TRANSFORMERS

The TURNS RATIO of a transformer is the ratio of the number of turns of wire in the primary winding to the number of turns in the secondary winding. When the turns ratio is stated, the number representing turns on the primary is always stated first. For example, a 1:2 turns ratio means the secondary has twice the number of turns as the primary. In this example, the voltage across the secondary is two times the voltage applied to the primary.

TURNS RATIO

Q10. What is the name of the emf generated in the primary that opposes the flow of current in the primary?

A10. Self-induced or counter emf.

When voltage is applied to the primary of a transformer, exciting current flows in the primary. The current causes a magnetic field to be set up around both the primary and the secondary windings. The moving flux causes a voltage to be induced into the secondary winding, countering the effects of the counter emf in the primary.

EXCITING CURRENT

A transformer used to match the impedance of the source and the impedance of the load.

IMPEDANCE-MATCHING TRANSFORMER

A transformer used in a radio-frequency circuit to transfer rf signals from one circuit to another.

RADIO-FREQUENCY TRANSFORMER—

Q1. What is meant by "transformer action?"

A1. The transfer of energy from one circuit to another circuit by electromagnetic induction.

Q11. What causes a voltage to be developed across the secondary winding of a transformer?

A11. The magnetic lines generated by the current in the primary cut the secondary windings and induce a voltage into them.

A transformer with a single winding in which the entire winding can be used as the primary and part of the winding as the secondary, or part of the winding can be used as the primary and the entire winding can be used as the secondary.

AUTOTRANSFORMER

The basic transformer is an electrical device that transfers alternating current energy from one circuit to another circuit by magnetic coupling of the primary and secondary windings of the transformer. This is accomplished through mutual inductance (M). The coefficient of coupling (K) of a transformer is dependent upon the size and shape of the coils, their relative positions, and the characteristic of the core between the two coils. An ideal transformer is one where all the magnetic lines of flux produced by the primary cut the entire secondary. The higher the K of the transformer, the higher is the transfer of the energy. The voltage applied to the primary winding causes current to flow in the primary. This current generates a magnetic field, generating a counter emf (cemf) which has the opposite phase to that of the applied voltage. The magnetic field generated by the current in the primary also cuts the secondary winding and induces a voltage in this winding.

BASIC TRANSFORMER

The power and current ratios of a transformer are dependent on the fact that power delivered to the secondary is always equal to the power delivered to the primary minus the losses of the transformer. This will always be true, regardless of the number of secondary windings. Using the law of power and current, it can be stated that current through the transformer is the inverse of the voltage or turns ratio, with power remaining the same or less, regardless of the number of secondaries.

POWER AND CURRENT RATIOS

A transformer with two or more windings wound on a laminated iron core. The transformer is used to supply stepped up and stepped down values of voltage to the various circuits in electrical equipment.

POWER TRANSFORMER

The amplitude of the voltage induced in the secondary is dependent upon the efficiency of the transformer and the turns ratio. The efficiency of a transformer is related to the power losses in the windings and core of the transformer. Efficiency (in percent) equals Pout/Pin ⋅ 100. A perfect transformer would have an efficiency of 1.0 or 100%.

TRANSFORMER EFFICIENCY

Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measureable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.

TRANSFORMER LOSSES

Q12. What is the phase relationship between the voltage induced in the secondary of an unlike-wound transformer and the counter emf of the primary winding?

A12. In phase. Remember, the cemf of the primary is 180 degrees out of phase with the applied voltage. The induced voltage of the secondary of an unlike-wound transformer is also 180 degrees out of phase with the primary voltage.

Q13. Draw dots on the below symbol to indicate the phasing of the transformer.

A13.

Q14. What is "leakage flux?"

A14. Lines of flux generated by one winding which do not link the other winding.

Q15. What effect does flux leakage in a transformer have on the coefficient of coupling (K) in the transformer?

A15. It causes K to be less than unity (1).

Q16. Does 1:5 indicate a step-up or step-down transformer?

A16. Step up.

Q17. A transformer has 500 turns on the primary and 1500 turns on the secondary. If 45 volts are applied to the primary, what is the voltage developed across the secondary? (Assume no losses)

A17. Es ÷ Ep = Ns ÷ Np or Es = EpNs ÷ Np = 45V x 1500 turns ÷ 500 turns = 135 Volts

Q2. What are the three basic parts of a transformer?

A2. Primary winding; secondary winding; core.

Q23. Name the three power losses in a transformer.

A23. Copper loss, eddy-current loss, and hysteresis loss.

Q25. Why should a transformer designed for 400 hertz operation not be used for 60 hertz operation?

A25. The inductive reactance at 60 hertz would be too low. The resulting excessive current would probably damage the transformer.

Q26. List five different types of transformers according to their applications.

A26. a. Power transformer b. Autotransformer c. Impedance matching transformer d. Audio-frequency transformer e. Radio-frequency transformer

Q27. The leads to the primary and to the high-voltage secondary windings of a power transformer usually are of what color?

A27. Primary leads-black; secondary leads-red.

Q28. What is the cause of most accidents?

A28. Carelessness.

Q29. Before working on electrical equipment containing capacitors, what should you do to the capacitors?

A29. Discharge them by shorting them to ground.

Q3. What are three materials commonly used as the core of a transformer?

A3. Air; soft iron; steel.

Q30. When working on electrical equipment, why should you use only one hand?

A30. To minimize the possibility of providing a path for current through your body.

Q4. What are the two main types of cores used in transformers?

A4. Hollow-core type; shell-core type.

Q5. Which transformer windings are connected to an AC source voltage and to a load, respectively?

A5. Primary to source; secondary to load.

Q6. A transformer designed for high-voltage applications differs in construction in what way from a transformer designed for low-voltage applications?

A6. Additional insulation is provided between the layers of windings in the high-voltage transformer.

Q7. Identify the below schematic symbols of transformers by labeling them in the blanks provided.

A7. a. air-core transformer b. iron-core transformer c. iron-core center tapped transformer

Q8. What is meant by a "no-load condition" in a transformer circuit?

A8. A voltage is applied to the primary, but no load is connected to the secondary.

Q9. What is meant by "exciting current" in a transformer?

A9. Exciting current is the current that flows in the primary of a transformer with the secondary open (no load attached).

A transformer used in audio-frequency circuits to transfer af signals from one circuit to another.

AUDIO-FREQUENCY TRANSFORMER—

When the secondary winding is connected to a load, causing current to flow in the secondary, the magnetic field decreases momentarily. The primary then draws more current, restoring the magnetic field to almost its original magnitude. The phase of the current flowing in the secondary circuit is dependent upon the phase of the voltage impressed across the primary and the direction of the winding of the secondary. If the secondary were wound in the same direction as the primary, the phase would be the same. If wound opposite to the primary, the phase would be reversed. This is shown on a schematic drawing by the use of phasing dots. The dots mean that the leads of the primary and secondary have the same phase. The lack of phasing dots on a schematic means a phase reversal.

PHASE

consists of two coils of insulated wire wound on a core. The primary winding is usually wound onto a form, then wrapped with an insulating material such as paper or cloth. The secondary winding is then wound on top of the primary and both windings are wrapped with insulating material. The windings are then fitted onto the core of the transformer. Cores come in various shapes and materials. The most common materials are air, soft iron, and laminated steel. The most common types of transformers are the shell-core and the hollow-core types. The type and shape of the core is dependent on the intended use of the transformer and the voltage applied to the current in the primary winding.

TRANSFORMER CONSTRUCTION—A TRANSFORMER