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So while I was trying to solve another problem on a different question, I found that my AC(Air Conditioner) Unit compressor waveforms were a bit confusing; As you can see on the scope view below there's 150 degree phase shift between current and voltage. There is also another thing that confuses me the current was wiggling around as you can see in this video. Why is that happening ?

My question is why is this happening ? shouldn't the phase shift be at most 90 degrees ?

Current = green x 10, voltage = yellow x 10 enter image description here

Update 1:

I'm using a differential probe with a current shunt of 0.1 ohms and normal probe for the voltage. The ground of both probes are connected to each other at one end of the shunt. normal probe is connected to mains and other side of the differential probe to the other side (load side) of the shunt.

enter image description here

Update 2:

I measured another AC unit and it's phase shift was 180 degrees. I don't understand what causes this shift Also non of the AC units are inverter types.

Isn't 180 degree phase shift between current and voltage make the power always negative ? I'm really confused.

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    \$\begingroup\$ Can you update your question to explain how you are probing the current? The wiggle looks to me like you might have averaging turned on. If so, try turning it off. Most likely you are probing the current backwards (like a current probe with the wire running through the wrong direction, for example). \$\endgroup\$
    – user57037
    Commented Jun 19, 2021 at 6:53
  • \$\begingroup\$ @mkeith, I will check the averaging on the scope and update the question. I have measured current of another AC motor with the same method and it was fine. I have explained and uploaded a schematic of scope wiring. \$\endgroup\$
    – Oli
    Commented Jun 19, 2021 at 7:12
  • \$\begingroup\$ The polarity for current should be correct using this method. I question whether it is safe or a good idea to connect probe grounds directly to AC neutral. I thought you were either using a current probe (hall effect probe) or a current transformer (CT) to safely get the current. \$\endgroup\$
    – user57037
    Commented Jun 19, 2021 at 7:40
  • \$\begingroup\$ Or maybe, are you using special probes that have built-in isolation? I know those exist. \$\endgroup\$
    – user57037
    Commented Jun 19, 2021 at 7:40
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    \$\begingroup\$ @mkeith, I have check it with an electric heater and there is no phase shift. \$\endgroup\$
    – Oli
    Commented Jun 20, 2021 at 11:28

1 Answer 1

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The current phase shift happens when we have a reactive load. The bigger the reactive load, the more the phase shift.

In single-phase motors, the 90-degree phase shift is between the current of the main winding and the current of the secondary winding, not the voltages.

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    \$\begingroup\$ I don't think that is correct, The phase shift of any pure inductor should be 90 degrees not more. \$\endgroup\$
    – Oli
    Commented Jun 19, 2021 at 8:53
  • \$\begingroup\$ Also there are single-phase motors that have a single winding with a starter capacitor. \$\endgroup\$
    – Oli
    Commented Jun 19, 2021 at 9:02
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    \$\begingroup\$ The starter capacitor is between the power line and the secondary winding. The capacitor is for creating the current phase shift between main winding and secondary winding. \$\endgroup\$
    – Saadat
    Commented Jun 19, 2021 at 10:03
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    \$\begingroup\$ It seems to me that you didn't understand my question; I know what the capacitor is for and where it is and I don't care about phase shifts between coils. In any case I don't think your answer is correct. \$\endgroup\$
    – Oli
    Commented Jun 19, 2021 at 10:12

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