A
study of the
transformer magnetic circuit
Electrical Engineering
irtJiv.
i.r.r
1
,1
J.,TB J?
.
of
oi3
ARY
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UNIVERSITY OF ILLINOIS
LIBRARY
Class
MrlO-20M
Book
Volume
Vv^V? } f^JK;
Electrical Engineering
College of Engineering
UNIVERSITY OP ILLINOIS
June
-
1900
V
V
The
DESIGN
CONSTRUCTION AND TEST
o
a
f
RECTIFIER
b y
Guy
Richardson Radley.
THESIS
for the
Degree of Bachelor of Science
in
Electrical Engineering
College of Engineering
UNIVERSITY OP ILLINOIS
June
-
1900
Digitized by the Internet Archive
in
2013
http://archive.org/details/designconstructiOOradl
TAELE OF CONTENTS.
CHAPTER
I.
CHAPTER
II.
PREFACE.
STATEMENT OF PROBLEM.
CHAPTER III.
D -SIGN.
CHAPTER
IV.
CONSTRUCTION.
CHAPTER
V.
CHAPTER
VI.
TEST.
CONCLUSIONS.
INDKX OF TABLES.
TABLE
I.
TABLE
II.
TABLE
III.
TABLE
IV.
TABLE
V.
TABLE
VI.
TA.BLH
VII.
TABLE VIII.
Page 39. Data for Magnetization Curve of Rectifier.
Page 40. Data for Short-circuit Characteristic.
Page 40. Data for Dynamic Characteristic.
Page 41. Data for Stray Pov/er.
Pages 42-43. Synchronous Characteristics.
Page 44. Efficiency Test - Separarely Excited.
Page 43. SjTtchronous Characteristic
Page 45. Efficiency Test
-
-
Self-excited.
Self-excited.
..
TABLE OP ILLUSTRATIONS & CURVES.
FIGURE
1.
Page 46.
Photograph of Rectifier.
FIGURE
O•
Pace 47.
Sectioned Elevation of Rectifier.
FIGURE
o•
Pace
o 48.
End Section of Rectifier.
FIGURE
4. Pace 49.
Magnetization Curves of Rectifier.
FIGURE
5a. Pare 50.
Short-circuit Curves of Rectifier.
FIGURE
5b.
FIGURE
(Zj
r>
Pag? 51
Short-circuit Curves of Rectifier.
6.
Pare 52.
Dynamic Characteristic of Rectifier.
FIGURE
7.
Pace 53.
Curves of Stray Pov/er.
FIGURE
8.
Pace 54.
Diagram of Connections for General Test.
FIGURE
9.
K •
Pace JO
Synchronous Characteristics,
er
53.3 Cycles.
80
"
100
tf
"
120
"
h
ff
140
tf
tt
FIGURE
10. Pace
56.
o
"
"
FIGURE
11.
Pace
o 57.
tt
tf
FIGURE
12. Pace
58.
o
"
FIGURE
13. Pace 59.
FIGURE
14. Pace 60.
FIGURE
15. Pace 61
Variation of the V/atts with the Frequency.
FIGURE
16. Page 62.
Variation of the
FIGURE
17. Page 63.
Efficiency Test. Non-inductive Load.
FIGURE
18. Page 64.
Efficiency Test. Electrolytic Load.
FIGURE
19. Page 65.
Synchronous Characteristics.
FIGURE
20. Page 66.
Efficiency Test. Electrolytic Load.
Po".
160
r
»«
tt
er-factor with Frequency.
Self-excited.
CHAPTER
I.
PREFACE
The electric light has now become quite popular, nearly every
city of a thousand inhabi tants or so having its electri? light
plant.
At the tine of the greatest activity in this line it
was the fad to install indiscriminately single phase apparatus
producing an alternating current having a frequency of from 100
to 140 cycles per second.
One of the objections to this system was the fact that elec-
tric fans and small motors could not be profitably operated, for
With a transformer system connected to the mains for 24 hours, the
core losses in the transformers might amount to as much as the
revenue from the current sold for these small powers.
A solution of the problem of securing the advantages of a
continually available suppljr
of'
current suitable for these small
motors in a town where the single phase system is used, led to
the design of the machine which is the subject of this thesis.
CHAPTER
II.
STATEMENT OP PROBLEM
In this design the following conditions were laid dovm as
essential features in the machine.
1.
Character of Current:-
The current delivered by machine
must be suitable for charging storage batteries without any
destructive chemical effects such as would happen should an alternating current pass through the cells.
2.
Capacity:-
Machine must be able to charge two cells of
battery in series at a 10 ampere rate, requiring for this from
six to ten volts.
3.
Reliability:-
Machine must be started by a simple switch
and begin work without hitch or failure.
It must continue in un-
interrupted service without requiring attention, automatically
opening the battery circuit on failure of the
a.
c.
supply, thus
preventing the battery discharging back through the machine.
4.
Efficiency:-
This must be as high as is possible com-
patible with considerations of economy in first cost.
iency of the transformation must be as high
The effic-
or higher than would
be obtained were the given cells charged by introducing resist-
ance in series on a 110 volt
d. c.
circuit.
CHAPTER III.
Choice of Type.
1.
In selecting a machine to fulfill the
conditions enumerated in Chapter
that may be used.
DESIGN.
II.
there are three general types
These are; (a) I.lotor-generator
,
(b) Synchronous
Converter and (c) a Rectifier.
a.
Lotor-generator :-
In this machine an ordinary
tor of the proper voltage, is directly coupled to an
either the induction or synchronous type.
d. c.
a. c.
genera-
motor of
The operation of this
combination would be both simple and reliable since both operate
under known conditions.
Starting would be easy, since if a non-
self-starting synchronous motor
ed to the
d. c.
as a
d. c.
motor.
a. c.
voltage and the
ere used current could be furnish
generator and the combination run up to synchronism
With this arrangement the ratio between the
d. c.
,
may be changed within wide limits with-
out serious losses in either reactance or resistance.
Against
it may be laid the high first cost, due to the necessity of having
two complete machines.
If an induction motor is used the power-
factor may be very low, especially in a machine as small as this is
proposed to be.
The efficiency of conversion will also be low
owing to the double losses, the efficiency of a motor-generator
being the product of the separate efficiencies of the motor and
generator.
Since these may both be quite low the combination as a
whole will have a very low total efficiency.
Thus if we take the
4.
efficiency of a motor of the required size to be about
by
B4x%
Ralph Bennett
50/o
as found
and assume the same figure for the genera-
tor, the total efficiency of conversion would be but 9%,
The regulation for the induction motor combination would also
be very poor since the speed would fall off with the load and the
armature reaction would also tend to decrease the voltage.
Sparking would occur on the generator, due to shifting of the
neutral point with changes of the load.
b.
This machine owing to its single
Synchronous Converter:-
winding will have a high efficiency, the armature resistance having small effect since during portions of the revolution the alter-
nating current flows in an opposite direction to the direct current
This same fact prevents armature reaction and thus the operation
is sparkless.
On the other hand there is always more or less
difficulty in starting a synchronous converter from the
It is usually done from the
d. c.
a. c.
side.
side, using a synchronizing de-
This would be very troublesome on a machine of but one-
vice.
tenth kilowatt capacity.
The operation is not always very
stable, with leading currents the tendency to pump and to fall out
of step being very strong.
This machine has a practically fixed
ratio of transformation, a change in the
ed by a change in the
c.
Rectifier:-
a. c.
d. c.
voltage being obtain-
supply voltage.
This may be of two kinds,
(l) A synchronous
motor wound for line voltage, driving an entirely separate commutator which receives current from a transformer furnishing the
(1). "Tests of Fan Motors'*.
Ralph Bennett,' 99.
Thesis in University Library, by
necessary
(2)
a. c.
voltage to give the required
d. c.
potential*
A synchronous motor having slip rings and commutator connected
to a winding suitable for the voltage to be delivered.
With the first arrangement difficulty would be experienced in
Winding a very small synchronous motor for 100 volts.
Starting
would be almost impossible without an auxiliary winding.
The spark
ing would be great, due to change in phase between the current run-
ning the motor and that supplied to the commutator, when the powerfactor of the load changed.
To insure sparkless running the brush
position would have to be changed with every change in load.
On the other hand, if the motor is to be run from the same
current that is rectified, the winding for this low voltage will be
simple and the wire of a convenient size.
It will also start easily
since the fields may be connected directly to the
d. c.
brushes.
The machine will then start up as an ordinary shunt motor, since
the direction of the current in the armature and fields is changed
during the revolution in such a way as to make torque always ih the
same direction.
When up to synchronous speed the fields will be
excited by a uni-directional pulsating current.
phase of the
e. m. f .
with respect to the current
Any change in
due to changes in
load, will not necessitate a change in the position of the brushes
since the armature automatically lags behind the proper amount.
With this type of machine as well as With the synchronous
converter the polarity of the brushes in starting is not fixed.
In starting, if the desired polarity is not obtained the
a. c.
circuit may be opened momentarily. During this period the armature
falls behind one half cycle, and yet gets in step when the current
is again supplied.
The proper length of time to hold the circuit
open can be found out by experiment.
Since the machine while running is liable to fall out of step
or the
a. c.
supply may fail, some arrangement is necessary to pre-
vent damage in such cases.
This may consist of a polarized elect-
ro- magnet through which the rectified current passes.
of this magnet would control switches in both the
circuits.
d. c.
The lever
and
a. c.
Thus if the rectified current ceased or reversed, the
armature of the circuit breaker as it might be called, would be released, opening both circuits and preventing either the discharge
of the storage battery back through the rectifier or the sending
of an alternating curr-nt through the batteries with the consequent
production of sulphate on the plates.
Having considered the advantages and disadvantages of these
three types of machines it was decided that the second type of
rectifier offered the best solution of the problem and the design
worked out as follows.
2.
Calculation:-
In the design of any piece of electrical
apparatus there are usually a multitude of ways in which the desi-
red end may be accomplished.
In such a case the aim of the des-
igner is to secure the best efficiency, durability and reliability
at the lowest cost.
The degree to which any machine approximates to the maximum
value of these quantities, depends upon the experience and judge-
ment of the designer as well as his theoretical knowledge.
In the cass of the author, his experience at the time of the
design was very limited indeed, so that he had to draw upon such
These were such books as "Dyn-
sources as were at hand for data.
amo electric Machines"
A. E. V/iener.
nating Current Machinery"
D. C.
,
"Alternating Currents and Altea?
J.P.Jackson,
&.
and "Dynamo Electric
Machinery" S.P.Thompson.
Type of Field:- Since nearly all alternators of modern con-
struction are of the radial inner-pole type, that form was adopted.
Its advantages are, a lew co-efficient of magnetic leakage, ease of
construction and ease of magnetic calculation.
Speed:- This was taken at 1875, which is required on 125 cycles
with an
8
pole field.
If a
6
pole field had "been chosen the speed
would have been 2500 which it was thought was too high to reach
when starting as an
a. c.
shunt motor.
On the other hand a 10 pole
field would have been quite slow in speed which would have necessit-
ated much more wire on the armature and much greater weight of
machine.
On the whole it is thought that the number of poles and
consequent speed was well chosen.
Armature:-
This was chosen of the drum type, 2" in diameter,
partly because Table X in Wiener,
p.
60., seemed to indicate that it
would be a good size and also because the ferrotype plates which it
was proposed to use for the core discs, would cut up into that size
without much wastage.
Commutator:eter.
It was thought that this should be 1^" in diam-
According to Wiener,
p.
175.
the current density for copper
brushes should not exceed 150 amperes per square inch of contact
surface.
Since something more than 10 amperes are to be carried
thin would indicate that the surface of each brush should be about
.1
square inch in section, or that a brush about
.1
inch thick, should be used.
With a commutator lj" in diameter
the peripheral speed will be 750 ft.
p.
179 of Wiener,
sq.
in.
= 1.51 #
The proposed surface is .2 sq.
pull will be .302
From formula 119
per minute.
the tangential pull will be, for this speed and
With a brush pressure of 1# per
contact surface.
inch wide and
1
per sq.
in.
,
in.
of
hence the
The work done in overcoming the friction of
#.
the brushes on commutator will then be,
750 x .302 =
226 ft.
lbs.
Assuming a collector ring diameter of 1"
will be 490
1.88" per
ft.
sq. in.
the peripheral speed
for which the pull per sq. in.
per min.
,
,
will be
making the work done by these brushes,
1.88 x .1 x 490 = 92 ft.
lbs.
This is allowing a density in the slip-ring brushes of double
that used on the commutator.
This is thought to be all right
since that density is used in a particular machine, in satisfactory
commercial operation, with which the author has had an opportunity
to become acquainted.
This makes a total of 318 ft. lbs.
per min.
or 7.1 watts required in the motor for brush friction alone.
Shaft:-
The diameter of this was taken as .2" at the bearing
portion and 3/8"
for the core portion.
agreement with Table LII.
p.
This is in substantial
187 of Wiener.
Taking the co-efficient
of friction of the journals as .07 and estimating the weight of com-
pleted armature as 2. 6
18 ft. lbs. per min.
m.
77"
,
makes the work required for the bearings
Adding this to the work required for brush
friction makes a total of 336
ft. lbs.
per min.
or 7
.
6
watts input
necessary to run the machine.
Taking a "factor of safety" of 2
makes about 15 watts, or a current of 2.5 amperes necessary at
6
volts.
Assuming a current density of 500 circular mils
Size of Wire:-
per ampere, an area of 1250 cir. mils will be required
.
This is
nearest to #19 but for commercial reasons #20 B&S was chosen.
The density in the air gap was chosen
Length of Conductor:as S000 lines per sq. in.
ation in Table VI
p.
,
a figure which was obtained by exterpol-
This density is very low, but
54 of Wiener.
that author seems to think that with multipolar machines with toothed armature and cast-iron field poles, a very low density is needed.
The percentage of polar embrace was fixed at 50 since that is
usual in alternators.
With a depth of winding of 1/4"
would be, for a 2" armature, 15.34
From formula 26.
p.
,
the mean conductor velocity
ft.
55 of Wiener,
per second.
it is found that the active
length of armature conductor must be 54.3 ft, or 81.5 inches per
armature slot.
Laying this out to scale as in Pig.
3,
it is seen that with a
slot .37" wide and .26" deep there will be room for 5 layers with
8
wires per layer, of #20 B&S.
A core 2" long will thus give 80"
of active conductor per slot.
Field Calculation:-
Having fixed upon the dimensions and wind-
ing of the armature it becomes necessary to design a field magnet
to supply the required flux.
This is usually done by the "cut and
try" method, a particular form of field being adopted and different
lengths of cores tried till one is found which uses the materials
to the best advantage.
For a bipolar machine,
Ml*.
Geo. T. Hanchett
by which the length of core may be determin
has given a formula
d
directly, after a sufficient number of variables have been fixed,
but the method offered no advantage in this particular design.
Since the core length was fixed as 2" and 40 wires could be
put in a slot, there are but 80" of active conductor instead of
81.5" as required by the formula.
Since the length is thus de-
creased, the density of the flux has to be increased in the same
This makes the air gap density 8150 instead of 8000
proportion.
Below is tabulated the assumed and derived
as first assumed.
constants which were finally used, two other sets having been worked out but found to not allow enough space for the field winding.
TABLE
OF
L1AGNETIC
DATA OF RECTIFIER.
For one magnetic circuit. Length Area Density Total
inches sq. in. sq. in. flux.
Air Gap
Cores
(
= 1.5)
Yoke
3330. 2550.
060
.406
8150.
1.125
.312
10700.
3330.
C. .
1.33
.437
11500.
5000.
20.
26.6
1.33
.625
8000.
5000.
16.
22.1
.
Armature Core
f (B)*Ampere
turns.
o
153.
e
o D
t.
O
Total
<o .
204.3
Thus it is seen that about 204 ampere turns are required to
produce the required density in the air-gap.
Field Winding:-
After considerable preliminary work trying to
determine the size of wire necessary to give the required no.
ampere- turns
(1)
,
of
and also to keep the watts lost per unit of radiating
"Dimensioning of Field Magnets",
Taken from Table LXXXVIII.
p.
E.
'
W.
29 April '99.
336 of Wiener.
surface within allowable limits, it was found that for a given no.
of anpere turns to be put in a given space, the watts lost are prac-
tically independent of the size of wire.
This having been dis-
covered it was decided for commercial reasons to use the same size
of wire on the field that was to be used on the armature viz.
#20.
Taking this size the spools were planned out as shown in Fig.
3.
Room was found for 4 layers of 13 turns each, on each field.
This winding requires 2 amperes to make the desired 204
ampere
The mean length of a turn was 5.375", making a total length
turns.
of 25.5 ft.
on each spool, or 204 ft.
on the whole field.
From
standard wire tables this length should have a resistance of 2.3
ohms at 50°
C.
The resistance as measured after the machine was
constructed, was 2.5 ohms, at 30°
0.
The outside or radiating surface of each spool is about 7.5 sq. in
o
The watts lost = C^R, are 9.2 watts.
60 sq. in.
,
These are radiated from
making the density of radiation .15 watt per
is very moderate for a
d. c.
sq. in.
machine.
Since the terminal volts are to be
6
and the field current is to
be 2, there must be a total resistance in the field circuit of
ohms.
which
,
The field winding has a resistance of 2.3
,
3
hence there will
be required a rheostat having a resistance for normal operation of
.7
ohm.
Actually there was provided one having very small steps
aggregating 4.1 ohms.
CHAPTER
IV.
CONSTRUCTION
Having completed the design,
I
constructed the machine during
the months of June and July, 1309.
The shaft was the first piece finished.
This as shown in
Pig. 2. was 7* inches long, and of machine steel.
This was turned
4
up to a uniform diameter of - inch, v/ith a shoulder ±- high in the
8
middle.
On this shaft was forced a hard maple block, using an
arbor press for the purpose.
3.2-
32
This block was then turned down to
which was 1- greater in diam. than the internal diameter of the
64
hard brass tube which was to form the commutator and slip rings.
The tube after being pressed on, was divided" into the proper
f
number of segments and * flat head brass wood screws put
in.
The segments were then milled apart, the milling machine index
doing this quite accurately, while the screws kept the bars in the
sa'Tie
relative position.
Red fibre strips were then driven in be-
tween the segments, being held in by friction and glue.
tsrfl
An end plate ^- thick was next slipped on the shai
t
32
ed to the commutator core as the section in Fig.
2.
and screw-
clearly shows.
This served to keep the commutator bars and the armature core in
the same relative position without the possibility of a change.
The armature core discs were next prepared ^being punched out
of old "Ferrotype" plates such as used by photographers.
The iron
of these plates is thought to be of the best quality since they
13
The japan served to in-
are made from a brand which is very soft.
sulate the discs and prevent eddy currents.
each disc
v.'as
The burr formed on
removed by light blows from a hammer.
Although this
undoubtedly hardened the iron in that locality, it is thought that
this did not affect the core losses of the machine since this part
I
was removed, the discs being punched to 2*
finished diameter was but 2".
it
diameter while the
A nut working on the threaded por-
tion of the shaft served to compress the discs between the endplates.
After these were as tight as possible the exposed portion
of the thread and a portion of the nut were turned off in the lathe
not however to the final diameter, which was deferred till after
the slots had been milled.
This was done to prevent the possibility
of not having the armature run true when finished, owing to a
slight bending of the shaft during the milling.
>zfi
on
.
The slots were milled with a £ cutter to depth of
Troughs of shellacked manilla paper
insulation.
winding.
^
thick were used as a slot
Double cotton covered copper wire was used in the
This was continuous from one si in ring to the other,
there thus being but a single path through the armature.
wire was of #20 B&S gauge.
This
The slip rings were connected by pieces
of -"16 wire to every alternate commutator bar.
The field ring is an iron casting.
This casting was annealed
by heatng to a bright red and imbedding in air slacked lime dust.
In turning up the ring it was first bored out to
8~
,
the re-
quired size, and forced on a mandrel, the remaining operations
then being done with ease while the yoke was made perfectly con-
centric with the bore.
J
14
The bod plate was so designed as to have the seats Tor the
pedestals and the field ring cut at one setting of the boring oar.
Since the pedestals were turned up as segments of a circle of the
same diameter as the field ring, the shaft of a necessity came con-
centric with the boxes.
While the armature was kept in its pro-
per position by a layer of paper of the right thickness, paste-
board ends were put on the shaft and the boxes filled with genuine
Babbitt metal.
On cooling this shrunk and gripped the shaft so
tightly that its removal was difficult and necessitated the scraping of the boxes to insure a good running fit.
At the .same time
the skrinkage loosened the box from the pedestal necessitating
Setting up with a punch to prevent looseness.
Oiling was effected by compression cups filled with "Helmet
Oil".
This was found better than liquid oil since no provision
had been made to prevent the oil working from the bearings to the
winding.
V/ire
gauze brushes 1" wide by l/9" thick, with a filling of
leaf copper were used on the commutator.
This was done with the
hope that the sparking would be less than With an all copper brush,
the advertised claims being very glowing in regard to the non-
sparking qualities of this particular make.
It was soon seen how-
ever that the brush was too thick, so the wire gauze portion was
removed leaving sheet copper about 1/16" thick for the collecting
surface.
Regular copper brushes 1/4" by l/8" thick were used on
the collector rings.
The brush holders were all provided With springs and pivoted,
giving the brushes a yielding pressure on the commutator.
15
CHAPTER
1.
V.
TESTS.
Nature of Tests.
In testing the rectifier the general characteristics of
the machine itself and also the efficiency of conversion were sought.
Specifically the following were determined :a.
Magnetization Curve.
b.
Short Circuit Characteristic.
c.
Field or External Characteristic.
d.
Stray Power Losses.
e.
Synchronous Motor Characteristics.
f.
Efficiency of Conversion.
2.
Procedure.
a.
Magnetization Curve:-
A grooved pulley 1" in diameter
was hushed to fit the shaft of rectifier and the machine was run
as an
1
H. P.
a. c.
U.S.
generator from a grooved cone pulley on the shaft of a
500volt motor.
ine served as a belt.
A 3/16" cctton cord rubbed with paraff-
In order to get the low speeds a large
amount of resistance had to be inserted in the armature circuit of
the 500 volt motor with the result that the machine slowed down
very much with an increase in the load. For this reason it was
thought best to simply excite the fields of the rectifier to a
certain value and then vary the speed of the motor by steps, taking
the value as it came rather than trying to kiep the speed constant
with this greatly varying load.
Speeds were taken with a rub-
ber tipped speed or revolution counter, recording the revolutions
for half a minute.
The field was separately excited from 10
cells of storage battery having a wire coil rheostat in series.
The field current was measured with a Whitney 0-5 double scale
ammeter -1949.
,
which
wa.s
always connected up so as to obtain read-
ings on the left hand scale as the calibration had shown that this
scale was correct for values of current of more than .5 amp.
The
meter
d. c.
volts were measured with a Weston portable volt-
4349, using the lower scale.
/L
To measure the
a. c.
volts
a Thomson inclined coil voltmeter was borrowed from the Superint-
endent of the Municipal Electric Light Plant at Sandwich, 111.
A tap was made which cut out the dead resistance in series with
the moving system.
This made the constant of the instrument
approximately .1, giving it a range of 13 volts.
A german-
silver coil was wound up non-inductively of such a value that the
range was increased to 23 volts when it was in series with the tap.
The total resistance of the instrument when used regularly
was 1533 ohms, about .9 of this being non-inductive.
When the
instrument is used without this in series the inductance forms
a much larger proportion of the total impedance than formerly.
The result is that on high frequencies the inductance may be suff-
icient to sensibly increase the impedance, thus causing the readings to be low for high values of the frequency.
Whether this
occurred in these tests was not determined as no standard low
range
a. c.
voltmeter was available, and in all cases it has been
17
assumed that the effect was well within the limits of accuracy of
the instruments.
h.
Short-circuit Characteristic:-
In taking this the recti-
fier was driven as for the Magnetization Curve, the slip rings
being connected to a Thomson
a. c.
ammeter #14010, 0-15 range, by
The ohmic resistance was
about 6" of doubled #16 flexible cord.
thus very low, while the inductance of the ammeter is surely
neglegible.
The excitation was measured by a Whitney ammeter
#1949 as before.
A fixed value of the field was maintained,
different values of the frequency being obtained by changing the
resistance in series with the driving motor armature.
taken for half minute intervals.
Speeds were
With field currents above two
amperes the varying magnetic reluctance made a large variation in
This resulted in the armature
the torque in passing the pole.
sticking fast with the poles in line, so strongly
'that the
belt
By getting the machine up to
slipped even when very tight.
speed before the exciting current was thrown on, no difficulty
was encountered from this effect.
It was noticed that for any given excitation the current de-
livered was independent of the frequency after a certain speed
had been reached.
c.
Field or External Characteristic:-
This curve shows the
potential at which any particular value of current can be obtained with a given excitation.
In its determination the motor was
run without resistance in the armature circuit so as to maintain
frequency
as nearly a constant speed as possible.
rectifier as generator up to 170.
This brought the A of the
Readings were taken of the
V
18
terminal volts and amperes load for several values of the load,
with a field current of
1
ampere and a second series with a field
of 1.35 ampere.
d.
These were measured on the
Stray Power Losses:-
the rectifier being run as a
d. c.
d. c.
side
shunt motor and the electrical
input for various speeds determined for different field strengths.
The curves plotted in Pig.
7.
are from the data given in
Table IV, which was taken when the brushes were in tie position
always used when running as a synchronous motor viz: with the toe
of the brush in the middle of the fibre commutator insulation when
the polf of the armature was exactly opposite the field pole.
e.
Synchronous Motor Characteristics:-
m
these tests the
relation between power factor and the excitation for varying voltb ;es was found,
the tests being made with frequencies from about
50 to nearly 200.
The field ammeter and instruments for
a. c.
measurements were those previously described with the exception
of the wattmeter.
This was a Thomson inclined coil instrument
#789 also obtained from the Electric Light Plant at Sandwich.
It Was tapped cut as was the voltmeter.
This made the capacity
of the instrument 150 watts or 15 amps at 10 volts.
Current was obtained from the ft'estinghouse Pony Alternator
#£3*31 which was driven by belt from a 3K.
V/.
500 volt Edison motor
receiving current from the University Power Plant.
The speed of the motor was regulated by a rheostat in series
with the motor armature.
Speeds were taken after they were
steady by means of a portable tachometer made by
berg, #
Whiles this
S chaffer
& Buden-
method is notoriously inaccurate, it was
;
thought that as only comparative- results were desired it would do,
as with the large number of observations required, the the use of a
revolution recorder or speed counter was out of the question owing
to the time that would be required.
The observations necessarily-
had to be made on the generator as the motor would be sensibly
affected by even a light pressure of the speed counter.
The method, of procedure was as follows:
Rectifier field was
connected to the commutator brushes without resistance in the field
circu.it.
The alternator was started up with a strong field, so
that about 4 volts
a. c.
were delivered to rectifier at a frequency
of about 15 Circles per second.
This started the rectifier, it
falling into step immediately and running with a large current.
While thus running the field was switched over from self-exciting
to separarately excited,
the arrangement being shown in Fig.
8.
The switch for this purpose was one especially constructed
for the purpose 'and made so that the one circuit was made before
the other was opened, thus preventing opening the field circuit.
This was found to be necessary as a sudden opening of the field
followed by a closing was always sure to cause the machine to
fall out of step.
The resistance in series with the driving
motor was then cut out slowly, the field of the alternator being
simultaneously cut down to prevent the
10 or 12.
a. c.
volte rising above
The field current of rectifier was then adjusted to
the desired value as well as the terminal
a. c.
volts.
When these
were steady the speed of the alternator was taken with the tach-
ometer and then the various instruments were read in order;
watts,
a. c.
amperes,
a. c.
volts,
d. c.
a. c.
volts and field excitation.
20
In starting the rectifier the polarity of the
d. c.
brushes of
the commutator was liable to change with each new start.
When
this occurred as shown by the field ammeter, it was necessary to
stop and begin over again.
In all these tests it was noted that the exciting current
possible with a given frequency was very much smaller than that required to generate a counter electro-motive-force equal to the applied volts.
In fact the range through t which the field could be
varied without falling out of step was from 1/4 to 1* amperes at
50 cycles, while at the frequencies above 120 stable running was
only secured with a field strength of from .5 to .9 ampere.
The stability of operation at a frequency of 60 or below was very
marked, field currents up to
7
amperes sometimes being used and at
though the machine did not
other times the field current was made
fall out of step, the armature serving to magnetize its own fields.
When running this way the watts input seemed to change but slightly
but the current was large, the power factor being very low.
f.
Efficiency of Conversion:-
In these tests the rectifier
was connected as shown in Fig. 8., a Weston 0-15 ammeter #1846,
serving to measure the
previously used.
d.c
output.
The other instruments were as
Tests were made using a non-inductive resist-
ance consisting of very open coils cf large iron wire.
C.
&
C.
series motor was used as a load.
took a large current.
Next a
It ran very slowly and
Three series of observations were then
made when using ordinary carbon-cylinder sal-ammoniac cells, having
large size zincs, as a load, the current depositing spongy zinc
from the solution on the the postive plates.
In the tests with rectifier running self-excited, the load
was two sal-ammoniac cells of the same kind as previously used.
A noticeable feature when thus running was that with a load having a counter-electro-motive-f orce
,
field excitations and voltages
may be used which would cause the machine to fall out of step on
either inductive or non-inductive loads.
This is probabls? due
-
to the fact that when self-excited a pulsating current having a
zero value for a considerable portion of the time, flows in the
field circuit.
'..'lien
the cells were on as load the current
could only drop to the value of this back
e. m.
f.
,
and hence the
variation in the field circuit was very much decreased.
22
CONCLUSIONS.
CHAPTER VI.
1.
Discussion of Data and Curves.
a.
The Static Characteristic or I.'agnetizat ion Curve :-
Data obtained in this test is given in Table
plotted in Fig.
I
and the curves are
For a given frequency the
4.
volts vary
a. c.
directly as the field current, starting from the origin.
The
d. c.
volts on the contrary do not start from the origin, due probably
to the fact that the
d. c.
brushes open the circuit
siderable portion of the time.
The
a. c.
for a con-
volt curve for 161
cycles shows a decide break between excitations of 2.5 and
This
v:as
3 amp.
due to a stoppage of the driving motor to change the volt-
meter range by adding the resistance in series with the moving
system.
The fact that the voltage was apparently higher after
the change than before, leads one to suspect that the inductance
of the instrument had begun to influence the readings.
The value
of the inductance increases with the increase of the deflection,
thus ihcresing the impedance and preventing as large a current, and
hence torque, as would flow did it not change.
When plotted on a frequency base a gradual bending over of
the curves for both
a. c.
and
d. c.
volts for frequencies above 125,
except for the last readings of about 190, is noted.
case they are in a straight line.
In the latter
In inking in the curves the last
point was considered to have more weight than those lying off the
,
line, because it was quite probable that the apparent drop is due
to the inductance of the voltmeter.
In the last reading a con-
siderable resistance was in series with the voltmeter so that it
This explanation does not account
was free from this error.
for the fact that the
d. c.
volts also drop off for these frequenc-
This fact leads one to suspect that the speed was not corre-
ies.
ctly read, which has some weight for these speed determinations
were made by another person, and it is well known that the personal]!
equation plays a great part in the readings obtained for a given
speed when taken with a tachometer.
The fact that the
d. c.
volts
for 190 cycles where the author again took the readings personally
are lower than indicated by a law of proportional it;/, may be expla-
ined by
t~ie
fact that small inequalities of the surface of the com-
mutator, which are known to exist owing to the unequal wear of the
brass and the fibre insulating strips, cause, at these very high
speeds a vibration of the brushes which causes the circuit to open
a greater portion of the time than for low speeds.
b.
Short Circuit Curve--
This curve shown in Pig.
5,
shows
that the amperes output for a given excitation with increasing
frequency, approaches a limit, which limit is reached sooner with
small values of field current than with large ones, the absolute
value of this limit being of course larger for the large values
of excitation.
This is shown also by the curves of Fig.
which are plotted on a field current base.
In table II are
given the internal volts taken from the curves of Fig.
corresponding field currents and frequencies.
5b.
4.
for the
From these and
the short circuit currents, the apparent impedances are calculated.
1
)
?A
This term called the "Synchronous Reactv.nce" by Stcinmetz
^
includes the effect due to the armature reaction in demagnetizing
the field and thus reducing the terminal volts, as well as the
inductance proper
,
it being thus an equivalent or effective react-
The values of the synchronous reactance are shown in Fig.
ance.
5a to increase in exact proportion to the fequency, as would be ex-
From these values the "apparent inductance" has been cal-
pected.
culated and is given in Table
This appears to be, within the
II.
limit of error of observation, a constant for all frequencies but
varying with the excitation.
For a 2 ampere field the value is
about 1.85 millihenries.
c.
Dynamic or Field Characteristic:-
values of the excitation,
1
This is shown for two
and 1.35 ampere respectively, at 170
cycles, for a non-inductive load, in Fig.
6.
,
the data being given
The determination of this curve for a given machine
in Table III.
whose magnetization curve is known has been the subject of much discussion in the electrical papers within the last few years. As being
instructive in showing which of the proposed formulae apply to the
machine in hand, it being a rather exceptional case owing to its
high inductance, the methods are here given in brief together with
the results of their application to the rectifier.
Alex.
A.
Rothert
'makes the statement "If the primary ( exciting
,
current for no load and the given
e. m.
f.
,
is combined with that for
short circuit with the civen current, the primary ( exciting) current
for full load and any power- factor is obtained".
(1)
"Alternating Current Phenomena".
(2) Rothert.
E.
V/.
& S.
Steinmetz. 2nd.
34:496. 30 Sept. '99.
ed.
p.
245.
From Fig. 4,5 &
1.
6,
can be found the following values;
Exciting current for 3*35 volte and
"
2.
"
w
3*35
*
"
"
0.00
"
Hence for case
1,
low by about 5/&
1
amp.
"
1
"
load =
ii
it
ampere.
1
—
rt
ii
—
o;
ii
the formula gives .95 ampere, a result
For another load,
4. Exciting current for 6.05 volts and 2 amp.
5.
"
"
"
6.05
"
"
6.
"
"
"0.00
"
n
Thus for case 4, we get 2.15 amp.
2
load = 1.35 amperes.
"
"
= 1.30
"
"
=
*
.35
instead of 1.35 or 16% high.
The same author had earlier given an enlargement of Behn-Eschen-
bergs theory^
^ as
follows:
The terminal voltage is the vector sum
of the ohmic and inductive drops, and the impressed E.M.F*
E. M. F.
istic.
This
is obtained from the magnetization curve or static character-
The inductive drop is obtained for any current from the
short-circuit curve.
Then for any given load and voltage and
angle of lag, the point on the characteristic is obtained thus:
From the magnetization curve as Fig.
ponding to E', as H*.
(1)
Electrical World.
1.
From the short-circuit curve, Fig.
32:160. 13 Aug.
"Hi
Fig,
select an excitation corres-
Fig.
Z
'98.
2.,
;
select an excitation H'^ corresponding to an external load I'
find from Fig.
the
1.
e. m. f .
E'_ corresponding to the value of the
,
field.
From point 0, Fig.
drop.
From A, and at right angles to 0, lay off A3 equal to E
lay off OA equal to I9R01 the ohmic
3*
From B draw a line in the direction
the inductive drop.
and
C
f
s
>
making
an angle $ v/ith the axis of abscissae, equal to the angle of lag
of the current Ig behind the
equal to E
f
,
the internal
CB is then the terminal
e. m.
e. m.
From
e. m. f .
f.
f.
with radius 0C
cut the line BC at
,
C.
The line
for the excitation H'.
This method when applied to the curves in hand gives the
following:
E
H\ =
1
= 8.3 volts. H* = 1.85 ampere.
.85 amp.
Whence
=
e
•
6.3
=3.8
E*
volts.
= 2 amperes.
= 1.72
R
I
while as measured it was 6.05
This is assuming unity power-factor.
of
I' 9
or 5^ high.
By taking 6.05 as the value
it shows that, assuming the method as correct,
e
there was an
angle of lag of 3° corresponding to a power-factor of 99.8$.
Again with a larger load;
E' = 8.8 volts.
H\K =
1.75
E
H' = 1.85 amp.
= 8.4 volts.
f
S
Whence
of the
e. m.
is less than
e
f.
I
&
1
= 4 amperes.
1
= 3.48
R
ft
owing to uncertain determination
of self-induction for such large values.
For a smaller field excitation,
ampere, the method gives
1
the foil ow i ng
E
H'
1
=4.3
= .3 am.Dere.
giving
e a
volts.
E'
H' = 1 ampere.
=1.4
volts.
I
R
value of 3.7 while as measured
I*
=
1 ampere.
= .87
e
=3.35
or 10% high.
.
~7
To make the calculated value agree with the measured, a lag
or a power-factor of 92.7 would have to be assumed.
angle of 22
This method then seems to give fairly good results if the
magnetization and short-circuit curves have been determined with
precision.
Mr.
B.
Behrend
A.
is another who has contributed
clearing up of the mystery^
to the
In a series of articles on the
design of alternators he says regarding Behn-Eschenberg
'
s
theory.
To determine the dynamic from the static and short-circuit
we can assume that armature reaction and
characteristics
armature leakage (self-induction) produce the same effect as an
induction coil placed in series with the machine.
Hence to get
a point of the dynamic characteristic we should simpl;/ have to de-
duct
e. m.
the
e. m.
consumed in the induction coil from the impressed
f.
of the alternator.
f.
If we take an ordinate of the static
characteristic, and divide it by the short-circuit current be-
longing to the same excitation, we get immediately the reactance
of the induction coil, which is the apparent reactance of the
armature, or as styled by Steinmetz, the synchronous reactance "
Applying this we have:
With
1
ampere output and
1
ampere field,
E
E
t
= 4- 8
X
x
=
1 ain P-
Z
88
s
2' 3
= 2.09
s
4.S
20 %
2.09 = 2.69 instead of 3.35 as observed, a result low by
.
(1) E. W.
&
E.
55:125.
27 Jan. '00.
For a 2 ampere load and 1.85 ampere excitation;
E
=8.8
E
I
t
= 2 amp.
x
I
.
s
s
Mr.
Q
= 2.01
'•
E
6.05, a result low by 21 %.
instead of
Steinmetz "^shows
'
X
8.8 - 2.01 x 2 = 4.78
R
t
= 4.25 amp.
determine the dynamic or field
hor/ to
characteristic when the value of the synchronous reactance is
known.
The value of this reactance, which is a resultant reactance
composed of the effects due to armature reaction and armature
self-induction, he assumes is constant for any given excitation.
As determined from Fig.
5a,
the synchronous reactance on short-
To compare this with
circuit with a 2 ampere field was 2.00 ohms.
the values obtained for this quantity as deduced from. the observ-
ed curve, the following method was used:
= open circuit e.m.f.
E
Let.
X
r
o
= synchronous
reactance,
J
= internal resistance of armature,
o
Then
i
=
y
Since the load used in determining the characteristic was non-
inductive, X=
and
r=
—*
15
Then from Fig.
6.
for the 1.85 amp.
I
excitation the following values were obtained:
E.
t
= 7.65 volts,
E o = 8.80
(l)
"
i»„
= .87
r
= 7.65
o
ohm.
/
X =
"
"Alternating Current Phenomena".
X =
o
2nd.
ed.
?
page 246.
X
-- -(r_+r)
°
=
o
=
2.21 ohms for a
ampere load.
1
I*
r = 3.03
E t+ = 6.05
•
= 3.85 ,r = 1.28
E.
t
*
E+" =
X
;
r = .175
.70
= 2.04
o
2
"
X„o = 1.99
"
3
"
= 1.93
"
4
"
.\X
o
"
"
In these observations the speed incresod 2.4 % from the begin-
ning of the time till the end, while the field decreased during
the same time 4.2
Assuming a straight line law for the magnet-
%.
ization curve the net decrease of the
these causes, 1.8
e. m.
should have been for
f.
Hence for the 4 ampere load the value of the
%.
e.m.f. was most probably .57 volt.
.142 ohms which would then make X
Q
The resistance would then be
= 1.95
Even with this cor-
rection it is seen that the synchronous reactance for the curve in
question decreases slightly with increasing armature current.
This is probably due to the increasing density of magnetization
of the iron and consequent lowering of the permeability, thus de-
creasing the self-induction.
of X
Q
5
E
amperes.
ampere field the values
1
being taken as 4.8 volts.
are as follows, E
Load
For a
E
r
XQ
4.15 volts.
8.3 ohms.
2.9 ohms.
1
If
<7
t!
B -9R-
N
O
12
tl
1.5
"
2.10
"
1.40
"
2.0
"
2.0
"
.60
"
.30
"
2.1
"
was taken as 4.8 volts and the curve drawn through that
point rather than through 4.1 for the reason that if 4.1 is taken
the synchronous reactance works out less than
0.
In searching for
30
the cause of this it was noticed that from the magnetization curves
for 170 cycles and 1.85 ampere field, the E q was 4.8 volts, so this
value was deemrd correct and used to the exclusion of the other.
d.
Stray Power Losses:-
In a direct current motor the losses
due to friction of the bearings and brushes, the hysteresis and
the eddy current loss, are taken together and called the "stray
power loss".
These losses are measured by simply measuring the
current required to run the machine as motor, at different speeds.
The current multiplied by the drop in volts across the armature,
In this test it was found
gives the watts lost for that speed.
that the speed varied directly as the impressed volts for a constant
field excitation.
The watts increase rapidly with the impressed
It was the intention
volts, as shown by the curves of Fig.
7.
to compare the stray power loss as a
d. c.
motor, with the watts
required to run the motor as an unloaded synchronous motor.
However, as a synchronous motor the machine broke down with high
frequencies before the field could be increased to
a
d. c.
motor it would not run on less than a
at a very high speed.
2.
As
2 amperes.
ampere field except
The consequence is that no value of the watt
input for the two methods of operation
and for the same frequency
and field current, can be found.
e.
Synchronous Characteristics:- A synchronous motor has a
great many characteristics
- some
of them are similar to those pos-
sessed by the mule - you cant tell what is going to happen next.
One of the objects of this investigation was to determine what
results would be obtained for
an;/-
given set of conditions.
The
principal points investigated were the influence of frequency and
field strength upon the power-factor and upon the stability of operation, these effects being observed for different values of the im-
pressed a.c. volts.
Observations were accordingly made of the
amperes and the a.c. watts, for impressed volts varying from
5
a. c
to 12
It was found that for
These were made with a constant field.
the high frequencies the machine would not stay in step for a wide
For this reason the
range of voltage with a strong field current.
series of tests were made with different field strengths for different frequencies, and hence cannot be compared directly.
These curves are shown in Figs.
9
to 14.
Fig.
15 shows the
variation of the watts with the frequency, for 7,9 and 11 volts,
from 100 to 160 cycles, with a constant field of .49 ampere.
The true watts are thus seen to decrease with the frequency, decreasjjing also for a decrease in voltage.
Fig*
16 shows how the power-
factor varies with the frequency for different vol'tages.
A peculiafl'
feature of this curve is the fact that all the curves seem to converge toward, a common point, indicating that for a frequency of abou£
90 and with a .49 ampere field, the power-factor v/ould be the same
for all values of the impressed volts and of a value of about 78
For e.m.f.s of more than
9
%•
volts the power-factor increases with
the frequency, reaching unity and then decreasing again, as shown
in the curve for
6
volts.
decreases with the frequency.
curve in general
,
For voltages above
9
the power-factor
Apparently then the power-factor
begins from near
for
frequency, rises to
unity for some particular value of the frequency and then decreases
again, becoming
again at infinity.
The value of the frequency
at which these curves cross is dependent upon the field excitation.
The synchronous characteristics of rectifier running self-
excited are given in Fig.
These exhibit the same general
19.
tendencies as to ampere and watt input, and to power-factor as when
run separately excited.
The value of the
increase in direct proportion to the
d. c.
volts.
a. c.
vclts is seen to
The large deviat-
ion of some of the points from the value indicated by the curve is
undoubtedly due to the fact that the relation of the
volts depends, for a given
brushes.
a. c.
d. c.
to a.
c.
voltage, upon the position of the
The wide insulation of the commutator taken in connection
with the narrow brushes, causes the
d. c.
circuit to be open a con-
siderable portion of the time and at a time when the
changing at a rapid rate.
e. m.
f.
is
A slight shifting of the brushes or a
slight variation of the pressure would hence cause the average potential as shown by the voltmeter to change considerably.
f.
Efficiency of Conversion:-
One of the requirements laid
down in the problem, was that the efficiency of the process of con-
version must be higher than that which would obtain if resistance
were in series on direct current mains of 100 volts of a value sufficient to reduce the current to 10 amperes.
is
10" x 10 4 100 x 10
= 10
33 % being attained for a
50 watts.
%
d. c.
Thie efficiency
This value was exceeded easily,
output of
5
amperes at 10 volts
or
Owing to lack of a suitable load, tests were not made
with the machine under commercial conditions (self-exciting), for a
greater output than this.
An output of 10 amperes was contemplate
in the design, but owing to troubles with sparking the thickness of
brush was reduced, diminishing the area of contact, which would undoubtedly prevent the satisfactory operation at rates as high as thi
33
A word might here be said in regard to sparking.
The prob-
lem in designing a rectifier of this type to be used with a load
having a counter-electro-motive-f orce , ia to have the brushes
change from one commutator bar to the next when the
just equal to the
armature.
e. m.
f.
c. e. m. f.
is
being generated at that instant in the
If this condition is fulfilled and in addition the
power-factor is unity, there will be no current at that instant
and hence no spark will be produced.
kept open till an
the
c. e. m.
f.
e. m.
f.
The circuit must then be
is being induced equal to or greater than
of the load, in order to prevent a useless flow of
current from the batteries into the armature, which will have to be
stopped and reversed.
It is thus seen that much depends upon the
In the mach-
relative' width of brushes and commutator insulation.
ine under consideration the brushes were first made of a width but
slightly less than the insulation, it being the desire to raise
the
d. c.
volts as high as possible by keeping the armature in cir-
cuit as long as possible and yet not short circuit the batteries
by allowing the brushes to bridge two segments at once.
in this manner the sparking was considerable.
T
,Yhen
set
By reducing the
brush width to half that of the insulation the operation was rendered sparkless up to loads of about 40 watts
d. c.
.
Fifty watts
must then be considered as the upper limit of the capacity of the
machine ? f or the sparking would soon destroy the commutator.
In the test given in Fig.
20, which is plotted from data in
Table VIII, it is seen that the efficiency is 33 % for a 50 watt
output, -this being with a field strength of 1.25 amp.
factor being 85 % under these conditions.
,
the power-
Tests were also made with separate excitation.
Fig.
curves for thin condition, the load being non-inductive.
constant impressed
a. c.
17 shows
For a
volts of 9.5 and a constant d.c. load of
23.9 watts, the familiar changes due to a change of the field, are
produced.
The amperes follow the
'.veil
factor reaches unity and then decreases.
known "V"
The
,
a. c.
while the porerwatts seem to
follow to a certain extent the same form of curve as the amperes.
This is perhaps due to the
fact that the iron losses as well as
the copper, increase with the current, thus increasing the watts
measured by the wattmeter.
The carves on the right side of Fig. 17 show the variation of
the power-factor, current, watts and efficiency, for an increasing
impressed voltage, the d.c.
load however increasing also.
From
this set of curves it is seen that unity power-factor is obtained
with 7.6 volts on a
1
ampere field.
From the curves on the left
volts
side of sheet, unity power-factor is obtained with 9«5Awith .7 amp.
field.
It is thus seen that for a given load the excitation must
be decreased for an increase of the voltage.
Since the increase
in voltage means an increase of the true watts input it may be de-
duced that to maintain a good power-factor with increasing load
the excitation must be decreased.
It was also observed that to
keep up the power-factor with increasing frequency and constant
volts, the field must be weakened.
Figure 18 shows the variation of the various quantities for an
electrolytic load of three sal-ammoniac cells.
load was constant and the
pressed a.c. volts.
d. c.
In this case the
watts hence increased with the im-
The maximum value of the output reached was
.
35
about 25 watts, the efficiency being some higher than was the case
With the non-inductive load for this value, while the power-factor
is considerably less,
the excitation being the same in both cases.
By increasing the load the efficiency increases as shown in
Table VI.
been plotted but the efficiency
the
d. c.
2.
Data in this series has not
for a load of two cells.
is
seen to rise with the value of
Watts output, reaching 69 % for 63 d.c. watts.
From outside sources.
As very often happens in thesis work, there are many things
that the author would like to investigate but time has not permitted.
Among these the question of whether stable running of a
synchronous motor is better with an armature having a large moment
of inertia, or the reverse, and also whether the self-induction
should be large or small.
Boueherot^
^
hias given a formula for the
periodic time of oscillation of alternators when run in parallel
which is as follows:
T
Where
N
r. p m
M
moment of inertia in kilogram-meters.
f
frequency.
.
normal effective voltage.
current on short-circuit with excitation
which gives normal volts on open-circuit.
(l) L'Sclairage Electri que.
21:121.
28 Oct.
'99.
36
He says that the ratio of the periodic time of oscillation of
the alternator, should be great as compared v/ith that of the prime
mover.
"be
For successful parallel operation this ratio should alv/ays
4 and the larger this is the more stable is the working.
Since
the parallel operation of alternators is very analogous to the op-
eration of a synchronous motor, this formula should hold for that
In this formula "M" appears in the numerator, hence
condition.
any increase in this would increase the ratio and tend to make the
operation more stable.
,f
Ij*
appears in the denominator.
To increas)|e
"T", this v/ould have to be decreased, which would mean an increase
of the value of the self-induction, since the short-circuit current
depends upon the impedance and this would on short-circuit be composed principally of the reactance.
The conclusion would then be
drawn from this formula that for stable operation of a synchronous
motor the moment of inertia should be great and the impedance also
great.
Directly opposed to this is the statement made by the
Well known English designer, Mordey, who states^hat self-induction
or anything that prevents the motor armature from instantaneously
following every slight variation of the frequency of the generator,
is detrimental to good working.
Kolbeiv
"
'
inclines to the latter
view since he says that a motor with a "weak armature field" (low
self-induction), and a short-circuit curve which rises rapidly for
a small increase of the excitation, will bear overloading.
(1)
J. I.E. E.
(London) 23:206. 1S94.
(2) Electrotechnische Zeitschrift.
51:802.
1895.
7
Another sub jest that the author would have liked to have followed up was this
In designing an alternating current generator
:
or synchronous motor, it is of vital importance to know what the
inductive drop will he.
Actual calculation of the co-efficient
of self-induction is almost impossible except perhaps with a smooth
core armature.
In looking up the subject there was found an
article by one of the leading lights in dynamo design
Hinnend) which with
was translated.
Fischer-
-
the assistance of Professor William E. Esty,
The article is a generalization of the theory
advanced by Behn-Eschenbarg, which has been referred to before in
this thesis.
This author points out the fact that the method of
deducting the ampere-turns of the armature from those of the field
to get the ampere-turns effective in producing the
e. m.
f.
,
is not
correct, since the magnetic reluctance of the armature is different for different relative positions of the armature coil and the
field pole, due to due to different phase-displacements.
This he
says may amount to 30 or 40 % 9 as shown by the following example;
A certain 600 KW alternator with a constant excitation, gave
the following;
Volts,
No load.
4400.
Short-circuit.
Amp.
0.
320.
Non-inductive load.
3900.
.
200.
The impedance of the armature then, for a power-factor =100$ is;
Z s
200
4400
- 3900
(l) L'Eclairage Electrique.
=
10 ohms.
13:495. Mar.»97.
And for short-circuit or a change in phase of 90
Zn =
m°
520
,
= 14 ohms.
The difference is about o0 %%
Starting with this as a gen-
eral value for the change, and assuming that it varies as a linear
function of the phase-displacement, he deduces a formula for the
design of an alternator.
This formula is quite long and will not
he given here, it expressing the relation between the diameter of
the armature and fourteen other variables.
of empirical constants.
There are four tables
Had more time been available,
I
would
have tested it by applying it to the rectifier to see how closely
the calculated drop would agree with that as actually measured.
o.
Acknowledgements:- Finally, expression should be made of the
help rendered by Professor William Esty in clearing up doubtful
points in the design of the rectifier, v/hile during the progress of
the tests many valuable suggestions were received from Professor
William Hand Browne, Jr.
The co-ordination and arrangement of
*
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