7.5HP Three Phase Motor Conversion

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Senior Member

Joined: 17/04/2008
Location: Canada
Posts: 196
Posted: 05:24am 24 Sep 2008      

When I discovered this motor in the discard pile, I swallowed my pride, and into the bin I went!

It weighs about 80 pounds, but that's not much more than the 3HP I converted before!

Toshiba 7.5 HP 3-phase induction motor
1740 RPM
19.4Amps @ 230VAC, or
9.7Amps @ 460VAC
Frame 213T

Once I got it open, more good news. A nice big rotor, 12 wires already in the connection box, rear shaft stub with a fan - I couldn't ask for more.

Okay, four poles isn't a lot, but I've been noticing that the size and weight of these motors goes up rapidly with the pole count. Why is that? Anyway, four poles works perfectly well. I have a stub shaft for a RPM pick-up, so trying to count pulses on the power line will be a thing of the past.

Before cutting anything apart, I started modelling it with FEMM. I like using FEMM both because it gives concrete results that I can use to predict performance, and because it's perrrrty.

Here's the FEMM output (GIF is a bit big to fit on this page)

Here's the solution when I modeled the conversion of the Toshi using a rotor turned down and flats milled for the 2x1x1/2 magnet blocks. I tried many other magnet combinations, but this worked the best and used magnets that are available. The end view allows me to see the flow of field lines through the stator and rotor, measure peak field intensities (you can find some saturation areas where it's the most pink). Most importantly, FEMM allows you to draw a boundary equal to the span of the windings in the motor, and measure the total flux through the winding. This number, with a bit of number crunching and educated guessing can give you a estimate of open-circuit volts-per-RPM. I also extrapolated the charging current in a 24V battery, but that's taking the guesstimate pretty far...

So the forcast is for the mill to reach cut-in for my 24V battery system at about 180 RPM. This should be pretty good for a 10-12 foot prop (a little on the fast side for a 12-footer).

So I put the rotor into the lathe at work and started to turn it down. This rotor's laminations are so thick that there was no risk of removing enough material for the laminations to split apart. A rotor without enough "meat" forces you to make a brand new rotor out of raw materials, or maybe one could press the laminations off the shaft and press on a replacement cylinder. In the end, I cut most of the way through the squirrel cage, but enough material was left to hold the cooling fins on.

At the time, I thought the cooling fins were a bonus. How wrong I was!

A few more machining steps involved milling the flat faces. You can see that there are two rings of flats destined for two rings of magnets. The offset of the faces will provide an anti-cog skew. As the magnets in each ring are attracted clockwise to the teeth of the stator, the magnets on the other ring will be attracted to counter-clockwise teeth on the stator. That's the theory, and test fitting of the rotor bears that out. (more on that later).

Each of the mounting holes are tapped. The holes fit 6-32 NC screws. Obviously one uses stainless steel screws for this job. Steel ones would reduce the strength of the magnets. I had tapped 21 of the 24 holes when I broke the tap in the hole! There was a tiny nub sticking out, which I could grab with the pliers to slowly work the end of the tap out of the hole!

That broken tap on the 21st hole was, it turns out, a warning of more trouble to come.

Before going too far with the magnets, I put on just four, and put it into the stator to get a "feel" for the de-cogging skew's effect. The rotor turned smoothly, but with a fair bit of resistance. It is called "iron loss", meaning the magnetic field dragging through the iron of the stator requires some force to be overcome. I hope the resistance doesn't scale up by a factor of 6 when I get all 24 magnets on. That could cause a big problem with start-up torque. Anyway, there wasn't much cogging so I satisfied myself with that result, and continued installing magnets.

I got to the 20th magnet. The 21st magnet WILL NOT GO ON.
It took a lot of force, luck, and several tries each to get magnets 12-20 mounted.

Previous projects that I have studied on the internet forums, either by zubbly, dinges, or other distinguished experimenters, where many magnets were installed on a single rotor, have all employed rotors made from scratch. I thought I was lucky, having found a rotor where I could use the original laminations and cooling fins! Argghh!

Installing magnets on a rotor without cooling fins is comparatively easy. I did that myself on my first motor conversion. You position the end of the magnet on the edge of the rotor flat, and slide it on. It takes some marshalling because the magnet does want to slide around a bit, but it's not that difficult, especially if you can screw each one down once it's in position.

With cooling fins, you either have to tip it up, beside it's neighbours, or push it down flat. All it wants to do, however, is flip over!

If I can find no way to install the remaining four magnets, I will have to disassemble the entire thing and go back to the shop to remove the cooling fins.

I am currently building a tool to force the magnet to remain straight as I force it into place. It's slow going, but I'll let you all know in a few days what does, or doesn't work.

Later, guys!
Steven T. Fahey