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What is the best way to restrict the current a motor draws? I am using a 12V DC motor which unrestricted (i.e. no current restriction) draws around 18 amperes. I need this to be limited to 12 amperes maximum. Is pulse width modulation (PWM) the only solution?

EDIT: This for a portable water pump unit, which is designed to be powered by the cigarette lighter socket in a car. However, not all cars can handle 18a through this circuit, hence I would like to adjust the current draw. If possible I would like to avoid having to change the motors (for a lower current option), hence I am searching for a cost-effective alternative to essentially reduce the power of the motor. It does sound like PWM is the only solution, potentially in-line on power cord, away from water?

I appreciate your comments!

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    \$\begingroup\$ An unrestricted motor I take to mean an unloaded motor and, if your 12 volt DC motor takes 18 amps when unloaded, what does it take when it's doing its job. Restricting it to 12 amps appears to be swallowing a spider to catch a fly. \$\endgroup\$
    – Andy aka
    Commented Nov 22, 2020 at 12:05
  • \$\begingroup\$ If we knew why it needs to be limited to 12 A then perhaps we could come up with some ideas. \$\endgroup\$
    – Andrew Morton
    Commented Nov 22, 2020 at 12:11
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    \$\begingroup\$ Does "... which unrestricted draws around 18 amperes" mean "which draws 18 A at full load"? \$\endgroup\$
    – Transistor
    Commented Nov 22, 2020 at 12:30
  • \$\begingroup\$ You may use power supply with current limiting. \$\endgroup\$
    – user263983
    Commented Nov 22, 2020 at 14:12
  • \$\begingroup\$ A motor generally draws more current when it is starting up, than when it is operating at its rated full load capacity. Limiting the current may prevent the motor from reaching operating speed. What kind of load is the motor driving? \$\endgroup\$
    – Math Keeps Me Busy
    Commented Nov 22, 2020 at 14:40

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A PWM motor controller is the best solution. It needs to be mounted where is is not subject to splashing or designed with a sufficiently sealed enclosure. Mounting electrically insulated power devices to a heatsink that has fins protruding from a sealed enclosure is one approach. The size of the enclosure must be large enough to safely dissipate heat from the lower-power components.

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