The History of HRDL: My High Rate Data Logger!

One of the ways I gather high speed data in my FunkWorks Lab...

Everyone,

When I first started supervising electric motor installations back in the ‘70s I knew very little about the causes of inrush current. That’s the extra kick that many electric motors (especially capacitor start ones) needed to get running up to speed. But it wasn’t until perhaps 5 years ago that I needed to understand exactly what was going on with air conditioner compressors that contributed to the light-dimming surge that happened during startup.

What it’s not!

Contrary to the general understanding of how motor starting capacitors work, they do not get charged up and then release a burst of current. Here’s a basic diagram of how they’re hooked up to the start winding in an induction motor.

It’s all about the phase shift

Basically, there’s a start winding that’s connected to the starting capacitor for a fraction of a second until the motor comes up to speed. And that capacitor introduces a phase shift that creates a rotating magnetic field for a few hundred milliseconds until some sort of control (such as a centrifugal switch) disconnects power from the start winding. After that, the rotor (spinning part of the motor) is on its own and keeps rotating without the start winding. But much more on that much later.

How I learned the inner workings of capacitor start motors

I needed to take a deep dive into how this all worked, but didn’t have a digital storage scope at the time, so I built my own from junk parts in the lab. Once I designed and built HRDL (High Rate Data Logger), my understanding of what was going on inside of a air conditioner compressor jumped enormously. Below is one of my first diagrams from the data that HRDL was able to record.

So take a trip back 5 years to my design and development of HRDL and how it taught me what was going on inside of capacitor start electric motors. And I’ll go more in-depth into the physics of phase shift in a future article.

Read the full story HERE.

Let’s play safe out there… Mike