AC line generator (60 hz) characteristics

[ge]

We have a customer who's trying to run some of our uP-based equipment
on a 15KW gas-driven generator. Things are not going well. (Like,
our box won't boot. Oops.) We're still trying to get information
from him, and we're going to get a generator here, etc. But, there's
the big rush; so, in the meantime, I thought I'd enquire here.

The generator in question is "a step above consumer-grade." We know
the frequency is not dead-on 60 hz, but we don't think that should
create the problems they're seeing. Transients might, depending on
their characteristics - particularly at the zero-crossing point.

If anyone wanted to share their experience with the output quality of
these things, I'd appreciate it.

TIA,
George

 

[Roger Lascelles]

It is a brave act to plug sensitive equipment into a generator, without
first checking the situation.

The generator can kill your equipment too - zap it in a nasty way so even
when you switch to good power, you continue to get failures over time.

Of course, the basic solution is a UPS.

Roger

 

[ChrisGibboGibson]

Assume output voltage will only be nominal and related to engine speed.
Assume output frequency anywhere from -10% to +40%. Assume waveshape will
have only only a passing resemblance to a sine wave. Assume third harmonics
will be present along with higher frequency components (eg 360Hz) that can
ride on the main waveform and upto 10%-20% of the output voltage. Depending
on generator quality, assume a non-linear series output inductance of up to
50mH. Bear in mind that generators have limited power capabilities and poor
power factors that can be tolerated when using a normal mains supply can
cause problems when a generator is used.
I.e just assume that the generated power is guaranteed to be of a crap
quality and design accordingly .

I'll second that.

One other thing I've seen on many generators is a dip in the waveform down to
around 10% voltage for 1 to 2mS duration at around 60 and 120 degrees.

Not nice.

Gibbo

 

[Joerg]

Hi George,

Large gas generators are usually pretty good. I operated lots of stuff
including computers with generators for days on end. Never had anything
behave erratically.

What these non-Diesel generators do though is create quite a bit of
noise from the magneto circuit. But if your system doesn't even boot
that isn't likely the cause.

Have them measure the voltage with a good RMS meter or an analog meter,
to see whether it in the proper range for your equipment. There should
also be a "base load" on the generator. If your gear draws little power
there should be some other constant load in parallel. Since they use
15KW I assume other stuff is on this generator. What is it? Could it
present a wildly varying load? Hopefully it isn't a large commutated
motor or an arc welder....

Regards, Joerg

 

[nospam]

We have a customer who's trying to run some of our uP-based equipment
on a 15KW gas-driven generator. Things are not going well. (Like,
our box won't boot. Oops.) We're still trying to get information
from him, and we're going to get a generator here, etc. But, there's
the big rush; so, in the meantime, I thought I'd enquire here.

Are they turning on your equipment by starting the generator? A slow
voltage buildup from a starting generator is something you would never see
on a mains supply.

 

[john jardine]

We have a customer who's trying to run some of our uP-based equipment
on a 15KW gas-driven generator. Things are not going well. (Like,
our box won't boot. Oops.) We're still trying to get information
from him, and we're going to get a generator here, etc. But, there's
the big rush; so, in the meantime, I thought I'd enquire here.

The generator in question is "a step above consumer-grade." We know
the frequency is not dead-on 60 hz, but we don't think that should
create the problems they're seeing. Transients might, depending on
their characteristics - particularly at the zero-crossing point.

If anyone wanted to share their experience with the output quality of
these things, I'd appreciate it.

TIA,
George

Assume output voltage will only be nominal and related to engine speed.
Assume output frequency anywhere from -10% to +40%. Assume waveshape will
have only only a passing resemblance to a sine wave. Assume third harmonics
will be present along with higher frequency components (eg 360Hz) that can
ride on the main waveform and upto 10%-20% of the output voltage. Depending
on generator quality, assume a non-linear series output inductance of up to
50mH. Bear in mind that generators have limited power capabilities and poor
power factors that can be tolerated when using a normal mains supply can
cause problems when a generator is used.
I.e just assume that the generated power is guaranteed to be of a crap
quality and design accordingly .

 

[Joerg]

Hi,

Are they turning on your equipment by starting the generator? A slow
voltage buildup from a starting generator is something you would never see
on a mains supply.

Usually the big commercial units have a hold-off so that the outputs
aren't enabled until the engine has stabilized.

This is off topic here but I have seen it happen: A guy had fun revving
engines so he tried that on a generator by grabbing that little link
lever on the governor. "Vrooooomm". Everything connected to this
generator died instantly.

Regards, Joerg

 

[John Woodgate]

I read in sci.electronics.design that Joerg <notthisjoergsch@removethisp

This is off topic here but I have seen it happen: A guy had fun revving
engines so he tried that on a generator by grabbing that little link
lever on the governor. "Vrooooomm". Everything connected to this
generator died instantly.

We have had problems in rural areas where the grid supply is down and
the supplier provides a temporary generator. The agricultural lads spray
a tractor starting aerosol into the air intake, which makes the diesel
engine speed up faster than the governor can compensate. Of course, they
are rarely caught in the act, but there are a lot of damaged appliances
as a result. The EMC filters usually undergo 'spontaneous high-energy
dis-assembly'.

 

[Jeff]

I read in sci.electronics.design that Joerg <notthisjoergsch@removethisp


We have had problems in rural areas where the grid supply is down and
the supplier provides a temporary generator. The agricultural lads spray
a tractor starting aerosol into the air intake, which makes the diesel
engine speed up faster than the governor can compensate.

The governor can't compensate much, since diesels usually only control the
fuel amount for speed, and not the air quantity. Dump in uncontrolled, large
amounts of fuel into the intake, and the engine governor could only cut the
diesel fuel supply to an minimal amount (about idle amounts). Dump enough
fuel into the intakes, and the engine can uncontrollably produce more then
rated power, depending on the fuel, and how much the injector pump holds
back at wide open throttle (for several reasons, mostly pollution). Fuel
introduced this way is also very hard on the engine, due to uncontrolled
fuel delivery timing, which causes detonation and preigntion. Hot spots may
also form.


Of course, they

 

[ChrisGibboGibson]

"Jeff" levy_jeff wrote:

[snip]

The governor can't compensate much, since diesels usually only control the
fuel amount for speed, and not the air quantity. Dump in uncontrolled, large
amounts of fuel into the intake, and the engine governor could only cut the
diesel fuel supply to an minimal amount (about idle amounts). Dump enough
fuel into the intakes, and the engine can uncontrollably produce more then
rated power, depending on the fuel, and how much the injector pump holds
back at wide open throttle (for several reasons, mostly pollution). Fuel
introduced this way is also very hard on the engine, due to uncontrolled
fuel delivery timing, which causes detonation and preigntion. Hot spots may
also form.

Welcome to sci.electronics.design.diesels

Sorry. Couldn't resist.

I actually found that quite interesting.

He's also correct. As I can confirm as the owner and maintainer of a vintage
Gardner diesel.

Gibbo

 

[John Woodgate]

The governor can't compensate much, since diesels usually only control
the fuel amount for speed, and not the air quantity. Dump in
uncontrolled, large amounts of fuel into the intake, and the engine
governor could only cut the diesel fuel supply to an minimal amount
(about idle amounts). Dump enough fuel into the intakes, and the engine
can uncontrollably produce more then rated power, depending on the fuel,
and how much the injector pump holds back at wide open throttle (for
several reasons, mostly pollution). Fuel introduced this way is also
very hard on the engine, due to uncontrolled fuel delivery timing, which
causes detonation and preigntion. Hot spots may also form.

I feel sure that the perpetrators worry about this a great deal. (;-)

Yes, my experience with starting reluctant tractors certainly confirms
the detonations. But you have to get the ba***rd going if you want to
get the hay in.

 

[ge]

We have a customer who's trying to run some of our uP-based equipment
on a 15KW gas-driven generator. Things are not going well. (Like,
our box won't boot. Oops.) We're still trying to get information
from him, and we're going to get a generator here, etc. But, there's
the big rush; so, in the meantime, I thought I'd enquire here.

The generator in question is "a step above consumer-grade." We know
the frequency is not dead-on 60 hz, but we don't think that should
create the problems they're seeing. Transients might, depending on
their characteristics - particularly at the zero-crossing point.

Thanks for the various replies. A few particulars:

If your gear draws little power there should be some other constant load
in parallel. Since they use 15KW I assume other stuff is on this generator.
What is it?

It's supposed to be incandescent lights. But, fairly often, it's just
some rather light SMPS. That's what they're testing with.

I only have 15KW on hearsay, but from a reliable source. Even
worst-case, I'd expect the actual load to be more like 2.5KW. It
would be unfortunate if light loading is a problem, since the end user
has reportedly bought a rather large number of them.

I wouldn't think the magneto noise would bother us, but it is
something we hadn't thought of. So, thanks for that.


This statement ...

One other thing I've seen on many generators is a dip in the waveform down to
around 10% voltage for 1 to 2mS duration at around 60 and 120 degrees.

This actually might cause us a great deal of trouble. Is it common
among generators? It seems fairly astounding. Do you know the
mechanism?

Thanks again,
George

 

[ChrisGibboGibson]

This statement ...


This actually might cause us a great deal of trouble. Is it common
among generators? It seems fairly astounding. Do you know the
mechanism?

Yes it's *very* common. Many phase controlled circuits have *real* trouble
because of this. I've never looked into what causes it so I have no idea of the
mechanism. It does tend to diminish the higher the load.

Gibbo

 

[Joerg]

Hi John,

Yes, my experience with starting reluctant tractors certainly confirms
the detonations. But you have to get the ba***rd going if you want to
get the hay in.

That is especially fun with the one-cylinder variety where the piston is
probably the size of a regular bucket. Remember the Porsche tractor and
the Lanz Bulldog?

Once they ran they kept going though. Thumpah - thumpah - thumpah....
and don't ever let the clutch snap.

Regards, Joerg

 

[Joerg]

Hi George,

It's supposed to be incandescent lights. But, fairly often, it's just
some rather light SMPS. That's what they're testing with.

SMPS can cause grief with a generator. The source resistance of the
power is higher than the utility would be and noise from the SMPS can
interfere with your product's supplies.

I only have 15KW on hearsay, but from a reliable source. Even
worst-case, I'd expect the actual load to be more like 2.5KW. It
would be unfortunate if light loading is a problem, since the end user
has reportedly bought a rather large number of them.

It doesn't have to be a problem. If they bought these large generators
there must be a reason and certainly they aren't after the better fuel
efficiency of a smaller model. At least for a test I'd hook up some
substantial load, for example a space heater that does not operate via a
thermostat.

This actually might cause us a great deal of trouble. Is it common
among generators? It seems fairly astounding. Do you know the
mechanism?

I only know older generators, the classic engine with synchronous
generator type. There I have never seen anything irregular about the
waveform. It resembled a sine pretty well. After all, we sometimes ran
all our electronics, computers, coffee maker and other gear with
generators. Except for fouled up spark plugs or a forgotten fuel level
check there were never any problems.

Regards, Joerg

 

[john jardine]

[-]

This statement ...

One other thing I've seen on many generators is a dip in the waveform down to
around 10% voltage for 1 to 2mS duration at around 60 and 120 degrees.

This actually might cause us a great deal of trouble. Is it common
among generators? It seems fairly astounding. Do you know the
mechanism?

Thanks again,
George

The new alternators are shite as compared to the older stuff. Use of finite
element field calculators has allowed many makers to trim the amount of
expensive Copper wire and steel lams down to a absolute bare minimum. Result
is alternators that are barely capable of supplying a unity power factor
resistive load, never mind bulding in a 20% overload margin or 0.8 power
factor capability. The generator market is cut-throat and to get good
performance means paying out more cash. Few do. How many engineering buyers
will (or can) discuss power factor aspects with their suppliers?.

Stingy design results in loose magnetic fields, hence a high series
inductance that varies markedly over each 360degrees single turn of the
motor.
Because of this oddball inductance, a total load as trivial as the impulse
current taken by a normal Power supply diode charging up it's capacitor
twice each cycle can be enough to cause sharp dips in the incoming waveform.
As always, there's no obvious, easy solutions and its down to detailed
design work to provide the compatibility.
regards
john