AC Soft Start- How is this possible?

[mbrooke]

Just saw this video of a soft start on an AC unit:

How is it physically possible to reduce 140 amps down to 6 amps? I want to say the meter is missing something... this feels to dramatic to be true. What am I missing?

 

[mivey]

I would say the meter inrush measuring interval was too short and captured the event during the current rise time to peak. In other words, the current was still ramping when the meter ended its inrush period.

You need to measure the current until run current is reached. I suspect it spiked higher than 6 amps, and probably higher than run current.

You need a PQ meter or oscilloscope to get an accurate measurement.

 

[mbrooke]

I would say the meter inrush measuring interval was too short and captured the event during the current rise time to peak. In other words, the current was still ramping when the meter ended its inrush period.

You need to measure the current until run current is reached. I suspect it spiked higher than 6 amps, and probably higher than run current.

You need a PQ meter or oscilloscope to get an accurate measurement.

I suspect the same. Several Youtube videos by the manufacturer of a soft start device show a 50% reduction.

 

[mivey]

I suspect the same. Several Youtube videos by the manufacturer of a soft start device show a 50% reduction.

You could probably do better but I don't know at what cost. I know some soft starts will cut down to 25-33% of normal inrush so you could get to 1.5-2 X run current. But not 1/4 of run current and that should have been a red flag to the videographer.

IIRC, a written-pole motor can keep start current below run current until full speed is reached.

 

[mivey]

I looked at the written poles and their inrush is 1.5-2 X run current. But for a single-phase motor to have 1/4 the inrush current of a 3-phase motor of the same size is pretty cool (the run current is higher of course).