Dynamoelectric machine :
A dynamo is a machine that converts either mechanical energy into electrical energy or electrical energy into mechanical energy. When a dynamo is driven mechanically by a prime mover such as by a diesel engine, steam engine, steam turbine or water turbine and supplies electrical energy, it is called a generator . when the dynamo draws electrical energy from the supply mains and drives mechanical devices such as line shafts and machine tools it is called a motor. In other words It can be
stated that the generator converts mechanical power ωt into the electrical power EI and motor converts electrical power EI into mechanical power ωt
stated that the generator converts mechanical power ωt into the electrical power EI and motor converts electrical power EI into mechanical power ωt
The motoring and generating a
ction is basically a reversible process i.e. the same machine is capable of operating either as a generator or as a motor depending upon the whether the supplied power is mechanical or electrical. An electro mechanical energy conversion device is a link between an electrical and mechanical system and electromechanical energy conversion needs the presence of natural phenomena which interrelate electric and magnetic
fields on one hand and mechanical force and motion on the other .
fields on one hand and mechanical force and motion on the other .
EMF INTRODUCED IN A COIL ROTATING IN A MAGNETIC FIELD
Consider a single turn rectangular coil rotating in counterclockwise direction with angular velocity ω radians per second in a uniform magnetic field of flux density B teslas (Wb/m²)
When the coil is rotated in a uniform magnetic field with uniform angular velocity , an emf is induced in its coils sides. The magnitude of emf induced in the coil depends on
The flux density of the main magnetic field .
The effective length of coil side in the magnetic field .
number of turns on the coil .
Velocity of the coil side is perpendicular to the field.
The coil sides do not always cut the lines of flux in perpendicular direction while rotating in a magnetic field.
Power losses and efficiency
Electromechanical energy conversion is necessarily accompanied by a certain amount of irreversible conversion of energy to heat in the conversion device or machine. These energy losses arise because of circuit
Resistances, because of existance and from mechanical factors. although
They plan essentially no basic role in the energy conversion process , losses are nervertheless important factors in the practical application of machines.
Treatment of machines losses is important for three reasons
Power losses determine the efficiency of machine and appreciably influence its operating cost.
Power losses determine the heating of the machine and hence fix the rating or power output that can be obtained without deterioration of the insulation because or overheating.
Voltage drops are associated with ohmic or electrical losses while current components , like core-loss current, pertains to the iron loss in electrical machines. Obviously , the losses associated with voltage drops or current components must be properly accounted for in a machine representation so that the analysis of the electrical machine can be carried out as desired.
The efficiency of an electrical machines, like that of any other apparatus , is defined as the ratio of useful power output to the input power, the two being measured in the same unit .
Consideration of machine parts in which power losses can occur will show that the following list includes all the possibilities within the machine itself.
1.COPPER OR ELECTRICAL LOSSES: These losses occur in the rotor and stator windings. In synchronous and dc shunt machines , the field copper loss is constant
because such machines are normally operated with constant field current . In series and induction machines, the losses in both windings vary as the square of the line current.
because such machines are normally operated with constant field current . In series and induction machines, the losses in both windings vary as the square of the line current.
2. CORE OR IRON LOSSES: These losses consist of eddy current and hysteresis losses. In synchronous and induction machines, these losses are confined essentially to the stator iron, and in dc machines essentially to the rotor iron, although in both cases a small core loss will be present in the other member Because of small flux variations caused by the slots. the iron in the offending
member is laminated to reduce the eddy-current loss. In all except series machines , variable-speed shunt motors, and to a lesser degree, compound motors, the air-gap flux and hence the core losses are sesibly constant regardless of load.
member is laminated to reduce the eddy-current loss. In all except series machines , variable-speed shunt motors, and to a lesser degree, compound motors, the air-gap flux and hence the core losses are sesibly constant regardless of load.
3.FRICTION AND WINDAGE LOSSES: These losses are constant unless the speed varies appreciably . The sum of the friction and windage and core losses is called the rotational losses.
4. STRAY -LOAD LOSSES: These are additional
hysteresis and eddy-current losses arising from any distortion in flux distribution caused by load currents. These losses are difficult to measure and are ususally about 1percent of the machine output.
hysteresis and eddy-current losses arising from any distortion in flux distribution caused by load currents. These losses are difficult to measure and are ususally about 1percent of the machine output.
ESSENTIAL PARTS OF DC MACHINE:
1> FIELD SYSTEM: It is to provide a uniform magnetic field , within which armature rotates. Electromagnets are preferred in comparision with permanent magnets on account of its greater magnetic effect and its field
strength regulation , which can be achieved by controlling the exciting current.
strength regulation , which can be achieved by controlling the exciting current.
Field magnet consists of four parts
> yoke
> pole cores
> pole shoes
> magnetizing coils .
YOKE is meant to provide mechanical support and a cover for the entire machine and carry the magnetic flux produced by the poles .
POLE CORE is usually of circular section and is used to carry the coils of insulated wires carrying the exciting current.
POLE SHOE acts as a support to the field coils and spreads out the flux in the air gap.
MAGNETIZING OR FIELD COILS are to provide, under the various conditions of operation, the number of AT of excitation required to give the proper flux through the armature to induce the desired pd.
There are several field constructions adopted according to the type of excitation . In shunt field
