MEP 802/803A QUAD winding & voltage regulator A1 protection

[asilitch]

Thank you all very much for the wealth of information and experience compiled on this site regarding these generators:

I have recently acquired a low-hour early-production 803a.

I am trying to understand the design-flaw that leads to the rectifier bridge shorting out in the voltage regulator (and thus to the destruction of the QUAD winding -- unless a fuse is added to the circuit).

At this point it seems to me that the fundamental problem is that the field flash circuit can provide excessive voltage to the field windings, and cause the QUAD winding to put out voltages much higher than the design voltage. Is this more or less correct?

The TB suggests that the voltage regulator should be re-designed with diodes in the quad circuit bridge that can withstand a higher PRV. The suggestion in these posts is to add an MOV across the QUAD windings to limit the voltage seen by the rectifier bridge. However, if the root problem is excessive voltage from the field flash function, why not add a clamping circuit between the field flash resistor (R14) and diode (CR2)?

If I am making a gross mistake here, or if this might be an effective solution, I would be grateful to feedback from those with more understanding and experience.

Thanks!

Alec

 

[DieselAddict]

I'm sure Kurt will be able to answer your question better than I but I'll take a swing at it. From my understanding its the failure mode of the transistor that drives the quad windings. When it fails, it shorts and causes full upstream power to get dumped into the coils. Its more than they were designed for and it will burn them out.

The field fix was to add a fuse so when the regulator did eventually fail, the fuse would blow and save the windings. The MOV was added to improve the lifespan of the output transistor.

The true fix is to replace the stock VR with the one that @kloppk builds. He designed his so if it failed it would NOT take out the windings.

 

[Digger556]

I'm sure Kurt will be able to answer your question better than I but I'll take a swing at it. From my understanding its the failure mode of the transistor that drives the quad windings. When it fails, it shorts and causes full upstream power to get dumped into the coils. Its more than they were designed for and it will burn them out.

The field fix was to add a fuse so when the regulator did eventually fail, the fuse would blow and save the windings. The MOV was added to improve the lifespan of the output transistor.

The true fix is to replace the stock VR with the one that @kloppk builds. He designed his so if it failed it would NOT take out the windings.

That's pretty close, my understanding was the voltage rating of the diodes in the rectifier bridge was too low and quad winding can burn them out under certain conditions. The fuse protects the quad winding if the regulator fails, but Kurts MOV mod clamps voltage transients from the quad winding, preventing failure of the diodes.

 

[kloppk]

My understanding is that during cranking the Quad winding in the gen head can output some voltage spikes that exceed the PRV rating of the diodes in the VR causing them to short out. Exactly how I don't know. The flashing circuit is voltage limited by R14 and this only supplies voltage to the static exciter during cranking to cause the stator to output voltage on its 6 output windings and the Quad winding.
However, both the VR output to the static exciter and the flashing circuit are providing power to the static exciter during cranking. The flashing voltage is switched in by relay K15 when the engine RPM is below about 900 RPM and S1 in is START.

Basically the factory A1 VR's input diode bridges PRV rating is too low and they can short out if the see any spikes exceeding their PRV. The VR's should have been made with higher PRV rated input diodes. Some choose to swap in diodes with higher PRV ratings to prevent this failure.

The military's "fix" was the "Fuse Mod" to add a fuse between the Quad Winding and the A1 VR. If the input diodes in the VR short out the fuse pops to protect the Quad winding from burning up wrecking the entire stator. The A1 VR is still toast.

The MOV, as mentioned, is an added item I came up with years ago to clip the voltage spikes from the Quad winding to prevent them from causing the A1 input diodes from failing and shorting out.

 

[kloppk]

... He designed his so if it failed it would NOT take out the windings.

It has both input and output protection by way of a pair of fuses. It does not utilize the power from the Quad winding so it's not affected by Quad winding voltage spikes. Also the VR is switched out of the circuit during Flashing.

 

[asilitch]

Thank you all very much, and especially @kloppk ! I really appreciate your going over this for me, it was very helpful.