Transformer : Relevant Terms

TRANSFORMERS – RELEVANT TERMS

· Turns Ratio

Voltage induced into the secondary winding depends on the turns ratio of the transformer. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. If the turns ratio of a transformer is 4:1, the induced voltage is lower compared to that of primary and the transformer is called step down transformer. On the contrary, if the turns ratio of a transformer is 1:4, the induced voltage is higher than that of primary and the transformer is called step up transformer.

If the turns ratio and the input voltage are known, the output voltage can be determined as follows

Where, E₁& E₂are primary & secondary voltages, N₁& N₂are the number of turns in primary & secondary winding.

· Power Ratio

When a transformer steps up the voltage, it steps down the current by the same ratio, thereby input and output electrical power remains constant (neglecting losses). Transformer, being a passive component, cannot produce more power from the secondary winding than what is applied to the primary.

· Load Voltage

Load Voltage is same as the secondary voltage and is equal to the voltage delivered to the load.

· Line Voltage

Line Voltage is the primary voltage and the voltage available at the primary from the source.

· Coefficient of Coupling

In practice, no transformer is 100 percent efficient, i.e., output power is slightly less than that of input power. Since all the magnetic lines of force in the primary do not cut across the turns of the secondary coil, certain amount of flux leaks out of the magnetic circuit. The degree of how well the primary flux is coupled into the secondary is called the “coefficient of coupling”, this in turn, determines the efficiency of the transformer. Efficiency is the ratio of the output to the output plus the losses.

· Transformer Regulation

The regulation of a transformer is the change in secondary voltage from no load to full load. It is generally expressed as a percentage of the full-load secondary voltage.

The regulation depends upon the transformer design and the power factor of the load.

· Transformer ratings

Transformers are rated at their kilovolt-ampere (kVA) outputs. Transformers are generally rated on the kVA load which the transformer can safely carry at the ambient temperature, at rated load voltage and at rated frequency.

· Transformer Losses

· Copper Losses

These losses occur due to the finite resistance of the wire of the windings. When current flows through the windings, power = I²R dissipates in the form of heat and are treated as winding losses or I2R losses or copper losses.

· Hysteresis Losses

When alternating current reverses, once during each cycle, tiny magnetic domains are reversed and these physical changes in the core consume some amount of energy. These losses are referred to as hysteresis losses.

· Eddy Current Losses

With iron core, varying primary current sets up electromagnetic field in the secondary and also sets up EMF in the core, causing losses referred to as eddy current losses. These currents in the core oppose magnetic field changes in the core and hence must be kept very small.