Lesson 30 AC Generators I 1 Learning Objectives

Lesson 30: AC Generators I 1

Learning Objectives • Understand the operation of a single phase two pole AC generator. • Describe the operation of a simple AC generator. • Identify and define the components of a three phase two pole AC generator to include rotor, stator, armature. field windings, slip rings and brushes. • Understand the effects of applying a DC voltage power supply to a two pole rotor's field windings via brushes and slip rings. • Understand the induced effects that result from rotating the rotor's electromagnetic field past the armatures (Faraday's Law). • Given the armature coil sequence and their physical location, plot the induced AC voltages for a three phase two pole AC generator as a function of time and as phasors. • Understand the relationship between the number of poles and rpm of the rotor to the induced AC current's frequency. 2

Producing Electricity • A generator is a machine that converts mechanical energy into electrical energy. − Motors and generators perform exactly the opposite function. − However, motors and generator are essentially the same device. 3

Advantages of AC Power • Motors: − AC is ‘natural’ for rotary motors. • Voltage Transformation: − AC transformers allow efficient changing of voltage to enable better power transmission. • Power Transmission: − AC power can be transmitted hundreds of miles. − DC transmission limited to ~1 mile. 4

Motor to Generator: Rotating DC • Armature current (Ia) produces force (Fd) in the armature causing rotation. • What if we remove the voltage source (VT) and we provided the torque? Equivalent circuit representation 5

Motor to Generator: Rotating DC • What if we remove the voltage source (VT) and the torque was provided? 6

Basic Single-Phase AC Generator • Turning the armature results in induced emf (e. AA ) across the load (Faraday’s Law). • The voltage e. AA will be single phase AC given as: e. AA = Vm sin t [V, volts] • What determines ? Rotor 7

Three-Phase AC Generator • What if we added two additional armature coils? Single Phase Three Phase 8

Three-Phase AC Generator • The three-phase generator has voltages as a function of time: 9

Three-Phase AC Generator • Phasor representation of the three-phase generator: 10

Phase Sequence • The phase sequence is the time order in which the voltages pass through their respective maximum values. • Phase sequence is important because it determines the direction of rotation of a connected motor. 11

Positive Phase Sequence (ABC) • The ABC sequence or positive sequence, is produced when the generator rotates counter-clockwise. 12

Negative Phase Sequence (ACB) • The ACB or negative sequence, is produced when the generator rotates clockwise. 13

Large AC Generator • Unlike the generator model with a fixed magnetic field and rotating armature, it is more practical to fix the armature windings and rotate the magnetic field on large generators. • Rotating armatures require brushes and slip rings to conduct current from the armature to the load. • The fixed armature advantage is that the generated voltage can be connected directly to the load with no slip rings or brushes. • The voltage applied to generate the rotating field is a small DC voltage called the field excitation voltage. 14

Generator Stator • The stator is the stationary part of induction motor. A stator winding is placed in the stator of induction motor and the three phase supply is given to it. • Stator is slotted with integer multiple of 6 slots. • Three pairs of slots contain identical coils of wire, each with NS turns. • These windings are called the armature. 15

Generator Rotor • Rotor contains rotating electromagnet called the field winding and is connected to the mechanical load through a shaft. • The electromagnet is powered by a DC current via slip rings and brushes. • Unlike in the DC motor application, brushes are not commutating and are not as subject to wear (less friction). 16

Slip Rings • Allow DC current to flow to the field windings on the rotor to produce the magnetic field. 17

Generator Output • The amplitude of voltage output is a function of the current supplied to the field windings. • The stronger the current, the larger the magnetic field, the larger the output voltage. http: //people. ece. umn. edu/users/riaz/animations/alternator. html 18

Generator Frequency • The frequency f (in Hz) of the AC voltage is a function of speed of the rotor N (in RPM): N = 60 f [RPM] • If the rotor contains multiple number of even poles (2, 4, 6, etc. ) then: 19

Synchronous Speed • Synchronous Speed (speed of rotation of B) versus Poles for a 60 Hz Machine: P (poles) 2 4 6 8 10 N (RPM) 3600 1800 1200 900 720 20

Example Problem 1 a) For a 4 pole, 60 HZ generator, what is the speed in rpm of the rotor? b) What would be the frequency of a 6 pole machine spinning at the same rpm? 90 Hz 21

QUESTIONS? 22