Handout_3_types_of_dc_generators (1).pdf

CHAPTER 2

TYPES OF DC GENERATOR Generators are usually classified according to the way in which their fields are excited. Generators may be divided into;

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Separately-excited Generators Are those whose field magnets are energized from an independent external source of d.c. current. It is shown diagramatically in Fig. 2.1. This type of DC generators are generally more expensive than self-excited DC generators because of their requirement of separate excitation source. Because of that their applications are restricted. They are generally used where the use of self-excited generators are unsatisfactory.  Because of their ability of giving wide range of voltage output, they are generally used for testing purpose in the laboratories.  Separately excited generators operate in a stable condition with any variation in field excitation. Because of this property they are used as supply source of DC motors, whose speeds are to be controlled for various applications. ExampleWard Leonard Systems of speed control.

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Fig. 2.1 2.

Self-excited Generators

Modern DC generators with field coils are self-excited generators which get started with the initial current in the field coils. When generator is switched off, a small magnetism is developed in rotor iron which induced electromotive force in the armature due to which current is produced in the field windings. Initially, weak magnetic field creates less current in the coil, but to sustain self-excitation, the additional magnetic flux increases the electromotive force in the rotor, due to which voltage keep on increasing until the machine takes the full load. There are three types of self-excited generators named according to the manner in which their field coils (or windings) are connected to the armature. a) Shunt wound The field windings are connected across or in parallel with the armature conductors and have the full voltage of the generator applied across them (Fig. 2.2). The application of shunt generators is very much restricted for its dropping voltage characteristic. They are used to supply power to the apparatus situated very close to its position. These type of DC generators generally give constant terminal voltage for small distance operation with the help of field regulators from no load to full load.  They are used for general lighting.  They are used to charge battery because they can be made to give constant output voltage.  They are used for giving the excitation to the alternators.  They are also used for small power supply.

Fig. 2.2

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b) Series Wound In series wound generators, field winding and the armature winding are connected in series so that current that passes through external circuit and through field windings, passes from armature as shown in the fig. 2.3. The field coil of series wound generator has low resistance, consist of a few turns of thick wire. If the load resistance decreases, then current flow increases. As a result magnetic field and output voltage increases in the circuit. In such type generator, output voltage varies directly with respect to load current which is not required in most of the application. Due to this reason, such types of generators are rarely used. These types of generators are restricted for the use of power supply because of their increasing terminal voltage characteristic with the increase in load current from no load to full load. We can clearly see this characteristic from the characteristic curve of series wound generator. They give constant current in the dropping portion of the characteristic curve. For this property they can be used as constant current source and employed for various applications.  They are used for supplying field excitation current in DC locomotives for regenerative breaking.  This types of generators are used as boosters to compensate the voltage drop in the feeder in various types of distribution systems such as railway service. In series arc lightening this type of generators are mainly used.

Fig. 2.3

c) Compound Wound It is a combination of a few series and a few shunt windings and can be either short-shunt or long-shunt. Among various types of DC generators, the compound wound DC generators are most widely used because of its compensating property. Depending upon number of series field turns, the cumulatively compounded generators may be over compounded, flat compounded and under compounded. We can get desired terminal voltage by compensating the drop due to armature reaction and ohmic drop in the in the line. Such generators have various applications. Cumulative compound wound generators are generally used for lighting, power supply purpose and for heavy power services because of their constant 3

voltage property. They are mainly made over compounded. Cumulative compound wound generators are also used for driving a motor. For small distance operation, such as power supply for hotels, offices, homes and lodges, the flat compounded generators are generally used. The differential compound wound generators, because of their large demagnetization armature reaction, are used for arc welding where huge voltage drop and constant current is required. At present time the applications of DC generators become very limited because of technical and economic reasons. Now a days the electric power is mainly generated in the form of alternating current with the help of various power electronics devices. 

Short Shunt Compound Generator

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Long Shunt Compound Generator

DC Generators Performance Curves Performance curves of a DC generator is that curves which shows the ability of delivering output voltage of a DC generator with the change in load current from no load to full load. These are also called characteristic curves. From the performance curve we can get a clear idea about the voltage regulation of various kind of DC generators. The lower the voltage regulation will be, the performance of the generator will be better.

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Performance Curve of Separately Excited DC Generator Though this type of DC generators are rarely used due to its cost for the separate excitation but the performance of these DC generators are quite satisfactory. In separately excited DC generators, the terminal voltage as the load increases and the load current started to flow. There is slight drop in the terminal voltage due to armature reaction and IR drop but these drop can be eliminated by increasing field excitation and then we can get constant terminal voltage. In the diagram below, the curve AB is showing this characteristic. Performance Curve of Series Wound DC Generator In series DC generators, the terminal voltage at no load will be zero because there is no current flowing through the field winding. When load increases then output voltage also increases. Because of its series field with the armature, its terminal voltage varies widely with a little increase in load current. Though in series DC generators the output voltage is lower than the generated voltage due to armature reaction and some ohmic drop in the armature winding. In the diagram below, the curve OC is showing this characteristic. Performance Curve of Shunt Wound DC Generator In shunt wound DC generators, there is always some no load voltage due to the existence of shunt field winding. As the load increases, the terminal voltage of this type of DC generators decreases very quickly. It has very large demagnetizing armature reaction and armature resistance drop. Because of this drastic reduction in the terminal voltage, the load current also decreased after a certain point. The performance of this type of generators are very poor. In the diagram below, the curve DE is showing this characteristic. Performance Curve of Compound Wound DC Generator At no load, the performance curve of this type of DC generator is same as that of shunt field generators because at no load, there is no current in the series field winding. When the load increases, then the terminal voltage drops due to the shunt DC generator, but the voltage rise in the series DC generator compensates the voltage drop. For these reason the terminal voltage remains constant. The terminal voltage can also make higher or lower by controlling the amp-turns of the series field winding. In the diagram below, the curve FG is showing this characteristic.

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Sample Problem 2.1 If the NL (no load) voltage of a separately excited generator is 120 V at 1250 rpm,what will be the voltage a. if the speed is increased to 1600 rpm?

b.

if the speed is decreased to 1100 rpm?

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Sample Problem 2.2 A 200 KW, 230 V compound generator is connected long shunt. If the shunt field resistance is 30Ω, what is the series field current at full load?

Sample Problem 2.3 A long shunt compound generator has a shunt field with 1200 turns per pole and a series field with 4 1/2 turns per pole. If the shunt field and the series field AT are 1200 and 196 respectively, calculate the power delivered to a load when the terminal voltage is 220V.

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Sample Problem 2.4 A 10 pole, simplex lap wound generator is wound for 110V, 600A capacity. It has an armature resistance of 0.0072Ω, and is wound in 160 slots, 2 coil per slot and 3 turn per coil. a. Find the resistance per coil. b. Find the generated emf c. If the winding is change to simplex wave, all other conditions unchanged. Find i. new Ra ii. new Eg

Sample Problem 2.5 A 10 pole, simplex wave dc generator has 3 coil/slot, 2 turn/coil, no. of slots = 161, ϕP = 1.5x 106 lines, speed of rotation is 500 rpm. Find the generated emf and resistance per coil if the armature resistance is 0.36 Ω.

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Sample Problem 2.6 A 5 KW, 120V compound generator has an armature resistance of 0.05Ω and a shunt field resistance of 60 Ω. Assume a long shunt connection and a voltage drop at the brushes of 2V. Calculate the power generated by the armature.

Sample Problem 2.7 A short shunt compound generator has a shunt field resistance of 77Ω , a series field resistance of 0.008Ω, a commutating winding resistance of 0.005Ω, an armature resistance of 0.02Ω. When the armature current is 128A, the generated voltage is 235V. Calculate the power delivered to the load.

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Sample Problem 2.8 The following information is given for a 300KW 600V compound generator. Shunt field resistance is 75Ω, armature resistance including brushes is 0.03Ω, commutating field winding resistance 0.036Ω. Calculate the voltage and power generated by the armature, a. at rated load. b. at half load.

Sample Problem 2.9 A long shunt compound generator delivers a load current of 50A at 500V and has an armature, series field and shunt field resistance of 0.05Ω, 0.03Ω and 250Ω, respectively. Calculate the generated power. Allow 1V per per brush contact drop.

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Sample Problem 2.10 An 8-pole d.c. shunt generator with 778 wave-connected armature conductors and running at 500 r.p.m. supplies a load of 12.5 Ω resistance at terminal voltage of 50 V. The armature resistance is 0.24Ω and the field resistance is 250Ω. Find the armature current, the induced e.m.f. and the flux per pole.

Supplementary Problems. 1. A long-shunt compound generator, rated at 100kW and 500V DC, has an armature resistance of 0.03ohm, a shunt field resistance of 125ohms, and a series field resistance of 0.01ohm. The diverter carries 54 amperes. Calculate the diverter resistance at full load and the generated voltage at full load. 2. Calculate the flux per pole required on full load for a 50 kW, 400 V, 8 pole, 600 r.p.m. dc shunt generator with 256 conductors arranged in a lap connected winding. The armature winding resistance is 0.1 ohm, the shunt field resistance is 200 ohms and there is a brush contact voltage drop of 1 V per brush. 3. A short shunt compound generator delivers a load current of 30 A and has Ra = 0.05 ohm, Rse = 0.3 ohm and R sh = 200 ohms. Calculate the induced e.m.f. and Ia. Allow 1.0 per brush as contact drop. 4. Repeat the above problem for a long shunt compound generator. 5. A 4 pole, lap wound, long shunt generator has flux per pole of 0.07 Wb. The armature winding consists of 220 turns and the resistance per turn is 0.004 ohm. Calculate the terminal voltage if the resistance of shunt and series field are 100 ohms and 0.02 ohm respectively; when the generator is running at 900 r.p.m. with armature current of 50 A. Also calculate the power output of the generator.

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