VFD: How does the load change with the frequency.

[Keri_WW]

If you have a VFD and a known load at a known Hz, how can you determine what the load will be at another Hz value?

For example a 50kW exhaust fan at 60 Hz will be controlled at 50 Hz. Is there even any relationship between the Hz and power?

Thanks,
Keri

 

[rcwilson]

For an ideal centrifugal fans, power required varies with the cube of the speed.

50/60 Hz =0.833. (0.833)^3 = 0.579.

At 50 Hz the fan power will be around 58%, call it 60% of the power at 60 Hz.

For centrifugal fans and pumps, torque varies with the square of the speed.
Since horsepower = k x torque x speed, horsepower varies with the cube of the speed.

Losses and specific fan/pump characteristics will cause some variations.

 

[Jraef]

And if it is not a centrifugal fan or pump, i.e. a "Constant Torque" load, the power requirement varies directly as the speed reduction. So a 60Hz designed machine will require 83.3% of the power (HP or kW) when operated at 50Hz.

 

[Besoeker]

For an ideal centrifugal fans, power required varies with the cube of the speed.

50/60 Hz =0.833. (0.833)^3 = 0.579.

At 50 Hz the fan power will be around 58%, call it 60% of the power at 60 Hz.

For centrifugal fans and pumps, torque varies with the square of the speed.
Since horsepower = k x torque x speed, horsepower varies with the cube of the speed.

Losses and specific fan/pump characteristics will cause some variations.

I agree with that.
Maybe a word of caution for the OP is that the temperature of the exhaust air/gas can affect the loading quite a bit. Cooler gasses are more dense and that would load the fan more.
We have some fairly big fan drives (over 2MW) in cement mills and temperature makes a significant difference to loading.

 

[ron]

And if it is not a centrifugal fan or pump, i.e. a "Constant Torque" load, the power requirement varies directly as the speed reduction. So a 60Hz designed machine will require 83.3% of the power (HP or kW) when operated at 50Hz.

Jraef,
What do you think of this quote:
Variable Torque. Centrifugal pumps, fans,blowers, and
compressors are good examples where torque varies as the square of the
change in speed and the horsepower (work) varies as the cube of the change in
speed.

from http://www.yaskawa.com/site/dmdrive.nsf/link2/MNEN-5JFQNW/$file/AR.AFD.02.pdf

 

[Jraef]

Jraef,
What do you think of this quote:
Variable Torque. Centrifugal pumps, fans,blowers, and
compressors are good examples where torque varies as the square of the
change in speed and the horsepower (work) varies as the cube of the change in
speed.

from http://www.yaskawa.com/site/dmdrive.nsf/link2/MNEN-5JFQNW/$file/AR.AFD.02.pdf

I have seen this exact sentence pulled out several times in duiscussions and taken out of context.

It's fine, as long as you understand that the word centrifugal is meant to be in front of ALL of those examples, which in my opinion makes it a little bit poorly worded. The problem is there are pumps, blowers and compressors that are NOT centrifugal loads (not so much with "fans"). For example a squirrel cage blower is a centrifugal load, but a "Roots" type lobed blower, as is very common in WWTPs, is not, it's a Constant Torque load. Centrifugal compressors, such as scroll type, are VT: Reciprocating and Screw compressors, CT. Positive Displacement pumps are CT etc. etc. This is mentioned in the second paragraph of that paper, but for some reason many people don't seem to notice it.