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Author Topic: Looking for a variable speed motor capable of 16 to 2k RPM.  (Read 1019 times)
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Anyone have suggestions on a variable speed motor that is capable of 16 to 2k (or higher) RPM?  I figure a stepper motor would probably be the best bet, but I can't find any that spin fast enough without lots of voltage.

What I'm wanting to use the motor for:  I replaced the transmission in my car.  The old transmission had a cable driven speedo, the new one sends a digital signal.  Eventually I plan to replace the instrument cluster with a digital speedometer, but for now I just want to make a little device to read the digital signal from the transmission and spin the speedometer cable the appropriate revs to match.  I know that the speedometer is 1000 revs per mile.  So to work from 1 mph to 120 mph I need to spin from 16 RPM to 2000 RPM.  And I'm limited to 12v or less since this is in a car.

Thanks for any suggestions!
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Analog speedo's aren't usually at all accurate below 5mph, so perhaps you only need the range 100..2000 rpm.  A DC motor that's not struggling ought to be controllable over that range with PWM, just make sure it can provide a good factor extra torque than the speedo+cable needs so that the speed/voltage curve is close to linear.  Find a motor that does about 2500rpm at 12V so you have some headroom.  You'll need to correct for battery voltage when derivng the PWM ratio.

Stepper motor at 2000rpm isn't trivial, does sound like overkill.
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The problem I've seen with standard DC Motors is most of them will spin to fast at their minimum voltage to even get it to spin.  The ones I've been able to find at least are like that.  I also haven't been able to find any with an acceptable voltage range to be able to range it that much with any precision.  That is why I was thinking a stepper motor, much more precision and much easier control over that precision.  Also stepper motors usually can handle torque resistance a little better from what I've seen which there may be some of it in a cable going to the speedo.

If you know of any DC motors that can actually be modulated in this range though I'd take a look at them.
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You will need a decently over-speced motor to make static friction and dynamic friction negligible - there are quite a lot I've seen on eBay which are pretty decent - I've a 12V gear motor that will run down to about 0.5V (starts up between 0.7 and 1.5V from cold.  Its a cheap and cheerful chinese model about 40mm diam, 60mm long (motor part), can't remember the power rating but perhaps 50W at a guess.

A smaller motor would struggle more to start against static friction, but medium sized DC motors are good value these days, I think aimed at robot traction market, mine runs well and feels v. smooth.  I'd suggest its worth a try.

Something like this http://www.ebay.co.uk/itm/12V-DC-HIGH-TORQUE-High-power-Reversable-Electric-Motor-2-RPM-Gear-Box-/140860012644?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item20cbe93864 - but with gearbox removed.
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