1. Electrical Machines and Energy Conversion
Unit 4 Deck 1
Transformer Principles

2. Construction of Power and Distribution Transformers
Core Type
High voltage
Shell Type
Less leakage flux
Type of Cooling
Ventilated Dry-Type Transformers
They are cooled by natural air convection.
Gas-Filled Dry-Type Transformers
Cooled with nitrogen or other gases
Liquid-Immersed Transformers
Hermetically sealed tanks with insulated liquid (mineral oil, silicone oil)

3. Principle of Transformer ActionDC
The same flux (mutual flux) exists in both coils.
Flux is generated by i1 (right-hand rule).
The induced emf e1 and e2 are generated to oppose the buildup of flux in its window.
i2 is generated by the induced emf e2.
In DC, the induced emfs are transients, in steady state: AC
Assuming:
Core permeability constant
No leakage flux

4. No-Load Conditions I0 exciting current (no-load current)
Core-loss component that supplies the hysteresis and eddy-current losses in the iron.
Magnetizing component that establishes the mutual flux ΦM
No-load Ampere-turns
core losses
eddy currents
VT applied voltage
IP primary current
EP voltage induced in the primary
RP resistance of the primary winding

5. Transient Behavior When Loading and Unloading
Placing the load
The secondary current established a flux opposite to the direction of the mutual flux:
The decrease of flux causes a decrease of EP and therefore an increase of IP.
The additional primary current is called load component of the primary current (IP,load).
The primary current will increase until Npip,load = NSiS, at which point ΦM and Ep will return to essentially the same values they had before the switch was closed. Ep will remain unchanged but an additional current will be established.

6. Effect of Leakage Flux on the Output Voltage of a Real Transformer
Φlp leakage flux associated with the primary coil
Φls leakage flux associated with the secondary coil

7. Ideal Transformer Turns Ratio Input/Load Impedance
High Side (HS) High Voltage winding
Low Side (LS) Low Voltage winding
Nameplate voltage ratio
Induced voltages
Assuming the primary is HS :

8. Leakage Reactance and the Equivalent Circuit of the Real Transformer
Primary
Secondary

9. Leakage Reactance and the Equivalent Circuit of the Real Transformer (2)
since
Leakage reactance

10. Equivalent Impedance of a Transformer
Parameters refereed to Primary
Resistance of secondary referred to primary
Leakage reactance of secondary referred to primary
Parameters refereed to Secondary
Resistance of primary referred to secondary
Leakage reactance of primary referred to secondary

11. High-Side, Low-Side
Step-down
Step-up

12. Voltage Regulation Secondary (Step-down low side)
Secondary (Step-up high side)
Enl Voltmeter reading at the output terminals when no load is connected to the transformer
Vrated voltmeter reading at the output terminals when the transformer is supplying rated apparent power
Rated low-side current at specified power factor
Rated low-side voltage (output V, breaker closed)
No-load low-side voltage (output V, breaker open)
Equivalent impedance of the transformer referred to low side
Loading the transformer to its rated value at the desired power factor is seldom easy to accomplish, an equivalent circuit on the secondary can be used to calculate the voltage regulation

13. Per-Unit Impedance and Percent Impedance of Transformer Windings
Vrated and Irated are also called base voltage and base current

14. Calculating Voltage Regulation from Per-Unit Values
Lagging power factor loads
Leading power factor loads

15. Transformer Losses and Efficiency

16. Efficiency From Per Unit-Values

17. Determination of Transformer Parameters
Open-Circuit
Short-Circuit

18. Electrical Machines and Energy Conversion
End of Presentation
Electrical Machines and Energy Conversion