Alternator tutorial

Thread Starter


Joined Jun 5, 2007

This tutorial claims something I suspected: that a car alternator could be run as a low voltage induction motor, by shorting the field to itself.

I am curious if anyone has tried this, and got a feel for the performance of the alternator running as a motor? It seems torque could be somewhat limited without a re-winding of the rotor, but I could be wrong (I am a CPE not a EE :)).




Joined Apr 19, 2007
Hi..I can just about remember working on something like this years ago.The guy used it to turn over a jet engine..... He could run the alternator up to about 60,000 RPM, but the coils were driven by MOSFETS in a three phase config...... I can't off hand remember the details, and am not sure if I still have the info.... I will have a look....Daniel.
hi,its a machine which has two parts
stator,& rotor
stator which is stationary and rotor is moving part
stator ,from which we get supply and we give dc supply to rotor


Joined Dec 27, 2007

I believe you are correct in that assumption. Why not apply DC to the field coils, then the 3-phase to the motor? You will be able to produce higher amounts of torque. If you put a local 3-phase rectifier plus a bulk capacitor, you will have a supply for the field winding.



60KRPM would destroy any car alternator! There are two big problems when spinning up to that kind of speed: natural rotor frequencies, and inertia on field windings. The natural frequency, I am willing to bet, is much lower than 60KRPM. So, you're resonance would probably self-destruct the motor pretty quickly. Secondly, all of the inertia on the windings could cause them to lift out of the rotor, also self-destructing the alternator.

Don't alternators have bushings instead of bearings??



Joined Aug 11, 2008
Steve, by "bushings" do you mean plain bearings, and by "bearings" do you mean ball bearings? Keep in mind that the name bearing was used long before roller or ball bearings were invented.

Actually I've never seen plain bearings used on an automotive alternator, maybe on earlier generators.

This peaked my intrest as to how fast these actually spin, so I measured one up, here is my findings;
Alternator sheave (pulley) is 2 3/4"
Crank sheave is 7"
Max engine RPM is 6000
So 7/2.75*6,000=15,273, that little hummer goes pretty good.

But 60,000 is very fast for an alternator, maybe he geared up the turbine like 4/1, turbines dont require much torque to start.


Joined Dec 27, 2007

That's what I meant, a rotary plain bearing / bushing. I am not an automotive tech, so I don't really know what is in there. Do they have ball-bearings at both ends then? You need to watch the friction coefficient, since as we should know, heat is proportional to both speed and this coefficient. There will be a rated temperature for this, which you shouldn't exceed.

I would even say that 15000 RPM is pushing it for an alternator. I wouldn't run it past what it would run in a vehicle. It takes precision machining and a PM DC motor to do 60KRPM.