ventilation said:
I have a question concerning the electric motor that drives a fan. A fan can experience different loads depending on the pressure build-up in the ducting behind the fan and depending on the air density. Do fans keep their revolutions per minute constant if the pressure build-up or air density is varied? Or do "speed controls" exist that keep the torque constant which causes the revolutions per minute to decrease if for example the air density increases?
First thing, not all fans are the same, whether or not they maintain their rotating speed would heavily depend on what type of electric machine, what is controlling the machine etc etc. I will assume since you are talking about range hoods, its a single phase induction machine squirrel cage style blower, unlikely they would use a universal motor here due to brush life issues.
I would say generally there is no speed control happening in most range fans. But... Most line connected fans are running single phase induction machines, these are asynchronous, that is they are not locked exactly to the electrical frequency. But their output torque is linked to the difference between their rotating speed and the line frequency, this is known as slip. So an unloaded 2 pole induction machine running from 60Hz will rotate at very slightly below 3600rpm. Since an induction machine builds the rotor magnetic field by induced currents in the rotor generated by the difference between rotor rotation speed and the rotating field in the stator (locked at the driving frequency, ie 60Hz in this case), as load is applied to the fan it must slow a little bit to generate the required field to produce torque.
So in short a single phase induction machine is not exactly constant speed, but it does not change speed by that much as it is loaded (unless overloaded!), slip is in the range of 1% or so for large machines, more like 5-10% for smaller machines.
Eg this blower motor (note loaded rpm is 3400rpm, ie ~6% slip):
So as the fan is loaded, the machine will slow a little, but not much.
Even if machine speed is constant, air density, pressure, humidity etc will have a large impact on flow rates. Which is why any standard or test specification worth talking about will list the environmental conditions required to repeat the test, eg ambient temp, humidity, elevation etc etc.