5KW 3PH AC brushless generator output regulation and load-dump protection

[positivebalance41m]

I am tasked with the job to design a regulator circuit for a brushless 5KW
3PH AC burshless generator that gets it's output rectified to produce a
340VDC power source.

This is my first expereince with a generator.

Coming from a background in linear and switchmode power supplies, the first
thing I notice is how slow a generator responds to changes in excitation
coil current; and how awfully large the output ripple with a rich splash of
harmonics relating to the number of poles in the output and exciter
windings.

The big issue I am facing is load dump transient response. The generator
takes several hundreds of milliseconds to reduce it's output voltage after a
15 amp load is removed from the 340VDC rectified output.

Odd I find because the excitation coil is shut off by the regulator circuit
alomst instantly and I can see it decays to nothing in just under 100ms.

I notice the generator output can handle going from no load to full 15 amp
load in less than 100ms.

I am told the rotating excitation coil inside has some diodes on it, and
that this is a typical construction of a brushless generator.

Why is the load dump response time so slow, and what can be done to speed it
up? What type of output over-voltage protection schemes are typically used
in a generator based DC power supply? I can't have downstream electronics
modules connecting to my output getting blown-up! Yet to clamp or limit or
otherwise regulate or filter 5KW of power for almost 1/2 second long
transients where the outptu voltage can double, seems like an expensive
prospect?

If anyone knows a better news group or this type of question please do
suggest. I appreciate this is borderline power electrical and not entirely
electronics. However, I'm looking for answers that are more creative than
just to to use a large series filter inductor, shunt capacitor bank, and
over-voltage cut-out relay. My application is cost and size and weight
sensitive.


Suggestions anyone?

[email protected]

 

[Ian Stirling]

I am tasked with the job to design a regulator circuit for a brushless 5KW
3PH AC burshless generator that gets it's output rectified to produce a
340VDC power source.

This is my first expereince with a generator.

The big issue I am facing is load dump transient response. The generator
takes several hundreds of milliseconds to reduce it's output voltage after a
15 amp load is removed from the 340VDC rectified output.

Odd I find because the excitation coil is shut off by the regulator circuit
alomst instantly and I can see it decays to nothing in just under 100ms.

I know almost nothing about this, so in the best traditions of usenet,
I'll stick an oar in.
Where are you measuring the current in the exciter?
It seems to me that it's at least possible that the current is removed,
the coil current is continuing to flow, and there is a diode in there
to clamp it.
The relatively low voltage across the coil when the diode is conducting
means that the current decays slowly.

If you could increase the clamping voltage, to something that is safe
for the inductor wiring/... it would decay much faster.

 

[Tony Williams]

I am tasked with the job to design a regulator circuit for a
brushless 5KW 3PH AC burshless generator that gets it's output
rectified to produce a 340VDC power source.

[snip]

I have some information, based on testing regulators Terry,
but from the pov of never having designed a regulator myself.

The two-stage brushless alternator effectively has two field
coils, and therefore has two main time-constants, one for each
field circuit. Unfortunately these time-constants are not
fixed, they vary with load current and (to a lesser extent) with
rpm. In general, the time-constants are far longer at no load
than at full load.

Note that the low-current fixed field is totally under external
control. ie, You can drive the field from a low impedance in both
directions. However, the rotating field can only be driven
upwards through the rectifying diodes, the speed at which the main
flux reduces is dependant on the demagnetising effect of the load
current.

Note also that the Gain of the generator (Field-I v AC o/p)
varies heavily with load current, by as much as 5x. See the
remarks at the end about factoring the load-current as a
variable into the Field-I driver.

I do not have enough knowlege to be more informative than
that. You will need to delve quite deeply into alternator
dynamic performance for more accurate descriptions.

[snip]

Odd I find because the excitation coil is shut off by the
regulator circuit alomst instantly and I can see it decays to
nothing in just under 100ms.

I notice the generator output can handle going from no load to
full 15 amp load in less than 100ms.

Yes, those varying field time-constants means that the
response is always quicker at full load than at no load.

[snip]

Why is the load dump response time so slow, and what can be done
to speed it up? What type of output over-voltage protection
schemes are typically used in a generator based DC power supply?
I can't have downstream electronics modules connecting to my
output getting blown-up! Yet to clamp or limit or otherwise
regulate or filter 5KW of power for almost 1/2 second long
transients where the outptu voltage can double, seems like an
expensive prospect?

You might need to have a dummy transient load, maybe only
about 10% of full load. Also sense the 3-phase output current,
and have a 3-phase output contactor.

IF (Overvoltage) AND (Field-I=Zero) AND (I-Out=Zero) THEN
Dummy-Load = ON.

If that state lasts greater than a certain time, then open
the output contactor.

Umm.... best of luck with your regulator, not a job for
the faint-hearted..... oscillation of the servo loop, at
certain speeds and loads, seems to be the main thing that
will bite you in the bum.

You might find it handy to characterise the Gain and the Response
of the alternator at various rpms and loads. Sense the rpm and
output current, and factor those variables into the field-coil
controller. The aim is to get a control that is automatically
nearly right in open-loop, so that the closed loop voltage servo
has as little to do as possible.

 

[N. Thornton]

I am tasked with the job to design a regulator circuit for a brushless 5KW
3PH AC burshless generator that gets it's output rectified to produce a
340VDC power source.

This is my first expereince with a generator.

Coming from a background in linear and switchmode power supplies, the first
thing I notice is how slow a generator responds to changes in excitation
coil current; and how awfully large the output ripple with a rich splash of
harmonics relating to the number of poles in the output and exciter
windings.

The big issue I am facing is load dump transient response. The generator
takes several hundreds of milliseconds to reduce it's output voltage after a
15 amp load is removed from the 340VDC rectified output.

Odd I find because the excitation coil is shut off by the regulator circuit
alomst instantly and I can see it decays to nothing in just under 100ms.

Yes, but the generating windings are inductive too.

I notice the generator output can handle going from no load to full 15 amp
load in less than 100ms.

Ah, then your prolonged decay may be due to freewheeling diodes. Or, I
dont know how youve got it all wired, but might the generating wind be
feeding a little power to the exciting wind somehow? If so, that may
slow the decay.

I am told the rotating excitation coil inside has some diodes on it, and
that this is a typical construction of a brushless generator.

Why is the load dump response time so slow, and what can be done to speed it
up?

A huge power zener. Not literally a zener, but something to do the
same job.

What type of output over-voltage protection schemes are typically used
in a generator based DC power supply? I can't have downstream electronics
modules connecting to my output getting blown-up! Yet to clamp or limit or
otherwise regulate or filter 5KW of power for almost 1/2 second long
transients where the outptu voltage can double, seems like an expensive
prospect?

Does it? a 5kW bank of halogen bulbs costs little. You then have to
switch them in/out with power trs and control cicuitry, plus add
passive protection to cover the tiny time delays involved in switching
the halogens on, and the small inductive kickback from them.

There are other options, depending on what you're powering. Options
like a slower power down of the load, and so on. As ever, more info
would be good.


Regards, NT