Modern dynamo electric machinery

M O D E R N D Y N A M O ELECTRIC MACHINERY.*

BY

ALEXANDER GRAY, M.Sc., Professor of Electrical Engineering, Cornell University, Ithaca, N. Y.

ALT~nATI~G-CURR~nT OEN~.~ATORS.

We have already seen that direct-current machinery had become somewhat standardized by I89o, and that the Hobart machine, described on page 22, has design constants and a type of mechanical construction which do not differ much from those of

Fro . 32.

Belt-driven single-phase alternator, 90 k.w., I6,OOO alternations.

similar machines of recent date, but at that time, 27 years ago, there was no one type of alternator that was considered superior to all others.

The first Niagara machines, shown in Fig. 3, had an external rotating field system, this type of construction being considered necessary to give the large flywheel effect required to maintain

* Continued from page 48, July issue. 205

2 0 6 ALEXANDER GRAY. [J. F. I.

the speed :approximately constant with changing load. Rotat- ing armature machines, such as that shown in Fig. 32 , were still listed in 19o3, and inductor alternators of the type shown in Fig. 33 were on the market as late as 19o 7, but the advan- tages of the present type of internal rotating field construction were such that by 19oo most of the other types had been dis- carded, so that Guilbert, in describing the alternators that had been on exhibit at the Paris Exposition in 19oo. states as follows :

" One of the most striking features was the triumph of the three-phase system even for lighting; of equal interest was the

Fw,. 33.

Inductor alternator with vertically split armature.

fact, as had been foreseen before the opening of the Exposition, that the inductor type of alternator was being abandoned."

Many data have been published on these Paris machines. 1~ From these data, and from some additional inforlnation published by Rothert, 1~ the seven machines in Table III have been selected as typical alternators of 19oo, a;nd these we shall use as a starting point in our study of modern machines. Let its

l*L'12clcdrge Electrique, vol. 29, p. 276, November 2,3, 19Ol; Electrical World, vol. 37, PP. 113, 154, 194, 231, 274, 302, 352, 398, January to March, 19Ol.

"~L'l~clairage I~lectrique, vol. 29, p. 307, November 30, 19Ol. See also " Engineering Evolution of Electrical Apparatus," Lamme. Electric Journal, vol. II, p. 73.

Aug., IOl7.] ~'~ODERN I)VNAMO t{LECTRIC 5IACHINERY. 207

first, however, take up the various limitations in the polyphase alternators.

design of

T A B L E I I I .

K v . a . . . . . . . . . . . . . . . . . . . . 215 800 8 0 0 8 6 0 lOOO 133o 14o0 V o l t s . . . . . . . . . . . . . . . . . . . 220 2 2 0 0 2 2 0 0 2 4 0 0 5000 5 5 0 0 3 0 0 0

A m p 4 r e s . . . . . . . . . . . . . . . . . 565 2 1 0 210 207 I I 5 140 270

P h a s e s . . . . . . . . . . . . . . . . . . . 3 3 3 3 3 3 3

F r e q u e n c y . . . . . . . . . . . . . . . 32 42 .5 5 ° 50 50 25 5o

R . p m . . . . . . . . . . . . . . . . . . . 12o 8 0 79 9 4 9 4 75 72 P o l e s . . . . . . . . . . . . . . . . . . . . 32 6 4 76 64 6 4 40 8,1 I n t e r n a l d i a m e t e r of s t a t o r

in i n c h e s . . . . . . . . . . . . . . . lO6 236 236 197 230 214 252 F r a m e l e n g t h i n i n c h e s . . . . i o IO lO.7 i i 11 .8 17 IO

m . P o l e p i t c h i n i n c h e s . . . . lO .4 11 .6 9 .8 9-7 11..35 16 .8 9 . 4 n. A i r - g a p 21earance in

i n c h e s . . . . . . . . . . . . . . . 197 .312 .275 ,275 .312 .43 .235

R a t i o m / n . . . . . . . . . . . . . . . 52 .5 37 35 .5 35 36 39 40 S l o t s p e r p o l e . . . . . . . . . . . . . 3 6 6 3 6 6 6

S i z e of s l o t in i n c h e s . . . . . . 1.85 d i a . .87 X I . 4 .6 X I . I 5 1.7 X 2 . 5 I . I d i a . I X 2 . 7 .83 X 2 . 2

C o n d u c t o r s p e r s l o t . . . . . . . 9 6 5 7 6 9 6

C o n n e c t i o n . . . . . . . . . . . . . . . Y Y Y Y d e l t a Y Y Y Y Y d e l t a T o o t h / s l o t . . . . . . . . . . . . . . . - - 1 .24 1 .72 .9 - - 1.8 I .O M a x i m u m t o o t h d e n s i t y a t

n o l o a d in l i n e s p e r s q u a r e

i n c h . . . . . . . . . . . . . . . . . . . lO4,OOO 1 1 o , o o o 6 5 , 0 0 0 II5,OOO lO4,OOO 7 7 , o o o IOO,OOO (2ore d e n s i t y a t n o l o a d . . . . 2 9 , 0 0 0 3 0 , 0 0 0 2 1 , o o o 3 0 , 0 0 0 18 ,ooo 8 4 , 0 0 0 2 6 , o o o P o l e d e n s i t y a t n o l o a d . . . . 1 1 8 , o o o 1 2 o , o o o I I I ,OOO 1 1 8 , o o o lO9,OOO 9 0 , 0 0 0 1 1 4 , o o o a. A m p 6 r e c o n d u c t o r s p e r

i n c h of p e r i p h e r y . . . . . 365 375 3 2 0 4 5 0 3 7 0 4 5 0 570 b. C i r c u l a r m i l s p e r a m p 6 r e , 74 ° 4 3 0 4 5 0 6 2 0 725 lO6O 4 8 0

R a t i o a / b . . . . . . . . . . . . . . . . . . 49 .87 .71 .72 .51 .42 1 .18 P e r i p h e r a l v e l o c i t y of r o t o r

i n f e e t p e r m i n u t e . . . . . . 3 3 2 0 4 9 5 0 4 9 0 0 4 8 5 0 5 6 5 0 4 2 0 0 4 7 5 0 c. A m p h r e t u r n s p e r p o l e a t

n o l o a d . . . . . . . . . . . . . . 5 3 8 5 7 6 0 0 4 6 0 0 6 9 5 0 5 1 7 5 8 0 5 0 6 6 7 5 d. A r m a t u r e a m p 6 r e t u r n s

p e r p o l e . . . . . . . . . . . . . 1 9 2 0 2 2 0 0 1 6 0 0 2 2 0 0 2 1 0 0 3 8 0 0 2 7 5 0

R a t i o o l d . . . . . . . . . . . . . . . . 2.8 3 .45 2 .87 3 15 2 .46 2 .12 2 .42 P o l e e n c l o s u r e . . . . . . . . . . . . . 57 .5 .8 . 6 t .66 .65 .64

O u t p u t f a c t o r X i o o . . . . . . . 1 .64 1.82 2 . i 2 2 .15 1.72 2. 3 3 I

V o l t s p e r p h a s e = 2 . 2 2 X c o n d u c t o r s p e r p h . )4 e# X f X I O -s

T o o t h a r e a p e r p o l e = m i n i m u m t o o t h X s l o t s p e r pole× n e t i r o n i n f r a r a e

l e n g t h . X qJ P o l e d e n s i t y = g a p f l u x X l e a k a g e f a c t o r / p o l e a r e a

A m p S r e c o n d u c t o r s p e r i n c h of p e r i p h e r y = c u r r e n t p e r c o n d u c t o r X c o n d u c t o r s p e r p h a s e X p h a s % rr X s t a t o r i n t e r n a l d i a m e t e r

A m p e r e t u r n s p e r p o l e a t n o l o a d = o p . P i g . 38. A r m a t u r e amp i~ re t u r n s p e r p o l e = c u r r e n t p e r c o n d u c t o r X c o n d u c t o r s p e r p o l e

2 O u t p u t f a c t o r = v o l t a m p & r e s

r p m X D ~ L

P E R F O I I M A N C E CHARACTERISTICs.--Fig. 34 is a section of a three-phase alternator showing the conductors of only one phase. \Vhen this machine is supplying current, these conductors are encircled by the lines of force of the alternating magnetic fluxes

V O L . I 8 4 , N o . 1 1 o o - - I 6

208 ALEXANDER GRAY. [J. F. I.

q,x and ¢r. The former, often called the armature leakage flux, generates a voltage of self-induction in the conductors which is proportional to the current and is equal to IX where X, called the leakage reactance, is constant, since the flux g,~ is proportional to. the current which produces it, the reluctance of the

FIG. 34 .

Armature fluxes produced by one phase of a three-phase alternator.

Pla. 35-

N ~ MOTION Armaturef ie ldofa three-phaseal te rna torwhenthecurrent lags 90 degrees.

P IG . 35 A.

Ptt.l PH.2 PH.3

Currents in three phases.

leakage path being practically all in the air part of the path. The magnetic flux ~r enters the poles and so modifies the main field.

Fig. 35 shows a section through the same three-phase machine and shows the current distribution in the conductors when the power factor of the load is zero and the current lags 9o degrees

Aug., 1917.] MODERN DYNAMO ELECTRIC MACHINERY. 209

behind the terminal voltage. Under these conditions the current is a maximum in the conductors that are between the poles, because in these conductors no electromotive force is being generated. The three fluxes q,,., produced by the three phases, com- bine to give a gliding field exactly as in an induction motor, which field is proportional to the current flowing and moves at synchronous speed in the same direction as the rotating poles. The phase relation between this armature field and that produced by the poles depends on the power factor of the load and, as shown in Fig. 35, directly opposes the main field at zero power factor.

Fi6. 3 6.

Eo

/ _ ~ ~ x F / I

Vector d iagram per phase of a polyphase al ternator .

These effects of armature reaction can best be indicated by a vector diagram, as shown in Fig. 36 .

The no-load m.m.f. Fo produces an alternating flux in the winding of each phase, and the voltage Eo generated in each phase lags Fo by 9 ° degrees.

The m.m.f, of the armature produces a synchronous gliding field of constant magnitude and causes an alternating flux to thread the windings of each phase. It may be seen from Fig. 35 that the armature flux threading phase I is a maximum when the current in that phase has its maximum value; thus in Fig. 36 we have :

2 I O A L E X A N D E R GRAY. [J. F. I.

F o is the no load m.m.f. E o is the voltage per phase due to F o. I is the current per phase. F a is the m.m.f, of armature reaction which, as pointed out in the last

paragraph, is in phase with 1. Fg is the resultant m.m.f, due to field and armature. ~, is the re'suiting synchronous flux. Eg is the voltage per phase due to q6 m 9!,, is the armature leakage flux per phase. .. I X is the leakage reactance drop. Et, the terminal voltage per phase, ~ E , j - I X - I R taken as vectors.

T h e vol tage drop ce due to the combined effect of leakage reactance drop and drop due to a r m a t u r e react ion is general ly called the synchronous reactance drop.

1.4

]-2 L9

1-0 CD

_ J

0.8 o 06 F-- 0-4

02

]PIG. 37, FIG. 38.

/ ~ >"Y.je[.--u / ~ . ~ ~ " -

? > 7i

~ / / -';~'~ l

_ i i I

c AMP. TURNS P. POLE

Characteristic curves of an alternator.

1.4

12

10

08

0.6

F--- 0.4-

02

Ea

I

? ~i"_:

i

',2~.-'" ~:..--T

L ) " z . 0 j ~ " ' o

1 ,0~N F-

0 c P q AME TURN5 P. POLE

T e s t c u r v e s of a I s o - k w . , 6 0 0 r . p . m . , three-phase alternator.

In the par t icular case when the power fac tor is zero and the cur rent lags the te rminal vol tage by 9 ° degrees, the leakage reactance drop is subtracted direct ly f r o m the genera ted voltage, and the m a g n e t o m o t i v e force of a r m a t u r e react ion is subtracted direct ly f r o m the exci t ing m.m.f . The vol tage drop due to a r m a t u r e reaction, however , decreases as the poles become satu- rated, because the same m.m.f , produces a g radua l ly decreasing reduct ion in the flux. These two effects are well shown in Fig. 37 , where curve I is the no- load saturat ion, ab is the m.m.f , to overcome the demagne t iz ing effect of a r m a t u r e reaction, and be

Aug., I917,] MODERN DYNAMO F.LECTRIC ~[ACHINER¥. 2 I I

is the voltage drop due to this effect, bc is the leakage reactance drop, and c is a point on the full-load saturation curve at zero power factor.

It is found in practice, however, that the two curves are not parallel to one another as shown, but gradually separate as in Fig. 38, which gives test data on an actual machine. The additional voltage drop is due to the pole leakage. The m.ln.f, required to generate a voltage Eo on no load is op, Fig. 38, but, when the alternator is loaded and the power factor is zero, the m.m.f, for the same generated voltage is oq: the flux crossing the air-gap is unchanged, but the leakage flux from pole to pole is increased in the ratio oq/'op, and, under these conditions, an increased excitation is required because of the higher densities in the pole core. The relative amounts of these three effects can be shown as follows:

The distance ab, Fig. 38, is determined from the formula : Demagnetizing amp6re turns per pole = o.35 Z lc/p.

where Z/p is the total conductors per pole, L. is the current per conductor,

and the distance bc must be the leakage reactance drop; this can be checked approximately by calculation. If the triangle abc be now placed so that the full-load saturation curve found by actual test is the locus of the point c, then the locus of point a is what designers call the no-load saturation curve figured with the full- load leakage factor, and the excitation ad is the additional excitation l,~r the poles over that required at no-load for the same air-gap flux.

LINITATIONS IN DESmN.--If it is desired to have good regulation, pa.rticularly with low power factor loads, it is necessary to keep the synchronous reactance drop fc, Fig. 38, small com- pared with the normal voltage. To accomplish this result:

I. Keep the leakage factor small and the pole density below saturation.

2. Keep the leakage reactance small : this, as shown in Fig. 39, means limiting the number of amp6re conductors in the phase belt .r or, what is equivalent, limiting the number of amp6re conductors per inch of armature periphery.

3- Keep the demagnetizing amp6re turns per pole a small

212 ALEXANDER GRAY. [J. F. I.

fraction of the exciting ampgre turns per pole and sa.turate the magnetic circuit so as to make the voltage drop be small.

One may well ask why the output of this particular machine is limited to 15o kilowatts. Why would it not be possible to increase the main excitation and thereby allow a larger current to be carried by the armature without any sacrifice of regulation?

FIG. 39.

1 I i"x-'i

Winding of one phase of a three-phase alternator, showing the leakage flux.

Fm. 40.

P'C p

¢9

ll!r / / \ \ \ ¢.

The main field and the pole leakage field of an alternator.

Fig. 40 shows part of a machine to scale. Of the flux q~p which enters the pole, the leakage flux q~ at no load is 2o per cent., and the useful gap flux Cg is 8o per cent., under which conditions the pole density at the root is ioo,ooo lines per square inch, and the tooth density 95,oo0, while the length of the pole is just sufficient to give the necessary radiating surface to the field coils.

Aug., I9x7.] MODERN DYNAMO ELECTRIC ]~ACHINERY. 2I 3

If, then, the field excitation is increased 5 ° per cent., the pole length will increase in the same ratio to dissipate the additional heat, the air-gap will be increased to use up the increased m.m.f., since ~g, limited by the tooth density, remains constant, but the area as well as the m.m.f, of the leakage path will be increased, so that the leakage will become (1.5) ~, or 2.2 5 times the original value, and the flux density at the root of the pole will be excessive.

FIG. 41 .

120C

110C

100C

90C .w-

600 z

7OO cO

600 b -

2 500 7 " 0

400

500

"< 200

100

KV-A.OUTPUT l 2 5 ~- 5 6 7 8 9 lOqO00

Values of ampere conductors per. inch of periphery for slow-speed alternators,

For a satisfactory machine, therefore, all of the dimensions in Fig. 4o must increase as the ampfire turns per pole are increased, so that one constant in design is the ratio amp6re turns per pole/pole pitch, the value of which ratio can be increased either by raising the permissible temperature rise of the field coils or by improving the ventilation. It is of interest to note, however, that, in order to obtain a certain desired voltage regulation, the armature ampere turns per pole are made a definite fraction of the main excitation, so that the constant used by the designer is rather the ratio of armature amp6re turns per pole/

214 A L E X A N D E R G R A Y . [J. F. I.

pole pitch, of which the ampere conductors per inch of armature periphery are a definite measure. This constant, therefore, which we found to be of great importance in direct-current design, is of equal importance in the design of alternators, but, since its value depends on so many factors, the writer who gives a table of values is subject to criticism. Let us, however, ex- amine a few machines and see what values are found in practice.

TABLE I V .

K v . a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I o o o

P h a s e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

F r e q u e n c y . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 °

R.p.m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4 P o l e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4

Date 19oo I n t e r n a l d i a m e t e r of s t a t o r in i n c h e s . . . . . . . . . . . . . 2 3 0

F r a m e l e n g t h in i n c h e s . . . . . . . . . . . . . . . . . . . . . . . . . 11.8

1917 130

13.5 C e n t r e v e n t d u c t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N o n e 3 o f ~ i n c h

E n d v e n t d u e t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N o n e 2 of I i n c h

m . P o l e p i t c h in i n c h e s . . . . . . . . . . . . . . . . . . . . . . . . . I 1 .35 6-4

n . A i r - g a p c l e a r a n c e in i n c h e s . . . . . . . . . . . . . . . . . . . 5 / 1 6 3 / I 6

Ratio m / n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 6 3 4 S l o t s p e r po l e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6

S ize o f s l o t i n i n c h e s . . . . . . . . . . . . . . . . . . . . . . . . . . . I . I d i a . . 5 X 2 . 7 5

T o o t h s l o t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 .8 1.12

M a x i m u m t o o t h d e n s i t y a t n o l o a d in ! i n e s p e r sq . in . lO4,OOO IOO,OOO

C o r e d e n s i t y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I8,OOO 5 o , o o o

P o l e d e n s i t y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lO9,OOO 9 5 , o o o

a . A m p S r e c o n d u c t o r s p e r i n c h . . . . . . . . . . . . . . . . . . . 3 7 o 9 o o

b. C i r c u l a r m i l s p e r a m p e r e . . . . . . . . . . . . . . . . . . . . . 725 7 5 °

R a t i o a / b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 1.2

P e r i p h e r a l v e l o c i t y o f r o t o r i n f e e t p e r m i n u t e . . . . 5 7 0 0 3 2 0 0

c. A m p e r e t u r n s p e r po l e a t n o l o a d . . . . . . . . . . . . . 4 5 o o b y t e s t 3 7 o o

d . A r m a t u r e a m p S r e t u r n s p e r p o l e . . . . . . . . . . . . . . 2 1 o o 2 8 8 o

R a t i o c / d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 .15 1.28

P o l e e n c l o s u r e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o . 66 0. 7

O u t p u t f a c t o r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o . o i 7 2 o . o 4 7

G u a r a n t e e d t e m p e r a t u r e r i se . . . . . . . . . . . . . . . . . . . . 4 ° ° C . 5 °0 C .

Table III gives data on several of the machines that were exhibited at the Paris Exposition; the i4oo-kv.a, machine is of special interest because Rothert claims that it is so much superior to other machines of that time. The values of amp,~re conductors per inch found in these machines are plotted in Fig. 41, and on the same diagram are plotted figures given by S. P.

Aug., ~9~7-] M O D E R N ] ) Y X A M O ]TLECTRIC M A C l l [ N E R Y . 2 I 5

Thompson in I9o5; these indicate the improvement in ventilation in five years. The figures of I9Io are higher because oper- ating engineers were willing to accept poorer regulation in order to get a cheaper machine, while the enormous increase in the last seven years is due partly to the fact that the larger machines are now rated at 5 °0 C. rise with no overload guarantee, but more particularly to the fact that regulation has been thrown to the winds and the voltage maintained at all loads and power factors by voltage regulators such as these first invented hv Tirrell.

FI(;. 4 2.

/ 'rlS.5~

<

b <

1000 KV-AI ,v 1000 ffV-A. 94- R.RM.[ / 94 R.P.M. oo 164 P0LESl/ POLES 7" JDAT~.-1900[/ DATE-1917 i,,"

/L Comparison between a machine built in I9oo and one built in I917 for the same output.

Perhaps the best conception of the progress made in design, and of the entire change in performance characteristics, may be obtained from Table IV, which gives comparative data on the ~ooo-kilovolt-amp&re machine exhibited at the Paris Exposition and on a unit as designed to-dav for the same output. The two machines are drawn to scale in Fig. 42, and the performance characteristics are given in Fig. 43. With an excitation neces-

2 1 6 A L E X A N D E R G R A Y . [ J . F . I.

sary to give normal voltage on no-load, the current on short- circuit is 2.75 times full-load current in the early machine and is only i. 5 times full-load current in the more recent design. In the case of alternators of large output, when the air-gap clearance is not limited partly by mechanical clearance, the short-circuit current with normal no-load excitation does not greatly exceed the full-load value.

The regulation guarantees expected at various times have been :

Date Regula t ion at IOO per cent . Dower factor

Per cent.

I 9 0 0 . . . . . . . . . . . . . . . . . . . . 5

1910 . . . . . . . . . . . . . . . . . . . . 8

1917 . . . . . . . . . . . . . . . . . . . . 20

Regula t ion at',8o per cent. power factor

Per cent. :~

15 20

4O

The method of obtaining good regulation in the early machines was, as shown in Fig. 43a, to saturate the magnetic circuit and to use a small number of amp6re conductors per inch of periphery. Since it was not possible to use high tooth densities as in direct-current machines because of the high frequency and the stationary armature, it was usual to run the pole density above saturation, and this often caused an excessive drop in voltage, due to pole leakage. The machines, therefore, would not give an overvoltage if this was desired: the iron going into the machine had to be carefully tested for permeaMlity and the leakage fluxes, and saturation curve closely calculated; the machines also were large for their output. The tendency in America until almost I9oo was to build a cheaper machine, of poorer in- herent regulation, and to compound or compensate the machine for voltage drop ; series excitation was obtained by SOlne kind of rectifying device such as that shown on the end of the shaft in Fig. 32 . This scheme worked well on the small single-phase units used for lighting service, because the power factor was approximately constant, but it gradually lost favor as the polyphase system came into general use.

The extent to which alternators have been gradually rated up is well indicated by the curves of output constant given in Fig. 44- It is of interest to note from these curves that, contrary to general opinion, the core dimensions of an alternator are greater than those of a direct-current machine of the same output. This

Aug . , 1917.] ~ ' I O D E R N D Y N A M O E L E C T R I C ~ ' I A C H I N E R Y . 217

is due principally to the fact that, whereas in direct-current machines tooth densities of 15o,ooo lines per square inch are not exceptional, values of IOO,OOO lines per square inch cannot safely be exceeded in 6o-cvcle alternators, because the loss at that fre-

FiG. 43 a

1'2 ~ / ~ ' ~ LO < 1 0 I

' \ Z 0

06 • 5'0 z U

S ~: 04 2.0 j / ,.'o 7-- ~ o - r

0~-/ / / I )°~_ / L ~-

2 5 4 5 6× 1000 AMP.TURNS PER POLE

Performance characteristics of the machine built in t9oo.

Fie;. 43B.

1-2 / f

1.0 z

o 08 I / F- <-J // "'~ z:: c¢: (..)

N 0"6 '" ' = ~

0-2 " 1O_z I--

/ 1 2 3 4- 5 6~'1000

AMP. TURN5 PER POLE Performance characteristics of the machine built in I917.

quency is large, while the ventilation of a s ta t ionary core is not nearly so effective as is that of a rota t ing armature.

VENTILAmmx.--Manv of the earl}, a l ternators were so large for their output that no special provision was necessary in order

2 1 8 ALEXANDER GRAY. [J. F. I

to keep the temperature rise down to a safe value. The machine shown in Fig. 42 , for example, does not even have vent ducts in the stator. Even in the case of modern machines, the slow- speed alternator does not require that any special precautions be

Fro. 44-

O08 & 0-07. ~ ; - - , ~ , ~ , ~ ~ ~ ~

0 0 6 ~ / @ I . ~ \ ~ , ~ > ~ t t I

==__ 0'05 / -'~R~

~ 0 0 4

0.03 "" _ I . . . . I Mp 40N. ]90 ' / 5.P.THO ,, -

o ~ 002 / f K : VOLT-AMPERES

0.01 D~ix'RF'M , , o 7 o ,ox,ooo

KV-A. OUTPUT

Output constants of slow-speed alternators.

taken to keep it cool, and it is only as the speed for a given output increases, and therefore the dimensions decrease, that the problem becomes difficult.

watts loss T h e t e m p e r a t u r e r i s e is p r o p o r t i o n a l to ~DL(a + b v) w h e r e

¢r D L is the armature surface V is the peripheral velocity of the armature a and b are radiation and convection constants respectively.

. . . . . D2L X rpm The output m ~v. a . . . . (DL V) X a constant,

K • watts loss V

therefore temperature nse=-(a @~V) × outPut X a constant

[ V \ = (per cent. loss)X ta + b V )

so that, for a given output and a given efficiency, the higher the peripheral velocity the more difficult it is to keep the machine cool. But note further that

V =~D X rpm I 2

7rD I2o f = X

T2 p = I0 X pole pitch X frequency

Aug . , 1917.] ~ , I O D E R N ] ) Y N A M O E L E C T R I C ~ , ' L , \ C H I N E R Y . 2 I 9

so that, for a given output and given frequency, the grea ter the pole pitch the more difficult it becomes to keep the machine cool. This is shown in ra ther a s tr iking way if we compare two water-

Fro. 45.

I. I.

rm r'~

/ /

I

Comparison between two IO,Ooo-kv.a. water-wheel generators, one of IOO r.p.m, and the other of 6oo r.p.rn.

wheel generators , one to operate oll a high head and the other on a low head: compare, for example,

K v . a . . . . . . . . . . . . . . . . . . . . IO,ooo IO,Ooo

R p m . . . . . . . . . . . . . . . . . . . . 6oo I0O

P o l e s . . . . . . . . . . . . . . . . . . . 12 72

Probable dimensions are :

~ n t e r n a l d i a m e t e r o f s t a t o r in i n c h e s . . . 80 250 F r a m e l e n g t h in i n c h e s . . . . . . . . . . . . . . . 37 26 P o l e p i t c h in i n c h e s . . . . . . . . . . . . . . . . . . 21 lO. 9 P e r i p h e r a l v e l o c i t y of r o t o r in f e e t p e r

m i n u t e . . . . . . . . . . . . . . . . . . . . . . . . . . i2 ,5oo 65oo P e r i p h e r a l v e l o c i t y a t r u n - a w a y s p e e d . . 22,5oo 11,7oo

2 2 0 ALEXANDER GRAY. [J. F. I.

These machines are drawn to scale in Fig. 45, and it can now be readily seen why the high-speed machine is so difficult to ventilate: it has not the radiating surface of the slow-speed machine.

In slow-speed machines the air is stirred up around the stator by the rotor itself, no fan blades being necessary. The whole construction is open, as shown in Fig. 47, and also in diagram A, Fig. 46. With this type of ventilation the air is

Fro, 46.

A

Different methods of ventilating alternators.

not always directed where desired, and the air streams are rather unstaNe, so that one side of the machine is often cooled better than is the other. With moderate-speed machines it becomes necessary to properly direct the air, which result is generally accomplished by the addition of fans to the rotor, while the housings are often made solid, as shown in Fig. 48, and also in diagram t3, Fig. 46, so as to deflect the air over the back of the stator windings and core.

In the case of high-speed water-wheel units, under which

Aug., ~9~7.] ~/[0DERN DYNAMO ELECTRIC ~{ACHINERY.

FIG. 47"

2 2 I

Generator with open end bells.

Fm. 48.

Generator with solid end bells.

2 2 2 A L E X A N D E R G R A Y . [J . F. I.

class would come the 6oo-r.p.m. machine in Fig. 45, the frame is generally long in order to keep the peripheral velocity down to a safe value, and in such cases it is necessary to create an air- pressure in the end bells to force the air between the poles and out through the vent ducts in the centre of the stator core, as shown in diagram C, Fig. 46, and also in Fig. 49; comparatively small openings are left at D, Fig. 46, to ventilate the end bells.

MECHANICAL CONSTRUCTION.--0ne feature of interest is the growing popularity of the vertical shaft or umbrella type of

lqc , . 49.

Generator with enclosing end bells and forced ventilation.

water-wheel generator. This has been due partly to improve- ments in the design of this type of water-wheel, but also in large measure to the development of a reliable thrust bearing. The Kingsbury bearing makes use of that feature of the horizontal shaft bearing on which its ability to carry large pressures depends; namely, that, as shown in Fig. ~o the oil is drawn into ,

the high-pressure space as a wedge, by the rotation of the shaft itself. ~Phe stationary thrust plate of this type of bearing is made of a number of segments, as shown in Fig. 52, these seg-

:\ug., 19~7.] -~'IODERN DYNAMO ELECTRIC ~,{ACHINERY. 223

i

FIG. 5 ° .

°1 < 0

~ O I L FILN Oil film in the bearing of a horizontal shaft.

FIG. 51.

°1 < 0 _1

MOTION I

~. s s ,a~ . ~ . ~ . s ~ s J ~ j j f f ~ . ~ t J f J f J

OIL FILM

Oil film in the Kingsbury thrust bearing.

FIG. 52.

Parts of a Kingsbury thrust bearing.

VOL. 184, No . 11oo- -17

224 A L E X A N D E R GR AY. [ j . F. t .

ments being free to tilt, so that when the upper plate rotates in the direction shown in Fig. 5I, the lower plates tilt and the oil wedge is formed as indicated. Such a bearing will carry an average load of 35o pounds per square inch, which is about seven times the safe value for an ordinary thrust bearing.

Fig. 53 shows the arrangement of the guide and thrust bearings of such a vertical generator, Fig. 54 shows the very sub- stantial support used to carry the weight, and Fig. 55 shows

FIG. 53.

Section of Westinghouse generator with two guide bearings.

one method adopted to force the air into the centre of the long core of these machines. The core length of water-wheel generators of large output is generally longer tl~an it would be for engine-driven units, because the diameter has to be kept small so that the stresses due to centrifugal force will not be excessive should the generator run away, due to faulty operation of the governing mechanism of the water-wheel.

It is generally assumed that the cost and weight of a machine for a given kilowatt output go down as the speed is increased.

Aug., 1917.] MODERN D Y N A M O ELECTRIC ~ I A C H I N E R Y .

FiG. 54.

225

Three 78oo-kv.a., I44-r.p.m. vertical generators with Kingsbury thrust bearings.

FIG. 55.

Rotor for a io,ooo-kv.a., i44-r.p.m, vertical generator.

226 I\LEXANDER GRAY. [J. F. 1.

I t is t rue that the value of D'aL decreases, but when the speed reaches such a value that a radical change in the type of con- s t ruct ion becomes necessary, as for example when one has to change f rom the simple cast-steel spider with bolted-on poles to the expensive construct ion shown in Fig. 45, where the ro tor is built up of steel plates, then the labor co,st becomes a larger pro- port ion of the total cost, and the fac tory cost of the machine will not always be reduced. 1G

(To be coutinued.)

America's Longest Railway Tunnel. A>~ox. (The Contract Record, vol. 31, No. 21, p. 449, May 23, I 9 I T . ) - - T h e Rogers Pass Tunnel, officially known as the Connaught Tunnel, on the line of the Canadian Pacific Railway through Selkirk Range of the Rocky Mountains in British Columbia, was placed in service on December 9, I916. The completion of the Connaught Tunnel is without doubt the most notable achievement in the art of tunnelling ever accom- pl'ished on the American continent. To complete a rock tunnel, five miles long, in practically three years, under conditions by no means easy, is an undertaking that might well be termed a brilliant success. The growing traffic congestion of this division of the Canadian Pacific Railway lines made the greatest speed construction essential. To meet the urgency demanded by the railway, methods somewhat removed from the conventional were obviously a necessity.

The rapidity of headway was a result of the combination of two elements--a new mode of heading attack and a revised plan of enlargement. In both of these distinct progress is evident upon the practices heretofore accepted. First, a pioneer bore, inaugurated as an experiment, was looked upon with distrust by many authori- tative tunnelling engineers, because it did not follow the precedent set by prior rock tunnelling. The enlargement operations further demons t ra ted the success of progressive ideas. Th e adoption of a central heading of such size as to permit radial drilling for the final enlargement proved its value, as the speed of driving, averaging 16 to 20 feet daily, constituted a record. Blasting in rings on planes perpendicular to the tunnel axis, although an innovation, contributed to phenomenal progress. Engineers throughout the world have expectantly watched the progress of this experiment. The experiment "has been successful, stereotyped processes are at least doomed to revision, and the achievement at Rogers Pass will be an incentive for still fur ther progress in this important branch of engineering.

l"Behrend, Eieclrical Re~ezv, New York, vol. 45, September io, 19o4 (P. 375).

Documents

Modern dynamo electric machinery