alternator power factor driving a rectifier

[brutus1955]

if we have an alternator, it produces AC
and assume it is 3 phase for the sake of discussion

attached to the alterntor is a bridge rectifier to convert the AC to DC

the power factor of a circuit driving a rectifier is reportedly something less
or often much less than unity

now granted an alternator that has its rectifiers mounted internally has the worlds shortest transmission lines (from alterntor to rectifier load), but

the winding of the alternator also take the hit for poor power factor

reciculating or reactive power produces no useful work, but
it does take up ampacity of the transmission lines and the stator windings as well.

in a perfect world an alterntor should be near unity power factor driving the rectifier...

now the question

how would one determine the power factor of an alterntor, between the stator and the rectifier?

if the stator three phase leads are also externally accessable at the rear of the case, could one simply start adding a bit of capacitance while watching for the field current to dip, while while supplying a fixed load?

it would appear that one could start adding capacitance and watch for the field current to start to drop, which would be evidence of the stator voltage starting to climb due to power factor being overunity. one could then reduce the capacitance sufficiently to assure that the power factor was comfortably and safely below unity.

there is a reason that i need to know this, i am slicing things very thin right now looking for the last percent or two in efficiency of a system.

poor power factor has no cost in power in as much as no useful work is done, therefore no work is needed as input power, but
reactive power takes away ampacity of the system, and in doing so creates heat
heat takes input power to generate.

anyone got some thoughts on this topic? or see anything wrong with the theory of adding capacitance?

bob g

 

[itsmoked]

You may be confusing distortion power factor with leading/lagging power factor.

There is a difference between driving a capacitive or inductive load and driving a non-linear load that actually alters the shape of the current flow.

Running a rectifier bank does the latter since power will be drawn in pulses. This is distortion. Adding caps may mediate it a bit but it also can greatly increase the distortion. As you add more capacitance the power is drawn in smaller and smaller pulse periods which increases the distortion. Less capacitance may improve the distortion power factor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[brutus1955]

ok that makes sense

would it be beneficial to filter the power
to clean up some of this distortion?

perhaps a filter that would pass the fundamental freq
which is ~500hz and scrub up some of the rest?

any text on the subject that i can be directed to would
be very much appreciated.

thanks
bob g

 

[waross]

If you are struggling with an automotive type alternator, don't try to apply power alternator concepts.
The field is DC so power factor does not apply. If you are intending to connect capacitors to the DC output, the DC does not have a power factor to start with.
If you are intending to connect the capacitors internally between the stator winding and the diodes, the diodes create distortion power factor and capacitors are used to correct displacement power factor.
"in a perfect world an alterntor should be near unity power factor driving the rectifier..."
The alternator responds to the load. If a load has a poor power factor, the alternator will suffer a similar power factor.
The winding current will be limited by the I²R of the conduction period and the cooling mechanism. However there is a large nonconducting period when the I²R is zero. The RMS of the entire cycle is much less than would be indicated by the current peaks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brutus1955]

Bill:

yes it is a modified automotive alternator,

it is producing right at twice its original ouput in watts

and targeted to 2.5 times oem design

i realize the field is DC and power factor does not apply, just as the DC output has no power factor concerns

my question revolves around the interaction between the stator 3phase AC at 500hz and the rectifier bridge.

"The alternator responds to the load. If a load has a poor power factor, the alternator will suffer a similar power factor."

by load i would assume in this case the load is the rectifier? and if the load presents a poor power factor how would one go about improving it?

there is little written, at least that i can find on design considerations of the automotive lundell alternator. i would
really like to get my hands on a good book that covers in depth this design.

bob g

 

[waross]

Forget power factor. Look at increased cooling, or rewinding with fewer turns of heavier wire. You can makeup the drop in volts by spinning it proportionately faster.
If the stator is star or wye connected you can gain a curent factor of 173% (root three) by reconnecting in delta. If you are out of volts, increase the speed by the same factor. At the higher speed the cooling may be more efficient allowing more than 173% of the original current.
When a power alternator is suffering from a rectifier load the first suggestion is often to double the size of the alternator. Somehow I feel that this solution is not in your game plan. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter