Energy savings with Inverter drive
Energy savings with Inverter drive
(OP)
Hello, I need an approx calculation of energy savings for a fan application. Currently the 44hp motor is run at line voltage and a damper is adjusted to get approx half the rated air flow from the fan (1/2 load). We want to install an frequency inverter to adjust the fan speed to deliver the needed cfm - eliminating the need to adjust the damper.
would moving 1/2 the cfm amount to approx 1/2 the load current to the motor? I know that the motor magnatizing current will remain constant at all loads and that amounts to approx 30% of rated current for a standard AC 3-ph ind mtr.
Mag I = 44A x .3 = 13.2A
therefore load current = 44-13.2 = 30.8A
So really, I'm asking - what does a standard fan curve relating cfm to load (hp) looks like? linear?
Thanks
CAF
would moving 1/2 the cfm amount to approx 1/2 the load current to the motor? I know that the motor magnatizing current will remain constant at all loads and that amounts to approx 30% of rated current for a standard AC 3-ph ind mtr.
Mag I = 44A x .3 = 13.2A
therefore load current = 44-13.2 = 30.8A
So really, I'm asking - what does a standard fan curve relating cfm to load (hp) looks like? linear?
Thanks
CAF





RE: Energy savings with Inverter drive
The magnetizing current should not neter into your energy savings calculation. It is a reactive current and does not consume active energy. There are some losses assiciated with it, but you can forget that for now.
The amount of energy you save with a VFD vs Damper is Head*Flow(Dmpr)-Head*Flow(VFD). So, if your head is 50 mm water and your flow is just right when you have adjusted the damper and flow is just right and head 30 mm water with the VFD, you have saved 40 % energy.
There are so called affinity laws and there are fan curves. Your ventilation supplier should have documentation on this. The curves are not linear. Not at all. Mostly quadratic, but there are deviations. More cfm means more head. And head usually goes up as cfm squared, if you run variable speed fans.
BTW, you cannot add magnetizing current to active current the way you do it. Since they are perpendicular, you shall use SQRT(Active^2 + Magnet^2) for total current.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Energy savings with Inverter drive
However, if you are only reducing the speed by 1/2 and not looking at actually varying the speed, a change of pulley ratio or motor speed (2 speed motor) would bring the same energy savings but a faster return on investment. and this is coming from a guy whose company manufacturers VFDs...prepare to be shot at dawn by the salesman firing squad!
RE: Energy savings with Inverter drive
RE: Energy savings with Inverter drive
RE: Energy savings with Inverter drive
These savings are often greatly overstated by salesmen, but there are real savings to be had in many situations.
RE: Energy savings with Inverter drive
RE: Energy savings with Inverter drive
RE: Energy savings with Inverter drive
CAF
RE: Energy savings with Inverter drive
RE: Energy savings with Inverter drive
PF on a "standard" VFD will not be 1.0. I think you are looking at the fundamental PF and not the total PF. The total PF will be something between 0.6 and 0.92, depending on the topology of the VFD. Harmonic distortion will predominantly determine the total PF; the >the THD the lower the PF. Not all VFD's are the same.
RE: Energy savings with Inverter drive
They'll probably catch up soon however. The net effect on their equipment is the same.