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TUTORIAL 2 DC MACHINES EMT 113 FUNDAMENTAL ELECTRICAL ENGINEERING UNIVERSITI MALAYSIA PERLIS Tutorial 2 - DC Motor Question 1 a) Sketch the equivalent circuit of Series and Cumulatively Compounded DC Motor. Compare torque-speed characteristic for both motors. b) A Shunt motor rotating at 1500 rpm is fed by a 120 V source. The line current is 51 A and the shunt-field resistance is 120 Ω. If the armature resistance is 0.1 Ω, calculate the following: i. ii. c) The armature current, IA The mechanical power developed by motor A 220 V, 7 hp series motor is mechanically coupled to a fan. The motor draws the current of 25A and runs at 300 rpm. The torque required by the fan is proportional to the square of the speed. Given RA = 0.6 Ω and RS = 0.4 Ω. Neglect armature reaction and rotational loss. i. ii. Determine the power delivered to the fan and the torque developed by the machine without external resistance connected to the armature circuit (Rext = 0) The speed is reduced to 200 rpm by inserting an external resistance (Rext) in the armature circuit. Determine the value of this resistance and the power delivered to the fan. Question 2 A Shunt generator has a field resistance of 100 Ω. When the generator delivers 100kW, the terminal voltage is 130V, while the induced voltage, EA is 143V. Determine: i. ii. SMI/EMT113/TUTORIAL2 The armature resistance Induced voltage when output is 40kW and terminal voltage is 150V. TUTORIAL 2 DC MACHINES Question 3 A six pole machine has a flux per pole of 30 mWb. The armature has 536 conductors connected as a lap winding in which the number of parallel paths (a) is equal to the number of poles (p). The DC machine runs at 1050 revolution per minute (rpm), and delivers a rated armature current of 225 A to a load connected to its terminals. a) Describe field winding and armature winding Calculate: b) Machines constant, K c) Generated voltage, EA d) Electromagnetic torque, τe e) The power developed by armature, Pdev Question 4 (Chapman Example 9-1, pg. 541) A 50hp, 250V, 1200 r/min DC Shunt motor with compensating windings has an armature resistance (including brushes, compensating windings and interpoles) of 0.06Ω. Its field circuit has a total resistance Radj + RF of 50Ω, which produces a no-load speed of 1200 r/min. There are 1200 turns per pole on the shunt field winding. a) b) c) d) Find the speed of this motor when its input current is 100A Find the speed of this motor when its input current is 200A Find the speed of this motor when its input current is 300A Plot the torque-speed characteristic of this motor Question 5 A 240 V shunt motor runs at 850 r/min when the armature current is 70 A. The armature circuit resistance is 0.10 Ω. Calculate the required resistance to be placed in series with armature to reduce the speed to 650 r/min when the armature current is then 50 A. The field current is constant. SMI/EMT113/TUTORIAL2 TUTORIAL 2 DC MACHINES Question 6 A 50 hp shunt dc motor has a 280V of armature current when 10 A of field current is supplied and have the followings values: VT =240 V RA = 0.065 Ω RF = 10 Ω Radj = 14 Ω At a speed, ωm of 125.67 rad/s of , the rotational loss is Ploss = 1450 W. If this motor drive a load that demand a torque τout = 250 Nm-1independent of the speed, calculate the motor speed and efficiency. - DC Generator Question 7 A 100 kW, 230-V, shunt generator has RA = 0.05Ω and RF=57.5Ω. If the generator operates at rated voltage, calculate the induced voltage at: a) Full load b) Half load -Neglect brush contact drop Question 8 A 10kW, 125 V- compounded generators has rotational losses amounting to 580 W. The shunt field resistance is 62.5 Ω, the armature resistance 0.12 Ω and the series field resistance 0.022 Ω. Calculate: a) Full load efficiency for long shunt connection b) Full load efficiency for short shunt connection c) Compare your result between (a) and (b) Question 9 SMI/EMT113/TUTORIAL2 TUTORIAL 2 DC MACHINES A generator shown in figure 1 supplies a 20 kW load at 100 V through cables of resistance,R = 200 mΩ. If the field winding resistance, Rf = 40Ω and the armature resistance, RA = 50 mΩ, calculate the terminal voltage and the electromagnetic force generated in armature, EA. SMI/EMT113/TUTORIAL2