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The liquid-pump/motor is a miniature one from ebay with its operating voltage rated as "6-12V".

Even at 6V (the minimum of that specified range), it's pumping liquid too fast for my requirement.

Will there be any problem if I instead supply just 1 V or 2 V (assume long-term usage) so as to reduce the pumping power?

If that is not the recommended approach to reduce the pump's motor speed, what other method should I use?

I'd like to avoid using a different pump, because this one has dimensions just small enough for my application.

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    \$\begingroup\$ Sure go ahead and feed it less voltage. There's probably a simple DC motor in the pump and these do not mind if you give them a lower voltage. Do make sure that you're above the voltage where it stops though so that you can be sure it will start up every time. So if it stops below 1 V, I would at least feed it 1.5 V or even better 2 V. \$\endgroup\$ – Bimpelrekkie Jan 27 '16 at 12:49
  • \$\begingroup\$ If it runs unevenly or doesn't start reliably, that's too low a voltage. Otherwise, it'll be fine. \$\endgroup\$ – user_1818839 Jan 27 '16 at 13:29
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    \$\begingroup\$ It would be better to drive it with PWM than to feed it 1-2Vdc. \$\endgroup\$ – Matt Young Jan 27 '16 at 13:43
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N α K Eb/ɸ

Where

N = Speed of Motor

Eb= Back Emf

ɸ = flux per pole

From the above relationships the speed of DC Motor is directly proportional to Eb(Back emf) and inversely proportional to flux produced by field. (General DC Motor -Dc shunt)

So by reducing the Input voltage obviously( Back Emf) will reduce the speed of the motor. I am guessing there is no provision to control field.

Before running the motor in low voltage continuously for long time, check for the current taking by the motor. If it is more current than specified level then adjust the voltage such that it should not take more than specified level.

More current cause to lead the winding temperature to predefined level so it will damage the motor

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I don't know if you need variable speed or not, and if you are parts or design constrained, but you could look at PWM techniques to control the motor with the rated voltage.

Even though many applications of PWM use it as a way to alter speed (or brightness, or...) in a controlled and accurate manner, it is also a very good way to maintain a fixed output that isn't necessarily symmetric with input voltage. In this case we can make sure we run the motor at its happiest voltage, but alter the duty cycle so the RPMs are adequate for our needs.

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  • \$\begingroup\$ PWM may be better even without variable speed. A DC motor is less likely to stall when driven at a higher voltage with PWM than it is just being fed with a lower voltage. \$\endgroup\$ – Simon B Jan 27 '16 at 17:22
  • \$\begingroup\$ I agree. I realize my answer implies you want PWM just for variability, which was not my intention. \$\endgroup\$ – user65586 Jan 27 '16 at 20:28
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yes dc motors accept lower than spec voltages however the result may come some undesired effect as well, example: heat build up may occur thus resulting in added resistance in the motor causing it to lag or cease.

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  • \$\begingroup\$ Why would heat build up? Are you thinking that it is fan cooled? \$\endgroup\$ – mkeith Jan 27 '16 at 20:08

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