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7D dissident maps by Ernst Dieterich and Lars Lindberg

By Ernst Dieterich and Lars Lindberg

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1 per cent, while to produce this change the ampere-turns must be reduced by about 50 per To cent. 26, because the former reduced to one-half. This is 30 per cent latter ratio is The corresponding increase in cross ampere-turns, acting collectively with the increased back ampere-turns, causes excessive excessive. 46 SPEED CONTROL OF SHUNT MOTORS. 47 Hence a 30 per cent increase of speed with an ordinary shunt motor cannot be obtained without objectionable standard sparking. sparking. 28, an increase of 262 watts or 68 per cent, producing heat for the armature insulation to stand for any consider- able time.

Box Contact Points STARTING SPEED AND CURRENT CURVES. be directly connected across the line terminals, the current would be so excessive that it would tend to injure or destroy it. , so that the entire voltage would have to be consumed by the fall of of the supply circuit potential in the armature resistance 33 and brush contacts (Ia Ra + D b ). 34 ELECTRIC MOTORS, THEIR ACTION AND CONTROL. 28 816 amperes compared with rated current of only 37 D amperes. f. equal to zero. is an extreme value, because the fuses or circuit-breaker would act to prevent it; but a very excessive armature current would flow at least momentarily with injurious electrical as well as mechanical effects.

35 volts, so that the drop in the starting box would have to be 230 volts. 3 amperes. , was passed, when the armature would suddenly jump to rated speed, a change much too sudden and For this likely to injure the motor or the machine driven by it. box to all load to have the conditions, reason, and starting adapted the American practice is to use a box designed to start the motor under load. , total) resistance is sufficient to limit the armature current to a reasonable value and until next to the last contact point the speed does not rise more than about 3 per cent above rated value.

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