Inrush current from an HVAC point of view

[channellxbob]

I have a couple of very basic questions about inrush and motor starting current. I know you guys will know the definitve answer here.
I used three different clamp meters and measured some starting currents, and one of the meters gave decidedly different readings.
To summerize, the meters gave the following readings.
Fieldpiece SC440 18.82A   100mS
Klein CL450  19.41A    100mS
Amprobe ACD14PRO  25.5A    80mS

The three meters give virtually the same readings in normal current measurement, it is only the inrush that varies.
I'm smart enough to know that it's the timing of the trigger circuitry that is making the difference, my questions are.
1) which is the correct (or accepted) timing for inrush measurement?
2) Which of these readings would represent locked rotor current?
3) How do motor/compressor manufacturers test and rate Locked Rotor Amps?

[trobbins]

Do each of the meters specify the same definition for their measurements - eg. maximum peak of the inrush waveform (ie. typically the first peak), and is the time measurement related to say when the waveform peaks have reduced to 50% or 10% or ? of the initial maximum peak?

PS. the 2 meters with on-line operating instructions don't identify exactly what is being measured, and have more than a 10% inaccuracy rating, so presenting measurements with decimal points is a little pointless, as would be comparing results from different meters.

[tkamiya]

They could ALL be correct.

If you are measuring compressor motor starting current, initial starting current varies greatly depending on coolant temperature, pressure, position of pump, etc, at the time of measurement.  Also, unless you are using calibrated meter, they are close enough, that I would simply read it as 20 Amp for 100 ms. 

Keep in mind, in rush current is not a square wave.  It's not that 20 amp is consumed for the whole 100 ms.  So depending on how it is sensed and threshold is set, they are (again) close enough for practical use.  If I were doing what you are doing, and it is critical, I would use current sensing probe along with an oscilloscope.  Or, go further and use a shunt and measure the voltage, but that's going quite far, unless there is a very specific need.

For rest of your questions, the best thing you could do is to read off the manufacturer and part number off the motor and look for spec sheets.  I'm sure they test that.  If it's public information or not is a big guess. 

[threephase]

I don't believe that either NEMA or IEC actually specific a time point for motor starting current, they specify a locked rotor current to be within a range dependent upon the hp rating of the motor. This would be the maximum starting current, but as tkamiya has stated actual starting current will be dependent upon the load at the time of starting.

Manufacturers have therefore opted for their own definition of the inrush current with measurement over 100ms being one of the more popular values.

If you have the motor nameplate details then you would be able to look up the kVAr rating to calculate it from the NEMA code. Alternatively, the manufacturer's data sheet will usually contain the information, or they can be approached with the motor details and they would provide it.

As the motor is not stalled, it is highly unlikely that any of the readings you have will be the locked rotor current, but they should all be below it.

Motor manufacturers usually have a test procedure to measure the locked rotor current at a low voltage and then extrapolate this onto a performance curve to obtain it for higher voltages. The rotor is physically locked during the test, so the windings would heat up very quickly and be damaged if the test is done at full motor voltage.

Kind regards.