Handbook of Electric Power Calculations, Third Edition
A powerful calculations tool for electric power engineers and technicians, this book offers essential, step-by-step procedures and examples for solving a wide array of electric power problems.
TABLE OF CONTENTS Handbook of Electric Power Calculations, Third Edition
Section 3: DC MOTORS AND GENERATORS
Lawrence J.Hollander, P.E.
Dean of Engineering Emeritus
Union College
DIRECT-CURRENT GENERATOR USED AS TACHOMETER, SPEED/VOLTAGE MEASUREMENT
A tachometer consists of a small dc machine having the following features: lap-wound armature, four poles, 780 conductors on the armature (rotor), field (stator) flux per pole=0.32 10 ?3 Wb. Find the speed Wb. Find the speed calibration for a voltmeter of very high impedance connected to the armature circuit.
Calculation Procedure
1. Determine the Number of Paths in the Armature Circuit
For a lap winding, the number of paths (in parallel), a, is always equal to the number of poles. For a wave winding, the number of paths is always equal to two. Therefore, for a four-pole, lap-wound machine a=4.
2. Calculate the Machine Constant, k
Use the equation: machine constant k= Np/a ?, where N=780/2=390 turns (i.e., two conductors constitute one turn) on the armature winding, p=4 poles, and a=4 parallel paths. Thus, (390)(4)/(4 ?)=124.14 and k=124.14.
3. Calculate the Induced Voltage as a Function of Mechanical Speed
The average induced armature voltage, e a (proportional to the rate of change of flux linkage in each coil)= 
where 
per pole in webers, ? m=mechanical speed of the rotor in radians per second, and e a is in volts. Thus, 
(124.14)(0.32 10 ?3 Wb)=0.0397 V s/rad.
4. Calculate the Speed Calibration of the Voltmeter
Take the reciprocal of the factor e a / ? m , finding...
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