Feasibility of building a scale model salient-pole alternator/synchronous motor

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
107
Just a late night idea for a metalworking & electrical project in need of a reality check. I've always been fascinated by the old open-frame style salient pole synchronous motors and AC generators. I wouldn't mind building a working model of one at some point, probably to be coupled with a suitable 2-4 cylinder inline engine or air compressor as the case may be.

See Here, here or here for visual examples.

My question is how feasible this sort of project really is from a hobbyist perspective. Obviously the stator is going to require a laminated core with the correct number of slots for three phase windings. Short of having a stack custom made, maybe an old cieling fan motor or alternator core might suffice? As for the rotor, would laminations be strictly neccesary? Does the alternating field extend into the pole shoes at all or are they purely DC? Damper windings might require a bit of thought.

Are there any good resources to look into as far as the winding calculations? Particularly for figuring how many turns of what gauge for the DC field windings.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
21,964
The third example looks like some AC synchronus motors I was involved with some years ago.
The motors were about as large, but 4000v operating, the motors were induction motors with also a rotor winding with a pair of brushes and slip rings.
The motors were ran up as normal induction motors and the generated field in the rotor was output through the slip rings in order to detect the slip frequency, when it was up around 4 to 5 cycles, DC was injected into the winding and the rotor came up to synchronism.
Max.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
107
Another example just for giggles. I guess they just used an old motor core and some permanent magnets - no field windings. The slip rings are purely cosmetic.


Out of curiosity, anyone have a general idea of what it might cost to have a short run of custom stator and pole-shoe laminations laser cut? These sorts of motors are fortunately relatively thin in cross-section so it wouldn't require a very large stack. I figure maybe a 5-6" O.D core would make for a reasonably sized model. Big enough to handle at least half an amp at 120/208 3Ø 4W Y but not so large as to be difficult to machine.

Failing that, I'm just trying to think of other ways to obtain a suitable stator core without having to section a 1/2~1hp induction motor.
 

shortbus

Joined Sep 30, 2009
8,171
The price of the laser or water jet cutting would probably be less than the cost of the steel. One of my projects I wanted to do was making a switched reluctance motor(SRM). When I started looking at doing it I was still working and had access to wire EDM, so started looking for silicone lamination steel. Could find no place to buy small quantity of it. Steel suppliers wanted you to buy it by the coil, since that was how it was sold.

So like you suggested I started looking for stator lamination's, there are places that will sell the bare lamination's but they are very expensive too or at least to me they were expensive. And the more I dug into both motor and transformer lamination's I found out there is more than just the correct steel and a die to cut them. They go through a heat treatment that puts an oxide(IIRC) layer on them after stamping. The oxide, if that is the right term, is to keep the eddy currents down when they are assembled.

So I finally settled on using either an existing motor or for my experimental one an alternator stator and the end bells to support the rotor shaft. But then came the next problem. That is cutting the unneeded poles out of the stator to get the salient poles so they could be wound. Though I have a small machine shop and a milling machine, cutting the excess poles out is not an easy thing. The silicon steel is harder than mild steel and it also since the lamination's are thin they distort as you try to cut them. I also tried cutting them out by welding a saw blade in the center of the stator to cut them in my metal cutting band saw. The steel just rips the saw teeth to pieces. After ruining 2 stator's I put the project on hold and started working on another one of my on going projects. A lawn mower size crawler tractor. To get the SRM work done I need to finish yet again one of the on going projects, my sinker EDM machine. So many projects and so little life left.
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
107
Maybe this is a stupid question, but is there a general standard for the number of stator slots in a motor given it's phase and pole count? Or is this dependent on how each individual motor is engineered?

Just thinking if I can find a core with an odd number of slots (even number of protrusions), I could simply wind it for that number of poles. These types of machines tend to have very high pole counts (300-900 RPM) so it would actually be realistic.
 

shortbus

Joined Sep 30, 2009
8,171
No real *standard* number that I am aware of. It's up to the maker to how it's wound and the number of poles. Kind of like asking how high is up. This is kind of a way to show what I mean - https://www.pengky.cn/zz-generator-principle-and-structure/06-ac-motor-winding/ac-motor-winding.html Since most induction motors use a distributed winding pattern not the salient pole pattern, winding a distributed pole motor as a slient pole gets harder, do to the small amount of space in the slots in an induction motor.

There is a old book, that's still available, Electric Motor Repair, by Robert Rosenberg, that has probably the most information in one book. Was considered the "bible" of electric motors. I have a second edition that I bought from Abebooks. https://www.abebooks.com/servlet/Se...=all&sortby=17&sts=t&tn=electric motor repair
 
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