slowing down a DC motor?

To a first approximation, DC motor speed varies with voltage, so if you connect a series resistor of roughly the same resistance as the motor itself, this may do what you want. A down side is that the torque will also reduce, and you may not have enough power for your needs.

Chris

You use pulse-width modulation to control DC motor speed without reducing the torque so much I believe. You can buy DC motor speed controllers. Simon.

hmm torque is an issue, its going to swing a 4 foot length of thin timber (balanced with axis in the centre obviously)

The mind boggles!

You can slow them down by reducing the voltage, but this will also reduce the maximum torque too - likely to the point where it isn't great enough. The best way would be to use a PWM controller - but even building one will likely cost more than a suitable speed motor/gearbox. If you decide to investigate PWM control both Vellerman and Jaycar do kits for this. Others probably too.

If it was me, I'd try it with the length of timber in place. You never know, the air resistance might do the slowing down for you. If not, try putting a small diode (about 10p, from Maplins) in series with the battery. This will drop 0.6 or 0.7 Volts, irrespective of load, so will roughly halve the voltage to the motor. They do need to be the right way round, or no current will flow. However, with such a small battery voltage there may be some difficulties in getting the motor to start spinning, esp. with that load attached.

OK I'm wanting to rotate a lightweight length of timber or can be plastic if sturdy enough approx 4 foot length about it axis in the vertical direction a bit like a windmill this in turn will rotate about the horizontal (I've fixed this problem by standing a pole on a slow rotating shop window jewelry display turntable) I have seen motors with 2 axis shafts 90 degree to each other, this might be an option, rotating the whole thing at the top of the pole, but I've not ventured further into this idea.

The use for this contraption is photography based (as ever)

Thank you!

Cheap electric screwdrivers, high power motor and nice low RPM high torque planetary gearbox, seen them used for aerial rotators before.

Cheers Adam

and buy products that suit the purpose, rather than scrap..

OK I'll bite. What model are you making this time :-) It can't be anything to do with Halloween, can it?

Dave

Agreed. Or cordless drill motors; I ripped mine apart a few years ago (I hate cordless power tools) and built a little H-bridge driver and PWM circuit for the motor so I could electronically control the speed.

(The body and battery from the drill ended up with an old car headlight grafted to them which made for a nice little lamp to be used in underground photography, so no bits were wasted :-)

cheers

Jules

first mock up appeared to work, photo here

better link here

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