5KW brushless generator needs load dump overvoltage protection

[positivebalance41m]

I am tasked with the job to either find or commision the custom design or
design for myself... a regulator circuit for a brushless 5KW 3PH AC
burshless generator that gets it's output rectified to produce a 340VDC @ 15
amps 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; in particular to the excitation level dropping. Also I note
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. During this time
the generator output voltage is more than double!

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 and this way it has a fast transient response. My
output L/C filter cleans things up nicely. But going from full load 15 amps
to no load 0 amps is turning out to be a nightmare!

I am told the rotating excitation coil inside has some diodes on it, and
that this is a typical construction of a brushless generator. The rotating
excitation field coil freewheels it's current through these diodes and the
resulting field decay rate can be surprisingly slow.

What can be done to speed up a brushless generator's load dump recovery
time? 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 with voltage rises of up to
+100% which last for 1/2 second or more! Yet to clamp or limit or
otherwise switching regulate or filter 5KW of power for almost 1/2 second
long
seems like an expensive and tricky prospect?


Looking for pointers to off-the-shelf equipment suppliers???

Custom design expertise resources (companies and consultants)???

Suggested circuit topology and critical components one might use along with
their ratings???

Any layout and assembly issues which would promote reliabilty and avoid EMI
as much as possible???

This will be required for a rugged mobile application with wide range
temperature specification and high shock/vibration levels. Low cost is the
primary driving factor. Weight and compactness are not that much of an
issue.


Suggestions anyone?

Terry Moreau

[email protected]

 

[Ralph & Diane Barone]

I am tasked with the job to either find or commision the custom design or
design for myself... a regulator circuit for a brushless 5KW 3PH AC
burshless generator that gets it's output rectified to produce a 340VDC @ 15
amps 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; in particular to the excitation level dropping. Also I note
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. During this time
the generator output voltage is more than double!

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 and this way it has a fast transient response. My
output L/C filter cleans things up nicely. But going from full load 15 amps
to no load 0 amps is turning out to be a nightmare!

I am told the rotating excitation coil inside has some diodes on it, and
that this is a typical construction of a brushless generator. The rotating
excitation field coil freewheels it's current through these diodes and the
resulting field decay rate can be surprisingly slow.

What can be done to speed up a brushless generator's load dump recovery
time? 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 with voltage rises of up to
+100% which last for 1/2 second or more! Yet to clamp or limit or
otherwise switching regulate or filter 5KW of power for almost 1/2 second
long
seems like an expensive and tricky prospect?


Looking for pointers to off-the-shelf equipment suppliers???

Custom design expertise resources (companies and consultants)???

Suggested circuit topology and critical components one might use along with
their ratings???

Any layout and assembly issues which would promote reliabilty and avoid EMI
as much as possible???

This will be required for a rugged mobile application with wide range
temperature specification and high shock/vibration levels. Low cost is the
primary driving factor. Weight and compactness are not that much of an
issue.


Suggestions anyone?

Terry Moreau

[email protected]

Brushless generator? You're probably hooped, since you can't force the
field winding voltage negative in order to force the field current to decay
faster. I would put a buck converter (or an off the shelf SMPS) downstream
and live with the overvoltage.

 

[R.Legg]

I am tasked with the job to either find or commision the custom design or
design for myself... a regulator circuit for a brushless 5KW 3PH AC
burshless generator that gets it's output rectified to produce a 340VDC @ 15
amps 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; in particular to the excitation level dropping. Also I note
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. During this time
the generator output voltage is more than double!

When clamped, the voltage won't double, but the decay period might be
longer.

+100% which last for 1/2 second or more! Yet to clamp or limit or
otherwise switching regulate or filter 5KW of power for almost 1/2 second
long
seems like an expensive and tricky prospect?

2500joules - that looks like a 6inch long by 1inch diameter resistor ~
27 ohms 50W. You might have got by with a smaller part constructed out
of SiC (once refered to as carborundum), but they seem to be available
only on special order from the present manufacturers.

Control voltage with a 400V sensing circuit and a 600V mosfet switch
with hysterisis (on at 425V, off at 385V). Maybe tweak 'on' to respond
to dv/dt. You'd have to examine switch losses, if it spent much time
in a linear mode at turn-off. The turn-off time would have to be
geared to the generator's real response time to non-double voltage
output - takes some testing.

Looking for pointers to off-the-shelf equipment suppliers???

Kanthal Globar offers ceramic parts:

http://www.cesiwid.com/ec/tubular.php.html

Any layout and assembly issues which would promote reliabilty and avoid EMI
as much as possible???

No relevant emi effects for a single surge event.

Fairly simple configuration - should not affect MTBF, as it doesn't
normally function, but still one more power switch stressed to 60% at
380V, when off.

RL

 

[Winfield Hill]

R.Legg wrote...

Terry Moreau (positivebalance41m) wrote ...

When clamped, the voltage won't double, but the decay period might be
longer.


2500joules - that looks like a 6-inch long by 1-inch diameter resistor ~
27 ohms 50W. You might have got by with a smaller part constructed out
of SiC (once refered to as carborundum), but they seem to be available
only on special order from the present manufacturers.

Control voltage with a 400V sensing circuit and a 600V mosfet switch
with hysterisis (on at 425V, off at 385V). Maybe tweak 'on' to respond
to dv/dt. You'd have to examine switch losses, if it spent much time
in a linear mode at turn-off. The turn-off time would have to be
geared to the generator's real response time to non-double voltage
output - takes some testing.

Terry, the power resistor and MOSFET switch (with hysteresis) that
R.Legg suggests sounds uncomplicated and easy to implement. But you
may find several FETs wired in parallel are required to handle 15A.
For example the classic IRF840, IR's largest 500V FET in the old days,
is rated at only 5.1A when operating hot at 100C junction temp, which
is the temperature the junction would be after running a while at 5A.
http://www.irf.com/product-info/datasheets/data/irf840.pdf

Here's the calculation for that: Ron = 1.75 * 0.85-ohm max = 1.5 ohms
(fig 4 in datasheet), so P = I^2 R = 38.7W for 5.1A. With R_theta_JC
= 1.5C/W (w/ insulator) we calculate a 58C increase over the FET's case
temperature. Say the heat sink contact-point rises to 70C over a 22C
ambient, then Tj rises to its 150C limit. This would be a worst-case
equilibrium result for an indefinite-duration situation.

If the heat sink can be counted on to stay cooler for a short time,
say a 38C rise, then you're allowed a 90C junction-to-case gradient,
which works out to 60W and 6.3 amps max. So sorry, at least three
parallel IRF840 FETs (four would be better) would be required for a
safe 500V holdoff and 15A long-duration switching.

There are modern high-voltage FETs that can handle more current (but
they're often hard to get). For example, IRF's IRFPS40N60K is a 600V
FET that can handle 15A continuously with a safety margin, if properly
mounted. Allied Electronics has 39 in stock, selling for $12.60 each.
By contrast the IRF840 is widely available at low prices. Allied has
them at only $1.33 each. Digi-Key's website says they have 38543 !!
in stock at $2.05 each. Newark has the IRF840A, an advanced version
of the '840 requiring less gate charge, for $1.56 each.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Fred Bloggs]

I am tasked with the job to either find or commision the custom design or
design for myself... a regulator circuit for a brushless 5KW 3PH AC
burshless generator that gets it's output rectified to produce a 340VDC @ 15
amps 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; in particular to the excitation level dropping. Also I note
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. During this time
the generator output voltage is more than double!

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 and this way it has a fast transient response. My
output L/C filter cleans things up nicely. But going from full load 15 amps
to no load 0 amps is turning out to be a nightmare!

I am told the rotating excitation coil inside has some diodes on it, and
that this is a typical construction of a brushless generator. The rotating
excitation field coil freewheels it's current through these diodes and the
resulting field decay rate can be surprisingly slow.

What can be done to speed up a brushless generator's load dump recovery
time? 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 with voltage rises of up to
+100% which last for 1/2 second or more! Yet to clamp or limit or
otherwise switching regulate or filter 5KW of power for almost 1/2 second
long
seems like an expensive and tricky prospect?


Looking for pointers to off-the-shelf equipment suppliers???

Custom design expertise resources (companies and consultants)???

Suggested circuit topology and critical components one might use along with
their ratings???

Any layout and assembly issues which would promote reliabilty and avoid EMI
as much as possible???

This will be required for a rugged mobile application with wide range
temperature specification and high shock/vibration levels. Low cost is the
primary driving factor. Weight and compactness are not that much of an
issue.


Suggestions anyone?

Terry Moreau

[email protected]

There seem to be innumerable switching techniques that can be used. One
way would be to trigger a buck regulator on OVP, and the regulator
switch is normally bypassed by a polyfuse which is forced open by an OVP
crowbar. Some variation on this anyway- :
Please view in a fixed-width font such as Courier.





(+)o----------+--polyfuse---+ HYSTERETIC VOLT
| | CONTROL
| | |
+----+ +--+---+ | 375----------------
|OVP | switch | | | | / \ / \
| | -------> | IGBT | | | / \ / \
|CTL | | | | | / \ / \
+----+ +--+---+ | 340-------------------
| |
+-|>|---+-----|<|-----+
| )|| |
| )|| |
| )|| OVP
| )|| CRWBAR
| | |
| | |
| +---------------------> Vout
| | |
| === |
| | |
| | |
(-)o-+-------+-------------+------->

 

[Winfield Hill]

Fred Bloggs wrote...

There seem to be innumerable switching techniques that can be used.
One way would be to trigger a buck regulator on OVP, and the regulator
switch is normally bypassed by a polyfuse which is forced open by an
OVP crowbar. Some variation on this anyway- :

Acckk!! A 5kw regulator.

Please view in a fixed-width font such as Courier.

(+)o----------+--polyfuse---+ HYSTERETIC VOLT
| | CONTROL
| | |
+----+ +--+---+ | 375----------------
|OVP | switch | | | | / \ / \
| | -------> | IGBT | | | / \ / \
|CTL | | | | | / \ / \
+----+ +--+---+ | 340-------------------
| |
+-|>|---+-----|<|-----+
| | |
| )|| |
| )|| OVP
| )|| CRWBAR
| | |
| +---------------------> Vout
| | |
| === |
| | |
(-)o-+-------+-------------+------->

That's a more simple 5kW regulator than most. But the IGBT will get
hot! And it'll stay hot as long as the gnerator is in use, whereas
the switched-resistor and FET will be cool.

BTW, thanks for the insight; an IGBT would be much more suitable than
a power MOSFET to switch the resistor, because a single low-cost part
can do the job nicely. For example, an IRG4PC30F is a 600V IGBT that
likely dissipates under 25 watts at 15A, and costs $3.08 from Newark.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Yzordderex]

Terry,

I'm wondering where energy is coming from to send output up to 2X.
Could solution be simple as increasing your LC filter capacitor across
output of generator to snub energy stored in winding reactance or
filter inductance? If you double C and 1/2 L, what happens?

regards,
Bob

 

[GPG]

Try adding resistance in series with generator rotor to decrease Q of its inductance

 

[Winfield Hill]

GPG wrote...
It appears the generator output sags under its 15A max load,
and soars when the load is removed. Can the increase be due
simply to an inductance flyback?

Terry, does the engine speed slow automatically after the load
is removed, to bring the output voltage back down to spec.?

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Matthew Beasley]

I am tasked with the job to either find or commision the custom design or
design for myself... a regulator circuit for a brushless 5KW 3PH AC
burshless generator that gets it's output rectified to produce a 340VDC @ 15
amps 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; in particular to the excitation level dropping.

Is this a design where the field you are controlling is the field of a
pilot exciter? You may have found that without some way to control the
field directly on the rotating portion the decay is slow. This is why
some large generators have SCR controls on the shaft - particularly on
water turbine generators where the time constant can be longer.
Not really practical for a generator of this size.

Also I note
how awfully large the output ripple with a rich splash of harmonics relating
to the number of poles in the output and exciter windings.

Shouldn't be problem in a rectified output application, but yeah, you
should be able to see the number of poles, and if you look close you may
even be able to see the ripple from the stator slots.

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. During this time
the generator output voltage is more than double!

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 and this way it has a fast transient response. My
output L/C filter cleans things up nicely. But going from full load 15 amps
to no load 0 amps is turning out to be a nightmare!

I am told the rotating excitation coil inside has some diodes on it, and
that this is a typical construction of a brushless generator. The rotating
excitation field coil freewheels it's current through these diodes and the
resulting field decay rate can be surprisingly slow.

What can be done to speed up a brushless generator's load dump recovery
time? 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 with voltage rises of up to
+100% which last for 1/2 second or more! Yet to clamp or limit or
otherwise switching regulate or filter 5KW of power for almost 1/2 second
long
seems like an expensive and tricky prospect?

You may want to look at using a SCR rectifier bridge instead of a diode
bridge to produce the 340V DC. For normal operation the SCRs can be
gated full on. Under a load dump the SCRs can be turned off. I would
think that 1200V SCRs that could hold off the voltage rise are much
cheaper than any sort of clamp circuit. You could use a 1/2 controlled
rectifier module (3 SCRs for the positive side, and 3 diodes on the
negative side) for the rectification to hold the price down.
For an example, look at http://www.ixys.com/Appasp/pdrmod14.asp
Your L/C filter should be able to limit the dV/dt rate to a value slow
enough that the voltage rise won't be a problem for the 1 cycle response
time of the SCR bridge.
You could use 3 zero crossing SCR optocouplers to turn on the SCRs if
the voltage is OK, and turn off the opto's if the voltage climbs too
high; I would think that a simple hysteresis control would work to limit
the voltage rise until the regulator could regain control.

One thing to worry about is the startup of the regulator. Many
regulators use the residual magnetism of the exciter to provide the
power to start up the regulator and build up the output voltage. If you
intend to use this method, you will need to make the regulator operate
at low output voltages, often less than 5% of normal. Also be aware
that if the generator isn't used for awhile, it may not pick up at;
requiring "flashing" the field. (Briefly applying an external voltage
to the exciter field). The other option is to power the regulator off
of the engine starting circuit.

 

[Tony Williams]

I notice the generator output can handle going from no load to
full 15 amp load in less than 100ms and this way it has a fast
transient response. My output L/C filter cleans things up
nicely.

That L/C filter might also cause trouble, because at
No-Load it is a peak detector, and there is no means
of reducing the voltage on the C.

But going from full load 15 amps to no load 0 amps is turning out
to be a nightmare!

Have you experimented with other load reductions?

ie, Going from 100% Load to X% Load (in closed-loop or
switching down the primary exciting current appropriately),
where X is down around 10 or 20%. Is the transient Over-V
more acceptable for these steps?

Doing this experiment could point you towards the value
of the transient dummy load that is required to keep the
voltage across the C within spec.

Note that, as soon as you put a C on the output, you also
have to do a fast discharger of that C anyway, for safety
reasons.

 

[GPG]

Have sent email confirm several solutions easy

 

[Tony Williams]

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. During this time the generator output voltage
is more than double!

Vdc from a 3-phase bridge rectifier is roughly Vac(L-L)*1.35.
So 340Vdc requires a nominal 252Vac under normal operation.

A single half-cycle that charges that C to 650Vpk would be
equivalent to 0.707*650, or about 460Vac L-L. [see note]

Silly Question Time.... Terry, exactly what is the No-Load
terminal voltage of that alternator under the equivalent
of Full-Load excitation? Is it really as high as 460Vac?

Note: Sinewave sums are only approximations when applied
to small alternators, because the crest-factor changes with
loading. The waveshape is often 'peaky' at No-Load, with
the tips flattening on load, especially rectifier loads.