. The SCR should be very well heat sinked. I did not have a 25 amp scr so i used a 10 amp scr and it seams to work ok, but onley time will tell, it was free so no problem if i smoke it. i should put a fuse in line just incase. im going to try and replace the 10k pot with a 100k pot because i figuar you can never have to much pot, im hopeing it will smothe out the speed control selection adjustment.
 
well i was mistaken about the 100 k pot it made it more sensitive, meaning that onley like the last 1/4 turn did anything, so m asuming maybe like a 5-8 k pot would be perfect because i was onley using around 3/4 - 1/2 of a turn on the 10k pot. ill pick one up and see what i get going. it just takes some tweeking to get right.
 
O MAN i just found to key to this speed control. befor i was able to control from about half speed to full speed, and the speed controler would pulse the power to the motor giving you little bursts of speed but for the most part a steddy speed. but i was not happy with the not having total control. i made a change to the plans and o man i now have a perfect stedy speed from allmost not turning to full power, i mean i can make it so slow that i can watch a spot on the wheel go around. you redy for the change, here it is

newmotorcontrol.GIF


what i did was remove the full bridge rectfyer and just insert a fat hevy diode so i onley got a half wave dc out put, which spaces out the dc pulses to the scr making it easyer for the scr to tell there the pulse starts and stopes. i also added a diode to the positve line to the motor because the motor was creating some back voltage which was showing around 150 volts on my meter. just think with this change i just made the motor control around 5-6 bucks cheeper. o man im so excyted now i have a fully speed controled dc disk grinder with kick ass power. sweeeeeeeeeeeet, o ya and i reinstalled the 10k pot and its working just how it should, so its a keeper

What ever you do, DO NOT CONNECT A FILTER CAP TO THE MOTOR. if you do its like the speed control is not even there and the motor will spin up to full speed.
 
:thumbup: thanks for taking the trouble to post your results, this will probably come in handy when i get mine up and running, and the last comment was referring to the spelling of "finally" in the title of the thread:D dont worry, it happens to the best of us:p
 
ya sorry for for my spelling, i have never been that great with it. I am excyted to get some knives done with the speed control in place. i noticed that at full speed the disk grinder would burn the wood handle, but im hoping that with a slower speed that wont happen.
 
I would have never thought of going 1/2 wave, it would seem like you would lose amperage but it's cool that it improves it. Thanks!

How much heat does the control rectifier put out at full load? Do you have it mounted on a hefty heat sink?
 
How does this stress the motor it's controlling? If it was this easy to do, I'd think everyone would be doing it... there HAS to be a drawback. Please don't be offended by this, I just can't feature a circuit this simple NOT being used if it was this straightforward, you know?
So are there any drawbacks to this?
 
One problem with these kinds of circuits is that they can cause a lot of electrical noise. If you are interested in finding out more about motor speed control, try this link: http://www.electhai.com/E_link/Link_Motor.htm

I gave up reading about these scr circuits when I discovered that my cheapo grinder motor doesn't have brushes. :(
 
Pixelhack is correct about the electrical noise issue. The circuits described are quite likely to cause interference in other electronic equipment in your house or your neighbors house. So don't be surprised if you get some complaints. I have worked on the design of a circuit similar to this and we put in a lot of design and testing time (and many other components) to take care of the noise and to make it a reliable and safe product.

I would not recommend building the circuits shown in the website that Pixelhack has linked to unless you acually have enough electronic design experience to refine them. Quite a few of the circuit ideas are from magazines and/or from chip manufacturers and are mainly meant to sell the chip featured. If one is a qualified designer, one can get some good ideas of how to design something, after throwing out the parts of the design that won't work in your application. These circuits are not usually tested for safety and reliability. I have "let the magic smoke" out of many parts by just copying circuits that I have found in such sources. The articles about how motors work are good though.


Peter

I would immagine that your motor "that uses a starting cap" is a single-phase AC motor. Look at the nameplate on the motor. The circuits posted by JT would not work for an AC motor.


Back to work.
 
yes i do have some electrial noise from the motor, but for what it cost me to make it thats ok.
 
well its still just looseley wired but when put on a board no biger than a deak of playing cards. now i do loose some power from usind falf wave vrs a full bridge but the trade of is worth the amount of control. i should have it all asembled on a board by tonight. i will put up some pictures.
 
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