Chapter 16: Synchronous Generators Lecture 30 Electro Mechanical System

Electro Mechanical System Introduction Three-phase synchronous generators are the primary source of all electrical energy. They convert mechanical energy into electrical energy. Power ratings up to 1500 MW Stator construction is the same as and induction machine Commercial synchronous generators Commercial synchronous generators are built with either; Stationary magnetic field or Rotating dc magnetic field. Lecture 30 Electro Mechanical System

Commercial synchronous generators Commercial synchronous generators are built with either a stationary or a rotating dc magnetic field. A stationary field synchronous generator has: The same appearance like dc generator The salient poles create the dc field, which is cut by a revolving armature. The armature possesses 3-phase windings whose terminals are connected to three slip rings, enabling it to connect to 3-phase load. The armature is driven by a prime mover. As the armature rotates a 3-phase voltage is induced, the voltage value depends upon the speed of rotation and dc excitation current in the stationary poles. Stationary field generators are used when the power output is less than 5 kVA. For bigger generator it is cheaper, safer and practical to have revolving dc field. Lecture 30 Electro Mechanical System

Commercial synchronous generators A revolving field synchronous generator has; A stationary armature called a stator. The 3-phase stator winding is directly connected to the load, without going through unreliable large slip rings or brushes. A stationary stator makes it easier to insulate the winding, because they are not subjected to centrifugal forces. It is also known as alternator. Field is excited by a dc generator, usually mounted on the same shaft Brushes of the commutator are connected to another set of brushes riding on the slip rings to feed the dc current Ix in the revolving field Lecture 30 Electro Mechanical System

Commercial synchronous generators Lecture 30 Electro Mechanical System

Electro Mechanical System Number of Poles The number of poles on a synchronous generator depends upon the speed of rotation and frequency we wish to produce. The alternator frequency is give by: f = (p n)/120 Where: f = frequency of the induced voltage [Hz] p = number of poles in rotor n = speed of rotor or synchronous speed [rpm] Example A hydraulic turbine turning at 200 rpm is connected to a synchronous generator. If the induced voltage has a frequency of 60 Hz, how many poles does the rotor have?   P = 120f/n = 120 x 60/200 = 36 poles or 18 pairs of N and S poles. Lecture 30 Electro Mechanical System

Main features of the stator From an electrical standpoint the stator of a synchronous machine and an induction machine are identical. The windings are usually connected in a wye configuration because: The voltage per phase is only 1/√3 or 58% of the line voltage, permitting a reduction in the amount of dielectric insulation. This reduction gives us more space in the slots enabling us to have a thicker conductor. A large conductor allows more current and Power output of the machine. Under load, the voltage can become distorted and no longer sinusoidal. The distortion is mainly due to third harmonic voltages (180 Hz). With a wye connection, the third harmonic voltages cancel With a delta connection, the third harmonic voltage add and appear on the line-to-line voltages Lecture 30 Electro Mechanical System

Main features of the rotor Synchronous generators has two types of rotors: Salient pole (slow speed) and   Cylindrical (high speed) rotors Salient pole rotor Most hydraulic turbines have to turn at low speeds (from 50 to 300 rpm) Rotor is directly coupled to water wheel, & a freq. of 60 Hz is required. To get 60Hz at low speed a large no. of poles are required on the rotor. A large diameter is required to provide necessary space for the poles. In addition to dc field winding, a squirrel-cage winding is embedded in the pole face. Under normal condition this winding does not carry any current because rotor turns at synchronous speed. If load on the generator suddenly changes, the rotor speed begins to vary. This variation induces voltage in the squirrel cage winding, causing large current to flow, which reacts with the magnetic field of the stator, producing forces which dampen the oscillation of the rotor. Squirrel cage winding is sometimes call damper winding. Damper winding also tends to maintain balanced 3-phase voltages between the lines. Lecture 30 Electro Mechanical System

Main features of the rotor Cylindrical rotors High speed steam turbines are more efficient than low speed turbines. The same is true for high speed synchronous generator. To generate the required frequency, we cannot use less than two poles, and this fixes the highest possible speed. On 60 Hz system it is 3600 rpm. The next lower speed is 1800rpm, corresponding to a 4-pole machine. Steam turbine generator possess either 2 or 4 poles. The rotor of a turbine generator is long, solid steel cylinder, which contain series of longitudinal slots. The high speed of rotation produces a strong centrifugal forces, which produces upper limit to the diameter of the rotor. Lecture 30 Electro Mechanical System