DC motor speed control over a large range

[Tim]

I am trying to design a small DC motor and gear arangement that allows
electronic control of the output speed at two discrete points of 4 RPM
and 200 RPM (50:1 ratio). I can find DC brush motors that can operate
over a 4:1 or 6:1 ratio of speed but not over a 50:1 range. Has
anyone seen a gear mechanism that supports two speeds that can be
electronically shifted?

Thanks

 

[Tim Wescott]

I am trying to design a small DC motor and gear arangement that allows
electronic control of the output speed at two discrete points of 4 RPM
and 200 RPM (50:1 ratio). I can find DC brush motors that can operate
over a 4:1 or 6:1 ratio of speed but not over a 50:1 range. Has
anyone seen a gear mechanism that supports two speeds that can be
electronically shifted?

Thanks

A DC motor will support operation from no motion up to it's maximum - you
just can't ask for more torque at 0 RPM than you can at 10000. Even really
cheezy toy motors can be made to work reliably at fractions of an RPM using
pulsed drive.

Have you tried this? Where did you encounter difficulty?

 

[Spehro Pefhany]

I am trying to design a small DC motor and gear arangement that allows
electronic control of the output speed at two discrete points of 4 RPM
and 200 RPM (50:1 ratio). I can find DC brush motors that can operate
over a 4:1 or 6:1 ratio of speed but not over a 50:1 range. Has
anyone seen a gear mechanism that supports two speeds that can be
electronically shifted?

Try a gear motor with a reduction ratio of perhaps 20:1 and make or
buy a DC motor speed control, preferably with tachometer feedback.

Best regards,
Spehro Pefhany

 

[Frank Bemelman]

I am trying to design a small DC motor and gear arangement that allows
electronic control of the output speed at two discrete points of 4 RPM
and 200 RPM (50:1 ratio). I can find DC brush motors that can operate
over a 4:1 or 6:1 ratio of speed but not over a 50:1 range. Has
anyone seen a gear mechanism that supports two speeds that can be
electronically shifted?

You could try a stepper, 200 RPM is reasonably easy to achieve
with a stepper, and you can go as slow as you want. Or a DC motor
with a tacho feedback, which can run very slow and very smooth,
and with a high torque as well.

 

[Rene Tschaggelar]

I am trying to design a small DC motor and gear arangement that allows
electronic control of the output speed at two discrete points of 4 RPM
and 200 RPM (50:1 ratio). I can find DC brush motors that can operate
over a 4:1 or 6:1 ratio of speed but not over a 50:1 range. Has
anyone seen a gear mechanism that supports two speeds that can be
electronically shifted?

While the DC motor works down to zero rpm, the control loop
at very low speeds needs some attention as it becomes highly
nonlinear. Beside that a minimum voltage is required for the
collector, a starting current that is higher than the unloaded
running current has to be anticipated.
But is is doable.

Rene

 

[Rene Tschaggelar]

I am trying to design a small DC motor and gear arangement that allows
electronic control of the output speed at two discrete points of 4 RPM
and 200 RPM (50:1 ratio). I can find DC brush motors that can operate
over a 4:1 or 6:1 ratio of speed but not over a 50:1 range. Has
anyone seen a gear mechanism that supports two speeds that can be
electronically shifted?

I forgot to add :
a brushless DC motor might be simpler than one with
a collector.

Rene

 

[Spehro Pefhany]

What benefit does tacho feedback confer on the user?

Closed loop control allows the speed to be maintained despite torque
variation. There are also two or three distinct sensorless methods,
plus open-loop, but they are not as good. Open loop is fine for small
speed variations, due to the way DC motors work, but in this case
would likely fail to give enough torque at low speeds because of the
winding resistance.

Best regards,
Spehro Pefhany

 

[Paul Burridge]

Try a gear motor with a reduction ratio of perhaps 20:1 and make or
buy a DC motor speed control, preferably with tachometer feedback.

What benefit does tacho feedback confer on the user?

 

[Jim Thompson]

What benefit does tacho feedback confer on the user?

Taco feedback causes heartburn ;-)

...Jim Thompson

 

[Spehro Pefhany]

Taco feedback causes heartburn ;-)

ROTFL. ;-) Mr. Burrrriito can do the same.

Best regards,
Spehro Pefhany

 

[GPG]

I forgot to add :
a brushless DC motor might be simpler than one with
a collector.

Rene

A capstan motor from a VCR would be worth investigating.
You should be able to get one free from a servicing outfit.