I am reverse engineering a multi-speed blower controller and perplexed by my results. The blower (230V single phase 250W rated) is the heart of range hood. The schematic on the controller end is clear (left side below) but the motor unit itself (right side) is beyond reach for dismantling.

  • I measured the resistance between RED (common) and all the other wires. These resistances are listed next to the colors.
  • It seems to me it's a 4-speed motor with Blue-White-Orange-Black tapping into the primary winding to provide different speeds. Put the speed assumptions next to the resistances.
  • The capacitor is a "6.3uF 1.27.6CC2 MKP 420V 470V AV ARCOTRONICS" rated for 10k hours and is not at the motor, it's at the controller PCB
  • It looks like a run cap, it's sized like a run cap (though small for 250W I think?)
  • I also measured different wire pairs using a LCR meter and did not find any additional capacitance with those measurements
  • The switches in the circuit diagram are relays driven by a PIC microcontroller, nyt physical switches.

And to the question.. Why is the 6uF cap in the circuit only at the slowest speed (blue wire energized to drive the full winding). Is it even a run cap? And does my guesstimate of the blower internals make sense? I have zero experience with AC motors... Motor controller and assumed motor windings

  • \$\begingroup\$ It seems to be only a start cap. \$\endgroup\$ Commented Dec 7, 2021 at 20:26
  • \$\begingroup\$ @TonyStewartEE75 my searches before posting indicated that a start cap for a 250W motor should be somewhere in the 60-80uF range which is 10x more than what's in the circuit. Or does the multi-speed nature of the motor somehow play into this is and the start cap can be much smaller? \$\endgroup\$
    – vaizki
    Commented Dec 7, 2021 at 20:54
  • \$\begingroup\$ yes a start cap at full speed would be 10x bigger but not at a low speed. \$\endgroup\$ Commented Dec 7, 2021 at 21:03
  • 3
    \$\begingroup\$ It's in circuit at all speeds, via the full resistor chain (blue winding). \$\endgroup\$
    – user16324
    Commented Dec 7, 2021 at 21:27
  • 1
    \$\begingroup\$ The impedance of 6uF at 100 Hz is about 250 ohms so it is a partial phase shifter at all speeds depending on motor impedance \$\endgroup\$ Commented Dec 7, 2021 at 22:51

1 Answer 1


Nothing is disconnecting the capacitor from the circuit,

The motors main winding acts as a transformer so the capacitor is always in circuit.

At higher speeds settings the motor's main winding steps up the supply voltage increasing the voltage on the capacitor and thus the current that flows through it to the aux winding.

If you're comfortable to make a safe measurement of the voltage across the capacitor with the blower running you will be able to witness this. If you're not comfortable doing this don't risk it.

  • \$\begingroup\$ Any idea why this arrangement i.e. how does the main winding in series with the cap affect the auxilliary winding? Also, I am reverse engineering this to implement a replacement speed control, the original PIC microcontroller seems to be dead. So I am wondering about start-up sequencing mainly to make sure I don't fry the motor. \$\endgroup\$
    – vaizki
    Commented Dec 8, 2021 at 8:35

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