DC Machines: DC Generators (part 1)

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Fundamentals:

DC machines are still employed in many applications because of their variable speed and torque characteristics. This unit is devoted to the study of DC generators. Although many industries no longer use DC machinery, the study of DC generators is very important because it’s the foundation for the study of AC generators or alternators. This article...

++ describes the basic methods for generating electricity and how mechanical energy is converted to electric energy.

++ explains many of the principles of induced voltage.

++ describes how AC is produced in the armature of all rotating machines and how it’s converted to DC before leaving the generator.

++ discusses the various components of a DC generator and the principles of magnetic induction.

++ explains the differences between series and shunt field windings and the characteristics of each.

OUTLINE:

TERMINOLOGY

  • Armature
  • Armature reaction
  • Brushes
  • Commutator
  • Compound generators
  • Compounding
  • Countertorque
  • Cumulative compound
  • Differential-compound
  • Field excitation current
  • Frogleg-wound armatures
  • Generator
  • Lap-wound armatures
  • Pole pieces
  • Series field diverter
  • Series field windings
  • Series generator
  • Voltage regulator
  • Wave-wound armatures

Learning Goals:

++ discuss the theory of operation of DC generators.

++ list the factors that determine the amount of output voltage produced by a generator.

++ list the three major types of DC generators.

++ list different types of armature windings.

++ describe the differences between series and shunt field windings.

++ discuss the operating differences between different types of generators.

++ draw schematic diagrams for different types of DC generators.

++ set the brushes to the neutral plane position on the commutator of a DC machine.

Intro:

Although most of the electric power generated throughout the world is AC, DC is used for some applications. Many industrial plants use DC generators to produce the power needed to operate large DC motors. DC motors have characteristics that make them superior to AC motors for certain applications. DC generators and motors are also used in diesel locomotives.

The diesel engine in most locomotives is used to operate a large DC genera tor. The generator is used to provide power to DC motors connected to the wheels.

--4 Induced voltage after 180 deg of rotation.

--5 The negative voltage peak is reached after 27 deg of rotation. Volts; Negative voltage peak

What Is a DC Generator?

A generator is a device that converts mechanical energy into electric energy.

DC generators operate on the principle of magnetic induction. Earlier, it w as shown that a voltage is induced in a conductor when it cuts magnetic lines of flux ( --1). In this example, the ends of the wire loop have been connected to two sliprings mounted on the shaft. Brushes are used to carry the current from the loop to the outside circuit.

In --2, an end view of the shaft and wire loop is shown. At this particular instant, the loop of wire is parallel to the magnetic lines of flux and no cutting action is taking place. Because the lines of flux are not being cut by the loop, no voltage is induced in the loop.

In --3, the shaft has been turned 90 degree clockwise. The loop of wire cuts through the magnetic lines of flux and a voltage is induced in the loop.

When the loop is rotated 90 degree, it’s cutting the maximum number of lines of flux per second and the voltage reaches its maximum, or peak, value.

After another 90 degree of rotation, the loop has completed 180 degree of rotation and is again parallel to the lines of flux. As the loop was turned, the voltage decreased until it again reached zero.

As the loop continues to turn, the conductors again cut the lines of magnetic flux ( --5). This time, however, the conductor that previously cut through the flux lines of the south magnetic field is cutting the lines of the north magnetic field, and the conductor that previously cut the lines of the north magnetic field is cutting the lines of the south field. Because the conductors are cutting the flux lines of opposite magnetic polarity, the polarity of voltage reverses. After 27 degree of rotation, the loop has rotated to the position shown and the maximum amount of voltage in the negative direction is being produced.

After another 90 degree of rotation, the loop has completed one rotation of 360 degree and returned to its starting position. The voltage decreased from its negative peak back to zero. Notice that the voltage produced in the armature (the rotating member of the machine) alternates polarity. The voltage produced in all rotating armatures is alternating voltage.

Because DC generators must produce DC current instead of AC current, some device must be used to change the alternating voltage produced in the armature windings into direct voltage before it leaves the generator. This job is performed by the commutator. The commutator is constructed from a copper ring split into segments, with insulating material between the segments. Brushes riding against the commutator segments carry the power to the outside circuit.

--6 Voltage produced after 360 degree of rotation.

--7 The commutator is used to convert the AC voltage produced in the armature into DC voltage. Commutator; Loop; Brushes

--8 The loop is parallel to the lines of flux.

--1 A voltage is induced in the conductor as it cuts magnetic lines of flux. N, S; Magnet Loop Sliprings Brushes

--2 The loop is parallel to the lines of flux, and no cutting action is taking place. Loop; Flux lines; Volts Loop Shaft

--3 Induced voltage after 90 degree of rotation. Voltage reaches peak value. Volts

In --8, the loop has been placed between the poles of two mag nets. At this point in time, the loop is parallel to the magnetic lines of flux and no voltage is induced in the loop. Note that the brushes make contact with both of the commutator segments at this time. The position at which the windings are parallel to the lines of flux and there is no induced voltage is called the neutral plane. The brushes should be set to make contact between commutator segments when the armature windings are in the neutral plane position.

As the loop rotates, the conductors begin to cut through the magnetic lines of flux. The conductor cutting through the south magnetic field is connected to the positive brush, and the conductor cutting through the north magnetic field is connected to the negative brush. Because the loop is cutting lines of flux, a voltage is induced into the loop. After 90 degree of rotation, the voltage reaches its most positive point.

As the loop continues to rotate, the voltage decreases to zero. After 180 degree of rotation, the conductors are again parallel to the lines of flux and no voltage is induced in the loop. Note that the brushes again make contact with both segments of the commutator at the time when there is no induced voltage in the conductors.

During the next 90 degree of rotation, the conductors again cut through the magnetic lines of flux. This time, however, the conductor that previously cut through the south magnetic field is now cutting the flux lines of the north magnetic field, and the conductor that previously cut the lines of flux of the north magnetic field is cutting the lines of flux of the south magnetic field. Because these conductors are cutting the lines of flux of opposite magnetic polarities, the polarity of induced voltage is different for each of the conductors.

The commutator, however, maintains the correct polarity to each brush. The conductor cutting through the north magnetic field will always be connected to the negative brush, and the conductor cutting through the south field will always be connected to the positive brush. Because the polarity at the brushes has remained constant, the voltage will increase to its peak value in the same direction.

As the loop continues to rotate, the induced voltage again decreases to zero when the conductors become parallel to the magnetic lines of flux. Notice that during this 360 degree rotation of the loop the polarity of voltage remained the same for both halves of the waveform. This is called rectified DC voltage. The voltage is pulsating or fluctuating. It does turn on and off, but it never reverses polarity. Because the polarity for each brush remains constant, the output voltage is DC.

To increase the amount of output voltage, it’s common practice to in crease the number of turns of wire for each loop. If a loop contains 20 turns of wire, the induced voltage is 20 times greater than that for a single-loop conductor. The reason for this is that each loop is connected in series with the other loops. Because the loops form a series path, the voltage induced in the loops add. In this example, if each loop has an induced voltage of 2 volts, the total voltage for this winding would be 40 volts (2 V 3 20 loops 5 40 V).

It’s also common practice to use more than one loop of wire. When more than one loop is used, the average output voltage is higher and there is less pulsation of the rectified voltage. This pulsation is called ripple.

The loops are generally placed in slots of an iron core. The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic lines of flux to increase the inductance of the loops and provide a higher induced voltage. The commutator is connected to the slotted iron core. The entire assembly of iron core, commutator, and windings is called the armature. The windings of armatures are connected in different ways depending on the requirements of the machine. The three basic types of armature windings are the lap, wave, and frog-leg.

--9 The loop has rotated 90 degree.

--10 The loop has rotated 180 degree.

--11 The commutator maintains the proper polarity.

--12 The loop completes one complete rotation.

--13 Increasing the number of turns increases the output voltage.

--14 Increasing the number of loops produces a smoother output voltage.

--15 The loops of wire are wound around slots in a metal core. Commutator; Slotted metal core; Shaft

--16A DC machine armature.

--16B Cutaway view of an armature.

--17 Lap-wound armatures have their windings connected in parallel. They are used in machines intended for high-current and low-voltage operation.

--18 Wave-wound armatures have their windings connected in series. Wave windings are used in machines intended for high-voltage, low-current operation.

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