just call it 2 phase

[Archimedes' Lever]

I do remember visiting the Sylmar Pacific Intertie facility. They had
some pretty large harmonic filter components out there. I don't remember
if they were there to absorb harmonics generated from the switching or
whether they compensated for reactive currents as well. I also do not
remember how many pulses they used to approximate a sine wave from the
switching banks.

Bill[/QUOTE]


Wow... you visited their rectification hall? It is immense. I'll bet
that was a cool field trip (I had to call it that)!

 

[Don Kelly]

The molex 'pins' were cheap crap that came on a reel, and were cut to
length. Then you plugged the IC into the strips & soldered them to the
PC board before breaking off the carrier strip. It was almost
impossible to solder them by hand, without wicking the flux into the
contact area. They were designed for wave soldering, and you were lucky
to pug an IC in two or three times before one broke, or became
intermittent. You might as well have just bent the pins over on the
back of the board & not bothered to solder them.

You are showing my age!! Any Heathkits I built had tube sockets (small
and large). They also had big heavy transformers and nasty voltages.
They did give a good product for a decent price and some ability to read
and follow instructions (and solder properly-). A good way to get a
decent radio and audio amp as well as basic test equipment at a decent
price. Sure their scopes weren't up to Tectronics but in many
applications that didn't matter when budgets had to be stretched.

 

[Don Kelly]

I do remember visiting the Sylmar Pacific Intertie facility. They had
some pretty large harmonic filter components out there. I don't remember
if they were there to absorb harmonics generated from the switching or
whether they compensated for reactive currents as well. I also do not
remember how many pulses they used to approximate a sine wave from the
switching banks.

Bill
[/QUOTE]
Basically harmonic compensation and the scheme would be roughly
equivalent to a 6 phase supply (12 pulse- often with a star primary and
delta and star secondaries feeding converters in series).Reactive would
be drawn from the systems at each end and would depend on the control
scheme used and the real power loading. Note that filtering will
generally be on both the AC and DC sides.

 

[Don Kelly]

The power system that I worked in has machines with traditional
excitation systems (DC generator, commutator, & slip rings)
ranging from 250V, 800A to 375V, 3653A on the exciter nameplates.

Rectified AC exciters both brushless with rotating diode wheels
and stationary rectifiers with slip rings were used on machines
built after about 1965 and carried ratings like 525V, 8381A and
550V, 6009A. The high current one is brushless and the other one
has slip rings. Something similar is probably used today on new
machines.

The largest traditional DC machine I worked with was 375V, 3653A.
It had all of the windings know to dc, including a pole face
winding extension of the interpoles which I'd never seen on any
smaller machine. I know that there were some larger DC machines
used in steel mills at that time which have been superseded by
variable frequency and adjustable speed AC drives. (VFD uses
synchronous motors while ASD uses induction motors.)

In the late 80's, I worked on variable frequency ID fan drives
that were rated 4500HP @ 945 RPM, 63Hz. They used synchronous
motors and were probably similar to what would have been used in
steel mills about the same time (late 1980's). The motor voltage
was 2300V and they had 2 windings. Each winding was driven via a
3 phase frequency converter from an 11kV system. The converters
were arranged to form a 6 pulse system for both the motor and
power system so that the 5th and 7th harmonics cancelled. I sold
them to the mechanicals as 'electron turbines'. ;-)

As I recall, the pole face windings in series with the interpoles were
used to counter the distortion of the field flux by the armature mmf.
The armature mmf tends to weaken flux on one side of the field pole and
increase it on the other side but some saturation can occur resulting in
a weakening of the total flux. It happens in all machines but generally
it isn't all that important in most cases. In the size of machine that
you mention, it could be of concern. This machine would have been near
the upper limit for conventional DC machines.

 

[Andrew Gabriel]

Reminds me of my own old Telequipment 555 true dual beam scope with 2
dual beam plugins...to make it 4 beams in all. Along with the seperate
power supply one sure didn't need a heater in the lab in the winter.
Loved Heathkit stuff...I have fond memories of their shop in Tottenham
Court Road, London during the '70's.

That brings back memories.
The last electronics shop in Tottemham Court Road was Proops,
which probably went around 20 years ago. Those electronics
shops all got replaced with PC shops, and now gadget shops.
Same thing happened in Edgeware Road, except I think Henry's
is still there (not been there for a long time though).

 

[Don Kelly]

I have not worked on this kind of machine. Tell me if I understand the
principles correctly.

The armature reaction does distort the field as you describe. That
shifts the neutral axis. That in turn means that the brushes now short
out commutator segments that have more voltage between them than would
be the case without armature reaction.

To get around these problems, the brushes can be rotated so to minimize
the voltage between the segments that do get shorted. To avoid rotating
the brushes, interpoles can induce an emf in the coils connected to
adjacent commutator segments to reduce the voltage between segments that
are shorted through the brushes. This avoids the necessity of rotating
the brush assembly.

Do I have that right?

Bill

Yes -you have it right. The interpoles cancel the effect of the armature
reaction only at the segments under commutation - eliminating the need
to shift brushes. They also insure that the net induced voltage in the
coil under commutation is 0. However they have no effect under the field
poles as they act only in a narrow region on the the neutral axis. The
distortion under the field poles really wouldn't matter except for
saturation which causes the increase in flux on one side of the pole to
be less than the decrease under the other side. The result is a
weakening of flux under load. This can be partially countered by
increasing the field current but,for high armature current or highly
variable loads, this distortion has an effect on commutation- tending
to increase arcing between segments of the commutator. Interpoles
alone don't completely handle this problem. Hence the "compensating
winding"- an attempt to cancel out armature reaction mmf.
In most machines it is not needed but there are cases where it is very
useful and worth the expense.