Wednesday, 20 April 2016

Induction Motor Adventures

I have plans to make this bandsaw. Ever since I saw the video I've wanted to make it. Matthias Wandel's design is super simple, lightweight and made almost entirely out of wood.

About 6 months ago I bought the plans and have been collecting all the parts I need as cheaply as possible. The frame will be from pine pallets that I've salvaged. I have tried to acquire a second hand motor but - it's been a challenge.

I managed to find this 2HP single phase induction motor on Gumtree for $15.

The guy was in the middle of nowhere and, based on our limited interaction, a little eccentric. He was adamant that "It had barely been used. The only problem is it needs a new start capacitor". For $15, I decided it was worth giving it a go. 2HP would be pretty much perfect for my home-made bandsaw. So I let my significant other know that if she didn't hear from me in an hour to send the police to the crazy man's place, and set off to pick it up. He had mentioned his aggressive people-hating dogs. More than once.

Annoyingly, the motor didn't have a wiring diagram, so being the diligent engineer that I am, I took to the internets to work it out. I first cracked open the terminal/capacitor enclosure to work out what I was dealing with. There were two capacitors. So far so good.

Rather than trying to measure capacitance with my crappy $20 multimeter (and general lack of knowledge/skills about exactly how to do this), the plan was to determine where to attach my power lead to the terminals, then check that the motor starts to see if the start capacitor really was gone. Not wanting to mess with 240V AC without a little more understanding, I looked around for some information about how the motor should be wired up. I also bought a Residual Current Device to avoid excessive current draw once I was ready to give it a go.

All About Circuits had a great article that explained a standard capacitor run single phase induction motor circuit. Based on this information, I plotted out the known connections and drew a diagram to work out where the primary and starter coils should be. I came up with this:

The left side shows the coloured leads and the existing capacitor connections on the motor's terminal block. The picture on the right was how I thought the schematic mapped to the terminals.

With this information, I should be able to connect my multimeter to the two red wires and read continuity, and the two black wires and read continuity. I had about 6 ohms between the two red terminals, but no continuity between the black terminals, which is where I thought the starter winding should be.

At this point things were not looking good. It was time to crack open main motor enclosure. I wanted to confirm the centrifugal switch was fine, although I was pretty sure my understanding of the circuit was correct. At this point, I had nothing to lose anyway.

The enclose was a little rusty and required some percussive persuasion to remove the end bells. Lots of persuasion. After a few hours (and much swearing), I pulled out the rotor and was greeted by this:

I'm no expert, but I was pretty sure the black wiring was not a good sign. A few measurements with the multimeter and some careful inspection showed that the winding was indeed burnt out and was causing the lack of continuity between the black terminals.There were a number of solder blobs too, so it looked like someone (I'm looking at you crazy Gumtree guy) had tried to repair it in the past.

On the plus side, I managed to confirm that my understanding of the switch placement was correct. I also got to finally have a play with one after reading about how they work - which is always nice.

So...still motor-less - but at least I learned quite a bit.