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Passive Power Factor correction

J

Johnny B Good

Jan 1, 1970
0
The message <[email protected]>
This is true. However, the torque required of the prime mover is roughly
proportional to the current flowing in the generator. With a low power
factor, the engine has to supply more torque and therefore more HP since the
speed is fixed. The excess HP ends up being dissipated as heat in the
generator windings.

Almost total bollocks! :) Yes, the I2R losses will be higher, but this
is unlikely to represent more than a few percent of the full load.
Small isolated generators (as opposed to large grid-connected
generators) are
somewhat of a special case regarding PF.

No, it's the same case only in microcosm.
 
J

Johnny B Good

Jan 1, 1970
0
The message <[email protected]>
from "stu" <[email protected]> contains these words:


I was following you until I got to this part.
I don't understand how PF correction would save much fuel. If the gererator
is loaded to maximum with a 0.50PF load, isn't still only generating the
REAL power?

Well spotted! That's exactly right. The I2R losses will be higher, but
these are normally only a small fraction (5% ?) of the generator's
output power.
 
D

daestrom

Jan 1, 1970
0
Johnny said:
The message <[email protected]>
from "stu" <[email protected]> contains these words:




Well spotted! That's exactly right. The I2R losses will be higher, but
these are normally only a small fraction (5% ?) of the generator's
output power.

One other small point, a very low pf means the voltage regulation won't be
very good. A lagging pf requires a larger increase in field current for a
given increase in load current. How much it affects the generator output
depends on details of the generator's exciter and regulator though.

daestrom
 
N

Neon John

Jan 1, 1970
0
Almost total bollocks! :) Yes, the I2R losses will be higher, but this
is unlikely to represent more than a few percent of the full load.

You wouldn't want to put a little wager on that would you? I'm REALLY hoping
that one of you alligators (all mouth, no brain) will back one of your claims
a little loot, as I could use some extra spending money.
No, it's the same case only in microcosm.

Really? So you're saying that since I can over-excite the field of a
grid-connected generator to supply leading KVARS when needed, then I can do
the same thing with a little isolated generator? What do you think happens to
the output of an isolated generator when its field excitation is increased?
After you explain that, please explain how that is the same only in microcosm.

Geez, man, I need hip waders anymore to wade through the BS in this group.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Sporadic E is the Earth's aluminum foil beanie for the 'global warming' sheep.
 
C

Cydrome Leader

Jan 1, 1970
0
daestrom said:
Sorry, that's NQR (not quite right). The inductive current and the real
current are 90 degrees out of phase. So the reactive current and the real
current form the two legs of a right-triangle and the total current is the
hypotenuse. If you apply Pythagorean Theorem,

Total^2 = Real^2 + Reactive^2

So in your example:
4^2 = 3^2 + Ir^2
Ir^2 = 16-9
Ir = sqrt(5) = ~2.2 amps.

At 120 V, to draw that you need a capacitive reactance of about:


So it works out to about 45uF total needed.

daestrom

you are correct.

I neglected the part about there being a real load (motor doing real
work), and not just cancelling out a purely inductive load with no real
power ( over simplified example).
 
J

Johnny B Good

Jan 1, 1970
0
The message <[email protected]>
from Neon John said:
On Tue, 14 Oct 2008 21:48:29 +0100, Johnny B Good
You wouldn't want to put a little wager on that would you? I'm REALLY
hoping
that one of you alligators (all mouth, no brain) will back one of your
claims
a little loot, as I could use some extra spending money.
Really? So you're saying that since I can over-excite the field of a
grid-connected generator to supply leading KVARS when needed, then I can do
the same thing with a little isolated generator? What do you think
happens to
the output of an isolated generator when its field excitation is increased?
After you explain that, please explain how that is the same only in
microcosm.

You can't make a generator produce 'reactive' VA, that's purely an
effect of the load characteristics. All "over-excitation" does is raise
the generator voltage which makes it drive more current into whatever
load it is seeing regardless of whether that load is purely resistive or
happens to include a portion of reactive loading. This, in turn, means
it will place a higher mechanical loading on its PM [1] due to it now
supplying more _power_.

PSUs [2] don't use individual gensets (or even whole power stations) in
this fashion to 'compensate' for lagging currents in the 'Grid', they
simply spread the load over whatever capacity they have on-line
regardless of the PF of the total load. In the end, it is the current
that sets the upper limit of operation.

Ignoring the secondary effect of increased I2R losses, the fuel
consumption of the PMs will actually drop when driving a low PF load.
Since the system is essentially limited by the current carrying capacity
of the wires and the associated circuit breakers, the PSUs penalise
commercial users with low PF loads since they can't sell as much power
without bringing more gensets on line which means a less profitable
operation.

Legislation of standards applied to domestic appliances is the
mechanism used to keep the PF above an acceptable minimum for the PSU's
domestic customers which is why they only ever meter for real power
consumption.

Medium to large commercial users, otoh, if allowed to, can present the
PSU with very low PF loads since the motors associated with any large
machinery are often custom builds or configurations where the PF
correction kit might be considered an 'optional extra'. The PSU will
find it worthwhile to also meter VA and peak VA demands as well as the
KWH consumption with appropriate penalty charges included in the
'contract to supply'.

However, in the context of this discussion, when you're generating your
own power, your main consideration is whether the VA loading is likely
to exceed the genset's VA rating for any protracted periods by more than
a few percent. Please, also keep in mind that a purely reactive current
with purely resistive current doesn't add up in a simple sum; for
example, a 4 amp 'wattless' current plus a 4 amp resistive current gives
a total current of 5.656 amps (a PF of .707). It's also worth
considering that correcting the PF to a 'perfect' figure of 1 is not an
economic exercise and correcting to a figure of 0.9 is deemed entirely
satisfactory in most practical cases (correcting that last 10% usually
involves a doubling or tripling up of PF correction capacitors).

[1] PM = Prime Mover

[2] PSU = Public Supply Utility (in this case, the Electric Power
Company or companies)
 
D

daestrom

Jan 1, 1970
0
Neon said:
You wouldn't want to put a little wager on that would you? I'm
REALLY hoping that one of you alligators (all mouth, no brain) will
back one of your claims a little loot, as I could use some extra
spending money.


Really? So you're saying that since I can over-excite the field of a
grid-connected generator to supply leading KVARS when needed, then I
can do the same thing with a little isolated generator? What do you
think happens to the output of an isolated generator when its field
excitation is increased? After you explain that, please explain how
that is the same only in microcosm.

Once again you don't know what you're talking about. Now your comparing
grid-tied to isolated systems, something Johnny B Good didn't even bring up.
Trying to cloud the issue or something?

When a generator is tied to an 'infinite bus' like the grid, raising
excitation makes it pick up more MVAR and supply more reactive power to the
bus.

On the other hand, if you take two small generators that are tied in
parallel supplying some common load without a tie to the grid, you can
increase the reactive loading and reduce the reactive loading on the other
at the same time by increasing its excitation. If you raise it far enough
above the other, you can have the first one supply reactive power to all the
reactive load on the bus and also force reactive power back into the
opposite generator. With two or more supplies tied into a bus, you can
share/shift reactive loading from one to another using the voltage
regulators (assuming they actually have a reactive droop characteristic
designed into them, something many small homepower units do *not* have).

An single, *isolated* generator cannot by itself change the amount of
reactive load on the bus. So changing excitation on an *isolated* generator
will only raise or lower the voltage.

Years ago in school, we used little (1 to 5 hp) generators to demonstrate
real and reactive load sharing between machines and between machines and an
'infinite bus'. Everything works substantially the same as the large MW
units, only on a smaller scale. If you actually studied and learned
anything about electric power systems, you wouldn't make such silly remarks.
But your hanging around with a couple of electricians makes you an
'expert'?? LOL

Your previous statements about a small generator...
....is wrong. While the current does of course increase with a lower pf, the
reactive current in the armature winding is not in the same phase
relationship with the field winding that created the EMF. So the reactive
component does *not* increase the torque load on the shaft and the engine
does not have to supply more HP to overcome that.

But a lagging pf will cause the terminal voltage to drop more and if the
unit has an active regulator, it will supply more current to the field
winding.

The exception to the rule is the small Honda inverter style generators that
are totally different in design and operation. They use a solid-state
inverter to create the output.
Geez, man, I need hip waders anymore to wade through the BS in this
group.

Considering that a lot of the BS is coming from you, well you deserve it.

daestrom
 
N

Neon John

Jan 1, 1970
0
Once again you don't know what you're talking about. Now your comparing
grid-tied to isolated systems, something Johnny B Good didn't even bring up.
Trying to cloud the issue or something?

Actually your buddy Johnny brought up the issue.

Me:
Johnny
No, it's the same case only in microcosm.

So according to Johnny, if a large grid-connected generator is the same as a
small one, only larger, then according to him I should be able to over-excite
the field of a small isolated generate and generate leading VARs just like I
can on a grid-connected generator. Absurd, of course. Almost as absurd as
you're being his sycophant.

But then I wouldn't expect your attention to detail to be any better in this
thread than any other.

BTW, still waiting for you to admit your error on where thermal sensors are
located in fractional HP motors. Or in larger motors, for that matter. I now
have some photos of a 1000 hp low speed motor we recently rewound - including
replacing the thermal sensors (notice plural). If you still insist on slot
placement, I'll be happy to post the photos and demonstrate yet another
example of your fabrication of "knowledge" and outright lying. Clue: They're
laid right in between the end turns just like on smaller motors.

John

--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
I love cats ... they taste just like chicken.
 
B

Bruce in alaska

Jan 1, 1970
0
daestrom said:
Years ago in school, we used little (1 to 5 hp) generators to demonstrate
real and reactive load sharing between machines and between machines and an
'infinite bus'. Everything works substantially the same as the large MW
units, only on a smaller scale. If you actually studied and learned
anything about electric power systems, you wouldn't make such silly remarks.
But your hanging around with a couple of electricians makes you an
'expert'?? LOL

Isn't it amazing what we remember from "Motor Generator LAB" in
the EE School......
 
C

Cydrome Leader

Jan 1, 1970
0
Neon John said:
Actually your buddy Johnny brought up the issue.

Me:

So according to Johnny, if a large grid-connected generator is the same as a
small one, only larger, then according to him I should be able to over-excite
the field of a small isolated generate and generate leading VARs just like I
can on a grid-connected generator. Absurd, of course. Almost as absurd as
you're being his sycophant.

But then I wouldn't expect your attention to detail to be any better in this
thread than any other.

BTW, still waiting for you to admit your error on where thermal sensors are
located in fractional HP motors. Or in larger motors, for that matter. I now
have some photos of a 1000 hp low speed motor we recently rewound - including
replacing the thermal sensors (notice plural). If you still insist on slot
placement, I'll be happy to post the photos and demonstrate yet another

Ah, so one motor that doesn't have something is proof that all others are
the same.

Tell use more about your chinese desk fan that has a motor that uses about
100 watts. Is that how much it uses when it's on fire, or running
normally?
 
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