High starting currents from A/C compressor.

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11bgrunt

11bgrunt

Pragmatist
Location
TEXAS
Occupation
Electric Utility Reliability Coordinator
I consider locked rotor current, LRA on the data plate, the same as the expected current draw of that motor at startup.
The customer complaint is the dimming of the lights when the A/C starts. The model number shows a 4.2 ton Trane compressor motor - dataplate reads 26.4 RLA & 134 LRA.
The starting current has been as high as 230 amps and always over 200.
The 70mfd capacitor checked good. Recently an A/C tech gave the 2015 system a clean bill of health and said there are no issues.
I had a recorder set for 5 days, 370 high current starts.
Electric service is 200A with a 100' of underground from the transformer pole to the house. The POCO transformer serving two houses is now 50kVA.

Is this compressor failing? What can cause the motor to draw such high start currents. When I talk about motor inrush, my rule of thumb for a conventional motor is there can be start currents 6-10 times the run current.
There was a time this unit did not dim the lights in the house, so the condition has not been since day 1.
 
11bgrunt said:
I consider locked rotor current, LRA on the data plate, the same as the expected current draw of that motor at startup.
The customer complaint is the dimming of the lights when the A/C starts. The model number shows a 4.2 ton Trane compressor motor - dataplate reads 26.4 RLA & 134 LRA.
The starting current has been as high as 230 amps and always over 200.
The 70mfd capacitor checked good. Recently an A/C tech gave the 2015 system a clean bill of health and said there are no issues.
I had a recorder set for 5 days, 370 high current starts.
Electric service is 200A with a 100' of underground from the transformer pole to the house. The POCO transformer serving two houses is now 50kVA.

Is this compressor failing? What can cause the motor to draw such high start currents. When I talk about motor inrush, my rule of thumb for a conventional motor is there can be start currents 6-10 times the run current.
There was a time this unit did not dim the lights in the house, so the condition has not been since day 1.
Click to expand...

What are you using to mesure the inrush current? Is it known to be accurate and reliable?

What have you done to troubleshoot so far?
Let’s start with the basic questions first then go from there…

10 times 26.4 is 264 amps. You are still well within your rule of thumb. What did it used to be when the system was new?

Do you have copper or aluminum wiring?

Have you checked all connections along the line to make sure there is nothing loose?

Do you have a hard start kit? Has it been tested and verified good?
 
What are you using to mesure the inrush current? Is it known to be accurate and reliable?
POCO used a PMI recorder in the meter socket for 5 days.
I had a HT PQA at the 100A subpanel serving the lighting and a Fluke 345 to measure inrush on the compressor wires at the breaker.

What have you done to troubleshoot so far?
Let’s start with the basic questions first then go from there…

10 times 26.4 is 264 amps. You are still well within your rule of thumb. What did it used to be when the system was new?
There is no PQ history before the complaint.

Do you have copper or aluminum wiring? 4/0 AL

Have you checked all connections along the line to make sure there is nothing loose? Thermal imaging at every panel.

Do you have a hard start kit? Has it been tested and verified good? A/C tech called all good and no hard start needed.

Voltage starts at 242-246
LEDs have been in the house since new.
Voltage drop is expected, the unanswered question; the VD at startup was not noticed and now it is the complaint.
Sags are definitely tied to compressor starts.
Does anyone else say the dataplate LRA is the same as expected starting current?
 
LRA always meant Locked Rotor Amps to me. It's what I expected to see when the compressor was just that, Locked. The current may have been that high at start, but not for long. Without confirming history, we don't know either. Something changed if the customer never noticed the VD before.

What does the voltage drop to at the SE as it starts? IF it's dropping 15 volts instead of 7, look for compromised connections or underground.
Put your own non motor loads online and measure the VD.

It wouldn't be the first time an AC tech has missed something.
 
A couple of observations:

1) Something has changed. The lights did not previously dim on startup, now they do.
2) I would check the compressor running load amps. If within spec, it’s likely OK.
3) It’s likely that a connection somewhere has degraded. Possibly even on the POCO side.
 
If you really want to pinpoint voltage drop, add a long wire to one of your voltmeter leads and measure the voltage from one service conductor to the same "phase" at the compressor during a start. Then do the same on the other phase.

If the drops are alike, I'd attribute it to the load. If they're dissimilar, the greater drop indicates the line with a poor connection. You can narrow it down by doing the same thing among any intermediate access points, like across a breaker.
 
It's not clear what the usual voltage is and what it drops to when the comp. starts (and where it was measured), but it sounds like the entire service voltage is sagging. In that case, don't discount the possibility that something changed on the PoCo side of things. But.....

Have you asked the Trane what to expect for starting current? If they say "Oh, 150 amps" and you're reading 230, that compressor has a problem.
 
11bgrunt said:
There was a time this unit did not dim the lights in the house, so the condition has not been since day 1.
Click to expand...

Since you know that the starting current is 200-230A you need to start from there. You say that, given the service provided, you would expect that amount of current to cause a sag that would produce the dimming noted. So you consider that normal and there is no problem with the service or wiring.

So the only thing left is the A/C system and compressor which I suggest is out of your realm of expertise. It's possible that the compressor is failing or the charge is not correct, neither of which I assume you are equipped to deal with. At this point I would have an HVAC guy look at it to give you an answer.

We could speculate here as a bunch of electricians but that would be useful as an HVAC guy giving us advice on how to properly size a service.

-Hal
 
High starting currents can be the result of starting under load.

Hermetically-sealed refrigeration compressors don't usually incorporate an unloader; they rely on the thermostat to give the system time for the small amount of leakage in the valves to equalize the pressures. Short-cycling (turning on again after too little time to equalize) can cause hard starting and high starting currents, though I wouldn't expect them to exceed LRA.

If the same high starting current persists after leaving it it off for ten minutes, or after seeing (on the service gauges) that the pressures have equalized, or manually equalizing them, this is a problem that you do not have.
 
hbiss said:
Since you know that the starting current is 200-230A you need to start from there. You say that, given the service provided, you would expect that amount of current to cause a sag that would produce the dimming noted. So you consider that normal and there is no problem with the service or wiring.

So the only thing left is the A/C system and compressor which I suggest is out of your realm of expertise. It's possible that the compressor is failing or the charge is not correct, neither of which I assume you are equipped to deal with. At this point I would have an HVAC guy look at it to give you an answer.

We could speculate here as a bunch of electricians but that would be useful as an HVAC guy giving us advice on how to properly size a service.

-Hal
Click to expand...

OP stated in post #6 that the HVAC tech gave the unit a clean bill of health.
 
A hard start kit likely increases the starting surge current, but also likely makes it last for shorter duration.

Either way LED lights, especially ones with cheap drivers are going to be most susceptible to about any change in voltage.
 
You can add a soft start for the compressor. They were marketed to permit a smaller generator to be used to backup the dwelling unit loads, including the AC compressor. There is a Mike Holt video on this showing the drop in starting current and the elimination of dimming lights. No idea of there cost.
 
retirede said:
OP stated in post #6 that the HVAC tech gave the unit a clean bill of health.
Click to expand...

But did he even check the starting current? I absolutely doubt that, normally they don't even check the running current unless they suspect a problem. And how long ago was the tech there and was it even having that problem then?

-Hal
 
As the electrician for an electrical+ hvac shop, I know that "hard start kits" INCREASE the starting current of the compressor. The "better" ( tho much more costly part would be this one:
www.zillerelectric.com

Sure Start SS1B16-32SN (16-32 RLA) – Ziller Electric

230V, 1 phase, 60/50Hz Fits motors and A/C scroll compressors between 16-32 RLA (Rated Load Amps) It is mandatory that the correct size Sure Start model be used Incorrect sizing will result in damage to the Sure Start and/or Air Conditioning unit View images to see a typical nameplate placard...
www.zillerelectric.com www.zillerelectric.com

These are mainly sold for generator installations, but the end result is a lower starting current.

Hope this helps,


Howard
 
g-and-h_electric said:
As the electrician for an electrical+ hvac shop, I know that "hard start kits" INCREASE the starting current of the compressor. The "better" ( tho much more costly part would be this one:
Click to expand...

But he doesn't have a starting problem. It starts fine except for what it draws when it does which is an abnormal situation. I wouldn't go putting a band-aid on something like this. Again, get an HVAC guy to check it out.

-Hal
 
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