DC MOTORS part3

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PROBLEMS:

1. A 250 V DC shunt motor has shunt field resistance of 150 Ω and armature resistance of 0.6 Ω. The motor operates on No-load with a full field flux at its base speed of 1000 rpm with armature current of 5A. If the machine drives a load requiring a torque of 100Nm, calculate the armature current and

speed of motor. If the motor is required to develop 10 kW at 1200 rpm, what is the required value of the external series resistance in the field circuit? Neglect saturation and armature reaction.

SOLUTION:

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2. A 4-pole series wound fan motor draws an armature current of 50A, when running at 2000 rpm on a 230 V DC supply with four coils connected in series. The four field coils are now connected in two parallel groups of two coils in series. Assuming the flux/pole to be proportional to the exciting current and load torque proportional to the square of speed, find the new speed and armature current. Neglect losses. Given: Armature resistance = 0.2 0, resistance of each field coil = 0.05 0.

SOLUTION:

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3. A shunt motor runs at 1200 rpm on a 420 V circuit and current taken in 30 A in addition to the field current. What resistance must be placed in series with armature in order that the speed may be reduced to 600 rpm, the current through the armature remaining same.ra = 3Ω given.

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4. A 4 pole 230 V series motor runs at 1000 rpm when the load current is 12 A. the series field resistance is 0.8 Ω and the armature resistance is 1Ω. The series field coils are now regrouped from all in series to two in series with two parallel paths. The line current is now 20 A if the corresponding weakening of field is 15 %. Calculate the speed of the motor.

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5. A 220 V DC shunt motor runs at 500 rpm when the armature current is 50 A calculate the speed if the torque is doubled. Given ra = 0.2 0.

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6. A 500 V, 50 BHP (37.8 kW) 1000 rpm DC shunt motor has, on full load an η of 90%. The armature circuit resistance is 0.24 0 and there is a total voltage drop of 2 V at the brushes. The field current is 1.8 A. Determine

(i) Full load line current

(ii) Full load shaft torque

(iii) Total resistance in motor starter to limit the starting current to 1.5 times the full load current.

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7. A 460 V series motor runs at 500 rpm taking a current of 40 A. Calculate the speed and % change in torque if the load is reduced, so that the motor is taking 30 A. Total resistance of the armature and field circuit is 0.8 0. Assume flux is proportional to the field current.

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8. A 220 V DC series motor is running at a speed of 800 rpm and draws 100 A. Calculate at what speed the motor will run when developing half the torque. Armature + field resistance is 0.1 0. Assume that magnetic circuit is unsaturated.

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9. An 8kW 230 V, 1200 rpm DC shunt motor has ra = 0.70. the field current is adjusted until, on NO load with a supply of 250 V, the motor runs at 1250 rpm and draws a armature current of 1.6 A. A load torque is then applied to the motor shaft, which causes the armature current to raise to 40 A and the speed falls to 1150 rpm. Determine the reduction in flux per pole due to the armature reaction.

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10. A 230 V, DC shunt motor runs at 800 rpm and takes armature current of 50 A. Find the resistance to be added to the field circuit to increase the speed from 800 rpm to 1000 rpm at an armature current of 80 A. Assume the flux to be proportional to the field current. Given ra = 0.150 and rf = 2500.

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