In synchronous motor out of the following losses, which one will have the highest proportion?
Right Answer is:
Iron losses have a high proportion of losses occurring in the synchronous machine.
Losses in synchronous Machine
A synchronous machine is used to convert mechanical energy into electrical energy or vice-versa. While doing so, the whole of input energy does not appear at the output but a part of it is lost in the form of heat in the surroundings. This wasted energy is called losses in the machine. These losses affect the efficiency of the machine.
The various losses occurring in a synchronous machine can be sub-divided as
- Copper losses.
- Iron losses.
- Mechanical losses
- Stray losses
1. Copper losses: The various windings of the synchronous machine such as armature and field winding are made of copper and have some resistance. When current flows through them, there will be power loss proportional to the square of their respective currents. These power losses are called copper losses.
In general, the various copper losses in a synchronous machine are:
(i) Armature copper loss = I2R
(ii) Field winding copper loss = I2fRf
(iii) Brush contact loss = I2Rb
The brush contact loss is generally included in the field winding copper losses.
2. Iron losses: The losses which occur in the iron parts of the DC machine are called iron losses or core losses or magnetic losses. Iron losses have a high proportion of losses occurring in the synchronous machine. Iron core losses in electrical induction machines operate with sinusoidal power supplies account for 15-25% of the total machine losses which are one of the major losses in electrical machines. These losses consist of the following:
(i) Hysteresis loss: Whenever a magnetic material is subjected to reversal of magnetic flux, this loss occurs. It is due to the retentivity (a property) of the magnetic material. The Hysteresis losses are proportional to the frequency and the maximum flux density Bm in the air gap.
The loss is basically due to the reversal of the magnetization of the armature core. It occurs in the armature (stator core). To minimize this loss, the armature core is made of silicon steel which has low hysteresis constant.
(ii) Eddy current loss: When flux linking with the magnetic material changes (or flux is cut by the magnetic material) an emf is induced in it which circulates eddy currents through it. These eddy currents produce eddy current loss in the form of heat.The eddy current losses are proportional to the square of the electrical frequency. The electrical steel used in the stator and rotor of induction machines
The major part of this loss occurs in the armature core. To minimize this loss, the armature core is laminated into thin sheets (0·3 to 0·5 mm) since this loss is directly proportional to the square of the thickness of the laminations.
3. Mechanical losses: As the field system of a synchronous machine is a rotating part, some power is required to overcome:
(i) Air friction of rotating field system (windage loss).
(ii) Friction at the bearing and friction between brushes and slip rings (friction loss).
These losses are known as mechanical losses. To reduce these losses proper lubrication is done at the bearings.
4. Stray losses:- In addition to the iron losses, the core losses are also caused by the distortion of the magnetic field under load conditions and losses in the insulation of armature and field winding, these losses are called stray lasses. These losses are also included while determining the efficiency of synchronous machines.