DC Generators

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 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 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 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.

Problem 517: Two identical shunt generators running in parallel supply a total load current of 2500 A. Each machine has a field resistance of 50 ohms and an armature resistance of 0.01 ohm. The induced emfs generated are 230 and 235 volts, respectively. Calculate the common operating voltage of each machine.

Problem 518: Three shunt generators operating in parallel supply a total load current of 2000 amperes. Each machine has a field resistance of 40 ohms and an armature resistance of 0.02 ohm. The emfs generated are 240, 242 and 245 volts respectively. Determine current delivered by each generator.

Problem 520: A shunt generator and a battery are connected in parallel serving a 500 A busload. The generator has an external characteristics such that the terminal voltage drops uniformly from 460 volts on no load to 440 volts on rated load while delivering 125 A of current. The battery has an open circuit emf is 420 V and internal resistance of 0.01 ohm. What percentage of the load current does the battery deliver?

Problem 521: A 50 kW, 220 V dc generator has a voltage regulation of 10%. It is connected in parallel with a battery of 115 cells, each with an emf of 2 V and internal resistance of 0.01 ohm. What current will the battery carry if by accident the load is trip-off from the bus.

Problem 522: Two identical 600 kW, 230 V dc generators are operating in parallel and take equal shares of an 800 kW, 250 V busload. The voltage regulation of each machine is 5%. If one of the generators is accidentally tripped off from the line, what is the voltage of the remaining machine?

Problem 523: Two 250 V DC generators are operating in parallel and taking equal shares of a 3000 A, 250 V bus load. Ratings: Generator 1 (2% reg, 500 kW), Generator 2 (4% reg, 400 kW). If the load drops to 2100 A, determine the new operating bus voltage.

A 120 V shunt motor has an armature resistance of 0.5 Ω and a field resistance of 60 Ω. At full load, the motor takes 10 A and the speed is 1,000 rpm. At what speed must this machine be driven as a generator to deliver 10 A to an external load at 120 V?

A 20kW, 200 v shunt generator is operated at rated load. If the driving engine is developing 30 bhp, determine the iron and friction losses. Armature resistance is 0.05 ohm and shunt field resistance is 50 ohms.

A 10 kW, 110 volts dc series generator has a 2 volts brush drop, an armature resistance \(R_a\) of 0.1 Ω and a series field resistance \(R_s\) of 0.05 Ω. Calculate the generated emf when delivering rated current at rated speed.

The field current of a 200 kW, 230 volts shunt generator is 8 A when running full-load at rated terminal voltage. If the combined brush and armature resistance is 0.03 Ω, solve for the electrical efficiency of the generator.

A 6-pole lap wound dc generator has 240 armature conductors, a flux of 0.04 Wb per pole and runs at 1,500 rpm. If the armature resistance is 0.8 Ω, determine the current through each conductor.