DC Motors

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 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 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.

Problem 565: The input current to a 220 V, short shunt compound motor at no load is 6 A. The shunt field circuit resistance is 220 ohms; the armature resistance is 0.10 ohm and the series field resistance is 0.08 ohm. What is the stray power loss?

Problem 566: The pony brake test of an elevator door drive shunt motor, the ammeter and voltmeter measuring the input read 34 A and 220 V. The speed of the motor is found to be 910 rpm and the balance on a 2 ft brake arm read 27.2 pounds. The tare weight of the arm is found to be positive 2.3 pounds. Determine the efficiency of the motor at this load.

Problem 567: In a brake test of an elevator door motor, the ammeter and voltmeter measuring the input read 34 A at 220 V. The speed of the motor is found to be 910 rpm and the balanced on a 0.61 m brake arm reads 11.9 kg. The dead weight of the arm is found to be 1.09 kg. What is the output hp of the door motor?

Problem 568: A shunt motor with an armature and field resistance of 0.055 and 32 ohms, respectively, is to be tested for its mechanical efficiency by means of a rope brake. When the motor is running at 1400 rpm, the longitudinal pull on the 6-inch diameter pulley is 57 pounds. The readings of the line ammeter and voltmeter are 35 and 105, respectively. Calculate the efficiency.

Problem 570: Calculate the force that will be exerted on the scale in a pony brake test when a 20-hp, 1,400 rpm motor is operating at full load. The length of the brake arm is 3 ft. and the tare weight of the brake is 3.75 lb.

A 400 V, 10 kW series motor drives a fan when running at 800 rpm. The motor draws 50 A from the supply. The resistances of the armature and series field are 0.2 ohms and 0.1 ohms, respectively. Determine the electromagnetic torque developed by the motor.

The shaft power of a shunt motor is 7.8 HP. Its armature draws 50 A from 120 V. The field winding draws 1.2 A. What is the efficiency of the motor?

A 240 V dc series motor develops a shaft torque of 200 N-m at 92% efficiency while running at 600 rpm. Calculate the motor current.

A 120 V dc shunt motor has an armature resistance of 200 mΩ. If the full-load armature current is 75 A, find the starting resistance needed to limit the armature starting current to 150% of the full-load value. (Take a 2 V brush drop.)

A dc shunt motor running at 1,200 rpm has an armature resistance of 0.15 Ω. The current taken by the armature is 60 A when the applied voltage is 220 V. If the load is increased by 30%, find the new speed.

A 120 V dc motor draws 225 A from the supply at a distance of 350 m using two copper conductors of diameter 0.5 inch and resistivity 10.4 ohm-cmil/ft. Find the voltage of the supply.