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2007-03-01
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62 lines
╘HE PERFORMANCE CAPABILITIES ARE SHOWN
AS A SERIES OF CURVES, ONE FOR EACH
DISTANCE OF INTEREST, PLOTTED AS TRAVEL
TIME VERSUS SPEED RATIO. ╘HE SPEED
RATIO HERE BEING THE RATIO OF THE MOTOR
SPEED IN RADIANS/SECOND DIVIDED BY THE
CORRESPONDING LINEAR SPEED IN METRES/
SECOND.
╞OR EACH DISTANCE UNDER CONSIDERATION
(FOR A GIVEN BOGIE SYSTEM) THERE EXISTS
A SPEED RATIO THAT GIVES A MINIMUM TIME
OF TRAVEL, ALL OTHER SPEED RATIOS
GIVING GREATER TRAVEL TIMES.
┴ LOW SPEED RATIO IS ASSOCIATED WITH A
LOW LINEAR ACCELERATION AND A HIGH TOP
LINEAR SPEED. ┴LTERNATIVELY, A HIGH
SPEED RATIO IS ASSOCIATED WITH A HIGH
LINEAR ACCELERATION AND A LOW TOP
LINEAR SPEED.
╘HE SPEED RATIO THAT GIVES THE MINIMUM
TIME OF TRAVEL DEPENDS ON THE DISTANCE
TO BE COVERED. ╠ONG DISTANCES NEEDING
LOW ACCELERATIONS AND HIGH TOP SPEEDS,
WHILST THE CONVERSE HOLDS FOR SHORT
DISTANCES.
╘HE MOTOR AND BRAKING TORQUES ARE BOTH
ASSUMED TO BE CONSTANT BETWEEN THE
SPEEDS 0 RPM TO 'MOTOR SPEED' RPM, THE
'MOTOR SPEED' IS THE MAXIMUM SPEED THAT
THE MOTOR CAN ATTAIN. ╘HIS IS A FAIR
APPROXIMATION OF THE BEHAVIOR OF A
SLIPRING MOTOR WITH ARMATURE RESISTANCE
WHICH IS REDUCED IN STEPS DURING MOTOR
ACCELERATION.
╘HE INPUT DATA IS IN ╙╔ UNITS, EXCEPT
FOR THE MOTOR SPEED WHICH IS IN RPM.
╘HE INERTIA IS THE SUM OF THE INERTIAS
OF THE MOTOR ARMATURE AND ALL ITEMS
CONNECTED TO THE MOTOR SHAFT (EG BRAKE
DRUM, COUPLINGS ETC). ╬OTE THAT THE
TRUE INERTIA IS REQUIRED. ╘HE COMMONLY
PUBLISHED VALUES OF ╟─^2 ARE A FACTOR
OF 4 TOO LARGE.
╘RACTIVE RESISTANCE IS OFTEN GIVEN AS,
SAY, 20 LB/TON WHICH MEANS THAT A
HORIZONTAL FORCE OF 20 LBS IS REQUIRED
FOR EVERY TON OF MACHINE WEIGHT TO
OVERCOME WHEEL LOSSES AND PROPEL IT
HORIZONTALLY AT A CONSTANT SPEED. ╘HE
PROGRAM ASKS FOR THE TRACTIVE
RESISTANCE RATIO, IN THE CASE OF
22.4 LB/TON = 22.4/2240 = 0.01 IS THE
VALUE THAT IS WANTED.