Hi folks,
This isnt going to be quick Please bare with me.
I have, or at least had, a small single phase generator, circa 2.3KVA with an electronic field control circuit ( AVR - Automatic Voltage Regulator)
The AVR module is dead, short, so the genny output is way up and uncontrolled.
To test the system I applied a field current from a bench supply, it draws about 1.2A DC at around 30V when the genny has a 500W load.
All the windings are OK, the main stator has a tap at aprox 10% which was connected to the AVR, I assume for voltage sensing, and there is a secondary stator winding that outputs about 100VAC, during the above test, which was also connected to the AVR unit and must be the source of the field current, via whatever control circuit used to be in place.
The broken AVR unit is completely potted, giving only the merest indication of a substantial bridge rectifier and one big electrolytic cap, everything else is hidden in solid opaque epoxy.
I have built a small circuit around a FET capable of driving the field from a PWN signal (It probably needs additional work on the gate drive to reduce switching losses but will do for now.)
I have a sawtooth generator, supplied from the power control board above, which is stable even when the generator is under load, and can control the field adaquatly by applying this signal and a DC level to a simple comparator ......
My first problem, more RE fet gate drives later, is getting a stable DC level that is representative of the AC output of the genny.
I currently have a half wave rectifier, on the 10% tap, feeding a cap via a simple resistive bridge. The problem's are:-
Large cap = smooth DC level but way too slow to be of any use and the whole system oscillates slowly.
Small cap = large ripple on the DC level and the PWM becomes wildly unstable with a very noisy gate signal. Overall however the final regulation is 'better', high frequency oscillations, but the fet would fry if driven in this way for long.
In addition the type of load, (chopping PSU verses water pump), makes a huge difference to relationship between the average DC level and the generator output. I think this is due to the 10% tap behaving partially as a current shunt and thus superimposing a voltage proportional to the ofset current waveform that the above reactive loads induce in the main winding.
Sorry that's long but it seemed reasonable to provide all available info before asking questions.
SO .....
Q1 Do you think my 'shunt' theory is correct, if so it is obviously intentional but why?
Q2 How can I remove / cancel the ripple on my existing DC level?
Q3 How would I get a DC level proportional to the peak of the AC.
Q4 How would I get a DC level proportional to the Average of the AC.
Q5 Can I measure RMS without a microprocessor given that the generator is highly unlikely to produce a clean sinusoidal output and will almost always be exhibiting a poor power factor.
Q6 Am I just going about this all wrongly, if so why?
Any help would be much appreciated, 'O' and dont pull any punches I am quite happy to look silly but have a working generator again.
Thanks for reading all this
Al
This isnt going to be quick Please bare with me.
I have, or at least had, a small single phase generator, circa 2.3KVA with an electronic field control circuit ( AVR - Automatic Voltage Regulator)
The AVR module is dead, short, so the genny output is way up and uncontrolled.
To test the system I applied a field current from a bench supply, it draws about 1.2A DC at around 30V when the genny has a 500W load.
All the windings are OK, the main stator has a tap at aprox 10% which was connected to the AVR, I assume for voltage sensing, and there is a secondary stator winding that outputs about 100VAC, during the above test, which was also connected to the AVR unit and must be the source of the field current, via whatever control circuit used to be in place.
The broken AVR unit is completely potted, giving only the merest indication of a substantial bridge rectifier and one big electrolytic cap, everything else is hidden in solid opaque epoxy.
I have built a small circuit around a FET capable of driving the field from a PWN signal (It probably needs additional work on the gate drive to reduce switching losses but will do for now.)
I have a sawtooth generator, supplied from the power control board above, which is stable even when the generator is under load, and can control the field adaquatly by applying this signal and a DC level to a simple comparator ......
My first problem, more RE fet gate drives later, is getting a stable DC level that is representative of the AC output of the genny.
I currently have a half wave rectifier, on the 10% tap, feeding a cap via a simple resistive bridge. The problem's are:-
Large cap = smooth DC level but way too slow to be of any use and the whole system oscillates slowly.
Small cap = large ripple on the DC level and the PWM becomes wildly unstable with a very noisy gate signal. Overall however the final regulation is 'better', high frequency oscillations, but the fet would fry if driven in this way for long.
In addition the type of load, (chopping PSU verses water pump), makes a huge difference to relationship between the average DC level and the generator output. I think this is due to the 10% tap behaving partially as a current shunt and thus superimposing a voltage proportional to the ofset current waveform that the above reactive loads induce in the main winding.
Sorry that's long but it seemed reasonable to provide all available info before asking questions.
SO .....
Q1 Do you think my 'shunt' theory is correct, if so it is obviously intentional but why?
Q2 How can I remove / cancel the ripple on my existing DC level?
Q3 How would I get a DC level proportional to the peak of the AC.
Q4 How would I get a DC level proportional to the Average of the AC.
Q5 Can I measure RMS without a microprocessor given that the generator is highly unlikely to produce a clean sinusoidal output and will almost always be exhibiting a poor power factor.
Q6 Am I just going about this all wrongly, if so why?
Any help would be much appreciated, 'O' and dont pull any punches I am quite happy to look silly but have a working generator again.
Thanks for reading all this
Al