Losses in a DC Machine

[vc_row][vc_column width=”2/3″][vc_column_text]Motors convert electrical power (input power) into mechanical power (output power) while generators convert mechanical power (input power) into electrical power (output power). Whole of the input power cannot be converted into the output power in a practical machine due to various losses that take place within the machine.

Efficiency η being the ratio of output power to input power, is always less than 1 (or 100 %). Designer of course will try to make η as large as possible. Order of efficiency of rotating d.c machine is about 80 % to 85 %. It is therefore important to identify the losses which make efficiency poor.

Major losses

Take the case of a loaded DC motor.

  • There will be copper losses in armature and field circuit.
  • The armature copper loss is variable and depends upon degree of loading of the machine.

For a shunt machine,

  • The field copper loss will be constant if field resistance is not varied.
  • Recall that rotor body is made of iron with slots in which armature conductors are placed.
  • Therefore, when armature rotates in presence of field produced by stator field coil, eddy current and hysteresis losses are bound to occur on the rotor body made of iron.
  • The sum of eddy current and hysteresis losses is called the core loss or iron loss.
  • To reduce core loss, circular varnished and slotted laminations or stamping are used to fabricate the armature. The value of the core loss will depend on the strength of the field and the armature speed.
  • Apart from these there will be power loss due to friction occurring at the bearing & shaft and air friction (windage loss) due to rotation of the armature.

To summarize following major losses occur in a d.c machine.

  1. Field copper loss:
    1. It is power loss in the field circuit and equal to 
    2. During the course of loading if field circuit resistance is not varied, field copper loss remains constant.
  1. Armature copper loss:
    1. It is power loss in the armature circuit and equal to .
    2. Since the value of armature current is decided by the load, armature copper loss becomes a function of time.
  2. Core loss:
    1. It is the sum of eddy current and hysteresis loss and occurs mainly in the rotor iron parts of armature.
    2. With constant field current and if speed does not vary much with loading, core loss may be assumed to be constant.
  3. Mechanical loss:
    1. It is the sum of bearing friction loss and the windage loss (friction loss due to armature rotation in air).
    2. For practically constant speed operation, this loss too, may be assumed to be constant.

Apart from the major losses as enumerated above there may be a small amount loss called stray loss occurs in a machine. Stray losses are difficult to account.

Power flow diagram of a d.c motor is shown in figure 1.

Figure 1. Power flow diagram of a DC motor

  • A portion of the input power is consumed by the field circuit as field copper loss.
  • The remaining power is the power which goes to the armature; a portion of which is lost as core loss in the armature core and armature copper loss.
  • Remaining power is the gross mechanical power developed of which a portion will be lost as friction and remaining power will be the net mechanical power developed. Obviously efficiency of the motor will be given by:

Similar power flow diagram of a d.c generator can be drawn to show various losses and input, output power (figure 40.2).

Figure 2. Power flow diagram of a DC generator

It is important to note that the name plate kW (or hp) rating of a d.c machine always corresponds to the net output at rated condition for both generator and motor.[/vc_column_text][/vc_column][vc_column width=”1/3″][/vc_column][/vc_row][vc_row][vc_column width=”2/3″][vc_column_text]AUTHORS
1.Bunty B. Bommera
2.Dakshata U. Kamble[/vc_column_text][/vc_column][vc_column width=”1/3″][/vc_column][/vc_row]

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