Q:Explian the various features of an ideal transformer on no load ?

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Q:Explain the various features of an ideal transformer .

**Ideal Transformer **

**Ideal transformer on no load: **An** ideal transformer** is one which has no losses i.e. its windings has no ohmic resistance, there is no magnetic leakage and hence it has no I^{2}R and core losses. In other words an **ideal transformer** consists of two purely inductive coils wound on a loss – free core.

Consider an** ideal transformer** whose secondary is open and primary is connected to voltage V_{1}. The **transformer** is at no load. This potential difference causes an alternating current in transformer to flow in the primary. Since the primary coil is purely inductive and there is no output. the primary draws the magnetising current I_{u} only. The function of this current in transformer is to magnetise the core, it is small in magnitude and lags V_{1} by 90^{0}. This current in transformer I produces an alternating flux **ϕ** , which is proportional to the current in transformer and hence in phase with it. This changing flux produces self induced emf E_{1} in primary which is equal to and in opposition to V_{1} and mutually induced emf E_{2} in secondary which is in antiphase with V_{1}. In an ideal no-load V_{1}= E_{1} and V_{2} = E_{2}.

**Actual Transformer **

Q: What is significance of magnetizing current Of a Transformer ?

**Actual Transformer on ‘NO LOAD’**: When no is connected across secondary winding, secondary current in transformer I_{2} is zero . Theoretically I_{1} must be zero but in practice, in transformer a very small primary current I_{0} (known as NO-LOAD current) flows in primary circuit. current in transformer I_{0} has to supply .

- iron loss in the core and very very small amount of loss and
- produces flux in the core i.e. to magnetise the core. The
**phasor diagram of a transformer**at no load is shown in figure.

**Current I _{0} is divided into two components:**

- I
_{0}sinϕ_{0}is in the direction of flux (ϕ) .This current maintains flux in the core. It is known as magnetising component ( I_{u}sinϕ_{0}A) . - Other component I
_{0}cosϕ_{0}is in phase with V_{1}and supplies iron loss plus small amount of primary Cu loss. This is known as active or working or iron components (l_{w}).

∴ I_{w} = I_{0}cosϕ_{0} A

∴ I_{0} =** √**( I_{u}^{2} + I_{w}^{2} )

The **no load primary current of transformer** has two components – **active component** and **magnetising component**.

The function of **magnetising component** is to sustain the alternating flux in the core of the transformer, which is mainly responsible for the emf in secondary winding of the magnetizing current in transformer is wattless component. As I_{0} is very small, the primary Cu loss can neglected and hence all the input power at **NO LOAD** is practically equal to the iron loss or core loss of transformer.

Iron loss = V_{1}I_{0}cosϕ_{0} watts. and p.f.= cosϕ_{0} (lag).

I_{0} always lags behind V_{1} by ϕ^{0} .