Dc Motors

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INSTRUCTION: Solve the following problems. Show your solutions. Submit your solutions to me at the Electrical Engineering Department.


Test I.

1. A 4-pole wave wound dc generator has 50 slots and 24 conductors per slot. The flux per pole is 10 mWb. Determine the induced emf in the armature, if it is rotating at a speed of 600 rpm.

2. A 4-pole, simplex-lap wound, direct current armature has 48 slots containing coils, each of two turns. The flux per pole, in the field structure for this armature, is 500,00 lines. Calculate the speed for this machine in order that its generated emf be 220 V.

3. When a generator is being driven at 1,200 rpm, the generated emf is 125 volts. What will be the generated emf if the field flux is decreased by 12% with the speed increased to 1,800 rpm?

4. A separately-excited generator, when running at 1000 rpm supplied 200 A at 125 V. What will be the load current when the speed drops to 800 rpm. Assume armature resistance = 0.04 ohm and brush drop = 2 V.

5. The output of a shunt generator is 24 kW at a terminal voltage of 200 V. Armature resistance 0.05 ohm, shunt field resistance 40 ohms. If the iron and friction losses equal to the copper loss at this load. Find the overall efficiency?

6. A short compound generator supplies a current of 100 A at 220 V. If the resistance of the shunt field is 50 ohms, of the series field 0.025 ohm, of the armature 0.05 ohm, the total brush drop is 2 V and the iron and friction losses amount to 1 kW, find the bhp of the engine driving this generator.

Test II.

1. A dc shunt generator has an armature resistance of 0.31 ohm. The shunt field resistance is 134 ohm. No-load voltage is 121 V at 1775 rpm. Full load current is 30 A at 110 V. Assume a constant flux. What is the full load speed?

A. 1750 rpmC. 1760 rpm
B. 1758 rpmD. 1764 rpm

2. A six-pole lap connected 230 V shunt motor has 410 armature conductors. It takes 41 A on full load. The flux per pole is 0.05 weber. The armature and field resistances are 0.1 ohm and 230 ohms respectively. Contact drop per brush = 1 V. Determine the speed of motor at full load. A. 582.3 rpmC. 606.6 rpm

B. 529.3 rpmD. 655.6 rpm

3. A 4-pole dc shunt generator with a shunt field resistance of 100 ohms and an armature resistance of 1 ohm, has 378 wave-connected conductors is its armature. The flux per pole is 0.02 Wb. If a load resistance of 10 ohms is connected across the armature terminals and the generator is driven at 1000 rpm, calculate the power absorbed by the load. A. 5024 WC. 5322 W

B. 5525 WD. 5153 W

4. A short shunt compound generator supplies a load of 20 kW at 500 volts. The armature, series, and shunt field resistances are 0.10 ohm, 0.05 ohm, and 100 ohms respectively. Find the power developed in the armature. A. 24.6 kWC. 22.8 kW

B. 20.5 kWD. 25.4 kW

5. A dc series generator supplies a load of resistance of 1.4 ohms through a pair of feeder conductors of total resistance 0.1 ohm. The voltage at the dc generator terminals is 120 volts. What is the pd at the load? A. 110 VC. 112 V

B. 114 VD. 115 V
6. The output of a shunt generator is 66 A at 100 volts. Armature and field resistances are, respectively, 0.04 ohm and 25 ohms. If the efficiency at this load is 88%, determine the core and friction losses. A. 402 WC. 285 W

B. 304 WD. 596 W

7. The output of a shunt generator is 500 A at a terminal voltage of 225 V. Armature resistance is 0.02 ohms; shunt field resistance is 50 ohms. Determine the emf generated. A. 239.3 VC. 240.7 V

B. 235.1 VD. 228.3 V

8. A 50 kW, 250 V short shunt compound generator has the following data: Ra=0.06 ohm, Rse=0.04 ohm and RF =125 ohms. Calculate the induced voltage at rated load and terminal. Take 2 V as the total...
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