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Old 08-23-2012, 06:40 PM   #1
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Default Start Current

Hi Guys

Can some one please explain, How to figure out the starting current on a
Motor ore a A/C Compressor?
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Old 08-23-2012, 06:46 PM   #2
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If the locked rotor current is listed (LRA) then that's going to be your starting current. Otherwise the rule of thumb is 6-10 times the full load current.

I've heard it said that because you're putting voltage to a load with very low initial impedance, that taking a resistance measurement and using ohms law will give you a worst-case estimate of locked-rotor current, but I don't know how true that is.

-John
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Old 08-23-2012, 07:05 PM   #3
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So you mean the LRA means the starting current ?
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Old 08-23-2012, 07:10 PM   #4
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LRA is the maximum current that the motor will ever draw. It's the current at the time when the rotor is in a stationary state and power is applied. Since the rotor is stopped, it will draw the LRA when starting for a very short period of time.
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Old 08-23-2012, 07:14 PM   #5
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How do i find out the LRA if it's not mentioned on the motor ore compressor
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Old 08-23-2012, 07:21 PM   #6
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Quote:
Originally Posted by Big John View Post
If the locked rotor current is listed (LRA) then that's going to be your starting current. Otherwise the rule of thumb is 6-10 times the full load current.

I've heard it said that because you're putting voltage to a load with very low initial impedance, that taking a resistance measurement and using ohms law will give you a worst-case estimate of locked-rotor current, but I don't know how true that is.

-John
My experience has been 4-8 times is closer to a realistic number though under certain circumstances we have seen 12 times.
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Old 08-23-2012, 07:22 PM   #7
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Quote:
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How do i find out the LRA if it's not mentioned on the motor ore compressor
Use a clamp on with motor in rush capabilities, use a high speed disturbance analyzer.

Contact the manufacture. Do a google search.
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Old 08-23-2012, 07:56 PM   #8
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Is there no way to calculate the LRA ?
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Old 08-23-2012, 08:12 PM   #9
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Is there no way to calculate the LRA ?
Only roughly estimate it.

Is the motor fully loaded, over loaded, lightly loaded, utilized per design, belts properly tightened.

Hence 4-8 times is a rough number

Motor draws 10 amps inrush/starting current is 40-80 amps but it could be higher 100-120 amps.
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Old 08-23-2012, 08:21 PM   #10
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Well i got two A/C units each are 18000btu same comp size. The first one is 5 years old and the second one is 1 year old

The old one draws around 80 amps when starting
and the other one draws 40 amps at start point

What i did to find out how much the start current is was by looking at the Ammeter
that i got at home when one of them kicked in

So i want to find out what is the right LRA for the Comp
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Old 08-23-2012, 08:41 PM   #11
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I'd bet the LRA is on a nameplate somewhere. That aside, it can be hard to get accurate starting current without a true RMS meter with a fast sampling time.

What model ammeter did you use?

-John
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Old 08-23-2012, 08:55 PM   #12
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LRA is not meantioned any where
I use a normal ammeter gauge

P.S

what does P1MAX and P2n on motors mean ?
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