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MG Midget and Sprite Technical - Slowing down an electric water pump

I'm running a Davis Craig electric water pump on a lotus tc. It's directly wired to 12v. I believe it's running too fast. I want to slow it down using a rheostat so I can adjust and note results. It is quite greedy and is rated at 7.5 amps.

Any recommendations? I'm close to marlins and RS components

Thanks
Dan Cusworth

Maybe something like the Velleman K8004 kit would do you. IIRC it as standard does up to 6A but I have seen that can be increased by changing the current sense resistors and maybe the transistor for a newer one with lower on resistance. This seems to be a common query regarding this kit on the net. I did look into it for a certain project but in the end it looks like things will be fine with the standard kit so haven't gone further.
David Billington

Why not use the Davis Craig controller?
Deborah Evans

David , I just googled it. What is a pulse width? Does it "chop" the dc to give a rms lower voltage? (I wish I had a better electronics teacher!)
Dan Cusworth

Dan,

Basically yes it chops the DC supply to give an effectively lower DC supply. You have a few options regarding the output control.


David Billington

"What is a pulse width? Does it "chop" the dc to give a rms lower voltage?"

It is when the voltage is in the form of pulsed. Pulse width refers to the amount of time that the voltage is on. The circuit that does this is called a pulse width modulator and is used for controlling the speed of high current devices. Short on periods and long off periods provides a lower average current, while long on periods and short off periods provides a higher average current. This is a particularly effective to control speed of a device under load, in that each time the pulse is on, it is providing full voltage/current, but by using short pulse widths, the average current, thus speed, is kept low. Cheers - Dave
David DuBois

Adding to Dave DuBois...

Feeding the motor with a pulse dissipates less energy in the controlling circuit, since:

- little energy/heat dissipated in the circuit during 'off' cycle
- large current but low resistance dissipates comparatively little energy during 'on' cycle
- avoids middle-ground whereby the controller has medium resistance and is passing reasonably high current (analogeous to old-school rheostat control) giving off large amounts heat energy for power delivered.

This makes the system more efficient and it requires smaller heatsinks, and so on; equivalent to Class-D operation power-amps.

A
Anthony Cutler

Cool. Sold. Thanks all!
Dan Cusworth

All excellent posts and very informative.
Bob Turbo Midget England

David, can you point to info on upgrading the K8004? I have now got one to play with!

Thanks
Dan Cusworth

This thread was discussed between 24/07/2011 and 25/07/2011

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