The efficiency of a motor increases with the increasing load of the current. The efficiency gets highest when the load current becomes equal to the value and then decreases.
Efficiency curve: It is the curve which is formed by plotting the efficiency measure of a machine with different power output. In this way, the efficiency measures can be noted from the zero to the maximum value. This graph is a typical curve.
Main highlights include:
EE= (p? Z N)/ 60a
EE= Emf generated
P=Number of poles
?= Flux per pole
Z= Total number of conductors
N= Rotational speed of armature
a= Number of parallel path in armature
9.1 Introduction
It is the fact that the output is always less than the input because there is always a loss of energy in the form of heat. The loss of heat increases the machine’s temperature and the surrounding air until the temperatures are obtained when the heat can be radiated at the faster pace as it was generate. The loss of heat is depended on the load. Thus the maximum permissible load can be determined if the maximum temperature is known. This raise in temperature risks to the insulation of the windings.
Machine loss is important due to the following reasons:
The efficiency of the machine is given by the relation,
Efficiency= Output/ input
It can also be written as Efficiency= (Input- Loss)/ input
9.2 Losses
In a DC machines the losses are classified as:
The rotational loss is further classified as core loss and the mechanical loss. The core losses are caused due to either Hysteresis or Eddy currents. Frictional losses are caused due to Bearings, brushes and Windage. Core loss and mechanical losses are given to the mechanical power that is developed by the machine. The core and mechanical loses are constant regardless of the input or output of the motor and the generator or motor runs at a particular while generating a specific voltage.
The electrical losses are caused due o the electrical power which are delivered to or generated by the machine.
Iron Losses: the iron losses are the function of flux as well as speed.
Hysteresis loss: it is the measure of electrical energy which is required for overcoming the retentivity of iron in the path of magnetic flux.
Ph=KhBxfV
V= volume of iron
Kh= constant of grade iron
B= flux density
F=frequency of reversal of flux
Eddy current losses: this type of losses occur in all types of conducting materials along with the dynamo iron and the flux path of rotation of the magnetic field. The eddy current loss Ps is mentioned as:
Ps= Kct2B2f2V
K= Eddy current constant
t=thickness of pole core and armature lamination
B=flux density
F=frequency of reversal of flux
V= volume of iron
Mechanical losses: There are several frictional forces which are needed to overcome. Each of the frictional force requires continuous energy and cause heating in the parts of the moving machine the friction loss is in the machine bearings, on the surface of the commuters and in the armature core.
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