Destratification Fan

[awemawson]

Some years ago I hauled a standard desk fan up into the gods of my workshop on a length of rope, and it was highly effective stirring up the air and reducing the heating costs. It was second hand when I got it and it lasted for several years. When it failed I replaced it with another used one, which lasted for about a year.

Today I installed a brand new proper destratification fan and an associated speed controller, and this set me thinking about what sort of motor it is.It's single phase 240v AC, and the speed controller is in series with the live leg. Now as I understand it this sort of speed control works by altering the phase angle firing an SCR or thyristor, and the motor needs to be 'universal' with a commutator and brushes.

The motor is a pancake shape and no sign of brushes - have things moved on in motor design - is it some other sort of animal ?
.

It's running 24/7 so I hope that it doesn't have brushes.

[Noitoen]

 Since the fan has a constant load, the controller reduces the voltage and the speed also reduces. If you remove the load (fan blades) the speed would remain fairly constant. Assincronous motors rely on the frequency. The motor is constructed in such way to permit this.

[PekkaNF]

Power absorbtion curve of the fan is bit of a parabel vs. assyncrous motor torque curve.

Essentially these motors are sort of "starved" with the SCR control and drops out of sync - then catches sync when rpm drops 8fan load decreases) repeatedly....sort of power control. You can hear that wow-wow noise that comes out repeated out/in sync insidence.

Pekka

[awemawson]

Ah - that makes more sense. Obviously it's a very low powered motor - about 40 watt iirc. It's survived it's first 18 hours so probably will last a while - these things tend to either have infant mortalities or run a long time