Finding complex impedance of AC unit...

[lakevillethor]

Fellow engineers,

I need some help calculating the impedance of an Air Conditioning unit. I have the RLA (run load amps), FLA (full load amps) and LRA (locked rotor amps). It is a 240 unit. Are these ratings complex? Moreover, are these ratings given in sqrt (R^2 + X^2) or is it just r? any help would be apprecaited. Thanks.
-Andrew Thoresen

 

[60hzhmmm]

You still have not confirmed your screwy values. Finding the complex impedence using just the RLA or FLA or any other kind of name plate amps without knowing at least the power factor is dubious at best and with your bizar values.....

Jabartos those are some sweet links there... thanks!

 

[jghrist]

lakevillethor,

You also have not said what you are using the complex impedances for. The RLA or FLA values are currents during normal operations. If you are going to determine the impedance for use in load flows, then you are using the wrong load model. Motors are not a constant impedance load, they are a constant power load. Well, not exactly, but that is how they should be modelled in a load flow.

 

[lakevillethor]

jghrist,

Can you elaborate a little more on what you were talking about, por favor. Thank you.
-Andrew

 

[jghrist]

In load flow studies, that is calculations of steady-state currents and voltages at different parts of a distribution system, three types of loads are normally modelled.

Loads such as resistive heating and lighting or capacitors are most closely modelled as constant impedances. The power is proportional to the square of the voltage and the current is proportional to the voltage.

Motors are normally modelled as having constant power or constant kVA, where the current is inversely proportional to the voltage. This is a simplification, but is usually close enough for load flows (as opposed to fault studies or stability studies).

Studies have shown that most distribution systems have a mix of loads where the power is proportional to the voltage (a 1% increase in voltage will produce a 1% increase in power) and the current is constant.

 

[jbartos]

Suggestion: To evaluate motor parameters, the nameplate rated values should be used, e.g. FLA, etc. If other than nameplate values are sought, then necessary data including curves should be obtained from the manufacturer, e.g. efficiency curve, torque-speed curve, power factor curve, etc.

I just would like to send thanks to manufacturers who support their product with suitable theoretical background and practical data or application notes that can be used by the customer and others, e.g. consultants, engineers, designers, students, or just plain enthusiasts/hobbyists.