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Can you help me with some circuit specs?

I am planning to build a driver control system for a variable speed DC motor. My initial design has shaped up as follows: AC voltage from a wall plug to a full-wave rectifier with a four diode bridge and a T or Pi output smoother. The rectifier would then be connected in series to the motor and a potentiometer, like a dimmer switch. Now the part that has slowed me down, the voltage range for driving the motor is 0V to 120V DC, which leads to the question. If I took out the transformer that is normal seen in a rectifier circuit to keep the higher voltage, what sort of specs would I need for the diodes, capacitors, and inductors in my circuit? Or is there a better way to obtain the desired Voltage range from an AC wall plug with out buying something costly?

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Prometheus8 years ago
What was suggested below was a workaround. Rectified light-dimmers can be hard on DC motors made for pure DC by eating up the brushes and commutator due to arcing. What you need to find is a discarded treadmill that uses a DC "permanent-magnet power-motor" (steal the motor too, they are some of the best you can get). Treadmills often yield some pretty useful SCR-drives already assembled, as well as some of the best DC power-motors you could ever hope to salvage. Don't forget the other half of the speed-control at the "helm" of the treadmill, that varistor will be important. If you see markings such as "H, W, L" on the board and the controller, then simply match them in connections. I have the speed-controllers from 3 different treadmills, and aside from one, they are fully-adjustable for maximum-current, initial reaction (reaction-time to change to the new speed setting), maximum voltage, and minimum voltage (baseline power when the circuit is energized). Older Sears models are golden with the most compact and powerful motor-drivers I have ever seen. One of the two I have has a variable programming loop via a sub-controller, so I can set speeds at 8 different intervals where the timeline varies by my "time" setting "at the helm", but the controller itself is "dumb", and will take any input it is given to it's limits. Treadmills are gold-mines for 120V motors and controllers! I strongly-recommend anyone looking for such start there. The excess junk and paying maybe $150 for one is so worth it if you find the right ones. Look for a speed-control, and if it has one, you will quickly see why even $200 is a steal for it without the belt. You cannot buy motors like these for less than $300, much less the variable controllers that match them. It doesn't matter if it's not useable as a treadmill, just so long as the motor and controller work. You cannot go wrong salvaging these....A $4500 treadmill with no belt was salvaged for the $1200 motor and a $350 full-wave SCR-drive controller that can run from 0-150VDC at 20 amps, because someone lacked interest in paying $200 for the belt-replacement. That was a 3kW controller for free, that could step-up the voltage..NEVER let one of these pass you by.
110100101108 years ago
you can try standard light dimmer on the ac side of the rectifier if you have really big capacitor after the rectifier then it may be close enough to dc for the motor to run acceptable pay attention to heating of the motor. it may not like the pulsing of ac after the dimmer for diodes use something that can stand 350 V (atleast. i'd look for something like 500 - 600) 100 HZ few times X the current rating of the motor (the more the better). it may tend to take more current when dimmed add a fuse to the same current as the diodes or a bit less to protect them
knotmuch (author)  110100101108 years ago
My diode knowledge is part of my weak point here. I understand some of the different types (in theory) but not how to read the symbology on them or check their ratings, etc
knotmuch (author)  knotmuch8 years ago
sorry forgot to mention that the motor is rated to 14.?? Amps
you need 4 standard diodes (rectifier diodes) or rectifier bridge (factory made component that contains 4 diodes inside) they should be rated for 350 V 30 A 100 HZ diodes with better specs are ok or better (especially the current specs) but dont overkill diodes may require cooling (attach them to cooler plate of appropriate size and add fan if needed) light dimmer is not enough for switching 14 A. you may want some more powerful control that makes the same function. maybe you can find it in place that deals with industrial machine supplies
frollard8 years ago
1686 below has a good solution - one thing, most dc motors dont mind pulsed DC current - a pulse width modulation driver is what you really want. Using a potentiometer simply burns off the excess voltage as heat, where pwm reduces the power that goes to the motor while always driving it at the rated voltage. Again, about the components - a safety factor of 2 or 3x the nominal values is a good plan. Nothing like letting out some capacitor smoke right when you need something done.