Upgrading generator control board

[Jester]

I'm attempting to repair and possibly upgrade the control board for a 7kW Onan generator.

The problem... The generator shuts down on over-voltage just as it starts to output voltage. I scoped the output and indeed the output very quickly climbs past 135V before being shut down.

The way it works (my best guess after poking around the board and capturing wave forms:

A dedicated winding on the generator (referred to as quadrature winding) drives a two SCR/two diode rectifier that is used to control the field. Best guess is that the uC monitors the output voltage and either stops or delays the turn on time of the gate pulses when the voltage is too high, and turns them on sooner in the cycle when the output voltage is too low.

I tried disconnecting the field drive from the controller and substituted in a DC supply and I can control the output voltage by varying the field voltage as expected.

Apparently these control boards have a history of failing, especially this SCR based portion of the circuit. I'm speculating it's the transients that do them in or or possible just degrade the breakdown voltage over time.

Both the SCR as well as the diode (2 of each) are D2PAK:

SCR is TN 1215800G           12A, 800V
Diode is  ISL9R18120S3      18A, 1200V

If I can replace these devices with similar parts (with same package) with better ratings it just might live longer next time. Any suggestions for upgraded SCR / diode

I beeped out the circuit and it appears to be as follows:

[Renate]

I have some (limited) experience with Onan generators.
The ones that I know have three windings on the stator: the main output, the ("quadrature") exciter and a pokey (1 Amp) battery charging winding.
The exciter winding goes to a full wave bridge and a big cap for ~150 VDC.
The field coil is PWM'ed in the kilohertz.
There is also "field coil flash" when it starts to throw 12 VDC to the field coil to get things started.

Yes, this is different than yours.

It's amazing how difficult they make it to get a grip on this stuff.
It's all proprietary, potted regulators.
They can't give you a general idea how the field coil is wired in.
They tell you polarity but not which side is switched.

I was looking at one point to have a µP controlled throttle.

A friend's generator fried out the quadrature winding from an internal short.
Sure, you can have it rewound for $500 or so.
I took it apart, ripped out the fried winding.
I ran an isolation transformer from the main output to the exciter input.

[moffy]

This might do for the SCR: TN1625-1000G :- 16A/1000V but 25mA trigger current instead of the 15mA for the original. You would need to check that it triggers correctly.
https://www.digikey.com.au/en/products/detail/stmicroelectronics/TN1625-1000G-TR/686419
It's D2PAK, but since they are surface mount with probably limited heatsinking it might be heat that is killing the devices not voltage or current.

P.S. If there are brushes involved in the field coil connection, check their condition and the slip ring in case they are inducing arcing.

[Jester]

I have some (limited) experience with Onan generators.
The ones that I know have three windings on the stator: the main output, the ("quadrature") exciter and a pokey (1 Amp) battery charging winding.
The exciter winding goes to a full wave bridge and a big cap for ~150 VDC.
The field coil is PWM'ed in the kilohertz.
There is also "field coil flash" when it starts to throw 12 VDC to the field coil to get things started.

Yes, this is different than yours.

It's amazing how difficult they make it to get a grip on this stuff.
It's all proprietary, potted regulators.
They can't give you a general idea how the field coil is wired in.
They tell you polarity but not which side is switched.

I was looking at one point to have a µP controlled throttle.

A friend's generator fried out the quadrature winding from an internal short.
Sure, you can have it rewound for $500 or so.
I took it apart, ripped out the fried winding.
I ran an isolation transformer from the main output to the exciter input.

Yes they have made an effort to make it difficult to repair, and they charge $2k for the replacement board (plenty in stock for quick delivery;-) )

DMM diode check of the SCR appears okay, however might be breaking down with higher voltage.

Your description matches how mine works, except no big capacitor.

I'm going to do another test and capture the field signal after the initial 12V blip.

Images of preliminary testing below.....

The last image is the interesting one. This test is with a heavy load and the generator NOT shutting down with error code 12 as it normally does with no or light load. I think this load is the sweet spot (enough to not trigger the over-voltage shut down but demonstrate the field drive problem. It looks like the field drive is only occurring during one half of the cycle (this makes sense the uC is trying to reduce the field drive current). However during the other half of the cycle it looks like the SCR is breaking down and turning on as soon as the voltage is too high.

If replacing the SCR's doesn't work, I'm half tempted to make a little circuit to monitor the output voltage and drive the field with a PWM based 60Vdc supply.

[Jester]

This might do for the SCR: TN1625-1000G :- 16A/1000V but 25mA trigger current instead of the 15mA for the original. You would need to check that it triggers correctly.
https://www.digikey.com.au/en/products/detail/stmicroelectronics/TN1625-1000G-TR/686419
It's D2PAK, but since they are surface mount with probably limited heatsinking it might be heat that is killing the devices not voltage or current.

P.S. If there are brushes involved in the field coil connection, check their condition and the slip ring in case they are inducing arcing.

There are brushes for the field. I cleaned them up with scotch-bright. I think they are okayish now.

Probably one of the following:
1) SCR is breaking down when voltage from quadrature winding builds up.
2) Brush arcing noise is getting back and false triggering the SCR's
3) Reference signal to uC is noisy and so uC is not acting as it should.

[Renate]

I don't see a free-wheeling diode in your diagram. I mean, there has to be one.