Using a DC motor, I can reach about 1200rpm using 1V/1A (laboratory style power supply)

However, if I use 3V input and a PWMized MOSFET, I need a duty cycle of about 90% to reach the same rpms and the power consumption is then 3V/1A

Why it appears that the motor needs 3W to reach the same speed using PWMized supply as compared with the 1W when not using it?

  • \$\begingroup\$ Let me make a wild guess. When you do PWM, you are actually asking the motor to move and stop, move and stop, and so on. The motor gets annoyed and refuses to work hard, scolding the PWM guy and wasting energy, ... \$\endgroup\$ – tlfong01 Aug 14 at 8:06
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    \$\begingroup\$ A circuit diagram or (inferior) very good text description is needed. Use CTRl-M when editing answer to open the excellent very intuitive cct editor.|| Is there a diode reverse biased across the motor? If not, add one, report back. What PWM frequency. What voltage drop across the PWM switch when on (if it is about 2V then your situation is expected.) \$\endgroup\$ – Russell McMahon Aug 14 at 9:17
  • \$\begingroup\$ Actually the pwm switch is a mos with low rds on so the drop is expected to be very low. I added a decoupling capacitor of 470uF after the hasher and now the motor will turn even with 30% pwm. However it still has the same over-consumption. There is no diode. However I thought that diode purpose was to avoid emf to go back in the circuit, can it reduce consumption as well? How? \$\endgroup\$ – Kleenex Aug 15 at 8:13
  • \$\begingroup\$ @Kleenex Lets's try that again: "... If not, add one, report back." Try it, you'll like it. \$\endgroup\$ – Russell McMahon Aug 15 at 10:49

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