Losses in dc machine:
The difference between the power input and power output at steady state conditions is called power loss in watts. The power input is always greater than power output. A dc generator is an electrical machine which converts mechanical energy into electrical energy and a dc motor converts electrical energy into mechanical energy. Both dc generator and dc motor is termed as dc machine. There is no change in the construction of dc motor and construction of dc generator. So their losses i.e, losses in dc generator and losses in dc motor are same so lets term it as losses in dc machine. In accordance with the law of conservation of power is given by
Power input = Power output + Power loss
Power output = Power input - Power loss
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 in dc generator arise because of circuit resistances ( copper or electrical losses) , because of their existence of alternating or fluctuating magnetic fields ( core or iron losses i .e hysteresis loss and eddy current loss) , and from mechanical factors ( friction and windage loss). Although they play essentially no basic role in the energy conversion process, losses are neverthless important factors in the practical application of machines.
Treatment of machine losses is important for three reasons:
- Power losses in dc generator determine the efficiency of dc generator and appreciably influence its operating cost. For example a machine with low efficiency has more power losses and therefore, increased 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 of overheating.
- Voltage drops are associated with ohmic or electrical losses while current components, like core loss components like core loss current, pertains to the iron losses 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 described.
Losses in dc generator are as follows:
1. Copper Losses in dc machine:
Also known as Resistance losses or Electrical losses or I2R losses. These losses occur in the rotor and stator windings.
In addition to I2R loss in dc machine there are brush contact loss at the contacts between the brushes and commutator. Brush contact loss in a dc machine is proportional to its armature current. In synchronous machines 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.
2. Iron losses in dc machine:
Iron losses are also called core losses or magnetic losses. Iron losses consist of eddy current and hysteresis losses.
Eddy current loss:
When the rotor rotates emfs are generated in the rotor iron exactly the same way as they are induced in the rotor conductors. The emfs generated in the solid iron give rise to circulating currents called eddy currents. The power losses due to eddy current is referred to as eddy current loss.
Pe = KeBmaxf2t2V
Where Ke is the constant depending upon the resistance of the core and system of units used.
Bmax is the maximum flux density in wb/m2
f is the frequency of the magnetic reversals in Hz
t is the thickness of lamination in meters.
V is the volume of the core in m3
Hysteresis loss is due to reversal of magnetization of armature core. When the core passes under one pair of poles, it undergoes one complete cycle of magnetic reversal. Hysteresis loss formula is given by
Wh = ηBmax1.6fV watts
Where η is the Steinmetz constant.
Bmax is the maximum flux density in wb/m2 .
f is the frequency of the magnetic reversals in Hz.
V is the volume of the core in m3
In synchronous machines and induction machines, these iron losses are confined essentially to the stator iron, and in dc machines especially 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 airgap flux and hence the core losses are sensibly constant regardless of load.
3. Stray losses in dc machine:
Stray losses are additional hysteresis and eddy current losses araising from any distortion in flux distribution caused by the load currents. Stray losses are difficult to measure. In small machines these losses are neglected but for large machines about 150kw or more they are usually about 1% of the rated output of the machine.
4. Mechanical losses in dc machine:
These losses are constant unless the speed varies appreciably. The sum of the friction and windage and core losses is called the rotational losses.
Brush friction loss and bearing friction loss includes the friction losses in dc machine.
Brush friction loss occurs in machines with brushes such as dc machines, synchronous machines, wound rotor induction machines etc. This loss depends on the brush pressure, coefficient of friction and speed. Bearing friction loss is approximately proportional to speed. This loss depends upon the type of bearings and their lubrication.
The windage loss includes the power required to circulate air through the dc machine and ventilating ducts. Windage loss is approximately proportional to square of the speed.
Mechanical loss can be determined by measuring the input to an unexcited and unloaded machine by running it at rated speed. mechanical losses comprise 10 - 20% of total losses.
The electromagnetic power is associated with the emf generated in the dc machine. The sum of no load rotational loss and stray load loss is sometimes called rotational loss. The rotational loss when added to, or subtracted from electromagnetic power gives respectively the shaft power input to dc generators and shaft power output from dc motors. The total ohmic losses when added to, or subtracted from electromagnetic power gives respectively the total electric power input to dc motor or electrical power output from the dc generator.
- Construction of dc machine with parts.
- Armature reaction in dc generator.
- Armature winding, Lap and wave winding.
- Commutation in dc machine and methods to improve commutation.
- Compensating winding and interpoles.
- Torque equation of dc motor.
- Efficiency and maximum efficiency of dc generator.
- Parallel Operation of DC generator.
- Starting of dc motor
- Three point starter