ET 301 ELECTRICAL MACHINE TOPIC 1 DC GENERATOR
CONSTRUCTION & PRINSIPLE DC MACHINE CONVERT GENERATOR MECHANICAL ENERGY ELECTRICAL ENERGY CONVERT MOTOR ELECTRICAL ENERGY MECHANICAL ENERGY
• DC generators are DC machines used as generation • this unit will discuss about 1. mechanism of DC generators, 2. method of excitation and 3. the techniques used in self-excitation CONSTRUCTION & PRINSIPLE DC MACHINE
Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. It underline the operation of generators, transformers, induction motors and solenoids. 1.1.1. Faraday’s Law DC MACHINE
• Michael Faraday discovered that an electric potential can be established between the ends of a conductor in the following three ways: i. By a conductor moving or cutting across a stationary magnetic field. (DC Generator) ii. By a moving magnetic field cutting across a stationary conductor. (AC Generator) iii. By a change in the number of magnetic lines enclosed by a stationary loop or coil. (Transformer) • This law emphasizes rate of change or rate or flux cutting rather than density or extent of magnetic field. • A voltage is induced any time there is relative motion between a magnetic field and a conductor. 1.1.1. Faraday’s Law DC MACHINE
In mathematical form Faraday’s Law : For the special case a coil of wire composed of N (total number of conductors) loops with the same area the equation becomes Faraday’s law, together with Ampere’s law and ohm’s law is Lens’s law. The EMF induced in an electric circuit always acts is such a direction that the current it drives around the circuit opposes the change in magnetic flux which produces the EMF. 1.1.1. Faraday’s Law DC MACHINE
determines the directions of magnetic force, conventional current and the magnetic field Lenz's Law states that the induced current always flows in the direction that opposes the change in magnetic flux. 1.1.2 Right & Lift Hand Rule DC MACHINE
1.1.2 Right & Lift Hand Rule DC MACHINE
Using your Right hand determines the direction of the magnetic field around a current-carrying wire. B = the direction of the magnetic field, thumb = the direction of the conventional current. 1.1.2 Right & Lift Hand Rule DC MACHINE
The Right-Hand Rules give only the direction of the magnetic field. In order to determine the strength of a magnetic field , some useful mathematical equations can be applied 1.1.2 Right & Lift Hand Rule DC MACHINE
1.1.2 Right & Lift Hand Rule DC MACHINE
1.1.4 MAIN PART DC MACHINE
main part of mechanical construction of DC generators such as the yoke, the armature, the commutator, the field pole and the brushes carbon. Figure 1.1: Cutaway view of DC generator 1.1.4 MAIN PART DC MACHINE
Figure 1.2: Cross section of a generator DC motor mechanism in different angle as example in Figure 1.2. It is show us the cross section angle. 1.1.4 MAIN PART DC MACHINE
1.1.4 MAIN PART DC MACHINE
1.1.4 MAIN PART DC MACHINE
FRAME/YOKE 1.1.4 MAIN PART DC MACHINE
•protect part inside machine •supports the field, poles mechanically & provides the necessary magnetic path between the pole •can be solid or laminated •is a circular steel ring. In many DC machines, the yoke also serves as the frame. 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE i. FRAME/YOKE
1.1.4 MAIN PART MAGNETIC FIELD DC MACHINE
- produces the magnetic flux in the machine Divided 2 field@ core Solid steel or stacked laminations coils@ windings copper •field coils are those windings, which are located on the poles and set up the magnetic fields in the machine. •They also usually consist of copper wire are insulated from the poles. 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE ii. MAGNETIC FIELD
•The field coils may be either shunt windings (in parallel with the armature winding) or series windings (in series with the armature winding) or a combination of both •The number of poles depends upon the physical size of the machine; the bigger it is, the more poles it will have. 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE
1.1.4 MAIN PART BRUSH CARBON DC MACHINE
a. Carbon brush and ultraflexible copper lead b. Brush holder and spring to exert pressure 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE
• Brushes conduct the current from the commentator to the external circuit. There are many types of brushes. • made of carbon because it has good electrical conductivity and its softness does not score the commentator • The brush pressure is set by means of adjustable springs 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE iii. BRUSH CARBON
• Each brush usually has a flexible copper shunt or pigtail, which extends to the lead wires. • A brush holder is usually a metal box that is rectangular in shape. The brush holder has a spring that holds the brush in contact with the commentator. • Often, the entire brush assembly is insulated from the frame & is made movable as a unit about the commentator to allow for adjustment. 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE
1.1.4 MAIN PART ARMATURE DC MACHINE
-rotating part to cutting the magnetic flux Divided 2 core@ stack steel laminations coils@ windings Lap Winding Wave Winding 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE iv. ARMATURE
Armature lamination Armature core Assembled armature The laminations are welded, riveted, bolted or bonded together 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE
armature winding usually consists of copper wire, either round or rectangular and is insulated from the armature stack. 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE
Lap Winding Lap Winding connected to commutator bars Simplex Lap Winding, 1.1.6 ARMATURE WINDING DC MACHINE
Simplex Lap Winding, Circular Form Number of current path (a) for lap windings: a = mP Where: m: plex of the windings P: number of poles on the machine 1.1.6 ARMATURE WINDING DC MACHINE
Wave Winding connected to commutator bars Simplex Wave Winding, Development Form Wave Winding 1.1.6 ARMATURE WINDING DC MACHINE
Simplex Wave Winding, Circular Form Number of current path (a) for wave winding: a = 2m Where: m: plex of the windings P: number of poles on the machine 1.1.6 ARMATURE WINDING DC MACHINE
1.1.4 MAIN PART COMMUTATOR DC MACHINE
• the mechanical rectifier, • changes the AC voltage of the rotating conductors to DC voltage. • It consists of a number of segments normally equal to the number of slots. 1.1.5 FUNCTION OF COMPONENTS IN MAIN PART DC MACHINE v. COMMUTATOR
• Excitations in DC generator can be divided into two major types; i. separately excited generator (separate excitation) ii. self-excited generator(Self-independent excitation) 1.1.9 Excitation method DC MACHINE
• is a generator whose field current is supplied by a separate external DC voltage source. • The equivalent circuit is shown in Figure below. • DC source connected to terminals a and b causes an exciting current Ix to flow. The internal generated voltage is Eo appears between brush R terminals x & y. A IA IL EO IX LX RX a b + _ x y load Armature current Ia=IL Terminal Voltage V=Eg-IaRa Electric power develop = EgIa Power delivered to load = EgIa-Ia 2Ra = Ia(Eg-IaRa)= VIa 1.1.9 Excitation method DC MACHINE
•There are three basic types of self-excited generator such as i. series generator, ii. shunt generator iii. compound generator • In this part, we will focus a shunt generator an example of the self-excited generator 1.1.10 Draw-excited machine DC MACHINE
THE BASIC TYPE OF SELF-EXCITATION CIRCUIT series field armature load series Machine shunt field armature load shunt Machine load armature series field shunt field series field Shunt field armature load Sort field compound Machine long field compound Machine 1.1.10 Draw-excited machine DC MACHINE
Series Machine Shunt Machine Compound Machine A series DC Machine is a generator whose field is connected in series with its armature A shunt generator is a machine whose shunt-field winding is connected in a parallel with the armature terminals A compound Machine is a DC generator with both series and shunt fields series field armature load shunt field armature load series field shunt field armature load Sort field compound 1.1.11 Determine the self-excitation circuit DC MACHINE
The internal generated voltage equations of real DC generator • The voltage out of armature of a real machine is equal to the number of conductor per current path times the voltage on each conductor. So the internal generated voltage in the machine can be expressed as 1.2.1 state voltage & terminal DC MACHINE
• When the machine is operating as a generator, the EMF (E) is MORE than the applied voltage (V), the armature current is biggest than full load current and shunt current. where: V: Applied voltage E: Back EMF Ia : Current armature Ra : Resistance armature a Ra V E I 1.2.1 state voltage & terminal DC MACHINE
a NZ P E 60 E: Generated voltage (V) N: Speed (rpm) Z: Total number of conductors : Flux of a pole P: Number of pole a: number of current path Number of current path (a) for lap windings: a = mP for wave winding: a = 2m for frog-leg winding: a = 2Pm Where: m: plex of the windings P: number of poles on the machine 1.2.1 stated voltage & terminal DC MACHINE
Total number of conductors (Z) Z = 2CNC Where: C is number of coil on the armature NC is number of turns of wire 1.2.1 state voltage & terminal DC MACHINE
Series Generator Shunt Generator series field armature load shunt field armature load Armature current(Ia)= IL Terminal Voltage (V) = Eg-Ia(Ra+Rs) Power developed in armature = EgIa Power delivered to load = EgIa-Ia 2 (Ra +Rs) = Ia((Eg-Ia(Ra-Rs)) = VIa @ VIL Shunt current(Ish)=V / Rsh Armature current(Ia)=IL+Ish Terminal Voltage = V=Eg-Ia(Ra) Power developed in armature = EgIa Power delivered to load = VIL 1.2.2 determine generated voltage & terminal DC MACHINE
Compound Generator –short shunt Compound Generator –long shunt series field shunt field armature load load armature series field shunt field 1.2.2 determine generated voltage & terminal DC MACHINE = − −
Copper loss i. armature Cu-loss = Ia2Ra ii. Field cu-loss = for shunt generator = Ish2Rsh = for series generator = Ise2Rse Field cu-loss is also constant for compound generator This loss is about 20%-30% of full load losses iii. The loss is due to brush contact is generally included in armature copper loss 1.2.3 type of losses DC MACHINE
iron loss Iron losses are constant for shunt and compound generator, as their field current are approximately constant. This loss is about 20%-30% of full-load loss Wind & friction loss i. Air friction or windage loss of rotating armature ii. Friction loss at bearings and commutator. These losses are about 10%-20% of full load losses. 1.2.3 type of losses DC MACHINE
voltage regulation The degree of change in armature terminal voltage due to application of load is know as voltage regulation. If there is little change in voltage from no load to full load, the generator is to have good voltage regulation % voltage regulation = x 100% 1.2.3 characteristics og DC generator DC MACHINE
Speed Regulation Speed regulation is the change in speed with the change in load torque, other conditions being constant. A generator has good regulation if the change between the no load speed and full load speed is small % Speed Regulation = x 100% SFL SNL SFL 1.2.3 characteristics og DC generator DC MACHINE