Transformer on load and no load

Transformer on Load:

We know that when the transformer is loaded, the current I2 starts to flow through the secondary winding of the transformer to the load producing the potential difference of V2. The main thing is that the output power of the transformer depends on the power factor and it is totally determined by the loaded nature.

We know that                                    

W = V2I2cosФ


cosФ = Power factor

The magnitude and phase of I2 are also determined by the loaded nature. When the load is the resistive one, I2 is in phase with the V2 which means that the resistive loads have a power factor of 1. When the load is inductive, I2 lags the V2. When the load is capacitive, I2 leads the V2.

There is one thing named magnetomotive force (mmf) which depends on the output current and turns of the transformer.

We can say that mmf = N2I2

Therefore, there exist a secondary magnetomotive force N2I2 due to which secondary winding sets up its own flux Ф2. This flux opposes the main flux Ф of the transformer produced in the core due to the magnetizing component of the no-load current Io of the transformer due to which magnetomotive force N2I2 is called demagnetizing ampere-turns.

The flux produced due to the magnetomotive force Ф2 momentarily reduces the main flux Ф produced in the transformer due to which primary winding induced emf also E1 reduces. Hence, the vector difference V1 – E1 increases due to which the primary winding of the transformer draws more current from the AC power supply. This additional current drawn by the primary winding of the transformer is due to the load connected to the transformer hence called load component of the primary current denoted as I2. This current I2 is in antiphase with the I2­ and this current I2 sets up its own flux Ф2 which opposes the flux Ф2 produced due to the magnetomotive force in the secondary winding of the transformer due to the load setting up on the output. And this flux Ф2 helps the main flux Ф in the transformer. This flux Ф2 neutralizes the flux Ф2 produced by the current I2 and the ampere-turns or magnetomotive force due to flux Ф2, N1I2balance the ampere-turns or magnetomotive force due to flux Ф2 which is N2I2. Due to which the net flux in the core of the transformer is gain maintained on the constant level.

The main thing to remember is that when the transformer is on any load condition which is from load to no-load, the flux in the core is practically constant. The load component current I2 neutralizes the changes in the load. As practically the transformer core flux is constant due to which core loss is constant for all the loads, the transformer is called constant flux machine.

As ampere-turns are balanced, we can write,

N2I2 = N1I2

I2 = (N2/N1)*I2


I2 = KI2

Thus, when the transformer is loaded, the primary winding current I1 has two components:

  1. The no-load current Io which lags V1 by angle Фo. It has two components Im and Ic which are the magnetizing and active components, respectively.
  2. The load component of primary winding current I2 which is in antiphase with the I2 current and phase is determined with the nature of the load.

Hence, the primary current is the vector sum of the Io and I2.


I1 = Io + I2

The verification of the ratios of the current can be determined as follow:

As no-load current Io is very small, so we can neglect Io and we can write

I1 = I2

Balancing the ampere-turns,

N1I2 = N1I1 = N2I2


N2/N1 = I1/I2 = K

Under full load conditions when Io is very small compared to the full load currents, the ratio of primary winding current I1 to the secondary winding current I2 is constant.

Transformer on no Load:

Actually, in the transformer, the iron core causes hysteresis losses and eddy currents losses as the transformer are subjected to the alternating flux. While the transformer is being designed the efforts are made to put to keep these losses minimum by,

  • Using high-grade material like silicon steel to reduce hysteresis losses.
  • Manufacturing the transformer core in the form of laminations or stacks of thin laminations to reduce or minimize the eddy currents losses.

Apart from this, there are iron losses in the transformer. And primary winding of the transformer has got certain resistance which subjects to small primary copper loss. Thus, the primary winding current under the no-load condition of the transformer has to supply the core losses i.e. hysteresis loss and eddy current loss and a small amount of primary copper loss. This current is denoted by Io known to be the no-load current.

Now the no-load input current Io has two components:

  1. A purely reactive component Im which known as the magnetizing component of no-load current which is required to produce the flux in the transformer. It is also known as the wattles component.
  2. An active component Ic which supplies total losses under the no-load condition of the transformer is known to be the power component of the no-load current. This is also called watchful or core loss component of I­o.

The total no-load current Io is the vector addition of Im and Ic.


Io = Im + Ic

In the transformer, due to winding resistance, no-load current Io is no longer at 90o with respect to V1 but it lags V1 by angle Фo which is less than 90o due to which cosФo is called no-load power factor of the transformer.

The magnetizing component Im lagging V1 is

Im = IosinФo

And the core loss component which is in phase with V1 is

Ic = IocosФo

The magnitude of the no-load current is

Io = (Im2 + Ic2)1/2


Фo = no-load primary power factor angle

The total input power on no-load is denoted as Wo and is given by,

Wo = V1IocosФo = V1Ic

The main thing to notice here is that the current Io is very small which is about 3-5% of the full rated current of the transformer on load. Hence primary copper loss is negligibly small due to which it is called the core loss component of Io. Hence, input power Wo on no-load always represents the core losses in the transformer and are constant for all the loads.  

Wo = Pi = V1IocosФo = V1Ic = Core losses    

Related Topics;

  1. Equivalent circuit of the transformer
  2. Characteristics of an ideal transformer
  3. Efficiency and losses of a transformer
  4. 3-phase transformer
  5. Open Circuit and Short Circuit Test of Transformer