You are to perform some of the calculations needed support to the Mechanical Des

Question

You are to perform some of the calculations needed support to the Mechanical Design of an armature-controlled DC Motor. The preliminary design requirements are: a) When supplied with 90 volts, the motor must be able to produce 2 horsepower at 1200 rpm This is called the "Design-operating-point" or DOP b) It must have a stall torque at least 70-percent higher than the DOP torque c) At the DOP, the friction torque in the motor must be no more than 5-percent of the DOP torque. Perform the calculations necessary to answeelowing auestions a) Estimate the value for the torque and voltage constants Kr and Kb, in units of ft-lb/A. b) Estimate the resistance in the motor windings in Ohms. c) Estimate the current required by the motor at the design operating point, in Amps d) Estimate the percent-efficiency of the motor at the design operating point.

Explanation / Answer

A)we have the formula Kt=torque/Ia=60/(2*pi*Kv).therefore Kv=rpm/volt=1200/90=13.33rpm/volt.therefore Kt=60/(2*pi*13.33)=0.716rpm/V

Kb=(volt*sec)/radian;rpm=1200 so converting speed in rpm to radian per sec=Vrad=(Vrpm*2*pi)/60 therefore speed in radian=(1200*2*pi)/60=125.66rad/sec. Kb=90/125.66=0.716V-S/rad

C)I=torque/Kt;torque=(hp*5252)/rpm=2*5252/1200=8.75N-M.I=8.75/.716=12.22amps

B)R=V/I=90/12.22=7.36ohms

D)efficiancy=(Poutput/Pinput)*100=(V*I/2hp)*100=(12.22*90/2*746)*100=73.71%

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