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Transcript
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 the brushes, 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 rightangles 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: ea + eb + ec + ed = 0 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° 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: E = B l v – 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 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 shortcircuits 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 by Z nΦ E0 = where 60 – 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 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 through all the armature conductors – the current carrying conductors are subjected to a force according to Lorentz’s law – the forces on each conductor result in a torque that acts opposite to the direction of rotation (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