How to Read HVAC Chiller Nameplate and Model in Electrical Analysis software for short circuit calculations

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OCPD

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I am working on short circuit calculations for a system. There is an HVAC unit (nameplate attached). I did verify (Unit Power Supply) the cable and upstream protective device do meet the minimum circuit ampacity and max circuit breaker size, respectively. I have the following questions about the nameplate and how should it be modeled in the study software :

1. I understand there are motors for compressors. Will this HVAC unit be modeled as an induction motor in study software?[/li]

2. How to determine the HP/kW rating of this unit from the nameplate?[/li]

3. Nameplate shows TWO systems on the nameplate what does it imply? Is one backup to another? [/li]
 
Looks to me like in total you have four compressors and four fans. I doubt they are backups to each other probably just how much cooling capacity is required and it is more cost effective to supply for compressors and four fans rather than one large compressor and one large fan.
 
petersonra said:
Looks to me like in total you have four compressors and four fans. I doubt they are backups to each other probably just how much cooling capacity is required and it is more cost effective to supply for compressors and four fans rather than one large compressor and one large fan.
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So, two systems, each with two compressors with 55.8 amperes at 200V and 2 fans with 7.6A. A total of 4 compressors and 4 fan motors? Will
 
Looks like a two-stage system. Starts on first stage then goes to 2nd stage when demand increases.
 
Why are you worried about fault contribution from this small of a unit? One rule of thumb is that a motor's fault contribution is close to its LRA.
 
OCPD said:
View attachment 2561148

I am working on short circuit calculations for a system. There is an HVAC unit (nameplate attached). I did verify (Unit Power Supply) the cable and upstream protective device do meet the minimum circuit ampacity and max circuit breaker size, respectively. I have the following questions about the nameplate and how should it be modeled in the study software :

1. I understand there are motors for compressors. Will this HVAC unit be modeled as an induction motor in study software?[/li]

2. How to determine the HP/kW rating of this unit from the nameplate?[/li]

3. Nameplate shows TWO systems on the nameplate what does it imply? Is one backup to another? [/li]
Click to expand...

1. Yes.
2. The controls are resistive so model or ignore, your choice. And they are separate. The motors are FLA 7.6 A at 200 V nominal, 3 phase. So that’s 7.6 x 200 x 1.732 / 1000 = 2.6 kW / 0.746 = 3.5 HP.
3. It implies 4 motors total.

This is where you need to take training on power system analysis and not blindly just punch things into some software program. You need to know and understand the rules contained in the ANSI and IEC standards. You do NOT just blindly put in every motor. For short circuit considerations we are looking at assymetrical and symmetrical short circuit ratings. IEC and ANSI are different but produce very similar results (see IEEE commercial power systems analysis standard). The problem is transient reactance. At the moment that we lose power, the motor reactance DC transient since a reactor does not allow current to go instantly to zero. From a modeling point of view it becomes a DC source with a time constant representing an LR circuit. Both standards give guidance on the values to use but the software calculates this. Using the ANSI or IEC methods you do not model every motor. The transient reactance of small motors (25 HP or less) is so short that it doesn’t last 3 cycles so you ignore them. ADD together the HP of all motors from 25 to 150 HP on a bus and model as one motor (reactance). Everything larger gets modeled individually.

So for this load I would model it as nominally 10.4 kW and model the fuses based on their actual name plate rating (from your inspection). You don’t need to model anything at all but this will give you the ability to verify the 65 kA SCCR. Or you can ignore it completely. Once you model even a small amount of cabling or even bus bar there is no way you will reach 65 kA unless you have say a 1000+ kVA 208/120 transformer and even then I doubt it will be that high at the load.
 
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