DC Motors

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 569: A shunt motor was tested by means of a pony brake having a length arm of 3.5 feet and a tare weight of 5.7 lbs. The current drawn by the machine from a 240 V line was 50.9 A when the scale reading was 24 lbs and the speed of the motor was 1215 rpm. Calculate the rotational losses of the motor. The armature and shunt field resistances of the machine are 0.25 \(\Omega\) and 120 \(\Omega\), respectively.

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 shunt motor has a rated armature current of 50 A when connected to 200 V. The rated speed is 1,000 rpm and the armature resistance is 0.1 Ω. Find the speed if the total torque is reduced to 70% of that at rated load and a 3 Ω resistance is inserted in series with the armature.

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 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.

The electromagnetic torque developed in a motor is 150 N-m. If the field flux is decreased by 20% and armature current is increased by 15%, find the new electromagnetic torque developed.

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 dc motor takes an armature current of 100 A at 230 V. The armature resistance is 0.03 Ω. The total number of lap connected armature conductors is 500 and the number of poles is 4. The flux per pole is 0.03 Wb. What is the torque developed by the motor?

Calculate the no-load current taken by a 100 kW, 460 v, shunt motor assuming the armature and field resistance to remain constant and equal to 0.03 Ω and 46 Ω, respectively The efficiency at full load is 88%

Find the horsepower rating of a motor running at 1,500 rpm and developing a torque of 105 lb-ft.

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.

A 230 v shunt motor has a no-load line current of 7 A and a full-load line current of 46 A. The armature resistance is 0.05 Ω and the shunt field is 75 Ω Determine the motor efficiency at full load.

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 Ω and 0.1 Ω, respectively. Determine the electromagnetic torque developed by the motor.