Min motor speed for HVAC applications 2

[MNMaint]

Hello,
I've been told that the minimum speed I can run my HVAC fans and pumps at is 20Hz or 1/3 speed. Often cited is the need for "motor cooling". I've looked and can't seem to find manufacturer's reccomendations for motor minimum speeds. These are variable torque loads, not constant torqu loads. Does anyone know of any literature out there that discusses this?

Thanks for your help in advance!

 

[DickDV]

For centrifugal loads like fans and pumps, the load torque falls off by the square of the speed while the motor cooling falls off slightly slower than that. As a result, motor cooling issues are pretty much negligible on these loads.

However, the real question that should be asked is "at what minimum speed does the motor/load system cease doing any useful work?" And, as you cite, that point on fans is generally around 1/3 speed. In my experience, that point on centrifugal pumps is closer to 1/2 speed.

By setting your drive minimum speed at these points, you increase the resolution of the speed control since the analog-to-digital conversion is now used over a smaller range. You also eliminate the wasted energy that goes into magnetizing the motor and running it so slow that nothing useful results.

A similar argument could be made for stopping at 60hz. As long as the fan/pump can be safely run above 60hz, why alway stop there. It seems better to me to run the load up to the point where the load hp equals the available motor hp. At least, that way you get everything you've paid for!

 

[MNMaint]

Thanks a lot. That was very helpful. Good point about the min "useful" speed as well.

 

[LionelHutz]

Don't forget to try for the best work/kW used too. For example, you might get more water moved per kW of hydro used running 1 pump full speed vs 2 or 3 pumps at a low speed.

I so often read about how the power used, as in the watts, was reduced using a VFD at a slower speed. To bad this is such a red herring and doesn't mean anything. The work/kW is the number that matters.

However, it you need to run at a certain speed to achieve the control you want then you might have to live with higher energy usage.

I've seen pumps require about 90% speed before they move any water.