As long as the added capacitor exceeds the peak applied voltage and can handle the wattage I see no issues. You may need to have a higher wattage for it than the one already in circuit as it may have a higher current flowing through it as it has the current from both legs of the circuit. Additional temperature protection may be needed where the capacitor is mounted.

The two likely failure modes of open circuit or short circuit are no really different to not having it there. When slow speed is selected in one case the fan will stop, in the other it will switch to high speed.

Martin C

I don't know, so this post comes from the comfort of my armchair!

One way the capacitor might slow the motor is by altering the impedance of the load so less current flows. This would be efficient because capacitors don't get hot like dropper resistors.

However, another possibility is that much the same current flows in the windings as before and the motor slows down because the windings are out of phase. That would cause the windings to heat up, more so if the motor is normally cooled by the fan.

Why not measure the AC current going into the fan with and without the capacitor? If the current is similar in both cases, check the windings aren't warming up. The windings may take a while to get hot if both effects apply.

Dave

This method is fine. Basically the extra capacitor is a wattless dropper reducing the voltage across the motor. this gives increased slip and reduced speed. Not perfect but better than a resistor or simple phase angle controller as used by some fan speed controllers. Capacitor should be a motor start or run type. If it fails you will just loose slow or fast selection depending on failure mode.

Robert.