ELET 143 Transformers

• Transformers are covered in ELET 143 Circuit Analysis II

Objectives

• To understand how a transformer operates
• To learn how they are rated
• To understand how they operate in circuit
• To know the differences between step-up, step-down, and isolation transformers
• To learn how the ratio of voltage, current, and the number of turns are related in a transformer
• To learn the different applications of a transformer
• To identify the different types of transformers

Electromagnetic Induction

• Transformers allow the transfer of an AC signal from one circuit to another
• The transfer can either step up, step down, or pass the voltage unchanged
• If two electrically isolated coils are placed close to each other and an AC voltage is put across one, a changing magnetic field results
• The changing magnetic field induces a voltage into the second coil, which is electromagnetic induction
• The device itself is called a transformer
• The coil containing the AC voltage is the primary winding
• The coil where the voltage is induced is the secondary winding
• Transformers are rated in volt-amperes (VA) instead of watts (W)
• If the load is purely capacitive, the reactance may cause excessive current
• Power rating has little meaning in this case, so volt-ampere rating identifies the maximum current it can handle
• Transformers are wound with tapped secondaries
• A center-tapped secondary has two windings, each with half the total voltage
• The center tap is used for power supply to convert AC to DC voltages
• Transformers may have taps on the primary to compensate for line voltages that are too high or low

Mutual Inductance

• When a transformer operates without a load, there's no secondary current flow but there is a primary current flow because the transformer is connected across a voltage source
• The primary current depends on the size of the primary windings which act as an inductor, with the exciting current being the low amount of current that flows
• The exciting current overcomes the AC resistance of the primary winding and supports the magnetic field of the core
• Due to inductive reactance, the exciting current lags behind the applied voltage, but these conditions change when a load is applied across the secondary
• When a load is connected across the secondary winding, current is induced in the secondary
• Transformers are wound with the secondary on top of the primary
• The magnetic field created by the primary current cuts the secondary windings
• Current in the secondary establishes its own magnetic field
• The expanding magnetic field in the secondary cuts the primary turns, inducing a voltage back into the primary
• This magnetic field expands in the same direction as current in the primary which aiding it and causes it to increase, and is called mutual inductance
• The primary winding induces a voltage into the secondary and the secondary induces a voltage back into the primary

Turns Ratio

• The turns ratio of a transformer determines whether it steps up, steps down, or passes voltage unchanged
• The turns ratio is the number of turns in the secondary winding divided by the number of turns in the primary winding: Turns Ratio = Ns / Np
• N = number of turns
• P = primary
• S = secondary
• A step-up transformer has a secondary voltage greater than its primary
• The amount the voltage is stepped up depends on the turns ratio
• The ratio of secondary to primary voltage equals the ratio of secondary to primary turns: Es / Ep = Ns / Np
• The turns ratio of a step-up transformer is always greater than 1
• A step-down transformer produces a secondary voltage less than its primary voltage
• The voltage stepped down depends on the turns ratio; the turns ratio is always less than 1 in step-down transformers

Example Calculations

• To calculate the voltage induced into the secondary:
• Given: A transformer has 400 turns on the primary and 1200 turns on the secondary
• If 120 V of AC current is applied across the primary, find the induced voltage in the secondary
• Es / Ep = Ns / Np, Es / 120 = 1200 / 400, Es = 360 V
• To calculate the turns ratio for matching a speaker to a source:
• Required a transformer to match a 4-ohm speaker to a 100-ohm source
• Zp / Zs = (Np / Ns)^2, 100 / 4 = (Np / Ns)^2, √25 = Np / Ns, 5/1 = Np / Ns so the turns ratio is 5:1
• To calculate the power loss of power lines:
• Given a power station produces 8500V at 10 Amperes and the power lines have 100 ohms of resistance
• Find the amount of voltage drop: I = E / R, 10 = E / 100, E = 1000 V
• Using E, find the power loss: P = IE, P = (10)(1000), P = 10,000 W

Phase Shift

• How the transformer is wounds is how it determines the phase shift or not
• A transformer can be used to generate a phase shift
• When both the primary and secondary windings are wound in the same direction, there is no phase shift of the output signal in relation to the input signal
• If the secondary winding is wound in the opposite direction of the primary winding, the output waveform will be shifted 180° from the input and represents a phase shift
• The application determines how important the phase shift is

Blocking DC Voltage

• A transformer can be used to block DC voltage
• If DC voltage is applied to a transformer, nothing occurs in the secondary once the magnetic field is established
• A changing current in the primary winding is necessary to induce a voltage in the secondary winding
• A transformer can be used to isolate the secondary from any DC voltage in the primary

Isolation Transformer

• An isolation transformer prevents electrical shock by isolating equipment from the ground
• They are used to isolate electronic equipment from 120-volts AC, 60-hertz power for testing
• The reason for using the transformer is to prevent shocks
• Without it, one side of the power source is connected to the chassis
• When the chassis is removed from the cabinet, the "hot" chassis presents a shock hazard, which is more likely if the power cord can be plugged in either way

Autotransformers

• An autotransformer is a special type used to step-up or step-down the voltage
• It is a type of transformer in which the primary and secondary windings are both part of the same core
• With fewer turns, the voltage is stepped down Figure B shows an autotransformer stepping up a voltage