VFD Blower Efficiency

You can hook up a clampmeter to the motor and check the current. I've watched the current consumption of motors driven by variable-frequency-drives and the current does go down with speed. However, at a certain low speed (the value of which escapes me for the moment), the current will start to rise. Go lower than that and the current will rise quite swiftly and eventually trip the overload.

Further, if you drive the motor at a low speed (even if the current hasn't risen yet), the motor will start to heat up due to the reduced air flow from the cooling fan. If your blower motor has a separate motor for cooling, this should not be a problem.

I will take a look at the current response - thank you!

The answer, I believe, lies on a plot of current and torque against speed. As the speed reduces to somewhere below synchronous speed the 'torque output' curve and the 'load torque' curve will cross over. The current then rises to compensate. The exact position of this relates to the motor and machine characteristic. As mentioned elsewhere, motor cooling can often be a problem long before this position is reached.

I know of a waste water treatment plant that runs three roots type blowers off VFD's and the lower frequency limit is around 35-40Hz. The system is difficult to control and at high air loads produces noise typical of a 'beat' spectrum and is very hard to attenuate.

The shaft power consumed by any fluid mover is the product of the volumetric flowrate times the increase in pressure, all expressed in consistent units. Does that help?

Yes that helps - Thank you

hello azwader,

all you have to do is to find out what is your compressed air consumption and this can happen only if you install an analyser that will be able to store the data of speed (blowers) pressure of the air main for at least a week. then you will be able to find out the upper and lower limits off your consumption and as well the duration of them.

This will help you to understand clearly if the sizing of your blower was correct.

To be honnest this that you are trying to find is the specific energy consumption of your system (kw/m3/min).

Every variable speed blower or compressor have its own curve ( you can ask it from the manufacturer ) x axxis specific energy consumption and y axxis m3/min.

all these will help you to understand in which are of HZ or rpm you blower is efficient and where not. Usually its efficient from 50%-80% of rpm. if the fluctuations of your compressed air consumption (due to the recording that you have to do ) are high then it should be wise to split your system , meaning that it would be better you to use theoritically an 120 hp vfd blower and an 80 hp constant speed blower.

with this splitting method the air main pressure will e optimizes by a controller that will start first with the vfd blower and when is needed will add the constant speed one. As you understand your efficiency will go higher as you will use less hp when th consumption is low and the needed when its high.

But always be carefull with the choise of the blower size!!!!!! The constant speed blower free air delivery MUST be lower than the difference between lower and higher f.a.d of the vfd machine.

e.g constant speed produses 500m3/h and a vfd 100m3/h to 700m3/h.

700-600 > 500.

ONLY THIS WAY YOU WILL AVOID THE CONTROL GAP!!!!

Hopefully i helped you enough.

If you want to ask some more about blowers of compressed air i'll be in touch

700-100> 500

so sorry about the language mistakes.

Nice Job Thnak you!