Chapter 4: DC Generators

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

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

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

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

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

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

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

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 =

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

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

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

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)

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

Homework

• Problems:4-13, 4-14, and 4.16