Alternator/regulator electrical question

[rizzz]

I have a permanent magnet alternator & voltage regulator on my VW engine very similar to the one on the AeroVee.
I also use a S704 relay on the DC output of my regulator to be able to cut it off from the rest of circuit if needed.

The alternator stator has an AC output (2 wires) that goes directly to the regulator and I tap off that to get an RPM reading. (See my other thread here: viewtopic.php?f=6&t=2294)
The strange thing is, when disconnect my alternator/regulator (with the S704 relay), I also lose my RPM reading, as if the whole alternator is turned off.
This in itself is not a problem but I just don’t understand why this is, how can cutting off the output from the voltage regulator/rectifier from the circuit render my alternator inoperative?
It’s not shorting the alternator is it?

Please note as mentioned in the other thread, I am currently using an optocoupler to “square” the signal coming from the alternator to pick up my RPM,
but before I was just getting the signal directly off one of the output wires trying to read the pigtails from the signal (which had its problems), I had the same behaviour back then losing all readings when the relay was turned off.

Do others get RPM readings when the alt relay is switched off?
If so can anybody enlighten me into how this works as my knowledge in this field is limited.

[Bryan Cotton]

rizzz,
My initial guess is that the AC from the unloaded PM alternator is high and your opto coupler is saturated. When it has the regulator connected it will load the circuit and the voltage will drop.

[tonyr]

I believe its a quirk of the type of switching regulator and open circuiting the DC output.
Same thing happens with my Jab system and I inadvertently leave the alternator switch off, (despite the bright yellow annunciator light!)
First noticed no rpm on the tach after start...oops
Wether or not its shorting the AC output to drop the volts to match the voltage sense (0v) Im not sure.. but probably is.
Haven't put my hand on the regulator afterwards to see if its hot!
It has been suggested to preferably put the relay in the AC side to remove all power from the regulator in case it fries itself, but the rpm indication will still be a problem if its selected off.

[rizzz]

rizzz,
My initial guess is that the AC from the unloaded PM alternator is high and your opto coupler is saturated. When it has the regulator connected it will load the circuit and the voltage will drop.

Thanks Brian,
Unfortunately, when I was trying to pickup the signal directly from one of the alternator outputs (which gave me very unstable readings) I had the same problem so I don't believe it's related to the optocoupler.

[rizzz]

I believe its a quirk of the type of switching regulator and open circuiting the DC output.
Same thing happens with my Jab system and I inadvertently leave the alternator switch off, (despite the bright yellow annunciator light!)
First noticed no rpm on the tach after start...oops
Wether or not its shorting the AC output to drop the volts to match the voltage sense (0v) Im not sure.. but probably is.
Haven't put my hand on the regulator afterwards to see if its hot!
It has been suggested to preferably put the relay in the AC side to remove all power from the regulator in case it fries itself, but the rpm indication will still be a problem if its selected off.

Hmmm, at least I'm not alone and it's not something in my electrical system.
I know most people have it set up like this but still, you've got to wonder, are these permanent magnet alternators even meant to be "turned off" in this way?
The motorcycles they come from (Kawasaki in case of the AeroVee, Harley in case of mine) do not have a switch/relay.

Perhaps putting the relay on the AC side as you suggest might be a better option indeed.

[Bryan Cotton]

Still, I would be curious what would happen if you put two 10 mohm resistors in series across the alternator and picked up your signal in between them. Are you still on a breadboard? This would cut the AC voltage in half.

I do not think there is an issue running a PM alternator unloaded. The voltage will be high and both voltage and frequency increase with speed. I have done it on my old Suzuki to diagnose a bad stator and we do it in the lab all the time, after we knock the voltage down.

[rizzz]

I've now got a proper circuit board but I still have the breadboard so I can give that a try.

Like you I can't see how having no load could be an issue but still, the only way I can see the voltage regulator rendering the alternator inoperative is by shorting it which can't be good.
I'm trying to find diagrams of the internals of these voltage regulators, so far I have found one:

he alternator has a permanent-magnet rotor that needs no external power source to activate it. There are four charging coils in the stator, connected in pairs in series. The two series-connected pairs are connected in parallel. Thus, the charging coils only have two yellow wires leading to the rectifier.

The rectifier and voltage regulator are built into one unit in these charging systems. The rectifier/regulator unit is a solid-state type and cannot be disassembled or adjusted. CAUTION: Do not turn the bolts and nuts between the cooling fins in this unit. They are not adjustments. They are heat-dissipation points for the internal components. If they are loosened, the components will overheat and the rectifier/regulator unit will fail. Although there are several semiconductor components in the rectifier/regulator unit, the special components that make up the regulator circuit are a feature of this unit. One is the Zener diode, which like any diode will conduct only in the "forward" direction unless a "reverse"' voltage great enough is applied. (Any diode will conduct in reverse if a great enough voltage is present. but the Zener diode will not be damaged by this kind of treatment.) The other unique component is the bidirectional-controlled rectifier, or BCR, a type of electronic switch. It has a third lead called a "gate" lead. Ordinarily, the BCR will allow current to flow through it in one direction only. However, when the BCR is "gated" (when it has had a voltage applied to the gate lead), it will conduct in either direction until the gate voltage is removed.

To combine these two special components into a voltage-regulated circuit, a Zener diode is chosen with a breakdown voltage of 15.5 volts. The Zener diode senses when the output of the alternator reaches the desired maximum charging voltage of 15.5. It then breaks down and gates the BCR. The BCR is used in place of an ordinary diode in the rectifier circuit. The other three diodes in the circuit are conventional. But the BCR, when it is gated, will conduct in both directions. and the rectifier circuit can only send half as much current to the battery as a result. The other half is sent back to the charging coils through the BCR, conducting in a reverse direction. As soon as the BCR is gated and the rectifier can send only half the alternator output to the battery, the battery voltage drops and the Zener diode stops conducting. The no-longer-gated BCR returns to normal diode function, and the rectifier again sends the full alternator output to the battery. The cycle starts all over again. By combining the rectifier and voltage-regulator functions in a single unit, Kawasaki has made the system less expensive to manufacture and more reliable.

Not quite like the ones we use but close, still does not tell me much though...

Here's another related question, do you star/stop the engine with the alternator on or off?
I've been starting with the switch off and turning it on after starting, then turning it off before stopping the engine.
Is this what others do? (given the issue above it might not be the best thing to do?)

[tonyr]

Hi Rizz,

I start with the ALT, BATT and EFIS switches on.
The over-voltage relay is what the ALT switch controls, and OV protection in is the circuit. It should take care of any transients that might damage the EFIS.
Dynon say that it is fine to have the EFIS on as it has some internal protection as well, and I use it to monitor the engine start parameters.
Arcing of the relay contacts is something to consider if it is switched on after start, especially if the battery is a bit low and the rpms are high (>1500) for warmup.
This over time could lead to issues with the relay failing.

[rizzz]

Hi Rizz,

I start with the ALT, BATT and EFIS switches on.
The over-voltage relay is what the ALT switch controls, and OV protection in is the circuit. It should take care of any transients that might damage the EFIS.
Dynon say that it is fine to have the EFIS on as it has some internal protection as well, and I use it to monitor the engine start parameters.
Arcing of the relay contacts is something to consider if it is switched on after start, especially if the battery is a bit low and the rpms are high (>1500) for warmup.
This over time could lead to issues with the relay failing.

Thanks Tony,
I think I'll do that from now on (and update my POH)