The Shop > Electronics & IC Programing
Mad Modder speed control for DC motors...
John Hill:
Suppose I have a 230V DC motor of a horsepower or so that I want to control the speed of (and I am too skint to want to buy a "real" DC speed control).
How about a lamp dimmer to a rectifier to the motor? :coffee:
Now the lamp dimmer operates by delaying the turn on at every phase reversal i.e 100 times a second (or 120 in some parts of the world) then when that goes through a rectifier we have pulse width modulated DC (with round cornered pulses).
I think it would also work with a transformer before the rectifier for lower voltage motors? :scratch:
Now then, do lamp dimmers have any smoothing circuits, inductors for example, to smooth the output which would have the effect of reducing the peak voltage. If so we would want to take those out so that the motor got full voltage peaks and hence better torque.
awemawson:
The old 'drill speed controllers' were phase delay controllers, and most mains drills are 'universal' ie ac/dc so if you drive your 'dc' motor from an ac source it will work if it is indeed 'universal' ie series or parallel comutated
http://en.wikipedia.org/wiki/Universal_motor
Andrew
Pete.:
John you need a phase angle device and a rectifier. I use this to control the field voltage on my 3hp DC motor. You'll only get about 190VDC though.
The CSR1004 is the phase angle chip and will output 0-240VAC at 10 amps. You feed 240v into the middle leg and the right leg into a 10A bridge rectifier, other side of the rectifier to ground. Hook a 250k 1w variable resistor between the left and middle leg and this will give you zero to full speed control. You don't need the three pots as shown on my drawing, that's just for limiting the output from 50-115v on my lathe. A single control pot will do.
Dawai:
I bought a KB 2hp "jumper selectable" drive off ebay for $16..plus shipping.. "Hard to compete with that home-made." It's the one running the cnc'ed english wheel in one of my youtube videos.
Large DC motors new are expensive. I got a cheap 3/4hp one and gearbox that came from under a treadmill.. drive was a pulse-follower. as the belt turned it spun a encoder pulsing the motor drive. you see them drives about too. ( free )
BaronJ:
Hi Guys,
One of the problems with universal motors is that they tend to accelerate to very high rpm's if not loaded. Another is the torque produced fall off very rapidly as the phase angle and hence the applied voltage reduces. One way of trying to overcome these problems is to separate the field winding and feed it from a dc source then by feeding the armature from a phase controller you have more influence over the torque and speed. I used to have a good application note all about motor control. Unfortunately I lost this and a lot of other useful data following a HDD failure. If I can find it again I'll post a link to it.
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