Reducing AC fan motor speed

I have an AC motor fan I wish to use to extract air from my solar power electrical room. The fan is a 220v 1amp AC motor , with a 10uF cap.

It works fine in hot weather with the solar inverters running hard, but in cooler weather its speed is excessive. Fitting a 10uF AC cap in series with the live lead slows it to a very suitable speed. Is this a safe practice? Are there any reliaibility issues I should worry about?

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

Thanks Martin - Will do!

Appreciate your comments.

Joe

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

Have a look around air based HVAC systems if you can find circuit diagrams. My hot air central heating, rather rare in the UK compared to the USA, used simple (big) dropper resistors to give about 8 speeds. I believe that when underpowered synchronous fan motors just slip which does heat them up, but hey, they are fans so cool themselves if the motor is in the airflow.

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.

Yep, tried it a short while ago and it works fine. A 10uf cap gave about 3/4 speed, and a 2uF cap around 1/4 speed. The latter took about 4 seconds to get to speed.

SOD - ...and the motor slows down because the windings are out of phase..

How would that be? The motor itself is inside the power feed circuit and does not know about any phase shift, ie, across the 'live' and 'neutral' connections to the motor a scope would show the same phase relative to the existing 10uf phase shift cap - the phase shift you speak of would be relative to the Live side of the added cap...

Thanks all for the guidance.

Joe