To discuss about losses in electrical machine at first the term “electrical machine” should be cleared. As electrical machine in this case we can understand those which use magnetic characteristics such as electric motor, generator, transformer etc. Losses in electrical machine can be divided in to two parts. 1. Losses due to magnetic characteristics. 2. Losses due to rotation, friction etc. In this post we shall discuss only losses due to magnetic characteristics.
To understand the losses in an electrical machine first we have to consider its physical structure. There are three considerable parts in an electrical machine. Firstly the electrical coils or bar and secondly the magnetic material structurethat holds the electrical coils. This magnetic material structure has its own advantages. And thirdly the physical structure of the machine that provides mechanical strength plus interior shape that can modify the magnetic flux characteristics. For all these three parts current flows only through the electrical coils. There is generally two parts of electrical coils called as rotor & stator.
The electrical coils in the armature are placed in some precise geometric location which is carefully calculated by engineers to get certain electrical, mechanical characteristics. These coils are placed in some structure which is made by magnetic materials. These magnetic materials provide support and clear path for magnetic flux linkage and have their own advantages.
Under normal working condition magnetic material structure also subjected to the varying magnetic flux. Thus a certain working voltage is induced in the electrical coil as per Faraday’s laws of electromagnetic induction theory. With this voltage induced current flows in the structure, which is called the eddy current.
This eddy current circulates & flows in the magnetic materials and opposes the change in magnetic flux density in the magnetic material. (The change in magnetic flux density is required to be smooth). To counter act this excitation current is to be increased.
Hysteresis loss is due to the residual magnetism of the magnetic material. Due to magnetic flux variation the magnetic flux cycle goes to one direction and then reverse again to opposite direction. That is it starts from zero goes to peak and then become zero again. And then in the next cycle it goes from zero to peak in the reverse direction and comes to zero again. This continues as a normal operation.
Now the magnetic material also affected by the
change in magnetic flux. With the change in magnetic flux there is some
magnetism in the magnetic material. If the magnetic flux is zero there is zero
magnetism in the magnetic material; the magnetic flux goes to peak also the
magnetism goes to peak; then the magnetic flux again becomes zero but some
magnetism left in the magnetic material.
These residual magnetism causes heat generation
in the magnetic material.