Fan speed at different loads

Main Question or Discussion Point

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?
 

Answers and Replies

russ_watters
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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?
If the pressure in the system varies and compensation is needed, fans use speed controllers and closed loop control to vary fan rpm to compensate. Rpm is a function of power frequency, but just holding it constant against changing pressure/torque would not keep airflow constant.
 
I am specifically interested in the case of range hoods that exhaust cooking fumes. Let's say the speed setting on the display panel is fixed. Can the revolutions per minute vary depending on the air density or static pressure?
 
russ_watters
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I am specifically interested in the case of range hoods that exhaust cooking fumes. Let's say the speed setting on the display panel is fixed. Can the revolutions per minute vary depending on the air density or static pressure?
Well, I said no, but that's really only true of 3 phase induction motors. Single phase motor rpm varies with voltage and load.

Do you know what the electrical service is to the fan? What is the "display panel" reading/what kind of speed controller is it?

And why is the static pressure varying?

Also, atmospheric pressure/density variation is typically ignored since it is only a few percent unless there is a strong storm.
 
Well, I said no, but that's really only true of 3 phase induction motors. Single phase motor rpm varies with voltage and load.

Do you know what the electrical service is to the fan? What is the "display panel" reading/what kind of speed controller is it?

And why is the static pressure varying?

Also, atmospheric pressure/density variation is typically ignored since it is only a few percent unless there is a strong storm.
I'm working on a test method for the performance rating of range hoods. For the same device the test should give the same result in any laboratory. The laboratories can be located at different altitudes and thus they test under different air densities. One part of the test is to measure the fan curve (flow rate vs pressure). The test is done at the highest speed setting by selecting the highest level on the display panel. Usually, the number of the level does not have any units but is just representative for the "intensity" of the speed setting. I was told (and read online) that a higher air density will lead to a proportionate higher pressure difference and to a proportionate higher power input. The flow rate is supposed to remain unaffected. An example of a fan curve for this scenario is shown below. Now the laboratory should simply measure the air density during the test and adjust their measured fan curve to a fan curve that would have been measured during reference air density. For example, if the air density during the test was twice as high as the reference air density, then the pressure of every measured point on the fan curve is divided by two to get the reference fan curve. The calculated reference fan curve is supposed to be the same no matter where the measurement was done.
However, I wondered if there could be range hoods on the market where the electric motor does not simply double the power input in order to double the pressure if the air density is doubled. If there were electric motors that decrease their rpm because of the heavier load, the suggested calculation to "how the fan curve would look like during reference air density" would be wrong.
fan characteristics.png
 
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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):

1573587160762.png


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.
 
Baluncore
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I am specifically interested in the case of range hoods that exhaust cooking fumes. Let's say the speed setting on the display panel is fixed. Can the revolutions per minute vary depending on the air density or static pressure?
Have you noticed that fan speed often increases when the flow is completely blocked. That is because power is flow multiplied by pressure. Without flow there is no work being done pumping air, so there is less load. Try momentarily blocking flow completely with a sheet of paper to the muffin fan on a computer power supply, on a vacuum cleaner, or on your extraction fan at different settings. You should hear the RPM increase.
 
Thank you everyone for your help. My question has been answered :smile:
 

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