AC MACHINES-1 (66761) Theory

The no load voltage is the terminal voltage when zero current is drawn from the supply, that is, the open circuit terminal voltage. Power supply performance is measured in terms of percent voltage regulation, which indicates its ability to maintain a constant voltage.

The no-load current is small because the primary links with its own magnetic field and electromagnetic theory explains that this will induce a back-emf to oppose the voltage applied externally to the coil. The open circuit transformer therefore acts as a highly inductive choke with a power factor of some 0.15 lagging.

This current is responsible for supplying the iron losses (hysteresis and eddy current losses) in the core and a very small amount of copper losses in the primary winding. The angle of lag depends upon the losses in the transformer. The power factor is very low and varies from 0.1 to 0.15.

The no-load current consists of two components:

• Reactive or magnetizing component I_m
  (It is in quadrature with the applied voltage V₁. It produces flux in the core and does not consume any power).

• Active or power component I_w, also know as a working component
  (It is in phase with the applied voltage V₁. It supplies the iron losses and a small amount of primary copper loss).

The following steps are given below to draw the phasor diagram:

1. The function of the magnetizing component is to produce the magnetizing flux, and thus, it will be in phase with the flux.
2. Induced emf in the primary and the secondary winding lags the flux ϕ by 90 degrees.
3. The primary copper loss is neglected, and secondary current losses are zero as
   I₂ = 0.
   Therefore, the current I₀ lags behind the voltage vector V₁ by an angle ϕ₀ called the no-load power factor angle and is shown in the phasor diagram above.
4. The applied voltage V₁ is drawn equal and opposite to the induced emf E₁ because the difference between the two, at no load, is negligible.
5. Active component I_w is drawn in phase with the applied voltage V₁.
6. The phasor sum of magnetizing current I_m and the working current I_w gives the no-load current I₀.From the phasor diagram drawn above, the following conclusions are made

This is all about transformer in no-load condition.