brushless alternator?

Brand? Model?

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Michael A. Terrell
Central Florida

I don't understand why you think an alternator would need brushes or slip rings. An alternator is an AC generator. The diode rectifies its output (serving a function similar to what brushes would do in a DC generator).

BTW, in some sense all good capacitors are "open circuits." However, an ohm-meter should initially show a low "resistance" (until the capacitor charges).

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Just honing in on your post - you say the capacitor is open circuit - how did you determine that? And how did you determine the diode is OK?

Just wondering whether you actually checked the capacitor with a proper capacitor tester or just a multimeter?

And did you check the diode in both forward and reverse modes? ie with the meter leads one way it should read almost a short circuit, and with the meter leads reversed it should read almost infinity

David - who is very sus about the diode

CJT wrote:

How is the field current controlled? Automotive alternators use a pair of slip rings to power the rotor, regulation is achieved by adjusting the current through the rotor as needed to keep the output voltage constant. I've never seen one that was brushless.

I'm very familiar with testing capacitors, I've been repairing electronics as a hobby for around 15 years now. I checked it with an ESR meter as well as compared it to a similar motor run cap I had on hand. The capacitor is bad, besides, the diode is good, there's no other active components I can see and it stopped producing power suddenly while under load so it isn't a matter of flashing the field.

I checked the diode out of circuit with my Fluke multimeter's diode function, so far it's never let me down, though the check does only pass a few volts through it.

As I said in another post, the capacitor was thoroughly and properly tested, trust me, it's open circuit. As far as I know, these oil filled caps are internally fused so my guess is the plates shorted and blew the fuse.

Does it matter? I'm just asking for general principals here, not advice on fixing this particular unit. Surely brushless alternators of this sort must be widespread and likely don't vary much from unit to unit.

Presumably the rotor is a series of magnets? Rotate any magnet(s) within a coil(s) and you can get a form of AC output. DC dynamos have commutators to give 'pulsed' DC from this. The slip rings in a car alternator are there so the current in the field windings can be varied thus the output.

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Have you tried Google? Searching on schematic "brushless alternator" yields a lot of hits, including a simplified description and schematic of a three phase brushless alternator at:

and more info at:

If the armature has windings, you need to connect to them. The only alternator that wouldn't require any brushes nor slip rings would be one that had just a permenant magnet on the armature.

Generators don't produce electricity they convert mechanical energy to electrical energy.

So replace the capacitor and find out.

How do you think it works, you turn it and the magnets move and break electrons free from their atoms and that creates a charge and therefore electrical current.

Maybe the armature is broken.

You said generator not transformer.

That generator sounds broken beyond repair.

Search for "induction generators"

There's a common trick of using an ordinary induction motor as an AC sine wave generator. It requires one cap across the winding and that is usually a non-polar type with a low dissipation factor (oil filled are ideal)

They won't start under load - have to be started then the load applied. They balk at running other induction motors unless the motor is ~1/6 the size of the generator.

No rectifier necessary. (but that may be part of some self-excitation scheme, to insure the iron keeps some residual magnetism)

They can lose excitation and stop producing even without any defective parts. They usually depend on some residual magnetism to kick things off - and that may not be there - in which case you connect a DC source to the generator when it is stopped to magnetize the iron.

gives some info on them.

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I would also question that . . . oil filled caps don't incorporate fuses, real oil filled caps are "self-healing" and damn near indestructible. It is possible, but not likely.

And all caps are open with a DC ohm meter - except lossy electrolytic types.

Electrolytic caps can also be used in induction alternators and they can and do fail - the caps have to withstand a high circulating current - uses no power (ideally) but the current has to flow through the cap and the cap has to be able to handle that current.

Home power alternators look similar to induction generators and work similar but incorporate permanent magnets in the rotor - no regulator except control the speed of the engine driving it to keep the frequency correct.

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James-

The field windings on the rotor have their own set of diodes (and resistors), often two, sometimes four. I assume these are the diodes you checked. There are no slip rings because the field coil generates AC, the diodes rectify it for DC field current.

(Some generators also have a separate stator winding and rectifier for use in charging a battery, in case that is the rectifier you checked.)

The capacitor is connected to an independent winding on the stator. I don't understand how it works, but as load current increases the capacitor's reactance shifts the magnetic field to increase generator output. That is the only form of regulation on some simple generators.

If you are interested, one source of information is the search function at

Fred

budgie wrote in news: snipped-for-privacy@4ax.com:

Interesting topic. In brief, it's possible to make an alternator without brushes, as follows:

On the same shaft, there are two separate rotor windings. In the case, there are two separate stator windings.

The first stator gets its current from the battery. Its rotor generates AC which is rectified by diodes which are rotating with it. This DC is then wired to the second rotor winding, which is of course also rotating with the diodes. Hence, no brushes.

This second rotor now has a DC current running through it, which makes it the field coil for the second alternator. Its stator generates AC which is rectified by diodes and is wired to the battery as usual.

One way to control the output is to regulate the current to the first stator winding, which will regulate the current to the second rotor winding.

Yeah I did and I saw that actually. This one is different though. There's a single set of windings on the rotor which has a single diode, though there may possibly be another diode buried at the motor end of the rotor but looking through the vents I didn't see anything. I didn't see any diodes in the stator either, though they might be hidden as well. I have another motor run cap of similar but not the same rating on hand which I'll try in the generator and see if it fixes the problem to the extent of producing some power, I'm absolutely certain the original cap is open but I'm not positive another problem doesn't exist.

Seems nobody here so far has a real solid understanding of how these work but I'll figure it out. I was just hoping for a nice concise ready made theory of operation to replace my deductions.

No, the rotor is electromagnetic, it looks just like a very large DC motor armature except it has only two poles and it is entirely self contained with no electrical connection to any of the stationary parts. The one diode I can find is mounted in the rotating assembly.

So this one I'm looking at then doesn't exist? It clearly says "Brushless Alternator" and there are clearly no brushes or slip rings, just as there clearly *are* windings rather than permanent magnets on the rotor. It's starting to sound like a magic trick.

Question it all you like, but the ESR meter shows this 35uF 370VAC capacitor as open, and a similarly constructed 50uF 370VAC capacitor as less than one ohm. Using the capacitance range on the DMM the 35 uF cap reads 15 nF, while the 50 uF cap reads 50.2 uF. I can't imagine any circumstances beyond an open connection, whether it be a fuse or just a broken wire inside the capacitor can that would cause this.

There's a way to do this, but it's rather inefficient.

You take your basic old furnace fan motor, which has stator windings, and a rotor with pseudo-windings (actually just angled strips of copper or aluminum.)

Now this kind of motor works just fine, used in billions of applications. You might wonder how it rotates with no permanent magnets in the rotor and no slip rings. It's done by induction-- the stator windings induce current in the rotor, which reflects back a magnetic field. This requires a bit of slip, so these motors are typically rated at "1725 RPM" instead of 1800.

You can do exactly the opposite-- rotate the motor and have the generated current feed back a magnetic field. Only glitch, you need a hefty capacitor across the stator windings to provide the out-of phase current. And the efficiency isnt too wonderful. And there's no easy way to adjust the current versus voltage versus hertz.