Unit - 2

DC Motor Drives

Q1) A 50 HP, 240 Vdc separately excited motor is operating at 1000 rpm. The motor draws 7800 watts from dc supply. The total armature resistance is 0.221 . Find:

a.) The emf constant, Ke of the motor

b.) The motor speed if the terminal voltage is reduced by 20% and the power drawn remains the same.

Q2) A separately excited dc motor is rated at 100 HP, 600 V at 1200 rpm. The total armature resistance is 0.24 ohms. When the motor is delivering 75 HP at 1200 rpm its efficiency is 88%. At the 75 HP output find:

a.) the motor armature current

b.) counter emf (Ea)

c.) torque at the shaft

d.) an estimate of the mechanical losses

Q3) A 15 Hp separately excited motor is operating at its rated speed of 1200 rpm Determine the rated torque of the motor in ft-lbs.

Q4) A 25 Hp separately excited motor is operating at a speed of 250 rpm. It is supplied from a 120 V supply and draws 5.6 A. The total armature circuit resistance is .473 ohms. Find the torque constant for the machine.

Q5) Draw and explain speed torque characteristics of three phase-full converters.

Q6) Explain single phase controlled rectifier with suitable diagram.

Q7) A 20hp, 250V shunt motor with Ra=0.22 Ω, Rf=170 Ω. At no-load and rated voltage, the speed is 1200 rpm and the armature current is 3 A. At full-load and rated voltage, the line current is 55A. What is the full-load speed?

Q8) A 230V shunt motor delivers 30hp at the shaft at 1120rpm. If the motor has an efficiency of 87% at this load, determine: a) The total input power. b) The line current.

Q9) A separately excited motor runs at 1045rpm, with a constant field current, while taking an armature current of 50A at 120V. The armature resistance is 0.1 Ω if the load on the motor changes such that it now takes 95A at 120V, determine the motor speed at this load.

Q10) A separately excited DC motor has the following specifications: Terminal voltage = 250 V, field voltage = 250 V, armature resistance = 0.03 Ω, field resistance = 250 Ω. Initially the motor was running at speed = 1103 rpm while supplied by the rated terminal voltage and the armature current = 120 A. While supplying constant torque, what is the speed of the motor if the terminal voltage is reduced to 200 V?

Q11) What is Braking? Explain regenerative braking.

Q12) Draw block diagram and explain current limit control of electrical drive.