DC Motors

Problem 528: A DC shunt motor has a full load rating of 15 hp, 230 volts, 57.1 amperes, 1400 rpm. The armature circuit resistance is 0.13 ohm and the field circuit resistance is 115 ohms. Neglecting the effect of armature reaction, determine the no-load line current.

Problem 544: The shaft power of a shunt motor is 7.8 hp. It draws 50 A from 120 V. The field winding draws 1.2 A. What is the efficiency of the motor?

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 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 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 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.

The greatest percentage of power loss in dc motor is due to

Which motor is used to start heavy loads?

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 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?

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

As the load is increased the speed of a dc shunt motor will

The armature current drawn by any dc motor is proportional to

The armature current drawn by any dc motor is proportional to

An external resistance is added in series with the field of a dc shunt motor. When the motor runs, the effect of resistance is

The primary reason for using carbon brushes over copper brushes in DC motors is

Power developed by a dc motor is maximum when the ratio of back emf to applied voltage is

When a dc motor has no load, what will happen to the back emf?

The current flowing in the armature conductors of a dc motor is

If the load current and flux of a dc motor are held constant and the voltage applied across its armature is increased by 10%, its speed will ___________.