DC Motors

Problem 545: A 500 V shunt motor running at 720 rpm takes an armature current of 50 A. The equivalent armature resistance is 0.4 ohm. What resistance must be placed in series with the armature to reduce the speed to 540 rpm while maintaining the same torque?

Problem 546: A certain shunt motor has an armature resistance of 0.05 ohm. It draws 50 A at a terminal voltage of 120 V. Assume other miscellaneous losses at 1%. Determine the output horsepower of the motor.

Problem 547: The armature and field resistances of a shunt machine are 0.2 ohm and 110 ohms respectively. While running as a generator, the generated emf is 250 V at 1200 rpm. If the machine is run as a shunt motor, it takes 4 A at 220 V. At a certain load the motor takes 30 A at 220 V. However on load, the armature reaction weakens the field by 2 %. Find the motor speed at this load.

Problem 548: A shunt motor, which has a field resistance of 220 ohms and an armature resistance of 0.8 ohm takes 26 A from a 260 V supply when running at 500 rpm on full load. In order to control the speed of the motor a 1.2-ohm resistor is connected in series with the armature. Calculate the speed at which the motor will run when supplying full load torque.

Problem 549: The input and output powers of a 220 V, shunt motor are 30 and 28 kW respectively. The field and armature circuit resistances are 40 ohms and 0.0125 ohm respectively. Determine the efficiency of the motor if the output power is reduced to 10 kW.

Problem 550: A 220 V series motor takes 15 A and runs at 700 rpm. The armature resistance including the resistance of the series winding is 0.8 ohm. At what speed will it run if a 2-ohm resistance is connected in series with supply voltage? Assume the motor is taking the same current.

Problem 551: The equivalent armature resistance of a series motor is 0.1 ohm. When connected across a 110-V mains, the armature takes 20 A and its speed is 1000 rpm. Determine its speed when the armature takes 50 A from the same mains, with the field increased by 10%.

Problem 552: A 400 V series motor has a field resistance of 0.2 ohm and an armature resistance of 0.1 ohm. The motor takes 30 A of current at 1000 rpm while developing full load torque. What is the motor speed when this motor develops 60% of full load torque.

Problem 553: A 10 hp, 230-V, 1200 rpm series motor having rated load efficiency of 85.5%. The armature resistance including brushes is 0.3 ohm. The field resistance is 0.25 ohm. Assuming the flux varies directly as the armature current, what value of resistance should be placed in series with this motor when starting, in order that the starting current maybe limited to a value that will exert a starting torque equal to 150% of its rated torque?

Problem 554: The field and armature resistances of a 220-V series motor are 0.2 \(\Omega\) and 0.1\(\Omega\), respectively. The motor takes 30 A of current while running at 700 rpm. If the total iron and friction losses are 350 W, determine the motor efficiency.

Problem 555: A 100-V series motor is used to drive a load through a pulley. This machine has an armature resistance of 0.2 ohm and a series field resistance of 0.25 ohm. When a torque of 25 N-m is applied to the pulley, the speed is 600 rpm. If stray power losses at this load is 300 W, calculate the armature current.

Problem 556: A 400 V series motor, having an equivalent armature circuit resistance of 0.5 ohm, takes 44 A of current while running at 650 rpm. What is the motor speed for a line current of 36 A?

Problem 557: The resistance of each of the two coils of a series motor is 0.04 ohm, armature resistance, 0.04 ohm. The motor takes 50 A at 100 V while running at 800 rpm and the coils are in series. What will it speed if the coils are re-connected in parallel while the load torque is doubled?

Problem 558: On full load a 500 V series motor takes 100 A and runs at 820 rpm. The armature and series field resistances are 0.1 and 0.04 ohm respectively. What will be its speed when developing half-full load torque and with a 0.08-ohm diverter connected across the series field winding. Assume flux is proportional to the field current.

Problem 559: A 400 V series motor working with unsaturated field is taking 60 A and running at 840 rpm. The total resistance of the motor is 0.1 ohm. At what speed will the motor run when developing half-full load torque.

Problem 560: A long shunt compound motor takes a current of 42 A from a 230 V source. The generator parameters are: Ra = 0.1 \(\Omega\), Rse = 0.2 \(\Omega\) and Rsh = 50 \(\Omega\). If the friction and windage losses amount to 400 W, determine the overall efficiency of the machine.

Problem 561: A 230 V, long shunt machine has the following parameters: Ra = 2.0 \(\Omega\), Rsh = 460 \(\Omega\) and Rse = 0.25 \(\Omega\). When the machine is run at no-load at its normal speed and rated voltage, the armature draws 0.6 A. Determine the armature current drawn if the machine delivers an output of 5 bhp.

Problem 562: A 25 hp, 250 V long shunt compound motor takes 5 A when running light and runs at 1150 rpm. Armature resistance, 0.15 \(\Omega\), Shunt field resistance, 200 \(\Omega\) and series field resistance, 0.05 \(\Omega\). At rated load the motor takes 86.5 A and runs at 720 rpm. Determine the ratio of torque developed from no-load to full load.

Problem 563: A 200 V shunt motor has armature and shunt field resistances of 0.15 \(\Omega\) and 125 \(\Omega\), respectively. When running light, it takes 6 A and runs at 1200 rpm. A series winding of 0.05 ohm is added to make it long shunt cumulatively compounded. This winding increases the flux per pole by 25% when the motor is taking its full load current of 100 A. Neglecting armature reaction, solve for the operating speed as a compound motor.

Problem 564: A 220 V DC motor has an armature resistance of 0.4 ohm, a shunt field resistance of 175 ohms and a series field resistance of 4 ohms. This machine is connected as a short shunt compound motor to 220 V mains. Calculate the power developed by the armature if the armature current is 30 A. Neglect the brush drop.