Correct Answer - Option 3 : Armature copper loss
Armature copper losses:
- Armature copper losses \(= I_a^2{R_a}\)
- Where Ia is armature current and Ra is armature resistance.
- These losses are about 30% of the total full load losses.
- Armature copper losses in a DC generator vary significantly with the load current.
Field copper losses:
- Field copper losses \(= I_{sh}^2{R_{sh}}\)
- Where Ish is field current and Rsh is field resistance.
- These losses are about 25% theoretically, but practically it is constant.
Magnetic losses:
- As the iron core of the armature is rotating in the magnetic field, some losses occur in the core which is called core losses or magnetic losses or iron losses.
- Normally, machines are operated with constant speed, so these losses are almost constant.
- These losses are categorized into Hysteresis losses and Eddy current losses.
Mechanical Losses: The losses associated with mechanical friction of the machine are called mechanical losses. These losses occur due to friction in the moving parts of the machine like bearing, brushes etc. and windage losses.
Windage losses occur due to the air inside the rotating coil of the machine. These losses are usually very small about 15% of full load loss.
Out of all these losses, mechanical losses are the least proportion in DC machines.
Mechanical losses = friction loss of bearings + friction loss at a commutator + windage loss.
Core losses = hysteresis loss + eddy current loss
Stray loss = mechanical loss + core loss
Constant losses = Shunt fled copper losses + stray loss
Constant losses are almost fixed and do not vary with the load current
The losses that occur in a DC Machine are divided into five basic categories.
Electrical or Copper losses (I2R losses):
- The copper losses are the winding losses taking place during the current flowing through the winding. These losses occur due to the resistance in the winding.
- In a DC machine, there are only two windings, armature and field winding.
- Thus copper losses categories in two parts; armature loss & field winding loss. The copper losses are proportional to the square of the current flowing through the winding.
Armature Copper Loss in DC Machine
- Armature copper loss = Ia2Ra
- Where, Ia is armature current and Ra is armature resistance.
- These losses are about 30% of the total full load losses.
Field Winding Copper Loss in DC Machine
- Field winding copper loss = If2Rf
- Where, If is field current and Rf is field resistance
- These losses are about 25% theoretically, but practically it is constant.
Brush losses in DC machine:
- Brush losses are the losses taking place between the commutator and the carbon brushes. It is the power loss at the brush contact point.
- The brush drop depends upon the brush contact voltage drop (VBD) and the armature current Ia. It is given by the equation shown below.
- The voltage drop occurring over a large range of armature currents, across a set of brushes is approximately constant.
- If the value of the brush voltage drop is not given then it is usually assumed to be about 2 volts. Thus, the brush drop loss is taken as 2Ia.
Core Losses or Iron Losses in DC Machine or Magnetic Losses:
- As the iron core of the armature is rotating in the magnetic field, some losses occur in the core which is called core losses.
- Normally, machines are operated with constant speed, so these losses are almost constant. These losses are categorized in two form; Hysteresis loss and Eddy current loss.
Hysteresis Loss in DC Machine
- Hysteresis losses occur in the armature winding due to the reversal of magnetization of the core. When the core of the armature exposed to the magnetic field, it undergoes one complete rotation of magnetic reversal.
- The portion of the armature which is under S-pole, after completing half the electrical revolution, the same piece will be under the N-pole, and the magnetic lines are reversed in order to overturn the magnetism within the core.
- The constant process of magnetic reversal in the armature, consume some amount of energy which is called hysteresis loss. The percentage of loss depends upon the quality and volume of the iron.
Eddy Current Loss in DC Machine
- According to Faraday’s law of electromagnetic induction, when an iron core rotates in the magnetic field, an emf is also induced in the core.
- Similarly, when armature rotates in the magnetic field, the small amount of emf induced in the core which allows the flow of charge in the body due to the conductivity of the core.
- This current is useless for the machine. This loss of current is called eddy current. This loss is almost constant for the DC machines. It could be minimized by selecting the laminated core.
Mechanical Losses in DC Machine:
- The losses associated with mechanical friction of the machine are called mechanical losses.
- These losses occur due to friction in the moving parts of the machine like bearing, brushes etc, and windage losses occur due to the air inside the rotating coil of the machine.
- These losses are usually very small about 15% of full load loss.
Stray Load Losses in DC Machine:
- There are some more losses other than the losses which have been discussed above. These losses are called stray-load losses.
- These miscellaneous losses are due to the short-circuit current in the coil undergoing commutation, distortion of flux due to the armature and many more losses which are difficult to find.
- These losses are difficult to determine. However, they are taken as 1% of the whole load power output.
