Alternator regulator field PWM frequency

[BjarneK]

Originally Posted by RaymondR

I used one of these in parallel with the alternator regulator on a previous boat to set the charge voltage and current. I think it was based of an LM494 switched mode regulator chip and ran at about 20K.

It worked well and if the excitation coil was overheating it was well and truly masked by the general overheating from the high ampage output.

Thanks for this data point. I would expect running a DC motor and a field coil would be quite similar and high frequencies seem more common for running motors.

[ktowers]

Back to basic electrical theory, impedance of a inductive circuit is proportional to the frequency.

XL = 2PiLf

Where:
XL - circuit impedance
Pi - 3.14159
L - inductance of the circuit
f - frequency


In general you want to keep the frequency as low as possible in order to reduce the impedance. This will allow you to pass more current through the field coil with your limited voltage. The higher the field current, the larger the magnetic flux. This allows to alternator to produce its maximum current output.

[nfbr]

As per above, really you want nice variable DC current for a variable magnetic field. Switching just makes noise. Luckily the large inductance of the rotor will mask that noise. Rotor is a big coil so impedance will be signifianct at higher frequencies
You could get real clever and go variable frequency to vary current as induncance limits it. .

[BjarneK]

Originally Posted by ktowers

Back to basic electrical theory, impedance of a inductive circuit is proportional to the frequency.

XL = 2PiLf

I think this equation applies to when if you are trying to drive the current up and down at a given frequency. What an alternator regulator does is to increase the current when it is on and let the current freewheel when it is off. In this case the current does not vary much at high frequencies and the effective impedance of the field coil does not vary according to this equation with the PWM frequency.

[Jammer]

Hope you're planning on smoothing out that PWM if you're going to be > 20 khz. The alternator field windings behave like an inductor and if you try to apply PWM to them you'll burn up the switching transistor before you know it from the switching spikes.


Essentially you'll be building a variable-voltage DC-to-DC if you want to use those kinds of switching frequencies. Which is possible and not a bad idea.


If you run PWM at lower rates, around 100 hz, you will in fact be able to get the field to turn on and off that fast, though there will be some spikes you have to smooth out.


Good luck, sounds like a cool project.