Alternator torque as a motor 1

[Nikiss]

Does anyone have experience with Alternator torque when used as a motor?

They have electomagnets on their rotors. I saw different opinions. One side tells that it is worse, while the other says that greater magnetic fields can be achieved using electromagnets.

I have a DC motor from car fan that is pretty chunky. It gives about 0.75 Nm of torque at 15 amps from the power supply 12V lines, driven by chopper h-bridge. Any ideas if the alternators would be stronger?

The motors would be stalled for the whole time, they would be used to drive a PC steering wheel. The wheel is mounted directly to the shaft.

 

[waross]

If you are asking about the three phase alternators that are found on diesel generators, then yes, the torque is about the same when running as a motor when supplied with rated three phase power.
Starting torque may be less, it depends.
On the other hand, powering a power alternator with a Variable Frequency Drive should work well.
FYI, large aeroderivative gas turbine generators use the generator as a motor to start the turbine.
If you are going to use an automotive alternator, It would probably develop more torque than the fan motor.
However, there are a couple of practical problems mitigating against the use of an automotive alternator as a stall-able motor.
A power alternator has a damper winding which is similar to the squirrel cage winding on an induction motor. If the alternator as a motor is pulled out of synchronism the damper winding will allow the machine to operate as an induction motor for a short period of time.
1. An automotive alternator does not have a damper winding.
It may still be used as a motor but;
2. You will need a source of three phase power that has both variable frequency and variable voltage.
Try a motor from power windows or power seats.
This thread may be moved to a more appropriate forum, or maybe not.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Nikiss]

I forgot to mention that incremental encoder will be attached to alternator somehow. In that way it will always be synchronyzed.
Btw I'm planning to make the controller myself, will just add another half bridge to the already existing h-bridge. Torque is the most important thing. There is no need to care about speed. If the motor is always stalled winding current just needs to be adjusted according to the rotor position (probably with sinusodial drive method).

If the aforementioned conditions are met I think that there are no problems with damper winding. To be honest this is the first time that I heard info about it.
Starting torque will be the max torque with given current because of encoder, so it shouldn't be a problem as well.

Thank you for the clear answer and interesting information!

PS : When this post was made, I was not aware of the motors section. Can I move it in there myself or moderator needs to do it?

 

[waross]

You need three phase AC power for the alternator. (Assuming a three phase alternator.)
There is an important parameter with many AC machines. This is the V/R (Volts per Hertz) ratio.
That means that as the frequency is reduced, the voltage must also be reduced.
Take my word for this. It will save a few pages of theory explanation:
A typical 230 Volt three phase motor developing full torque at stall will require a frequency of about 1.33 Hz and a voltage of about 5.11 Volts.
The torque will decrease to zero at about 40 RPM unless the voltage and frequency are ramped up as the motor speed ramps up.
If the window or seat motors are not suitable, try using components from electric power steering.
Forget home-brewing the alternator.

A three phase AC motor is supplied with three alternating currents displaced 120 electrical degrees.
A Phase 0 deg.
B phase 120 deg.
C phase 240 deg.
This is called the phase rotation.
To reverse the motor the phase rotation is changed from ABC to ACB.
In commercial Variable frequency drives the voltage, frequency and phase rotation is done by Pulse Width Modulation, switching in the low KHz range.
I have the feeling that your abilities may be well up the first peak of the Dunning-Kruger curve.
By the time that you get this working you will have made the humbling drop into the first valley and be well started up the second rise.
I know what has to be done, but not how to do it.
I would buy an off the shelf VFD.
My congratulations and respect if you succeed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Nikiss]

Yep, you are right.
The Dunning-Kruger curve feeling is very familiar to me. It was a lot of fun to drive a big motor at 100hz with pwm, while punching it with more than 15 amps ( Facepalm and deaf ears ). And have a mosfet die because its leg got torn off. All immediately after figuring how to make dead time and braking for 2 weeks on a microcontroller.

Back on topic. My plans are ultra simplified and I really only have basic knowledge about this. Your posts were very helpful and I'm very grateful.
But I cannot avoid saying that this thread turned into actually controlling motors and being a greenhorn from the question : are alternators stronger that dc fan motors from cars? (please forgive if this sentence sounded angry, it was not its intention, I'm not native speaker)

Something tells me that you mentioned 3 phase induction motors, because of constantly mentioning frequency?
From what I know alternators are classic BLDC / PMSM motors with electromagnets instead of a permanent magnet.

 

[waross]

are alternators stronger that dc fan motors from cars?

Do apples taste better than oranges?
Going back to your first post where it was suggested that an electromagnet may be stronger than a permanent magnet.
That is true.

An automotive alternator may work as a motor but the lack of a damper winding means that it cannot be started Direct On Line.
It may work as a motor if driven with a VFD, but it is overkill to use a 240 Volt VFD to drive a 12 Volt alternator as a motor.
There are many cheaper and easier ways to get a motor.
But, if you are comfortable designing PWM circuits you may be able to make it go.
Two suggestions:
1. Put rated current through one of the alternator coils. Energize the field. Measure the torque required to start to turn the rotor.
That will give you a ballpark indication of what to expect.
2. When working out your PWM pattern don't forget the V/Hz ratio. Use the number of poles and the speed at which the alternator just puts out 12 Volts to determine the frequency for the V/Hz equation.
Current limiting may limit the voltage for you.

It was a lot of fun to drive a big motor at 100hz with pwm,

I don't know if they are still on the market but at one time a compressor skid manufacturer was using a VFD to apply 480 volts at 120 Hz to 50 HP, 230 Volt, 60 Hz motors and getting 100 HP.

Torque is a function of the strength of the magnetic field, the diameter of the rotor. and the active or effective length of the rotor or stator, whichever is less.
The alternator has a greater diameter than the fan motor, but the stator may be narrower.




Bill
--------------------
"Why not the best?"
Jimmy Carter