Electrical Engineering ⇒ Topic : Transformer at No Load
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 no-load, 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 no-load primary input current will have two components as shown in the phasor diagram of Figure (a).
Figure (a) Phasor Diagram of a Transformer on No-load
In the phasor diagram, the quantity that is common to both the primary and secondary windings, namely the flux, (Φm)., is taken as the reference phasor. The induced emfs E1 and E2 in the primary and secondary windings lag the flux (Φm). by 90°. The primary applied voltage V1 is almost equal in magnitude but opposite in polarity to E1 and is therefore shown in the phasor diagram as leading the flux (Φm). by 90°
The no-load primary current I0 has two components (a) the active or power component IC,. which is in-phase 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, Im. which is in-phase with the flux ΦmThe two currents Ic,. and Im. (see Fig. (a)) can be expressed as
hence the no-load current, .................. (a)
power factor on no-load ...................... (b)
The following points should be noted.
The component Ic, is usually very small when compared with the component Im., such that the no-load power factor is usually very small.
The no-load primary current, I0 is also low when compared to the fullload primary current,
Since /0 is very small, the primary copper loss on no-load is also very small. this means that the no-load primary input is practically equal to the iron losses in the transformer core.
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 I0 flows in the primary. Here I0 is called no load primary current which is not wholly reactive. The primary input current I0 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 900 behind the primary voltage V1, but lags behind it by an angle Φ0 < 900. Therefore, input power at no load is given by
where cos Φ0 is primary power factor under no load condition. The phasor diagram for no load condition is shown in Figure (a).From Figure (a), I0 has two components, i.e., IW and Iμ. Here IW = IO cos Φ0 is in phase with V1 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 Iμ, = I0 sin Φ0 is in quadrature with V1 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), Ro represents the core loss. The current component IW passes through Ro and it supplies the core loss. X0 is the inductive reactance. It takes the reactive current equal to the magnetizing component Iμ, of actual transformer
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