Losses and Efficiency of DC Machines

 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:

• Basic DC machine is a kind of commutator. A commutator is a machine which converts AC into DC.
• Electrical generator converts mechanical energy into the electrical energy.
• DC machine have two main parts: Stationary parts and the rotating parts.
• Other parts of the DC machine include:
  • Frame
  • Commutating pole
  • Armature
  • Field posts
  • Commutator
  • Brush Gear
  • Armature windings
• The armature windings are divided into close coil winding and open coil winding. The close coil winding is further classified into Ring Winding and drum winding. The drum winding are of two types including lap winding and Wave winding.
• Lap winding is ideal for low voltage but high current generators. In this type of winding the end of each coil is connected to the beginning of the next coil.
• Wave winding is ideal for high voltage and low current generators. In this type of winding the start of a coil is away from the first one.
• The rated duty of the motor is characterized by certain values that are given on the machine which include: termed rated values and rating plate.
• m.f of the generator is

E_E= (p? Z N)/ 60a

E_E= 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

• On the basis of methods of excitation, DC generator are classified as: Separately excited generators and self excited generators
• The latter type of generators are of the following types on the basis of the field winding:
  • Shunt Wound generators
  • Series Wound generators
  • Compound wound generators

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 loss affects the operating cost of the machines
• It also determine the heating of the machine so that the power output can be determined
• Drop in the voltage should be lined with the supplying loses and it should be recorded properly for representation of the machine.

 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:

• Rotational losses
• Electrical losses

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.

P_h=K_hB^xfV

V= volume of iron

K_h= 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 P_s is mentioned as:

P_s= K_ct²B²f²V

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.