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bob58o

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Lawnmower Generator AC to DC sharing common ground / neutral???

Question is general because I am trying understand fundamentals. I will give specifics as to what I am trying to do so you can understand. I am adding a charging system and electric start to an engine that never had one. I replaced the flywheel with one from a different engine and got the stator associated with that flywheel.

The stator I am using has a single lead. I believe this means the other end of the coil is grounded to the engine block through the stator’s mounting bolts.

The output of the stator is AC. When tested, the voltage was reading about 28-32VAC (RMS) at around 3600 RPM.

If the output is ran through a diode for half wave rectification, I was measuring around 14.5VDC across the battery terminals at around 3600 RPM. This sounds great, just use the diode and be done. However, this engine may turn 5500 RPM, instead of 3600 RPM. I want to use a Rectifier/Regulator instead of a simple diode to regulate the charging voltage. Looking at Rectifier / Regulator options, I soon ran into issues wondering how to wire it with only one lead from the stator.

I am using a Honda GX style key-switch box and starter motor. I was told the starter motor is “case grounded” to the engine and uses DC. My stator is also “case grounded” to the engine block for AC. I believe the ignition coil is grounded to the engine block as well (not sure if that is DC as well?)

When I first did testing (using a diode for rectification), I ran a jumper from the neg side of the battery to a bolt on the crankcase. So here I’m sure the engine block is being used as DC ground.

How does the stator being “grounded” to the engine block interact with the battery and DC circuitry also being ground to the engine block?

Trying to find a rectifier/regulator I was told that I couldn’t get it to work properly because I was using the engine block as ground/neutral for both AC and DC. I was told I needed a 2 wire stator so that I could isolate the AC from the rest of the DC system. But even without the Regulator (when just using a diode) wasn’t I already using the engine block for DC ground and AC neutral? Wouldn’t problems have shown themselves there?

I then came across the B&S 5/9 Amp regulated charging systems. These systems use a single wire stator that produces 28-40VAC RMS (28V for the 5A system and 40V for the 9A system) at 3600 RPM. This is very similar to the output of my stator/flywheel combo. This system uses a 2 wire rectifier/regulator. The two wire regulator is mounted to the engine via a bolt through its center. I assume since there is only 1 input lead and only 1 output lead, the R/R is “case grounded” through this mounting bolt. I assume this is using the same point on the block as ground/neutral for AC and DC.

Can anybody explain to me how this Briggs 5/9A regulated system works? Is a common ground for AC and DC even an issue? Is there a concept I am missing? Is there a hidden isolated ground being used somewhere somehow?
 

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It's all about the configuration of the rectifier.

With a half wave rectifier, only the positive-going half-cycles of the AC from the stator deliver current into the battery. Therefore although the stator voltage is AC, the current is pulsating DC. Considering the rectifier and stator coil as a unit, the blocked negative half-cycles are never seen, i.e. there is arguably no AC system in operation at all. A pulsating DC source is charging a DC battery, of which the negative is grounded. The Briggs module contains a regulator but is still half-wave rectified, so nothing really changes. It needs to see ground to sense the battery voltage and operate its own electronics, but the charging current still passes unidirectionally from stator to battery and no AC circuit exists.

With a bridge rectifier comprising four diodes, both half-cycles of stator output are used to charge the battery, which can offer a higher charge rate from a given stator. Due to the action of the bridge rectifier, the polarity of the stator coil and the DC system reverse relative to each other once per cycle. Therefore they cannot share a common terminal i.e. the engine block, as the polarities would conflict on the negative half-cycles.

In theory it doesn't matter whether the AC or DC side of the bridge rectifier has one wire grounded.; a 1-wire block-return stator would work with a fully insulated DC system, and a 1-wire block-return starter would work with a fully insulated 2-wire stator. In practice the latter is much more practical, as there is only the stator coil on the AC side to insulate, but multiple devices on the DC side that can benefit from having a block ground connection.
 
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Thank you so much. If I think of the stator/rectifier/regulator as one pulsating DC source and I ignore the AC aspect it makes it easier in my head - even if I don’t totally understand what Briggs Module is actually doing. Maybe something like a diode to half wave rectify, then a step down DC to DC regulator?

Just make sure I understand how to finish wiring this circuit... with this type of rectifier/regulator, there is no need to place another diode in the circuit to prevent the battery from “back feeding” the stator, correct?

Once I connect the stator to the regulator and the regulator to the key-switch box, there is a dual AC / DC circuit breaker.

125-250V AC
32V DC
5 AMPS


My “alternator” system is only 36watts I believe.

I believe I can wire as is the diagram without any other components needed. The Briggs 2-wire RR will be here tomorrow and I will hook it up (basically just replacing the diode with the Briggs module).

I will test voltage across the battery from 1500 RPM to about 6000 RPM. I plan on testing using the electric start but first with no electrical accessories running, then with an 18watt light, and finally with the 18watt light plus a 1-2psi 1amp 12V (12watt) fuel pump. I’d like to see 13-15 Volts across the battery throughout the whole RPM range in all 3 cases (no accessories, 1 accessory, and both accessories). My battery is 18 Ah.

Hopefully nothing electric blows up while Im trying to make sure the Kohler Flywheel I am using doesn’t explode once I turn it up to 6000 RPM. Sometimes the “ignition magnet” on the outside of this style flywheel will let go.
 

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