THIRTEENTH LECTURE ELECTRIC RAILWAY: MOTOR CHARACTERISTICS mHE economy of operation of a railway system, station, lines, etc., decreases, and the amount of apparatus, line copper, etc., which is required, increases with increas- ing fluctuations of load ; the best economy of an electric system therefore requires as small a power fluctuation as possible. The pull required of the railway motor during accelera- tion, on heavy grades, etc., is, however, many times greater than in free running. In a constant speed motor, as a direct current shunt motor or an alternating current induction motor, the power consumption is approximately proportional to the torque of the motor and thus to the draw bar pull that is given by it. With such motors, the fluctuation of power consump- tion would thus be as great as the fluctuation of pull required. In a varying speed motor, as the series motor, the pull increases with decreasing speed; and the power consumption, which is approximately proportional to pull times speed, varies less than the pull of the motor. The fluctuation of load produced in the circuit by a series motor therefore is far less than that produced by a shunt or induction motor — ^the former economiz- ing power at high pull by a decrease of speed ; the series motor thus gives a more economical utilization of apparatus and lines than the shunt or induction motor, and is therefore almost ex- clusively used. The torque, and so the pull produced by a motor, is approximately proportional to the field magnetism and the armature current; that is, neglecting the losses in the motor, or assuming ioo% efficiency, the torque is proportional to the product of magnetic field strength and armature current 1 66 GENERAL LECTURES In a shunt motor, at constant supply voltage e, the field exciting current, and thus the field strength, is constant ; and the torque, when neglecting losses, is thus proportional to the armature current, as shown by the curve To in Fig. 38. From ^ ^ H "■" /7 if "" pf ft — __ "^ ■" ' __ e _ — — — — — _ — s> — — - — — — = — .^ — -= = ;? — — ~ j; T. ^ r ^ ^ -^ ^ ''I ^ ^ ^ ^ *^ ^ ** " <^ -^ ^ ^ f^ "" ^ t: ^ M ■ _ ^ __ ^ ^ ^ ^ ^ ^ ^ _ Fife. 38. this torque is subtracted the torque consumed by friction losses, coreloss, etc. (which, at approximately constant speed and field strength, is approximately constant and is shown by the curve Ti) thus giving as net torque of the motor, the curve T, Neg- lecting losses, the speed of the motor would be constant, as given by line So ; since at constant field strength, to consume the same supply voltage W FX T C >; a M Ml :t £■/ \n r/ a L f 1 ^ 5 $ ft J( ' 7 nf n ~ 7 n i r ' / / T 3 ft Q / 0 V J / >! J / s // rto ^ t (\ f / e» <% g^ _9 M ^ , '/ / 4f1 jj > 7 — — — < f\ J y / 1 /j / / <¥ 0 J / / \ I J / f\ N ^ \ / 7 N V \ / / ...J /to XI \ '^ •^ / N sV ^ -■i r^ J / — _ / 1 ^ n !&, ^ — — n /j 0 a ^ j^ ^ ^ ^ ^ / 55: -^ ^ — 1 S9 / ~ =3 / r ^ / B^ -. ^ H XL t 4A PL .R ts i a « 0 I 0 i 0 ' M^ r \A / H) i W ■■■ # i B» ^^ ^ ^ ^ ^ ^ ^ F4fe. 40. 170 GENERAL LECTURES lecting losses and saturation, the field strength is proportional to the current; the torque being proportional to the current times field strength, therefore is proportional to the square of the current, as shown by the curve To in Fig. 40. The supply voltage, however, has no direct effect on the torque; but with the same current consumption, the motor gives the same torque, regardless of the supply voltage. The speed, at con- stant supply voltage, changes with the field strength and thus with the current : the higher the field strength, the lower is the speed at which the armature consumes the voltage. Since the field strength — neglecting losses and saturation — is propor- tional to the current, the speed of the series motor would be inversely proportional to the current, as shown by the curve S« in Fig. 40. As the voltage available for the armature rotation decreases with increasing current, from eo to e, by the ir drop in the field and armature, the speed decreases in the same pro- portion, from the curve So to the curve Si. In reality, however, the field strength, as shown by the curve Mo, is proportional to the current only at low currents; but for higher currents the field strength drops below, by mag- netic saturation, as shown by the curve M ; and ultimately, at very high currents, it becomes nearly constant. In the same ratio as the field strength drops below proportionality with the current, the speed increases and the torque decreases. The actual speed curve is therefore derived from the curve Si by in- creasing the values of the curve Si in the proportion, Mo to M, and is given by the curve S ; and in the same proportion the torque is decreased to the curve Ti. From this torque curve the lost torque is now subtracted ; that is, the torque represent- ing the power consumed in friction and gear losses, hysteresis and eddy currents, etc. Some of the losses of power are MOTOR CHARACTERISTICS 171 approximately constant ; others are approximately proportional to the square of the current; and the lost torque, being equal to the power loss divided by the speed, can therefore be assumed as approximately constant : somewhat higher at low and high speeds, as shown by curve F. The net torque then is given by the curve T. As seen, it is approximately a straight line, pass- ing through a point to, which is the "running light current," and its corresponding speed, the "free running speed" of the motor. At this current io, the speed is highest; with increase of current it drops first very rapidly, and then more slowly; and the higher the saturation of the motor field is, the slower becomes the drop of speed at high currents. The single-phase alternating current motors are either directly or inductively series motors, and so give the same general characteristics as the direct current series motor. In the alternating current motors, however, in addition to the ir drop an ix drop exists ; that is, in addition to the voltage con- sumed by the resistance, still further voltage is consumed by self-induction; and the voltage e available for the armature rotation thus drops still further, as seen in Fig. 41. Since the self-induction consumes voltage in quadrature with the cur- rent, the inductive drop is not proportional to the current, but is small at low currents, and greater at high currents ; e therefore is not a straight line, but curves downwards at higher currents. The speed, Si, is dropped still further by the inductive drop of voltage, to the curve Si, and then raised to the curve S by saturation. The eflFect of saturation in the alternating current motor usually is far less, since the magnetic field is alternating, and good power factor requires a low field excitation, and therefore high saturation cannot well be reached. The torque curves are the same as in the direct current motor, except that the effect of saturation is less marked. 1^2 GENERAL LECTURES In efficiency, the shunt or induction motor, and the series motor are about equal ; and both give high values of efficiency- over a wide range of current. A wide range of current, how- ever, represents a wide range of speed in the series motor, and ■■■■ MM "■ ■^ ^* ■" \ ~ *"" "" 7 r ^ MBM /: a 4 r -£? J , y ^i /oj 7 / /7 » ^ -Q-- / , y 1 ^5 / / r ^ 'M (\i Tl 7i Nt 17 N ?< ;"// ?/ v^ Vi' / 1 'y 5=^ j^ 1 WL W' py M m 1f\ fc //? ?/ 11 r/y VTh ;^ / / § L 1 i 7 $ 1 i ' / / 1 / fj -idi Y> 1 / / io \ y A f 1 /, / f / 7 \ > , / '"i )0 f^ // ^/ fa v / / / r /^ ■5/5 / e ^£ (\f is<3 Vl X> 7 "*""■ ^ / iM AJ '-' ■— . "^ y / /,( K( "* •* / f JiC \. /n v,^ / h oi ' 70 k i ' 1 / '^>. / { n i5i 10 ^ n f / "V — < ^J )_ V 7 / ^ >i s C w/< jfi L / / ^'' ■' s 9 rl/ 1 ■X* U ^ X S s^ \ J / N V. fl 1l ) •'(?, QW ^ ^ > \ / V > ^ \J J r ^ 5: > k y X fJ s / ' V ^^ u, A J* '^ ^ — ^ <$< / / f ^ ■i^ ^ -- 5 / // *~ — i 3/ wl\ .j^s:! ^ ^ 1. ■*■ >^ ■""■ — -— ^ F "•^ - L. ^ ^_, Fig. 41. MOTOK CHARACTERISTICS 173 nearly constant speed in the shunt motor; therefore while the series motor can operate at high efficiency over a wide range of speed, the shunt motor shows high efficiency only at its proper speed. In regard to the effect of a change of supply voltage, as is caused, for instance, by a drop of voltage in feeders and mains, the series motor reacts on a change of voltage by a correspond- ing change of speed, but without change of current ; while the shunt motor and induction motor react on a change of supply voltage by a change of current, with little or no change of speed. As the limitation of a system usually is the current, at excessive overloads on the system, resulting in heavy voltage drop, the series motors run slower, but continue to move ; while the induction motor is liable to be stalled.