motor can act as an inductor?

[Alan Horowitz]

the scenario is a large vessel where service electric power is usually
supplied by either a straight diesel-driven alternator, OR a
mechanical power-take-off (from shaft main propulsion) driving a
(different, but identical) alternator.

the manual states that when PTO is selected (this is preferred for
fuel-economy, a "synchronous compensator", which is just a large motor
of its own, is in circuit to "add inductive reactance".

How can a motor do that, and why does the PTO's alternator need the
additional inductance thrown into the circuit?

 

[John Larkin]

On 1 Feb 2004 14:03:50 -0800, alanh_27@yahoo.com (Alan Horowitz)
wrote:

the scenario is a large vessel where service electric power is usually
supplied by either a straight diesel-driven alternator, OR a
mechanical power-take-off (from shaft main propulsion) driving a
(different, but identical) alternator.

the manual states that when PTO is selected (this is preferred for
fuel-economy, a "synchronous compensator", which is just a large motor
of its own, is in circuit to "add inductive reactance".

How can a motor do that, and why does the PTO's alternator need the
additional inductance thrown into the circuit?

An unloaded synchronous motor can act as a capacitor or an inductor
depending on whether its rotating field is over or under-excited
(can't remember which is which!) Utilities sometimes use them as
virtual capacitors for power factor compensation. I can't guess why
additional inductance would be desired... capacitance is what makes
most power systems happy.

Does the shaft speed have to be constant for the PTO alternator to
make 50/60 Hz? Is the main propulsion steam or diesel?

John

 

[Operator Jay]

"John Larkin"
<jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote in
message news:m05r105sv739daasvg9it0avlg9iu34nhm@4ax.com...

On 1 Feb 2004 14:03:50 -0800, alanh_27@yahoo.com (Alan
Horowitz)
wrote:

the scenario is a large vessel where service electric
power is usually
supplied by either a straight diesel-driven alternator,
OR a
mechanical power-take-off (from shaft main propulsion)
driving a
(different, but identical) alternator.

the manual states that when PTO is selected (this is
preferred for
fuel-economy, a "synchronous compensator", which is just
a large motor
of its own, is in circuit to "add inductive reactance".

How can a motor do that, and why does the PTO's
alternator need the
additional inductance thrown into the circuit?

An unloaded synchronous motor can act as a capacitor or an
inductor
depending on whether its rotating field is over or
under-excited
(can't remember which is which!) Utilities sometimes use
them as
virtual capacitors for power factor compensation. I can't
guess why
additional inductance would be desired... capacitance is
what makes
most power systems happy.

Does the shaft speed have to be constant for the PTO
alternator to
make 50/60 Hz? Is the main propulsion steam or diesel?

John

I believe over excitation makes the sync motor look like a
capacitive load. I agree that usually capacitance is added
to improve pf / voltage, etc. Anyone know if there's a
transient stability benefit by staying well away from
leading load pf?

j

 

[Bill Shymanski]

"John Larkin" <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote in
message news:m05r105sv739daasvg9it0avlg9iu34nhm@4ax.com...

An unloaded synchronous motor can act as a capacitor or an inductor
depending on whether its rotating field is over or under-excited
(can't remember which is which!) Utilities sometimes use them as
virtual capacitors for power factor compensation. I can't guess why
additional inductance would be desired... capacitance is what makes
most power systems happy.

As the field current goes up, the reactive power of a synchronous
machine goes from lagging (inductive) pf, through unity pf and minimum
line current, up to leading (capacitive) power factor. I had the chance
to try this out one idle day at the steel plant, turning the big knob
that regulated field current to a 30 MVAR synchronous condensor used in
the arc furnace circuit to compensate for lagging power factor. You can
plot line current vs. field current and come up with a "V-curve"
so-called because of the shape - the minimum of the V corresponds to
unity power factor. For loaded machines the "V" isn't very sharp, and I
found that on the machine I had access to, the ammeters weren't terribly
well calibrated for the low-current end of the range so it was hard to
accurately plot a V-curve.

Bill

 

[Paul Hovnanian P.E.]

Operator Jay wrote:

[snip]

I believe over excitation makes the sync motor look like a
capacitive load. I agree that usually capacitance is added
to improve pf / voltage, etc. Anyone know if there's a
transient stability benefit by staying well away from
leading load pf?

A leading load power factor reduces system stability margins. All other
thing being equal, the more leading KVARs added at the load, the less
the generator(s) will need to supply. For the same load voltage, this
requires less generator excitation. As the generator's excitation is
reduced, the peak power point (top of the generator's power curve) is
lowered. This reduces the margin between the normal operating point and
this peak, which makes it more likely that a fault or load transient
will cause the generator to pull out of sync.

--
Paul Hovnanian mailtoaul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
We are confronted with insurmountable opportunities.
-- Walt Kelly, "Pogo"

 

[Johan Lexington]

"Paul Hovnanian P.E." <Paul@Hovnanian.com> wrote in message news:<401EAC00.32332CFD@Hovnanian.com>...

Operator Jay wrote:

[snip]

I believe over excitation makes the sync motor look like a
capacitive load. I agree that usually capacitance is added
to improve pf / voltage, etc. Anyone know if there's a
transient stability benefit by staying well away from
leading load pf?

A leading load power factor reduces system stability margins.

No it doesn't. Where did you learn the swing equation?

All other
thing being equal, the more leading KVARs added at the load, the less
the generator(s) will need to supply. For the same load voltage, this
requires less generator excitation. As the generator's excitation is
reduced, the peak power point (top of the generator's power curve) is
lowered. This reduces the margin between the normal operating point and
this peak, which makes it more likely that a fault or load transient
will cause the generator to pull out of sync.

Nope. It's mass. This is an advanced subject that you don't seem qualified.
Johan, P.E.E. see I got an extra E.

 

[John Larkin]

On 2 Feb 2004 13:53:52 -0800, JohanLexington@webmail.co.za (Johan
Lexington) wrote:

"Paul Hovnanian P.E." <Paul@Hovnanian.com> wrote in message news:<401EAC00.32332CFD@Hovnanian.com>...
Operator Jay wrote:

[snip]

I believe over excitation makes the sync motor look like a
capacitive load. I agree that usually capacitance is added
to improve pf / voltage, etc. Anyone know if there's a
transient stability benefit by staying well away from
leading load pf?

A leading load power factor reduces system stability margins.

No it doesn't. Where did you learn the swing equation?

All other
thing being equal, the more leading KVARs added at the load, the less
the generator(s) will need to supply. For the same load voltage, this
requires less generator excitation. As the generator's excitation is
reduced, the peak power point (top of the generator's power curve) is
lowered. This reduces the margin between the normal operating point and
this peak, which makes it more likely that a fault or load transient
will cause the generator to pull out of sync.

Nope. It's mass. This is an advanced subject that you don't seem qualified.
Johan, P.E.E. see I got an extra E.

Obnoxious jerk, however many letters follow your name.

John

 

[Don Kelly]

"Johan Lexington" <JohanLexington@webmail.co.za> wrote in message
news:d69e3ae6.0402021353.3a34a3e0@posting.google.com...

"Paul Hovnanian P.E." <Paul@Hovnanian.com> wrote in message
news:<401EAC00.32332CFD@Hovnanian.com>...
Operator Jay wrote:

[snip]

I believe over excitation makes the sync motor look like a
capacitive load. I agree that usually capacitance is added
to improve pf / voltage, etc. Anyone know if there's a
transient stability benefit by staying well away from
leading load pf?

A leading load power factor reduces system stability margins.

No it doesn't. Where did you learn the swing equation?

All other
thing being equal, the more leading KVARs added at the load, the less
the generator(s) will need to supply. For the same load voltage, this
requires less generator excitation. As the generator's excitation is
reduced, the peak power point (top of the generator's power curve) is
lowered. This reduces the margin between the normal operating point and
this peak, which makes it more likely that a fault or load transient
will cause the generator to pull out of sync.

Nope. It's mass. This is an advanced subject that you don't seem
qualified.
Johan, P.E.E. see I got an extra E.
----

Actually, with all other things being equal, the stability limit of a
generator IS reduced at leading power factor because the excitation voltage
is reduced and this affects the power transfer - look at the right hand side
of the swing equation and what affects the transfer. Mass simply determines
how fast the machine swings. Also look at the steady state capability limits
of a synchronous generator and steady state limitations (mass not involved)
on leading operation. Paul is absolutely right.
For motor operation, leading pf will be the overexcited case but for
generator operation it is the underexcited case. Suck the excitation down
enough and the generator will fall out of synch below rated KVA.

--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

 

[Repeating Rifle]

I just want to point out that drawing leading or lagging current does not a
capacitor or inductor make. A reactor component, capacitor or inductor, is
determined by its impedance as a function of frequency. Thus the capacitance
of a reactance modulator (think early FM) does not make a useful energy
storage device.

Bill

 

[Johan Lexington]

----
Actually, with all other things being equal, the stability limit of a
generator IS reduced at leading power factor because the excitation voltage
is reduced and this affects the power transfer - look at the right hand side
of the swing equation and what affects the transfer.
How do you think the right side of the equation was developed....MASS

Mass simply determines
how fast the machine swings. Also look at the steady state capability limits
of a synchronous generator and steady state limitations (mass not involved)
on leading operation. Paul is absolutely right.

Well, you are wrong also.

For motor operation, leading pf will be the overexcited case but for
generator operation it is the underexcited case. Suck the excitation down
enough and the generator will fall out of synch below rated KVA.

Sorry, it's kVA. When you write things incorrectly more mistakes are
easier to understand.

This group is just too funny. I read it for the entertainment; not
the technical content.

 

[John Gilmer]

"Repeating Rifle" <SalmonEgg@sbcglobal.net> wrote in message
news:BC448340.88C4%SalmonEgg@sbcglobal.net...

I just want to point out that drawing leading or lagging current does not
a
capacitor or inductor make. A reactor component, capacitor or inductor, is
determined by its impedance as a function of frequency. Thus the
capacitance
of a reactance modulator (think early FM) does not make a useful energy
storage device.

Maybe no, maybe so.

Aren't there amplifiers based on "pumping" a long linear cap?

EMWTK

 

[sammmm]

a motor IS an inductor. also, if you overexcite the field on a synchronous
motor, it acts like a capacitor,
adding a leading P.F. reactance, even without a mechanical load connected.
sammmm


"Alan Horowitz" <alanh_27@yahoo.com> wrote in message
news:1e3670a7.0402011403.2f83fada@posting.google.com...

the scenario is a large vessel where service electric power is usually
supplied by either a straight diesel-driven alternator, OR a
mechanical power-take-off (from shaft main propulsion) driving a
(different, but identical) alternator.

the manual states that when PTO is selected (this is preferred for
fuel-economy, a "synchronous compensator", which is just a large motor
of its own, is in circuit to "add inductive reactance".

How can a motor do that, and why does the PTO's alternator need the
additional inductance thrown into the circuit?

 

[ron doctors]

alanh_27@yahoo.com (Alan Horowitz) wrote in message news:<1e3670a7.0402011403.2f83fada@posting.google.com>...

the scenario is a large vessel where service electric power is usually
supplied by either a straight diesel-driven alternator, OR a
mechanical power-take-off (from shaft main propulsion) driving a
(different, but identical) alternator.

the manual states that when PTO is selected (this is preferred for
fuel-economy, a "synchronous compensator", which is just a large motor
of its own, is in circuit to "add inductive reactance".

How can a motor do that, and why does the PTO's alternator need the
additional inductance thrown into the circuit?

The group seems confused in many ways .. The reason for a utility
to need a power factor correction ( using a motor as a "capacitor" by
overdriving it for example ) is that the utility has to generate more
current if the voltage and current are not in phase. A simple vector
diagram shows this. ( look up any old Alternating Current
generator/alternator/production book ) . The utility gererator has to
pay for this "excess" current, ( but you do not! ), so naturally they
do not want you taking power at anything but unity power factor. As
the main use of electricity is to produce a lagging PF the need arises
for a very large capacitor to to be placed to correct the PF.
A large synchronous motor running at sync speed but with an
overexcited winding looks electrically like a very large capacitor.
That's it folks!!

PS JUst becuase I mentioned overexcited please do not add to my SPAM
with vi**&#a mail!

 

[The Captain]

JohanLexington@webmail.co.za (Johan Lexington) wrote in message news:<d69e3ae6.0402021353.3a34a3e0@posting.google.com>...

"Paul Hovnanian P.E." <Paul@Hovnanian.com> wrote in message news:<401EAC00.32332CFD@Hovnanian.com>...
Operator Jay wrote:

[snip]

I believe over excitation makes the sync motor look like a
capacitive load. I agree that usually capacitance is added
to improve pf / voltage, etc. Anyone know if there's a
transient stability benefit by staying well away from
leading load pf?

A leading load power factor reduces system stability margins.

No it doesn't. Where did you learn the swing equation?

All other
thing being equal, the more leading KVARs added at the load, the less
the generator(s) will need to supply. For the same load voltage, this
requires less generator excitation. As the generator's excitation is
reduced, the peak power point (top of the generator's power curve) is
lowered. This reduces the margin between the normal operating point and
this peak, which makes it more likely that a fault or load transient
will cause the generator to pull out of sync.

Nope. It's mass. This is an advanced subject that you don't seem qualified.
Johan, P.E.E. see I got an extra E.

Wouldn't it be better to actually explain why you disagree, rather
than throw pointless jibes at those who are trying to help the
original correspondent.

Your extre "E" obviously doesn't include a grounding in grammar.
"This is an advanced subject that you don't seem qualified -
infinitive required."

Also, "see I got an extra E", should read; "See, I have an extre E."

You don't seem qualified to post to this group due to your lack of
grasp of the English language.

Cap

 

[jk]

Captain794@yahoo.com (The Captain) wrote:

Your extre "E" obviously doesn't include a grounding in grammar.
"This is an advanced subject that you don't seem qualified -
infinitive required."

Also, "see I got an extra E", should read; "See, I have an extre E."

You don't seem qualified to post to this group due to your lack of
grasp of the English language.

Cap

So what you are saying is that extra "E" came from some where in his
vicinity.
Leaving him an "E"s hole?

jk

 

[Jamie]

i am glad you cleared that up.
i my self remember years ago using sync motors
to create a third phase to run our equipment in the
old shop!
i think the thread got carried away with the
inductive staturation that takes place when you
overexcite the fields on a sync motor thus causing
them to saturate out of sync .
i will not get into the tech arrays of this because i
am trying to keep this simple for most to understand..
swing loads do how ever cause some problems with VF motors
if the drive is not programmed or not designed properly to
handle it
etc..



ron doctors wrote:

alanh_27@yahoo.com (Alan Horowitz) wrote in message news:<1e3670a7.0402011403.2f83fada@posting.google.com>...

the scenario is a large vessel where service electric power is usually
supplied by either a straight diesel-driven alternator, OR a
mechanical power-take-off (from shaft main propulsion) driving a
(different, but identical) alternator.

the manual states that when PTO is selected (this is preferred for
fuel-economy, a "synchronous compensator", which is just a large motor
of its own, is in circuit to "add inductive reactance".

How can a motor do that, and why does the PTO's alternator need the
additional inductance thrown into the circuit?


The group seems confused in many ways .. The reason for a utility
to need a power factor correction ( using a motor as a "capacitor" by
overdriving it for example ) is that the utility has to generate more
current if the voltage and current are not in phase. A simple vector
diagram shows this. ( look up any old Alternating Current
generator/alternator/production book ) . The utility gererator has to
pay for this "excess" current, ( but you do not! ), so naturally they
do not want you taking power at anything but unity power factor. As
the main use of electricity is to produce a lagging PF the need arises
for a very large capacitor to to be placed to correct the PF.
A large synchronous motor running at sync speed but with an
overexcited winding looks electrically like a very large capacitor.
That's it folks!!

PS JUst becuase I mentioned overexcited please do not add to my SPAM
with vi**&#a mail!

 

[Tony Williams]

In article <1e3670a7.0402011403.2f83fada@posting.google.com>,
Alan Horowitz <alanh_27@yahoo.com> wrote:

the scenario is a large vessel where service electric power is
usually supplied by either a straight diesel-driven alternator,
OR a mechanical power-take-off (from shaft main propulsion)
driving a (different, but identical) alternator.

The only difference I can see is that a large marine
engine could well have larger cyclic irregularities
every rotation of the crankshaft, with enough torque
out of the PTO to wobble both the frequency and voltage
of an alternator.

the manual states that when PTO is selected (this is preferred
for fuel-economy, a "synchronous compensator", which is just a
large motor of its own, is in circuit to "add inductive
reactance".

The words "Large (AC) motor" probably means a squirrel
cage induction motor. The relationship between the slip
and the rotor currents give the induction motor some
useful (and unique) features.

An induction motor running with no mechanical load will
run at very nearly sync speed. If the supply frequency
suddenly drops then the motor will go into negative slip,
and will generate power, at the new supply frequency.
The name usually given to an induction motor that is
generating is 'asynchronous alternator'.

With a cyclically wobbling supply frequency an induction
motor would smoothly move between being a motor and being
a generator (no switching needed).

So I wonder if this characteristic is being used as an
'electrical flywheel', helping to smooth the terminal
voltage of the alternator.

It would require a decent rotor inertia, so the word
"large" would be part of the suggested spec.

--
Tony Williams.

 

[The Captain]

JohanLexington@webmail.co.za (Johan Lexington) wrote in message news:<d69e3ae6.0402030823.70d20768@posting.google.com>...

----
Actually, with all other things being equal, the stability limit of a
generator IS reduced at leading power factor because the excitation voltage
is reduced and this affects the power transfer - look at the right hand side
of the swing equation and what affects the transfer.
How do you think the right side of the equation was developed....MASS

Mass simply determines
how fast the machine swings. Also look at the steady state capability limits
of a synchronous generator and steady state limitations (mass not involved)
on leading operation. Paul is absolutely right.

Well, you are wrong also.

For motor operation, leading pf will be the overexcited case but for
generator operation it is the underexcited case. Suck the excitation down
enough and the generator will fall out of synch below rated KVA.

Sorry, it's kVA. When you write things incorrectly more mistakes are
easier to understand.

This group is just too funny. I read it for the entertainment; not
the technical content.

The K in this case stands for Kilo, KVA being Kilo Volt Amps. Since
it is standard practice to write KV, KHz, etc, please explaine why, in
this particular case, a lower case k should be used.

If you read this group for entertainment, three questions arise:

1. Just how dull is your life? (Sorry guys, I know it's interesting
reading and writing in here, but there's a hell of a lot more to life
than this newsgroup!)

2. Why do you write to the group, providing no answers to
correspondents questions, and merely tell us how very, very clever you
are?

3. Just how insecure are you?

I don't normally get involved in these pissing matches, usually just
read and learn, but your attitude is obnoxious in the extreme and you
contribute nothing.

Cap.

 

[John Woodgate]

I read in sci.electronics.design that The Captain <Captain794@yahoo.com>
wrote (in <7199d521.0402071400.78c2da29@posting.google.com&gt about
'motor can act as an inductor?', on Sat, 7 Feb 2004:

The K in this case stands for Kilo, KVA being Kilo Volt Amps. Since it
is standard practice to write KV, KHz, etc,

But it ISN'T. The prefix for 'kilo' = 10^3 is 'k'. See IEC 60027-1. 'K'
is used, unofficially until quite recently, to mean 2^10 = 1024.

But it's overly pedantic to pick up just one minor error in an article.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[The Captain]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote in message news:<4c7cf793a3tonyw@ledelec.demon.co.uk>...

In article <1e3670a7.0402011403.2f83fada@posting.google.com>,
Alan Horowitz <alanh_27@yahoo.com> wrote:
the scenario is a large vessel where service electric power is
usually supplied by either a straight diesel-driven alternator,
OR a mechanical power-take-off (from shaft main propulsion)
driving a (different, but identical) alternator.

The only difference I can see is that a large marine
engine could well have larger cyclic irregularities
every rotation of the crankshaft, with enough torque
out of the PTO to wobble both the frequency and voltage
of an alternator.

the manual states that when PTO is selected (this is preferred
for fuel-economy, a "synchronous compensator", which is just a
large motor of its own, is in circuit to "add inductive
reactance".

The words "Large (AC) motor" probably means a squirrel
cage induction motor. The relationship between the slip
and the rotor currents give the induction motor some
useful (and unique) features.

An induction motor running with no mechanical load will
run at very nearly sync speed. If the supply frequency
suddenly drops then the motor will go into negative slip,
and will generate power, at the new supply frequency.
The name usually given to an induction motor that is
generating is 'asynchronous alternator'.

With a cyclically wobbling supply frequency an induction
motor would smoothly move between being a motor and being
a generator (no switching needed).

So I wonder if this characteristic is being used as an
'electrical flywheel', helping to smooth the terminal
voltage of the alternator.

It would require a decent rotor inertia, so the word
"large" would be part of the suggested spec.

It's been so many years since I studied this stuff, that I've
hesitated to contribute, but Alan's post rang a bell. If the
synchronous motor is swinging a large flywheel, then it seems to me
that it will act as an electrically coupled flywheel on the
driveshaft. If the shaft is driving a marine propellor which may
occasionally be raised out of the water by rough seas, then the
propellor will tend to run free with no torque on the shaft to control
engine speed. This will affect the power output of the generator and,
presumably, the frequency of the generated AC current. With the
electricaly coupled flywheel in place, the shaft speed will be
controlled by the back emf on the coupled generator, safer for the
mechanics and a method of controlling the frequency of the generated
power..

Since the flywheel, which may simply be the inertia of the rotating
motor itself, is rotating considerably faster than the shaft, the
coupled inertia will be multiplied by the rotational speed of the
motor/flywheel divided by the shaft speed.

So maybe this is simply a relatively easily installed shaft and
frequency controlling system. If so, it's quite clever, since it
performs two functions relatively simply and with minimum equipment.

Cap.