The design of a regenerative control system for electric vehicles

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The Design of a Regenerative

for Electric Control System Vehicles.

Submitted

by

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Ta-Le_ of Con tents,.

Page

Acknowledgment - - - I

Obj ect - - - 2

Preface - 3

Theory - - - 4 Advantages and Disadvantages of the Several

Systems which Afford Regeneration - - - 5 Choice of one System - - - 8

Schematic Diagram of the System - - - -I3 Determination of the Characteristics of a

Motor for the System - - - -I4 Characteristic Curves of the System- - - -29

Acknowledgment.

In presenting this thesis the writer wishes to express his apreciation for the services rendered by Prof. William E. Wickenden and by r. Donald W. Prin .

Object.

The object of this thesis is to design a regenerative

control system , for electric vehicles, with a view to in-creasing the mileage obtainable on a single charge of the

Preface.

Mouch research has been conducted on the relative

e-conomies of horse-drawn , electric and gasoline vehicles for different kinds of service and different lengths of

hauls. It has been shown that , for a route the daily

mileage of which is equal to or lower than that which an

electric truck can negotiate on a single charge , the

electric can operate at an equal or lower unit delivery

cost than the horse-drawn or gasoline truck. For a greater

daily mileage the gasoline truck is the most economical.

So that if the mileage per charge of electric trucks can

be increased their field of operation will be matlerally

increased. Of course the increased cost , incurred in the

method used to increase their mileage , must be less than the saving effected by their use. There are several ways

to increase the mileage per charge. The efficiency of the

motor and battery may be increased , or the efficiency of the entire system,including the control , may be bettered. The latter method , in which the control is so arranged that part of the energy going to accelerate the vehicle

or used in climbing hills is returned to the battery when

retarding or when descending hills , is to be developed here.

Theory.

Regeneration , when referred to the action of electric

motors installed in vehicles , is the process by which the

motor is made to serve as a brake and restore to its

pre-vious source a part of the energy used for acceleration

and the ascent of grades. For effective operation there

are two requisites. First , the driver must have means of increasing , at will , the back electromotive force of the

motor to a value above that apPlied , and second , there must be some power absorbing device either at the source

or between the motor and the source. If the driver could

not increase the back electromotive force of the motor to

more than that of the source no energy could be pumped

back to the source. If there was no power absorbing

de-vice on the line there would be no load on the motor and

it would take no energy from the moving vehicle. In the

case of the electric automobile the power absorbing

de-vice is always present in the storage battery so that

the method of control of the back electromotive force is

the problem.

60

Advantages and Disadvantages of the Several Systems which

Afford Regeneration.

The systems to be considered are : (a) a shunt motor.

with subdivided field coils to be connected in parallel

or series combined with field resistance. ; (b) a shunt

motor,with a single field circuit, combined with means

of varying the voltage on the armature ; (c) a compound

wound motor the series field to be made cumulativetwhen

the machine is either motor or generator,by the use of

a reversing switch ; (d)a motor with a double armature

winding ; (e) a motor with multiple field windings , in

parallel to operate as series motor and in series to operate

as shunt generator.

The use of a shunt motor with subdivided field coils

affords a good range of

speed control

and the control

lever can be set to give any speed within a wide range ,

the seed remaining constant at any one position of the

controller. A high starting torque , with a reasonable current demand , is obtained by paralleling the field windings. Only two taps are brought from the battery,

thus all cells are charged and discharged evenly. Only

the relatively small field current is taken to the

con-troller , which may be of light and inexpensive

construc-tion due to the small heating losses. However in order

to use the two field windings in series or parallel a

switch is needed and the inductive discharge at this switch

would wear it out quickly. The control is rather complicated

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and since it does not afford automatic regulation of the

torque, as a series motor does , but necessitates

regula-tion of it by the driver the system is not suited to

be-ing used by inexperienced persons.

By the use of a shunt motor with a single field

cir-cuit and means of varying the voltage impressed on the armature good speed control and constant speed at any one position of the controller are obtained. This system re-quires several tars from the battery and uses the battery

unevenly. The points of connection to the battery are a

source of corrosion , inconvenience and unreliability. The system does not afford automatic regulation of the torque

and therefore is not adapted to inexperienced drivers. The full line current is brought to the controller so that it

must be made heavy and would be expensive.

By using a compound wound motor automatic regulation

of the torque is obtained ,to a certain degree,for the

ser-ies field increases the flux considerably when carrying a a large current as is the case when starting or when a

grade is encountered. The range of current demand can be kept well within permissible limits. The series field

reversing switch can be automatically controlled by a polarized relay. This switch is thrown when carrying no current , hence it is not subject to destructive arcing.

The speed is controlled by use of shunt field resistance.

It is very well adapted to inexperienced drivers because

but two taps from the battery thus insuring its even use.

There are only small currents brought to the controller

thus it could be made inexpensive and light. The only

physical disadvantage is that the presence of the

automa-tic reversing switch might decrease the reliability of the

system.

A motor with a double armature winding gives very good

speed control and good regulation of the torque but not automatic regulation .The presence of but two tars from

the battery insures its even use. However , the reliability

is decreased by the use of the two commutators. The

commu-tator is the most unreliable part of any direct current

motor. In the switching operations necessary in this

con-trol system the circuit would be broken when the current is of considerable magnitude and this would cause the

switches to wear out quickly . This system would also

re-quire an expert driver for i ts satiafactory operation.

A motor with multirle field windings gives automatic

control of the torque and good range of current demand.

There are several taps from the battery for sneed control

and these are a source of corrosion and inconvenience and

cause the battery to be used unevenly. The inductive dis-charge on breaking the field circuit would soon wear away

the terminals and cause frequent renewals to be made. It

would necessitate more than one lever for the control.

This type is used in the electric railway system of the

Chicago, Milwaukee and St. Paul over the "Rockies" but

it is under the care of expert motormen.

Choice of a System.

After a careful consideration of the advantages and

dis-advantages of the several foregoing types of regenerative

control systems for installation in commercial electric

vehicles, that using a compound wound motor appears

to be the most advantageous , as far as its mechanical and

electrical features are concerned. It was therefore decided

to develop further the electrical design of this system. It

would be impossible to accurately determine the relative

cost for installation of this system as compared with that

of the plain series system without extensive experience with

motor installations in electric vehicles , hence no direct attention was given to this aspect of the roblem.

Referring to Plate I , the system consists essentially

of a compound wound motor with its series field terminals brought out to a reversing switch (S) which is actuated by a polarized relay (PP) in series with the shunt field, and a solenoid (C) in the line. The starting is effected by means of resistance grids(RA) in the armature circuit in

the usual manner and the speed is controlled by varying i +>Q _{Lo41t WJiLllA I} Q~~lln+ Pi 1 ^ ant;LII· _{-LU U Otre} I+ono

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the resistance grids and the shunt field resistance to one

control lever. A motion of this lever in the direction of

the arrow(A) first

uts the shunt field in , then closes

the armature circuit through a resistance and the motor

starts.

A further motion of the controller cuts out some

of this starting resistance and the car speeds up.After

all of the starting resistance is cut out , further increase

.

1

in the speed is obtained by increasing the resistance of the shunt field circuit. The controller may be set for a certain speed , on the level , and the car will keep this speed very nearly. If the load is increased the car will

slow down a little and thus take the increased load

with-out too great an increase in the current. If it is desired to slow downjthe controller is merely returned towards its original position. In such a case the action of the relay and the reversing switch is as follows. Assume the direc-tion of the current as shown by the arrow (B) an Plate I, when the car is pulling in the forward direction. Then. the

fluxes in the two coils (PP) and in the coil (C) are as shown by the small arrows and the movable relay is against the contacts(dd) . The reversing sitch (S) is in the

po-sition (X) for forward motion. Assume that most of the

atthe ; me

shunt field resistance is inserted it is desired to slow the car down. Now the back electromotive force of the motor is in the opposite direction to that of the battery but not quite as large because of the Ir drop in the ar-mature. If the resistance in the shunt field is suddenly

cut out by returning the controller towards its original

position the shunt field current will be very much increased

and thus the field strength will be increased, and since the speed will remain practically the same} for the moment,

the voltage of the motor will be very much increased . By proper adjustment of the shunt field resistance the voltage

of the motor may be readily raised above that of the battery

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by the process just cited . Then the current will decrease

to zero and start to flow in the opposite direction to what

it was previously flowing. When the current reaches zero

the flux in the coil(C) becomes zero and the springs (I) and

(2) move the relay out of contact with (dd) . As the current

starts to build up in the opposite direction the flux in the

coil (C) begins to build up in the opposite direction to

what it was before and this causes the relay to move to the

contacts (ee). This action will energize the nagnet(h')and

the switch (S) will be pulled to the position (Y). This

ac-tion reverses the series field and causes its flux to add

to that of the shunt field. The motor will then be pumping

energy back into the battery and drawing its driving rower

from the car, which slows down. The magnets (h)Ji and (h') are

not left in the circuit after they have pulled the switch

a short distance, for after the arm (S) has moved a short

distance the contacts (LL) or (MM) are separated and the

magnet circuit opened. The spring (3) is called upon,then,

to close the reversing switch after the magnet has pulled

it by the center. By this arrangement the magnet may be

de-signed to exert a large pull and still not be wasteful, be-cause it is in the circuit only a short time during the

operation of slowing down. The losses in the coils (PP) and

(C) may be made very small. If after slowing down for a

while it is desired to speed up again then the controller is moved in the direction of the arrow (A) and this decreases the- voltage of the motor. When the two voltages , that of the

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battery and that of the motor , just balance there is no current in the coil (C) and the springs (I) and (2) break the contact at (ee) .Then as the voltage of the motor still further decreases the current starts to flow in the direction

shown by the arrow (B) _. This causes the relay to be moved to the contacts (dd) and energizes the magnet (h) which

pulls the arm (S) to the position (X) and thus reverses the series field again. Thus it again adds its flux to that of

the shunt field and helps produce the necessary torque

re-quired to speed the car up. The movable relay must be

de-signed to be very sensitive, for if it does not close the

circuit through either magnet (h) or (h') , as required ,

as soon as the current just starts to build up in the

op-posite direction the action of the two fields will be

differential and the resultant field weakened when it is

!% desired to strengthen it . If the field is weakened in this

way when it is desired to slow down , the car will seed

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up because the speed is inversely proportional to the flux.

·

If the field is weakened when it is desired to sneed u it

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will ause a large current to flow through the armature in

an endeavor to create the necessary torque. For the torque

is proportional to the armature current and the flux and . ~ if the flux is small the current will have to be large in

*: ! order to make their product large. The srings (I) and (2)

are intended to increase the sensitiveness of the relay by

starting it towards the other position when the current in

the coil (C) becomes zero. The coils (PP) and (C) can be

so proportioned that the relay will operate with a small

current and thus will reverse the series field before

its differential effect on the shunt field is sufficiently

great to materially decrease the motors generated voltage.

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Determination of the Characteristics of a Suitable

Compound Motor.

An attempt was now made to determine the electrical

constants and operating characteristics of a typical

corndnlndl mntn-r i n q..cli h frnr t'he A fltr rni nfi nna

the constants and characteristics of a series motor

which is operating satisfactorily in electric truck

ser-vice. The series motor referred to is a 60 volt , 32

ampere and I280 revolutions per minute General Electric "

motor installed in a I000 pound General Vehicle " truck the total weight of which , with Edison batteries, is

4000 pounds and the rated speed is I2 miles per hour .The

armature resistance of the motor , including brushes _, is

.080 ohms and its field resistance is .032 ohms. The

motor has four poles and its fields, which consist of

52.5 turms per pole , may be put in series for starting

or series-parallel for normal running.

The compound motor was assumed to have the same

current , voltage and revolutions per minute rating as the series motor and the same armature resistance . The excitation at rated current voltage and speed was

assumed to consist of I/3 series ampere-turns and 2/3

shunt ampere-turns .

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To Determine the Field Resistances and Number of Turns.

Assume 3.5 % total field loss at rated current,volts and r.p m. 60 x 32 x .035 = 67 watts

Total (NI)f from Plate C-25I42 for I280 r.p.m. is

52.5 x I6 - 840 amp-turns.

I/3 series amp-turns 280

2/3 shunt amp-turns = 560

Now the loss in the shunt field alone is

(67.2/4) x 2/3 = II.2 watts = IfVf Therefore

If = II.2/ 60 = .I87 amps. And

rf = 60/.I87 = 32I ohms for normal position Then the number of shunt turns is

N - 560/. I87 = 3000 turns per pole.

-The series turns

N' = 280/I6 = 17.5 turns per pole.

Amount of shunt resistance Ist' step down = 25I ohms. 2n

d.

n

n

= 206

n n

"

"

3

rd.

n

"

=

I25.5

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The Determination of the Characteristic Curves of the

Compound Motor when Operating as a Motor.

The two curves to be determined are the torque-current

and speed-current curves. For each position of the

control-ler , for the compound motor , there will be one of each of the above curves . The first position will be that for

nor-mal running the total resistance of the shunt field

cir-cuit being 0,i ohms and the series fields are connected in series- parallel . For the second and third positions the resistances of the shunt field circuit are 25I and 206ohms

respectively and the series fields are still connected in series-rarallel . For the first position the resistance of

the shunt field circuit isI2Jt5ohms and the series fields

are in series . This is the condition for starting. The equation for the torque in a motor is T = K IaO

-K' Ia (NI)f where T

K',

Ia *

4

and (NI)f are the

torque in pounds feet , a constant , the armature current, the flux and the total field excitation in amnere-turns ,

respectively . Due to the saturation of the iron (NI)f is not directly proportional to the flux thus for different

values of (NI)f K' is different , but for constant (NI)f

~a4rlN

.and variable IajK' is,tconstant. Now (NI)f for the

com-pound motor consists of the shunt ampere-turns, which are

constant for any one position of the controller , and the

series ampere-turns which vary with the armature current.

The ampere-turns of the series motor consist only of the series field ampere-turns which are a product .of the

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mature current and the series turns . Now for any particular value of armature current , in the compound motor , and any

one position of the controller the excitation , in ampere-turns,is a fiied quantity . To produce the same excitation

in the series motor a perfectly definite value of armature

current is required , as determined by the number of series field turns. This armature current will produce a definite

torque . Then since the excitation of both motors is the same, the torque of the compound motor will be equal to that of the series motor multiplied by the ratio of the compound

motor armature current to the armature current of the series

motor. As an examnle , assume that the compound motor

con-troller is in the normal running position and the current being taken is 20 amperes. Then its excitation is equal to

700 + I0 x .08 = 875 amp-turns. In determining the shunt

field excitation the change in the battery voltage must be

allowed for . The value of series motor armature current which will produce the same excitation of 87'5 amp-turns is

875/52.5 = I6.7 amperes. The torque,for this current and 875

amp-turns excitation,is 4.3 pounds-feet. Then the torque for

the compound motor is 20/I6.7 x 4.3 = 5.2 pounds-feet. In this manner the torque,for the compound motor, for any value of

armature current and excitation may be found from the

torque-current curve of the series motor.

The equation for the speed in a motor is n = K Ea/

K' Es/ (NI)f where n , k , By and (NI)f are the

speed in revolutions per minute a constant , the

in-I8

-duced electromotive force the flux and the excitation

in ampere-turns respectively . Again for constant excitation

ner rlyj

but variable Ea K' is X constant , therefore the speeds

of two like motors of same excitation will vary directly

as the induced electromotive force . Again as an example

assume that the compound motor controller is in the normal

running position and the motot taking a current of 20

amperes . It was found in the preceding paragraph that for

equal excitations of both motors the series motor took 16.7

amperes. ow Ea for this value of armature current is equal

to the terminal voltage of the battery , when delivering

this current , minus the Ir drop in the armature. It is

equal to 76 - I6.7 x .08 = 74.7 volts and the speed is

I230

revolutions per minute.

Ea

for the value of 20 amperes,in

the case of the compound motor, is 75 - 20 x .08 = 73.4 volts. Then the speed of the compound motor is equal to

(73.4/74.7) x I250 = 2I0 revolutions per minute . Then,by this methodscurves of speed against line current for the

compound moter, can be determined from the speed-current

curve of the series motor.

Determination of the Volt-Ampere Characteristic

of the

Batt ery.

The volt-ampere characteristic

of the battery shows the

mean voltage of the battery for any value of current , charge

or discharge . The mean voltage,at the normal rate of currernt discharge is, 72 volts and this is taken as I00 %. Now,

from Pender's "Handbook" , the voltage at twice and three

To I

times the normal discharge rate is 92 % and 84 % of the

nor-mal voltage respectively . The characteristic is a straight line , because its equation is of the form of a constant plus

an effective Ir drop , therefore it can be extended to

in-dicate the voltages with different values of charging

Volt-Ampere Characteristic of the Battery

Dis charge Terminal

Current Vo ltage

45

72

90

66.3

135 60.5

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Tabulation of Curve Data for Motor Oeration.

Definitions of letters used.

Ic = compound motor line current.

I = series motor "

(NI)f = excitation in amp-turns. Tc = compound motor torque

T = series " "

Ec = compound motor generated voltage

E = series motor " n

nC = compound motor speed

ns_S = series motor "

23

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Values of Ic T_c and n_c for normal running position

Ic (±I)f Is Ts Tc | c ESEs

20 875 I6.7 4.3 5.2 73.4 74.7 I230 12I0

40 I027 19.5 6 12.3 70.8 73.4 I080 1040 60 II79 22.5 7.7 20.5 65.2 73.2 980 873 80 I330 25.3 9.2 29.1 6I.6 73 9 I0 768 I00 1483 28.2 II.2 39.7 57 71.7 850 675 I20 1633 31.1 13 50.2 52.9 7I.5 800 592 0 729 13.9 -- -- 78 75.1 I530 1590

Values fcr first step down.

IC_ | (I)

Ia

T_s Tc Ec E _s n n 20 1075 20.5 6.5 6.3 73.4 73.4 1040 I040 40 I222 23.3 8 I3.7 70.8 73.1 960 930 60 1361 25.9 9.6 22.2 65.2 72.4 900 810 80 1512 28.8 11.5 32 61.6 71.7 840 720 I00 I651 31.4 13.2 42 57 71.5 800 638 120 I803 34.4 I5.2 53 52.9 70.7 755 565 0 932 I7.7 78 1170 74. I230 24

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Values for the second step down. IC (TI) f IaI Ts Tc Ec Es n nc 20 1275 24.3 8.6 7.I 73.4 73.1 940 944 40 1410 26.9 I0.2 I5.2 70.8 72.3 880 862 60 I545 29.4 11.8 24. 65.2 ~7I.6 830 756 80 1690 32.2 13.7 34.I 61.6 71.2 780 675 100 I825 34.8 15.6 44.8 57 70.7 750 605 120 I963 37.4 I7.4 55.8 52.9 70 720 544

0 I137 2I.6 - - 78 73.3 1005 I070

Values for the starting position.

Ic (NI)f Is Ts Tc E E ns 20 2I50 41 19.8 9.7 73.4 69.4 685 725 40 2450 46.6 24 20.6 70.8 68.3 638 662 60 2730 52 28 32.3 65.2 67.2 600 582 80 3030 57.7 32 44.4 61.6 66.I 560 522 100 3310 63 36.4 57.8 57 65 530 465 I20 3600 68.6 4I 71.8 52.9 63.5 495 4I2 0 1865 35.5 - - 78 70.5 740 8I8

Determination of the Speed-Current Characteristics of the

Motor Operating as a Generator.

The speed-current characteristics for the motor operating

as a generator are determined in the same manner as those for

motor action . However,for generator action the generated

voltage of the compound motor is equal to that of the battery plus the Ir drop in the armature.

26 I

Values of I_c and n for Generator Action , the Controller

C

being

in

the Normal Running Position.

Ic

(NI)f

I

n

s

Ec

E

n

20 927 7.7 I I70 82. I 74.3 I293

40 I25 2I. 4 I000 86.2 73.3 II90

60 I324 25.2 9IO 90.3 72.8 II30

80 1523 29 840 94.4 71.9 II00

I00 1725 32.8 775 99 71.2 I080

I20 I923 36.6 728 I03.1 70.4 1065

Values for first step down.

Ic

(

TI)f

I

s

ns

Ec

E

s

nc

20 II37 21.7 I000 82. I 73.3 II20

40 1342 25.6 910I 86.2 72.7 I078

60 1548 29.5 830 90.3 71.8 1I044

80 I750 33.4 770 94.4 71 I024

I00 I962 37.4 720 99 70.2 IO17

120 2I64 412 680 103.1 69.4 10IO

I

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F

Values for second step down.

Ic ( NI) f Is ns EC Es n

20

I347

25.7

900

82.1

72.6

IOI7

40

I560

29.7

825

86.2

71.7

992

60

I772

33.7

763

90.3

71

970

80

I985

37.8

7I5

94.4

70.1

963

100

2I97

41.8

675

99

69.3

964

120 2409 45.9 642 103.1 68.3 969

Values for the starting position.

Ic (NI)| Is ns c Ec Es 20 2270 43.2 665 82.I 69 79I 40 2690 51.2 605 86.2 67.4 775 60 3090 58.8 555 90.3 65.8 760

80

3500

66.7

505

94.4

64.2

742

I00

4030

76.7

450

99

61.9

720

I20 4340 82.6 417 I03. I 60.9 705 28 9

29

mIr

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Discussion of the Results as Shown by the Curves.

Assume that the vehicle is running on a level road ,

with the controller in the normal running position , and

a down grade is encountered. The car will speed up ,

following along the curve (A) , until it reaches a speed

corresponding to about

I600

revolutions per minute of the

motorthen it will start to slow down still on the curve

(A) . If the controller

is

not moved the value of

the

current

will

, in all

probability

,

exceed the safe limit

before the speed drops below that at which generator ac-tion can be sustained by the motor . If the speed gets

below that at which generator action can be continued , at

a fixed controller position , there will be motor action It is very probable that somewhere,before motor action

starts , the current will become very large . However the

operator can change from the curve (A) to (B) when the current tends to exceed a safe value and repeat the oper-ation to the next lower curve and so on if the tendency

is still

present.

As far as can be predetermined

from the

curves the current will tend to exceed the safe limit

when on any one of the curves and for satisfactory braking

the driver would be obliged to watch the ammeter carefully.

This would be undesirable at the time *when one would de-sire to brake the car,for at that time one probably might

need to watch ahead somewhat carefully . If an appliance,

that would automatically change the controller from one

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curve to the next lower curve when the value of the current

equalled a certain safe limit , was installed , the system would operate without any attention from the driver but it

probably would mean that the car would be slowed down to

a very slow speed each time regeneration started . This would be very undesirable.

For satisfactory regeneration the motor is too

strongly compounded . Since however for motor action the strong compounding is a distinct advantage it would be

unwise to decrease it much . The best procedure would probably be to cause the switch (S) , on Plate I , to merely cut out the series field for generator action . This would give characteristics , for generator action , as shown by the dotted red lines on Plate 2 . Now ,

making the same assumtions as before , again the car will

speed up and follow along curve (A). When the seed

reached I600 r.p.m. , the series field would be cut out ,

and the dotted red curve would then be followed until

equilibrium,between the energy being returned to the car,

the losses and the energy in the moving car. is reached.

If the energy in the moving car decreased the speed would

decrease nad the current would decrease , the two

decreasing along the dotted red line (I). If it was desired

to decrease the speed below I600 r.p.m. the controller

would be shifted to the curve (2) and after this had been followed to about I400 r.p.m. again shifted to (3) and

so on .If the controller was not shifted from curve (I)

the speed would simply drop off along curve (A) and motor

action would start again . Therefore it is evident that the machine will simply follow along the curve(A) and (I),

for that position of the controller , and at what point

it is , on the curves,at any time ~ depends upon the equilibrium of the quantities concerned . One thing

necessary to change in this design,to adapt it to the

above actions, would be to make it posible to materially

increase the shunt field strength when it is desired to

stop quickly . In such a design I would retain the series field , in all its strength , in orier to afford automatic

regulation of the motor torque . This design would be not only simple but intuitive , for if a certain motion of a

lever causes a certain effect it is decide/dly intuitive to move the lever towards its original position if the

reverse effect is desired .