Chapter 4: DC Generators

Transcription

1 Chapter 4: DC Generators

2 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

3 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

4 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

5 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

6 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

7 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

8 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 e a and e c (e b and e d ) are therefore opposite at all times: e a + eb + ec + ed = consequently, no current will flow in the closed loop formed by the four coils the voltage between the brushes varies between e a at 0 and e a + e d at 45 0

9 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 = Bl 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

10 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

11 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

12 Calculating the Induced Voltage The peak voltage, E 0, induced between the brushes in a DC generator having a lap winding is given by E0 = Z nφ 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

13 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)

14 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

15 Homework Problems: 4-13, 4-14, and 4.16