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Chapter 4: DC Generators
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Creating an AC Voltage
• The voltage produced in a DC generator is inherently AC and only becomes DC after rectification
• Consider an AC generator, consisting of a coil on the rotor and a permanent magnet for the stator– a pair of slip rings and
stationary brushes provide a current path from the rotor to the external environment
– a load would be connected to thebrushes, x and y
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Inducing a Voltage
• An external prime mover provides a torque that spins the rotor– the coil revolve inside
the magnetic field
– as the individual conductors cut through the flux, a voltage is induced
– maximum instantaneous voltage appears across x and y when the coil is passing through the horizontal plane
– no flux is cut when the coil is passing through the vertical plane, resulting in a zero voltage across x and y
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DC Generation
• A unidirectional pulsating dc voltage can be generated by switching the brushes from one slip ring to the other every time the polarity changes at the zero crossing– one brush x would always be at a positive potential
– the other brush y would always be at a negative potential
• A commutator provides the crossover rectification process– a commutator is a single
slip ring split into two halves with each segment insulated from the other
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DC Generation
• The commutator revolves with the coil– voltage between the two segments is picked up by the brushes
– the voltage between brushes x and y pulsate but never change polarity
– the commutator acts as a mechanical reversing switch
– the alternating voltage in the coil is rectified by the commutator
– the constant polarity between x and y causes the current in the external load to flow in the same direction
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AC & DC Generator Differences
• The elements of the AC and DC generators are essentially the same and are assembled together in the same way– the basic operating principle is also the same:
a coil rotates inside a magnetic field between the poles of a magnet, and develops a ac voltage
• The machines only differ in the way the coils are connected to the external circuit– an ac generator used slip rings
– a dc generator uses a commutator
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Improving the Voltage Waveshape
• By increasing the number of coils to four, oriented at right-angles to each other, and dividing the commutator into four segments, the voltage waveshape is improved– the voltage pulsates but never falls to zero
– all four coils are identical
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Improving the Voltage Waveshape
• Coils A and C (conversely, B and D) experience the same flux but are traveling in opposite directions– the polarities of ea and ec (eb and ed) are therefore opposite
– at all times:
consequently, no current will flow in the closed loop formed by the four coils
– the voltage between the brushes varies between ea at 0° and ea+ ed at 45°
0dcba =+++ eeee
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Induced Voltage
• By increasing the number of coils and commutator segments, the DC voltage waveshape can have smaller ripples
• When the coils are rotated, the voltage E induced in each conductor depends upon the flux density and the rate at which it cuts:– because the cutting of flux
density in the air gap varies from point to point, the value of induced voltage per coil depends upon its instantaneous position
vlBE =
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Neutral Positions
• At times, a brush straddles two commutator segments that are connected to a coil– the brush short-circuits the coil
– however, the coil is not cutting through any flux and the induced voltage is momentarily zero
– no current will flow through the short-circuit of the brush
• Brushes are placed in the neutral position where short-circuits occur during momentarily zero induced voltage
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Neutral Zones
• If the brushes are located away from neutral positions– the voltage between the
brushes will decrease
– large short-circuit currents flow at the brushes, causing sparks
• Neutral zones are those places on the surface of the armature (rotor) where the cutting of the flux density is zero– at no-load operating conditions, the neutral zones are located
exactly half-way between the poles
– during loading conditions, armature reaction will cause the neutral zones to shift away from the half-way point
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Calculating the Induced Voltage
• The peak voltage, E0, induced between the brushes in a DC generator having a lap winding is given bywhere– Z = total number of conductors on the armature
– n = speed of rotation [rpm]
– Φ = flux per pole [Wb]
• Example– the armature of a 6-pole, 600 rpm generator has 90 slots
– each coil has 4 turns and the flux per pole is 0.04 Wb
– calculate the value of the induced voltage
600
Φ= nZE
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Generator under Load
• Under loading conditions, some fundamental flux and current relationships take place that are directly related to the mechanical-electrical energy conversion process– the current delivered by the
generator also flows throughall the armature conductors
– the current carrying conductorsare subjected to a force according to Lorentz’s law
– the forces on each conductor result in a torque that acts opposite to the direction ofrotation (counter-torque)
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Generator under Load
• To keep the armature of the generator turning in the given direction of rotation– a torque must be applied to the shaft to overcome the
opposing electromagnetic torque (the drive torque)
– the resulting mechanical power is converted into electrical power that is delivered to the load
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Homework
• Problems:4-13, 4-14, and 4.16