# Why does reciprocating compressor's motor starting winding have high voltage than running winding

I want to know why the starting winding of (2 ton) reciprocating compressor's motor have high voltage than running winding? I have attached the voltage that I have captured from oscilloscope.

This reciprocating compressor has potential relay with start capacitor and also have run capacitor. Start capacitor provide the start torque, but what is the purpose of running capacitor? I could see that, starting winding have high starting current. But when the starting capacitor is disconnected from potential relay, the voltage at starting winding to common become 275 V, which is very high but the point that is triggering the start winding current. Start winding current reduced to 0.1 A or more less. Why and how this can that the is high but current is almost zero? In this picture, you can see that the yellow is the starting voltage which is high. Blue the running voltage with respect to common. RMS voltage of starting winding is 276 V and RMS voltage of running winding is 226 V.

In this picture, yellow is the running winding current and blue is the starting winding current.

The current through starting is 0.1 A and the current through running winding is 12 A.

• Likely resonance between start winding inductance and run capacitance. If you can, measure both and add those values to the question. And crop those photos! Mar 5, 2021 at 21:32
• Bigger current, bigger drop voltage on wires? Mar 5, 2021 at 22:11
• Please upload photos of proper size and post a schematic that shows where your scope leads are connected. Mar 5, 2021 at 23:41
• @BrianDrummond Thank you for your reply. I just measure the voltage and edit the question. The voltage at Starting winding was 276 Vrms and 226Vrms at running winding.
– Fame
Mar 6, 2021 at 17:03
• What is a “potential relay” and how does it differ from a normal relay? Mar 6, 2021 at 18:32