Electrical Engineering ⇒ Topic : Transformer at No Load

Luis
 
In the discussion in the previous sections, we have assumed the transformer to be an ideal device, in which there are no core or copper losses. However,under practical conditions these losses are not negligible and have to be taken into consideration When a transformer is on noload, the primary input current has to supply the iron losses in the core as well as a very small amount of copper loss in the primary winding. Hence, the noload primary input current will have two components as shown in the phasor diagram of Figure (a).
Figure (a) Phasor Diagram of a Transformer on Noload In the phasor diagram, the quantity that is common to both the primary and secondary windings, namely the flux, (Φ_{m})., is taken as the reference phasor. The induced emfs E_{1} and E_{2} in the primary and secondary windings lag the flux (Φ_{m}). by 90°. The primary applied voltage V_{1} is almost equal in magnitude but opposite in polarity to E_{1} and is therefore shown in the phasor diagram as leading the flux (Φ_{m}). by 90° The noload primary current I_{0} has two components (a) the active or power component I_{C},. which is inphase with V1. This supplies the hysteresis and eddy current lossesi n the core and a small quantity of copper losses in the primary winding and (b) a reactive or magnetising component, I_{m}. which is inphase with the flux Φ_{m}The two currents I_{c},. and I_{m}. (see Fig. (a)) can be expressed as
hence the noload current, .................. (a) power factor on noload ...................... (b) The following points should be noted. The component I_{c}, is usually very small when compared with the component I_{m}., such that the noload power factor is usually very small. The noload primary current, I_{0} is also low when compared to the fullload primary current, Since /_{0} is very small, the primary copper loss on noload is also very small. this means that the noload primary input is practically equal to the iron losses in the transformer core.  
 
Gaurav
 
TRANSFORMER AT NO LOAD When the secondary winding of the transformer is open circuit, i.e., no load connected to it,transformer is said to be at no load condition. In this case, secondary current is zero. When an alternating voltage is applied to the primary, a small current I_{0 }flows in the primary. Here I_{0} is called no load primary current which is not wholly reactive. The primary input current I_{0} at no load condition provides (i) iron losses in the core, i.e., hysteresis and eddy current loss and (ii) a very small amount of copper loss in the primary. Under no load condition, there is no copper loss in secondary. In this case, the no load primary current is not exactly 90^{0} behind the primary voltage V_{1}, but lags behind it by an angle Φ_{0 }< 900. Therefore, input power at no load is given by ........... (1) where cos Φ_{0} is primary power factor under no load condition. The phasor diagram for no load condition is shown in Figure (a).From Figure (a), I_{0} has two components, i.e., I_{W} and I_{μ}. Here I_{W} = I_{O} cos Φ_{0} is in phase with V_{1} and it is known as active or working or iron loss component. It mainly supplies the iron loss plus small quantity of primary copper loss. The second component I_{μ}, = I_{0} sin Φ_{0} is in quadrature with V_{1 }and it is known as magnetizing current. It maintains the alternating flux in the core. The equivalent circuit is shown in Figure (b).
Figure (a) Phasor diagram at no load. Figure (b) Equivalent circuit In Figure (b), R_{o }represents the core loss. The current component I_{W} passes through R_{o} and it supplies the core loss. X_{0} is the inductive reactance. It takes the reactive current equal to the magnetizing component I_{μ}, of actual transformer  
 
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