Outcomes from this session At the end of this session you should be able to Understand what is meant by the term losses.
Iron Losses There are three types of iron losses Eddy current losses Hysteresis loss Leakage and fringing Metal used as the core of an inductor has a magnet circuit within it. The circulating current induced is called an eddy current. This is produced when there is changing flux as in an a.c. supply. This eddy current when in a material such as iron can cause an increase in temperature and subsequent loss of power. When we look at equipment such as transformer we want the magnetic effect, but not the heating effect that goes with it.
Reducing the Effects of Eddy Currents With transformers we want the magnetic effect but not the heating that goes with it. If we take the core material usually iron or a steel based material, and split it into thin strips called laminations, then stick the strips back together again, with thin layers of insulation between each strip, the eddy current are dramatically reduced. Eddy current loss is proportional to the square of the thickness of the laminations to a certain value. Eddy current losses are also reduced by the use of steel.
Hysteresis Loss This type of loss depends on the type of material of the core, as well as the frequency of the supply. Hysteresis loss occurs whenever the magnetic state within the material is changed and is directly related to frequency. Hysteresis losses are increased by the increase in frequency. If the frequency increases by just a small amount the loss, given off as heat, will also increase. When a coil has an alternating supply provided to it, the core material has a changing magnetic flux set up within it. Because the supply is alternating then the magnetic field is also being forced to change. There is a delay in the magnetic field magnetising in one direction and then back in the other direction. It is this lag which brings about the loss.
Initial Build up of a Magnetic Field Here the supply has just been turned on and the first peak of the a.c. supply has been reached. The magnetic core has been magnetised, but not in a nice straight line, it increases rapidly before settling down. The material is now saturated which means it cannot become any more magnetised than it already is.
Remanence The supply current to the coil has started to fall towards zero. When the current in the coil reaches zero there is still some magnetic flux density. This is called Remanence.
Coercivity The current is now moving into the negative half cycle. It is necessary to put in more current to force the magnetic material to a point where it is no longer magnetic. This point is called Coercivity.
Maximum Magnetisation The current has now moved to its maximum peak on the negative half-cycle, again it has reached its saturation level.
Complete Cycle Here is the total loop, it is symmetrical and will continue as long as the supply is switched on. The area of the loop is the measure of the loss, the larger the area the larger the loss. Eddy current and Hysteresis losses are known as iron losses. Copper loss is related to the heat given off when any conductor has current flowing through it.
Leakage and Fringe Losses The third and smallest type of iron loss is called leakage and fringing. Fringing doesn t occur in toroids (doughnut shapes) but where there are edges. This means that in all power transformer there will be some loss from fringing.
Leakage Leakage occurs when some magnetic flux fails to be focussed in the core material and is created in the surrounding air. To reduce this loss the two sets of windings are laid on top of each other.
Copper Losses Copper losses are never affected by frequency. They are solely dependant on the resistance of windings etc. Many losses cannot be changed. They are part of the make up of the equipment. The final type of loss is not caused by the magnetic circuit, but by the flow of the current in both the primary and secondary windings. This loss is related to the heat given off when any conductor has current flowing through it. Also called the (I 2 R) loss.