In this more or less regular series, I want to show interesting patterns of power measurements of appliances. The first one is an air compressor with 1-cylinder, powered by a 3-phase asynchronous machine with 2 pole-pairs and 50 Hz nominal frequency.
Steady state operation
The more interesting detail is the steady state operation, which shows a power oscillation. To do this analysis, I added a feature in the pqopen-lib to calculate the cycle-by-cycle values with a sliding window, updating each 1ms. This makes it possible to visualize the pattern of the oscillation and read details out if it.
Interesting learning: The current is not that much oscillating like the active power – this is because the reactive power is opposite oscillating, which means, the phase angle of the current is changing as well.
The oscillation is a result of the mechanical load due to the cylinder compressor, which needs more force in the compressing cycle than in the release cycle. This pattern is transparent in the power consumption!
Why is the oscillation cycle 41ms?
The asynchronous machine is a 2-pole rotor, which has a synchronous frequency of 1500 rpm. Due to the load, the speed drops due to the slip characteristics to a lower value. We can do the back-calculation:
41 ms ~ 24.4 Hz * 60s = 1464 rpm of motor speed
This was an interesting experiment, I always wanted to know, how the power profile of an air compressor looks like – this secret is now revealed. If you are interesting in doing your own analysis, write a short message and I’ll provide the point on wave data for post processing.