[oldgold70c10]

Originally Posted by davepl

Can someone explain cap-start? Does it store a bunch of surge energy in a cap for the initial inductive "kick" when the motor starts?

I decided not to monkey with my big compressor. I have a small desktop compressor I used for tires and air brushing that I will play with instead!

I built a switched outlet box with a 30A Craydon SSR in it to play with, we shall see. I searched and paid extra for a non-zero-cross relay. Normally they don't switch until the next time the AC crosses zero, which is OK as a switch but wouldn't work for AC slicing.

I've also been tinkering with a bunch of old retro displays and trying to interface them to modern electronics. Here are some of my results!

Little saga on the VFD display: it's a Japanese unit from Noritake and uses hand-coded timing loops to drive it that I could only get to work on the CPU they designed it for (which I'm not using because it's ancient). So for the $1.75 they cost, I dedicated an Atmel 328P (which it works with) just to controlling the VFD. It then exposes the standard LCD interface across the i2c bus so that the ESP32 can call it as if it were a local display. Works great, but was a bit of an investment.

I'm a software guy, so every one of these little wires is a victory ;-)

You have to have some way of setting up a rotating magnetic field in a single-phase motor. Otherwise, the field just alternates without rotating. In a cap-start motor, there are 2 windings, a run and a start. The run winding is across the ac line, the start winding is across the ac line with a capacitor in series with it. The capacitor causes a shift in that winding (along with the winding being wound slightly different than the run) so that the field will rotate, to start the motor. On many motors, once the motor comes up to about 75% or running speed, a centrifugal switch cuts out the start winding, This is a capacitor-start, induction run. For loads requiring more running torque, a capacitor start, capacitor run motor is used. These have two capacitors, one for starting and one for running. The run capacitor stays in series with the start coil all the time. Air compressors and table saws often use cap-start, cap run motors. Another type of capacitor motor for low-torque loads like fans omits the start capacitor and has just a run capacitor. These are called permanent split capacitor (PSC) motors and they are very efficient.