PWM field coil

I'm building a voltage regulator for an antique generator that's built like a DC motor. It has an armature with a segmented commutator. Bolted in the case it has a pair of pole shoes of ductile, solid iron. Each pole shoe has a removable coil (they are wired in series).

The original voltage regulator was mechanical. I am going to use PWM on a mosfet driving the field coil. Obviously the circuit will have a diode to carry the freewheeling current of the field when the mosfet is turned off.

I have to choose a frequency for the PWM. 20kHz would be good, to avoid causing an audible whine in a nearby radio or something. Would it work to drive the field, which is in effect an electromagnet, at 20kHz?

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Reply to
Michael Robinson
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You might see considerable eddy current losses, because of the solid iron in the pole piece. If you have a signal generator and an oscilloscope you may want to put a square wave at various frequencies onto the field winding while looking at the current, to estimate the losses.

Or just build a switching amplifier, complete with inductors, to drive the field. With smooth DC being delivered to the field windings, you can totally decouple the high frequency behavior of the field winding from your field amplifier. In fact, you may end up spending less time and money just doing that for a one-off than you would trying to thoroughly characterize the motor behavior.

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Reply to
Tim Wescott

Note that the original electromechanical regulator used a MUCH lower frequency, and probably did not cause much audible noise. The inductance of the generator field magnetic circuit makes it very unlikely that the field can change rapidly. So, if the PWM field regulator doesn't put a lot of noise into the battery supply circuit, you won't have a problem.

Jon

Reply to
Jon Elson

The trick is to find the base line frequency that will give you the closes constant current you can achieve.

You're going to find the need to change the base line frequency as load demand on the generator changes.

This can be done by monitoring the ripple on the field coil and increase the base line freq to compensate on the fly.

Jamie

Reply to
Jamie

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No you wont, the current will be dc.

because of the solid iron

Isnt that what he's doing?

complete with inductors,

The field is a winding on a iron core!!

ttdesign.com

Reply to
cbarn24050

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There is no trick required he has the bases covered allready.

No he wont

There will be allmost no ripple.

Reply to
cbarn24050

Guess you haven't monitored very many DC generator fields under vary loads, have you?

Argue all you want, proof is in the pudding and I've had plenty.

With that, I'll ignore you now.

Jamie

Reply to
Jamie

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why do you want a low frequency?

20kHz into a large inductor with a reverse diode the current ripple will be tiny

-Lasse

Reply to
langwadt

Well, no. Apply a square wave to a winding on a slug of iron and you'll see a lovely square wave current, probably with a teeny bit of inductive triangle wave action visible.

Tim

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Reply to
Tim Williams

l see

guess it depends on the definition of applying a square wave but where's the squarewave current?

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Reply to
langwadt

Perhaps in the real one, not in the one that you imagine models it?

Iron is conductive. A coil wound around an un-laminated iron core is properly modeled as a transformer with the secondary shorted.

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Tim Wescott
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Reply to
Tim Wescott

Which isn't exactly correct either; a (nonideal) transformer with shorted secondary has no losses, and inductance equal to the leakage (= L1 - M).

Somewhere inbetween zero ohms (short) and infinite ohms (open, a lossless inductor) lies a true lossy inductor.

Physically, the core (and its losses) are usually tightly coupled to the winding, so the leakage reactance is small with respect to the loss resistance, and thus, a nonideal transformer model isn't critical, and the losses can be modeled reasonably well as a parallel equivalent. A series equivalent is sometimes useful instead.

I have a nonideal, nonlinear (saturable), lossy transformer model I like to use for switching design. All the parameters are to hand, so you don't have to specify an equivalent, it does it from the differential equations directly.

To further expand on the concept of lossiness, general core materials (inductive and capacitive) are defined by an infinite number of parallel branches, consisting of series RLCs (or the reciprocal, an infinite series chain of parallel RLCs). This general approach allows one to model frequency-depending loss, dispersion, resonance, filtering and so on. Ferrites typically have a cutoff frequency, beyond which they appear resistive (i.e., imaginary permeability -- a "flux capacitor", as it were). Ceramic capacitors behave identically; the electric/magnetic domains in both materials take some time to "flip" to the new state and thus contribute hysteresis, eddy current (or equivalent) and resistance to the response. Some particularly interesting materials continue to do the flippity-flop out into optical bands, where some very interesting things occur, from simple coloration all the way up to cloaking (not that we have visible light metamaterials yet, but microwave materials have been fabricated!).

Tim

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Reply to
Tim Williams

I think that 20KHz is a bit too high, maybe 5KHz would be a workable upper limit. Well worth experimentation..

Reply to
Robert Baer

True, I oversimplified; I should have said there'd be some (unknown to me) parallel resistance. But that's why I suggested to the OP that he _measure_ his field coil at his intended frequency.

Having burnt my fingers on the coils of solenoids that were designed for DC and operated with PWM, I have no faith in the simple-minded notion that if it has a ferrous core, a coil will automatically reject all AC current flow.

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Reply to
Tim Wescott

The point is that a coil around a big lump of iron is not inductive on those frequencies where there may be eddy currents in the iron. 20 kHz is, for sure, enough to create problems with eddy losses. Usually, even laminated iron cores are problematic at these frequencies.

Why do you think that the TV line output transformers working at slightly lower frequencies are not on lamination core, but ferrite instead?

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Tauno Voipio

PS. Google for 'eddy current losses'
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Reply to
Tauno Voipio

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Just a quick follow-up. I read the responses and would like to clarify that the field will not be operating as a transformer, that is, it will not have an ac current imposed on it by an alternating square wave voltage. The voltage to the field will have zero volts imposed alternating with battery voltage imposed. The field will conduct a dc current with an ac ripple superimposed on it, not ac like a transformer. Duty cycle will determine the magnitude of the baseline dc current. Magnitude of the ripple will have more to do with the frequency.

If there are any problems with eddy losses in the iron, they would result from the ripple current. The ripple at 20kHz would be less than the ripple at the 200Hz. So the eddy current thing might not be a problem, because of the lower ripple amplitude at 20kHz. If I'm wrong about that enlighten me. Sure there could be something I overlooked.

I am also pondering stray inductance. There could be a lot of it around that field winding. How would stray inductance act at 20kHz versus say

200Hz? I haven't tried to construct a model for that and I don't have a generator available right now for testing, so I'm interested in having a discussion about that.

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Reply to
Michael Robinson

All the mentions of transformers that I saw or sourced weren't because we were thinking you meant to do that, or because we thought you should. It was because the way that you mathematically model such a beast is as a transformer with a mostly-shorted secondary winding, said winding being the actual iron pole-piece around which the field coil is wound.

Actually, the eddy current will be induced by the voltage, not the ripple current -- the ripple current _and_ the eddy current are both a consequence of the changing flux, which is a consequence of the voltage.

Dunno. That's why I suggested taking measurements.

The stray inductance may work for you -- if there's enough leakage inductance before your PWM voltage couples to your pole piece to make eddy current, then you're saved. But I couldn't say whether it'll help or not: you either need someone who has experience with this sort of thing to chime in, or you need to -- wait for it -- take some measurements.

You could always design your circuit with a spot for a honkin' big inductor and maybe a filter cap, then don't bother making/buying it unless you get too much loss in the generator field winding.

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Reply to
Tim Wescott

Fortunately for the induction heating business, it works for megawatts too :)

Tim

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Reply to
Tim Williams

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Can't do any testing. I have to build the regulator for a generator that I don't have access to...

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Reply to
Michael Robinson

l see

Maybe but thats not what he's doing.

Reply to
cbarn24050

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