fixed rpm motor

[oxford]

Does anyone know a simple design for pinning a motor to a precise rpm?

I want to be able to turn the motor on and for it to ramp up to the desired rpm <2000rpm.

Thanks

[bharbour]

Originally Posted by oxford Does anyone know a simple design for pinning a motor to a precise rpm? I want to be able to turn the motor on and for it to ramp up to the desired rpm <2000rpm.

We need a little more information. What kind of motor is it and how big: stepper, DC, 3phase... ?

What kind of a load is it driving?

How accurate do you need the speed to be?

Is this a one off project or a production project?

How much do you want to spend to do this?

Regards,
Bob

[oxford]

I would like to keep the device as cheap as possible.

I am not sure as to how much power I will need yet.

I will be rotating something roughly the size and mass of a frizbee.

I would like to keep the rpm +/-5% of 1160rpm

[bharbour]

If the load is constant and light, a simple regulated power supply would probably work. Setting the speed could be done with a tach sensor, or a strobe strip.

If you need more regulation and are up for some building, check out the note on page 3 for sensorless speed control:
http://www.national.com/an/AN/AN-292.pdf#page=3
As long as the load is not too variable, this works pretty well.

The treadmill speed controls are probably larger than you need but are more packaged if you are not interested in building something from scratch. They can be had from surplus places cheaply enough.

Bob

[Al_The_Man]

One very cheap accurate way is with an AC stepper motor. e.g. Superior Electric, about $25.00 on ebay, they require NO controller as they run directly off the 120vac supply they are very accurate as they are synchronous with the supply frequency, on 60hz they are 72rpm, so you would have to use 16:1 step up, which should not be a problem with that load.
But they have no ramp up, they are instant start and stop.
Typical accuracy less than 1%.
You can also apply a DC voltage when at rest for holding brake.
If they stall, there is no burn out etc.
Al.

[NC Cams]

Cheap and accurate can sometimes be counterintuitive.

For tacho feedback, you can use a gear mounted to an end shaft. Read teeth with a reluctor. Check Red Lion for these. YOu can turn these into a DC voltage via a LM2917 IC from National.

A speed based reference voltage can be created with a pot and a DC ref voltage fed into a non inverting Op amp..

Feed into any number of DC drive "speed controllers". Most have or supply schematics that you can patch the above signals into and the inversion or non inversion of the input signals can be addressed internally.

I say DC as these are pretty easy and cheap to do as opposed to AC or 3phase.