DC Motors

Problem 525: A simplex lap wound armature has 580 conductors and carries a current of 125 amperes per armature current path. If the flux per pole is 20 mWb. Calculate the electromagnetic torque developed by the armature?

Problem 526: A 6-pole lap wound shunt motor takes 300 A when the speed is 500 rpm. The flux per pole is 0.06 Wb and the armature has 870 turns. Neglecting the shunt field current, calculate the brake horsepower of the motor. Assume a constant loss of 4%.

Problem 529: At rated load, a shunt motor draws an armature current of 50 A from a 230 V dc mains while running at 1000 rpm. At no load the armature current drops to 5 A. If the effect of armature reaction has weaken the flux by 2% from no-load to full load, determine speed of the motor at no-load. Assume the armature resistance to be 0.15 ohm.

Problem 530: The nameplate rating of a shunt motor is 150 hp, 600 volts, 205 A, 1700 rpm. The resistance of the shunt field circuit is 240 ohms and the total armature circuit resistance is 0.15 ohm. Calculate the speed regulation of the motor.

Problem 531: A DC shunt motor runs at 600 rpm on a 240-volt supply while drawing a line current of 30 A. Its armature and field resistances are 0.5 \(\Omega\) and 120 \(\Omega\) respectively. What resistance should be placed in series with the armature circuit in order to reduce the speed to 400 rpm. Assume no changes in the armature or field current.

Problem 532: A 120-V shunt motor has an armature equivalent of 0.5 \(\Omega\) and a field resistance of 60 \(\Omega\). At full load, the motor takes 10 A and the speed is 1000 rpm. At what speed must this motor maybe driven as a generator to deliver 10 A to an external load at 120 V?

Problem 533: A belt-driven 150 kW shunt wound DC generator is running at 450 rpm and is supplying full load to a bus bar at 240 V. At what speed will it run if the belt breaks and the machine continues to run taking 8.5 kW from the bus bar? The armature and field resistances are 0.05 ohm and 85 ohms respectively. The brush contact drop is 1.5 volt per brush. Neglect armature reaction.

Problem 534: A shunt motor draws a current of 40 A from a 120 V source and runs at 1200 rpm at rated load. The armature and field circuit resistances are 0.1 \(\Omega\) and 60 \(\Omega\), respectively. Determine the speed of the motor at half load. Assume a brush drop of 2 V at rated load and 1 V at half load.

Problem 536: A 220-V shunt motor delivers 40 hp on full load at 950 rpm and has an efficiency of 88%. The armature and field resistances are 0.2 ohm and 110 ohms, respectively. Determine the starting resistance such that the starting armature current does not exceed 1.6 times the full load current.

Problem 537: A 120-V DC motor rated at 5-hp has a full load efficiency of 86%. The field and armature resistances are 60 ohms and 0.75 ohm, respectively. What starter resistance will be required to limit the armature current at starting to 200% of its rated armature current?

Problem 538: A 10-hp 230 V DC motor of 85% full load efficiency is located 450 ft from the supply mains. If the motor's starting current is 1.75 times the full load current, what is the smallest cross-sectional area of copper wire required when the allowable voltage drop in the feeder at starting is limited to 24 volts?

Problem 539: The nameplate rating of a shunt motor is 150 hp, 600 volts, 205 A, 1700 rpm. The resistance of the shunt field circuit is 240 ohms and the total armature circuit resistance is 0.15 ohm. If the motor is to be deliver full-load torque at 1,200 rpm, what value of resistance must be added to the armature circuit?

Problem 540: A 120 V shunt motor draws a current of 77 A at rated load. The armature and shunt field resistances are 0.2 \(\Omega\) and 60 ohms respectively. A multi-tap starter rheostat is inserted in the armature circuit to limit the starting current. If the rheostat has a resistance of 1.732 ohms, determine the percentage tap used such that the starting current will be limited to 150% of its full load value?

Problem 541: A 10-hp power 220 V DC shunt motor has an armature and field resistance of 0.25 ohm and 100 ohms respectively. The full load efficiency is 83%. Determine the value of starting resistance in order that the starting current will not exceed 200 percent of full-load value.

Problem 542: A 50 HP, 550 V shunt wound motor draws a line current of 4.5 A at no load. The shunt field resistance is 275 ohms and the armature resistance exclusive of brushes, is 0.3 ohm, the brush drop at full load is 2 V. At full load, the motor draws a line current of 84 A. Calculate the efficiency at full load.

Problem 543: A 100-volt shunt motor is developing 6 hp while operating at an overall efficiency of 86%. The armature and shunt field resistances are 0.06 and 50 ohms respectively. Determine stray power losses.

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 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.