# Using an alternator as a motor



## major (Apr 4, 2008)

firefly said:


> Can someone explain the details of using a car alternator as a motor? I've read in some sources that an alternator is the same as a 3 phase induction motor - but then why does it have brushes?


Hi firefly,

It has brushes because it is a synchronous machine (generator). It could be used as a synchronous motor. Induction motors are asynchronous machines, mostly motors but can be generators. Induction machines use "induction" to induce a field in the rotor, therefore no brushes. This requires a difference in speed between the mechanical frequency(RPM) and the electrical frequency, hence "asynchronous". Synchronous machines have the mechanical frequency equal to the electrical frequency and require excitation for the rotor, which can be done different ways, like with magnets, but alternators use brushes and windings on the rotor, which is the field. This allows for the adjustment of the field by an external regulator to keep the proper voltage for your car battery charge.



> Which is more efficient?


Theoretically, about the same. However automotive alternators are notorious for being inefficient due to being optimized for low weight and low cost.



> Could an alternator used as a motor be controlled with a standard AC induction voltage/frequency controller?


Yeah, I've seen some do this. You need to know what you're doing. Like provide field excitation, which is not needed for the induction motor control. And you end up with a sucky motor. You're better off using an induction motor.

Regards,

major


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## firefly (Nov 16, 2008)

Hi thanks for the reply - I was just about to add to the post, because I found a number of useful threads about this subject:

http://www.endless-sphere.com/forums/viewtopic.php?f=3&t=6584&st=0&sk=t&sd=a

http://www.rcgroups.com/forums/showthread.php?t=905411

There's some useful info from people who have been experimenting with alternators - it seems they have got fairly useful power by varying the current in the rotor - this allows the motor to run faster (albeit at lower torque) which is something that can't be done on permanent magnet or induction motors (I think?)

I think it might be worth experimenting with, just because they're cheap and easy to get hold of - and I can't find an induction motor that matches those criteria!

I want to try and design a controller (I'm a programmer with some knowledge of electronics) - for BLDC and AC motors - so maybe an "altermotor" would be a good cheap starting point.


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## major (Apr 4, 2008)

firefly said:


> - this allows the motor to run faster (albeit at lower torque) which is something that can't be done on permanent magnet or induction motors (I think?)


Running induction motors over base speed is easy. Brushless DC or PMAC motors are quite a bit trickier, but possible.

major


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## Kirk_Wallace (Nov 20, 2008)

Hello firefly,

My random thoughts ahead.

I tend to think of automotive alternators as being similar to a 3-phase stepper motor, except instead of permanent magnets on the rotor you have windings and rings to give you control over the amount of the rotor's magnetic flux.

The LRK group may be of interest:
http://groups.yahoo.com/group/lrk-torquemax/

One issue that might come up is that sensorless BL motors generally won't start with a load. You should decide what you want the motor to do and go from there.
-----------
Kirk
http://www.wallacecompany.com/machine_shop/


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