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

 

[busbar]

Could you post the application to verify that you are getting valid answers? Load power factor is needed to figure a complex impedance.

 

[jghrist]

Typical starting pf (for use with LRA) is 20% for motors under 1000 hp according to IEEE Std 399-1990, Industrial and Commercial Power Systems Analysis (Brown Book).

 

[rbulsara]

You need to calrify further as to what you are trying to do with 'impedance' values?

There are different types (values) of impedances depending up on the state of the motor such as subtransient X", transient X' and steady state X. Typical values of these are normally known or provided by the manufacture and I would guess indicated in some IEC or NEMA manufacturing standards.

A subtransient reactance, in per unit values, would be FLA/LRA and typical X/R value for small (5 HP or so) motors could be 10 or so. This is the only relvant impedance data I have had to know for small motors.

You can make the job on hand as complicated as you want by throwing in other relationship as stator R values and Rotor R values, most of which may be irrelavnt to most practical application of a A/C motor

I am not pretending to know all these formulas. (I never needed to know them).

 

[jOmega]

Andrew :

The impedance under what operating condition ..

-- starting ?
-- running at steady state ?

and..

single or three phase ?

tnx

 

[lakevillethor]

The grim reality is that the people I work with never understand what I am asking them - perhaps I am bad at articulating what I am trying to say. Okay, I appreciate all of your help. Here's the application. I am an electrical engineer who works in forensics; I investigate electrical fires. I have only been in this industry for about a year now so I don't understand what everything means. What I basically want to know now is what RLA, and FLA actually mean. I know that they mean "Run load amps" and "Full load amps" respectively but I don't know what, in practice, that actually means. can anyone help me out?

 

[lakevillethor]

single-phase - 240 V. Here's the stats on it:

Max fuse or HACR type breaker amps...60
LRA...130; FLA...1.7; RLA...27.8;

 

[lakevillethor]

Lastly, in application, this AC unit is thought to have triggered an electrical event which started the fire. There was a screw through a wire nut. The screw did not penetrate all of the insulation in the wire nut when it was installed. Instead, a year later, when the main was switched ON, I think teh voltage peaked (Ldi/dt) and broke down the insulation. Once teh insulation was broke down, teh conduit was energized and you had current flow through the grounding conductors. One of the grounding conductors started the fire. Anyway, it all started at the AC unit so I am trying to figure it out. Any help would be appreciated.
-AT

 

[PwrCntEE]

NEC 2002, 440.2 definitions may help

 

[lakevillethor]

Thanks for the heads up on the NEC.

 

[peebee]

It sounds like you're working as an "expert" witness in a lawsuit -- are you really an expert? Maybe you should hire someone who already understands these things?

 

[lakevillethor]

Bro, there is no lawsuit pending here and I am doing the grunt work for my boss who would testify.

 

[peebee]

Cool. Just checking.

 

[advidana]

My expenience with ac units that caught fire is that the compressor motor has an arcing fault to the frame. These units usually don't have a ground wires so the fault follows the metal conduits. The ac unit has flex conduit with a plastic cover since it is out door flex conduit. This plastic cover is not fire proof so the heat at the connectors of the conduits get hot from the arcing fault and caught the plastic cover of the flex conduit on fire. Then everything else caughtes fire. It can be ever quick, about five minutes.

 

[jOmega]

Andrew:

You said:

"LRA...130; FLA...1.7; RLA...27.8; "


Looks like the FLA is a bit light..... in fact, quite a bit lite... Typo ?



Got a better value ..for us ?

Thanks

Also, if I understand correctly, the site of the failure was the screw that penetrated the wire nut.
Was this one of those yellow wire nuts? with a thin plastic outer shell (as opposed to those rigid thicker types with a hard plastic shell)...

I suspect that over time, and with temperature, the insulation properties of the the plastic on the wire nut deteriorated and at the opportune time... broke down allow a path of conduction from the condutor to the screw. ..

If you still have the wire nut, and if it wasn't damaged by the fire.... you could recommend that it, along with an new wire nut of the same type, be sent to a lab and have the insulation properties of each determined.

That would be a nice piece of investigative analysis, and look good in the report.

 

[60hzhmmm]

Lakevillethor, I will attempt to answer your question as I understand it...

RLA Running Load Amperage.
This is the typical Amps required to just run the unit, the expected current.

FLA Full Load Amperage.
This is the maximum sustainable current draw. The circuit must be able to handle this all day on a hot day. Typically the max amps and hot day will coincide. The system breaker and wiring must be rated at this or higher.

LRA Locked Rotor Amperage
This is the current drawn when the contacts first close. A refrigeration compressor is just about the most brutal thing you'll come across with respect to starting inrush current. Often it is an inertial load, (crankshaft and piston), the piston almost always at bottom dead center, often with about 400PSI on top of it. This makes the motor work very, very hard to get things moving. Clearly one of the 6-10 times running current to get going motors you always hear about. This is part of the reason they actually list the LRA for refrigeration compressors.

In practice before electrical circuit breakers were used motor starter units had resistors hooked up with the fuses. The actual values of these resistors were semi-critical for the system to allow starting but also protect the circuit from running overloaded continuously. The LRA value was used for calculating the aforementioned resistors. Now days you generally look at the FLA bump up a few amps and then wire the system for that. If you have excessively long runs you need to start bumping up the wire sizes relatively quickly or the motor will have a shortened life as the LRA will be around roasting things much longer. One point is that the circuit breaker used MUST be rated for refrigeration. They look the same cost 30% more and work right. Often I see standard breakers in that service. They heat up and often cause nuisance tripping, lost produce, hair pulling.

Refrigeration is the most common large, often serious electrical load in business. They run long and hot. They WILL demonstrate this wherever the circuitry is sub par. They will melt things. They will explode things. You will find whole subpanels utterly destroyed, unsalvageable. You will find breakers backs burnt off. A poor wire nut connection WILL toast.

Is was this what you were looking for?

 

[jbartos]

Suggestion: The following references will be useful for your work:
1. IEEE Std 100 "Dictionary"

http://www.ieee.org

2. NFPA 70 National Electrical Code

http://www.nfpa.org

Various electrical glossaries available on Web, e.g.

http://www.iprocessmart.com/leeson/leeson_glossary_of_terms.htm

Motors basics also available on Web, e.g.

http://www.reliance.com/mtr/mtrthrmn.htm

http://www.ab.com/manuals/dr/Motors/1329M-UM001B-EN-P.pdf

http://www.sesco-ge.com/docs/techinfo.pdf

http://www.motordoc.net/downloads/Motor Management.pdf

http://www.bh.com/bookscat/samples/0750673516/0750673516.pdf

http://www.aosmithmotors.com/pdf/brochures/bulletin3100/Acdc.pdf

etc.

 

[lakevillethor]

You said "FLA Full Load Amperage.
This is the maximum sustainable current draw. The circuit must be able to handle this all day on a hot day. Typically the max amps and hot day will coincide. The system breaker and wiring must be rated at this or higher."

I don't understand how the FLA would be lower than the RLA then. Are you sure your defintions are correct. I appreciate your help.
-AT

 

[60hzhmmm]

Jbartos, Nice links!

Lakevillethor, hmmmm, Let me try to clarify this a little more.

The LRA is the expected full load amperage for the motor itself. The RLA is sometimes also called the Rated Load Aperage. If a motor has extra current demands they will be reflected in the RLA. That is if the motor has "extra torque" ability it will possibly have a higher RLA then FLA... Crazy I agree. Sometimes the RLA will reflect included loads not included in the FLA such as related system fans. Regardless, the two values are usually VERY CLOSE TOGETHER. Differing by a few percent. They are so close together that usually I find they only ever include one OR the other not both. The fact that your stated values are different by what? 1600% makes me wonder about the stated numbers...ALOT!

The RLA is the value the NEC directs one to use when selecting the circuit protection using the tables in the NEC "article440" stuff if I recall correctly. This is probably because RLA can include ALL of the current a "system" uses as apposed to the FLA "single motor". To be honest there is a general confusion in the refregeration service community about this RLA-FLA thing. The standard result is that the guy in the field looks for these values, finds one or the other and uses it. If both are found the highest is used. The wire size is chosen to be one size larger and the breaker used is a refrigeration rated breaker "the next size up", too. If the compressor being installed is 208 or 230 or 240 then the wire size is bumped up yet another size, because those motors have it rougher than the 460/480 ones when it comes to the start.

Keep in mind the values stated be it LRA,RLA,FLA are stated running from a stiff source provided at the highest value seen on the name plate. This means if the motor is rated say 240VAC and is run at 208VAC the stated currents will be lower then reality.... Also notice in most places the voltage is not what it is supposed to be anyway. It's usually low by some amount and of course that's at 3pm on the hottest, highest demand days.

 

[lakevillethor]

I appreciate your help. Then here's my new question, do I use the RLA to find the complex impedence of the device? Moreover, should I use the 27.8 figure corresponding to the RLA figure and calculate the Z value? If so, that would give me a Z value of 8.633? I think you see where I am going with this. Thanks again for your help.
-AT