Compressor start current

ChrisJ

I'm looking for typical start currents for a 3 ton scroll just out of curiosity.

My 2 stage Copeland Ultratech claims 83A LRA.

mattmia2

For induction motors the typical LRA is 5x-10x the RLA.

ChrisJ

mattmia2 said:

For induction motors the typical LRA is 5x-10x the RLA.

RLA at least on mine, is 15.3A

But what is the expected real life current during starting?

pecmsg

Starting current for any electric motor can be several hundred amps. A peak hold meter will show it. 

generally LRA plus 

JUGHNE

Also dependent upon pressure in system, temp of compressor, voltage drop while starting, and condition of run capacitor or start components.

Solid_Fuel_Man

It's a LOT. I agree with all of the above. 

This is why a standby generator has a hard time starting inductive (motor) loads. The transformer on the pole by your house may only be 10kva (10,000 volt amps) but can pass many times that rating for short times starting motors. The generator on the other hand will only produce its rating and that's it. 

Why do you ask @ChrisJ

ChrisJ

Solid_Fuel_Man said:

It's a LOT. I agree with all of the above. 

This is why a standby generator has a hard time starting inductive (motor) loads. The transformer on the pole by your house may only be 10kva (10,000 volt amps) but can pass many times that rating for short times starting motors. The generator on the other hand will only produce its rating and that's it. 

Why do you ask @ChrisJ

After working on it on and off for two years I finally got my generator together and home for testing.

It's pieced together from a home standby generator that was rated 10KW on LPG and 10-20% less on natural gas. I'm running it on natural gas. The actual alternator says it's rated for 12KW with a 10% overload for 2 hours out of 24 but the 570CC vtwin can't run it at full power.

To my surprise, it managed to start my 3 ton air conditioner although it did sound like a person getting punched in the gut. It definitely went WOT for a second or two.

But it did it, and didn't seem to cause any issues. I was also surprised my ECM blower ran fine on it as well as my ICM fan controller on the condenser fan.

Solid_Fuel_Man

I thought you might be dealing with generator stuff, which is why I went down the RABBIT trail. 

ECM motors (that is saying motors twice ya know) are generally not too picky about power that is a bit dirty. 

You must be super luxurious having A/C on generator power! 😁

In my area a 10kw generator will run pretty much everything in a home, but we dont have much residential aircon. A generator will run a mini-split quite easily though (ECM) again. 

Pat_Collins94

For comparison, I periodically work on a heavy direct drive fan on a 3HP 230V motor. The starting current exceeds 2000A (my meter maxes out at 2000).

If your compressor has any differential pressure across it, you may be in a similar circumstance.

I’ve had previous success converting std compressor starting system to “Capacitor Start, Capacitor Run” in situations where the high starting current lasts too long.

Good Luck
Pat

ChrisJ

Pat_Collins94 said:

For comparison, I periodically work on a heavy direct drive fan on a 3HP 230V motor. The starting current exceeds 2000A (my meter maxes out at 2000). If your compressor has any differential pressure across it, you may be in a similar circumstance. I’ve had previous success converting std compressor starting system to “Capacitor Start, Capacitor Run” in situations where the high starting current lasts too long. Good Luck Pat

I'm confused, how can a motor exceed it's LRA spec?  I mean, that's what it draws literally stopped.

JUGHNE

Time, it all happens within the fraction of a second.
Other factors come into play within that fraction if the motor is turning, perhaps the back EMF contributes to the very high momentary current draw.

If locked, compressor seized up, it might be a different animal.

EBEBRATT-Ed

Capacitor start-capacitor run AKA a hard start kit. One would probably make it start easier

ChrisJ

EBEBRATT-Ed said:

Capacitor start-capacitor run AKA a hard start kit. One would probably make it start easier

I had one on it right after I installed the unit and it seemed to like to "bang" on startup. Never figured out why, if it was due to it being a 2 stage compressor etc but I pulled it out and now my ICM fan controller occupies the space.