DC Generators

Problem 497: A DC generator has no-load output voltage of 120 volts. Its armature circuit resistance is 0.95 ohm and its field coils are separately energized. If the load is rated 2 kW at 115 V. Neglecting the effect of armature reaction, what power could be delivered to the load?

Problem 498: The armature of a 6-pole separately excited dc generator is lap wound with 534 conductors. This machine delivers power to the load at 250 V while being driven at 1000 rpm. At this load, the armature circuit dissipates 640 W. If the flux per pole of this generator is 30-mWb, determine the kW rating of the load served. Assume a total brush contact drop of 2 V.

Problem 499: A separately excited generator when running at 1200 rpm supplies 200 A at 125 V to a circuit of constant resistance. Armature circuit resistance is 0.10 ohm. Determine the terminal voltage when the speed is dropped to 1000 rpm? Assume that the field current is unaltered.

Problem 500: A dc series generator is supplying a current of 5 A to a series lighting system through a feeder of total resistance, 1.0 ohm. The terminal voltage is 2500 V. The armature and series field resistances are respectively 18 and 15 ohms respectively. A 30-ohm diverter resistance is shunted across the series field. Determine the power developed in the armature of the generator?

Problem 501: A 30 kW series generator has an efficiency of 88 percent when operating at rated load. If the stray power loss is 15% of the full load losses, calculate the maximum efficiency of the generator. Assume that the stray power loss is constant and the other losses vary as the square of the load.

Problem 502: The hysteresis and eddy current losses of a DC machine running at 1000 rpm are 250 watts and 100 watts, respectively. If the flux remains constant, at what speed will the total iron loss be halved?

Problem 503: The eddy current loss in a DC machine is 600 watts when the total flux is 2,000,000 maxwells per pole and the machine is running at 1000 rpm. Determine the loss when the excitation of the machine is adjusted to produce 2,500,000 maxwells per pole and the speed is increased to 1200 rpm.

Problem 504: In a DC generator, the iron losses at 1,000 rpm are 10 kW at a given field current. At a speed of 750 rpm and at the same field current, the total iron losses become 6 kW. Assume the hysteresis loss is directly proportional to the speed and the eddy current loss is proportional to the square of the speed. Determine the iron losses at 500 rpm.

Problem 505: A compound dynamo supplies a load of 20 kW at 400 V through a feeder having a resistance equal to 0.1 ohm. The shunt and series field resistances are 85 ohms and 0.06 ohm respectively. Determine the total electromotive force when the generator is connected long shunt. Assume a brush contact drop of 1 V per brush and an armature resistance of 0.1 ohm.

Problem 506: A long shunt compound generator has the following parameters: Ra = 0.1 \(\Omega\), Rse = 0.05 \(\Omega\) and Rsh = 120 \(\Omega\). This machine supplies power a aggregate dc load of 10 kW. At this load the armature generates 246 volts. Determine the terminal voltage.

Problem 507: A compound generator is to be operated, as a flat-compounded long shunt compound generator. To obtain the same rated voltage at full load and at no load when operated as a shunt generator, it is necessary to increase the field current by 0.20 A. The shunt field winding has 1000 turns per pole, and the series field winding has 4 turns per pole. If the full load armature current of the compound generator is 95 A and the series field resistance is 0.025 ohm, calculate the diverter resistance required.

Problem 508: The armature, series field and shunt field resistances of a 125 kW, 250-V long shunt cumulative compound DC generator are 0.025 \(\Omega\), 0.01 \(\Omega\) and 30 \(\Omega\), respectively. If the stray power loss at rated load is 5 kW, determine the motor brake horsepower required to drive this machine. Assume a brush drop of 2 V and a stray load loss of 1% of the output.

Problem 509: A long shunt compound generator delivers 100 A, 250 V at rated load. The resistances of the various windings are: Ra = 0.1\(\Omega\), Rip = 0.02 \(\Omega\), Rsh = 100 \(\Omega\) and Rse = 0.025 \(\Omega\). If the stray power losses amount to 1.8 kW, solve for the overall efficiency of the generator.

Problem 510: A certain 5.5 kW, 125-V DC long shunt compound generator was operated as a shunt motor at no load for determination of stray power losses. With 135 V equal to the generated emf at full load applied to the armature and at normal speed of 1700 rpm, the current input to the armature was found to be 2.5 A. The armature, shunt field and series field resistances are 0.2 \(\Omega\), 25 \(\Omega\) and 0.25 \(\Omega\), respectively. Determine the efficiency of the generator at rated load.

Problem 511: A compound dynamo supplies a load of 20 kW at 400 V through a feeder having a resistance equal to 0.1 ohm. The shunt and series field resistances are 85 ohms and 0.06 ohm respectively. Determine the total electromotive force when the generator is connected short shunt. Assume a brush contact drop of 1 V per brush and an armature resistance of 0.1 ohm.

Problem 512: A short shunt compound generator has the following has an armature, series field and shunt field resistances are 0.04 ohm, 0.03 ohm and 80 ohms respectively. The generator serves a 20 kW, 200 V load through a feeder with an equivalent resistance of 0.1 ohm. Solve for the power generated by the armature.

Problem 513: A short shunt compound generator has an armature, shunt field and series field resistances of 0.02 \(\Omega\), 80 \(\Omega\) and 0.03 \(\Omega\) respectively. The generator's induced emf is 510 V when the terminal voltage is 500 V. How much load current is delivered under this condition.

Problem 514: A short shunt compound generator delivers 150 A at a terminal voltage is 230 V. The shunt field current is 2.5 A. The resistances of the armature, series field and diverter are 0.032 \(\Omega\), 0.015 \(\Omega\) and 0.03 \(\Omega\), respectively. Calculate the electrical efficiency of the generator at this load.

Problem 515: Two shunt DC generators G1 and G2 are rated at 100 and 150 kW at 110 V respectively. Their external load characteristics may be considered as straight line. The drop in the terminal voltage from no-load to full-load is 10 volts for G1 and 20 volts for G2. Calculate the no-load voltages of these generators when they are operated in parallel to supply a load of 2000 A, which is divided between them in proportion to their ratings.

Problem 516: Two shunt generators operating in parallel are giving a total output of 320 kW. Generator A: armature resistance 0.02 ohm, induced emf 450 V; Generator B: armature resistance 0.025 ohm, induced emf 460 V. Find the bus bar voltage. Neglect shunt field current.