Transformers
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Transformers
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TRANSFORMERS A transformer is a device which transfers electrical energy from one circuit to another with no physical connection. Transformers do not change the frequency of the signal from input to output. Voltage and current may change. Mutual induction is the means that a transformer uses to transfer energy. Two or more coils are combined to produce a transformer. BASIC TRANSFORMER OPERATION Basic Transformer Elements The most basic transformer consists of a primary coil, a secondary coil, and a core. Element Functions The primary coil is supplied with AC and produces changing magnetic flux lines. The secondary coil receives energy from the primary coil. The core provides a path for the magnetic lines of flux. Core Characteristics Air-core transformers are made with the windings wrapped around plastic or cardboard. These are used for high-frequencies. Iron-core transfers are made with the windings wrapped around a high permeability magnetic material such as silicon iron or nickel alloys. Core Composition The most common iron core transformer is uses a core known as a shell core. Layers of E shaped and I shaped pieces of metal are lamented and bolted together. The laminations prevent electrical current flow between the layers. Transformer Windings The primary winding is wound around the core first. An insulating material is then wrapped around the primary winding. The secondary winding is then wrapped around the primary. Insulation is wrapped around the secondary winding. The E and I sections of the shell are then inserted into and around the windings. TRANSFORMER LOADING No-Load Condition When voltage is present at the primary coil, but there is no load connected to the secondary coil the transformer is in a no-load condition. Counter-emf (The magnetic field of the coil producing a voltage in the reverse direction of the source) keeps the primary coil current amount small. The primary coil current is called the exciting current. Load Condition Mutual inductance causes a voltage to be present at the secondary coil. When a load is connected to the secondary coil current flows through the load. As the secondary coil current increases the counter-emf of the primary coil is decreased so additional current also flows through the primary coil. The primary coil current is affected by the load connected to the secondary coil. TRANSFORMER RATIOS Voltage Ratio and Turns The voltage present at the secondary coil is determined by the voltage applied to the primary coil and the ratio of turns of the primary coil to the turns of the secondary coil. Np / Ns = Vp / Vs Np = number of primary coil turns Ns = number of secondary coil turns Vp = voltage applied to the primary coil Vs = voltage induced in the secondary coil A step-up transformer has a greater voltage induced on the secondary coil than is supplied to the primary coil. A step-up transformer will have a more secondary windings than primary windings. A step-down transformer has a smaller value of voltage induced on the secondary coil than is supplied to the primary coil. A step-down transformer will have less secondary windings than primary windings. Turns Ratio A step down transformer may commonly be indicated with a ratio of 10:1 meaning the primary coil has 10 times the windings of the secondary coil. Current Ratio and Turns The secondary current is equal to the primary current multiplied by the turns ratio. Is = Ip ( Np / Ns ) A 10:1 step down transformer has 10 times the current in the secondary coil as the current in the primary coil. A 1:2 step-up transformer has one-half the current in the secondary coil as the current in the primary coil. TRANSFORMER LOSSES All power (watts) of the primary coil is provided to the secondary coil except for loss within the transformer. No transformer is perfect, but many transformers have a nearly 100% efficiency. Copper Loss The DC resistance of the windings causes some power loss to be dissipated as heat. This loss can be reduced by using larger-diameter wire. Eddy-Current Loss Random electrical currents may flow in the core causing power loss to be dissipated as heat. These random currents are called eddy-currents. The laminated construction of transformer cores greatly reduces this loss. Hysteresis Loss As the magnetic domains of the core material reverse direction with each cycle there is molecular friction which causes some loss to be dissipated as heat. This is called hysteresis loss. TRANSFORMER SYMBOLS, POLARITY INDICATORS, AND RATINGS Symbols The symbol for a transformer is like two coils opposite each other. Iron core coils are shown with two lines between the coils. Taps may also be shown where the secondary coil or coils are taps to provide multiple voltage levels. Polarity Indicators Depending on the direction of the windows the secondary coil may be in-phase or out-of-phase with the primary. Phase-indicating dots on a schematic indicate connections of a transformer that are in-phase. They have the same instantaneous polarity. Ratings The maximum voltage to the primary windings must be observed. The current rating is usually only specified for the secondary winding. The power-handling capability of transformers is measured in volt-amps rather than watts. TYPES AND APPLICATIONS OF TRANSFORMERS Power Transformers Power transformers are common to step down the 120VAC supply to more useful levels for various devices. It is common for a power transformer to have multiple secondary windings to provide more than one voltage level. Autotransformers An autotransformer does not have two separate coils. A tap on one coil provides the effect of a transformer. The variac is an autotransformer. A movable tap allows the output voltage to be adjusted. Intermediate-Frequency (IF) and Radio-Frequency (RF) Transformers IF and RF transformers are used in high-frequency circuits to couple (connect) circuits. DC voltages are blocked. They only couple high-frequency AC signals. Impedance-Matching Transformers Impedance is the opposition to AC current flow. It is similar to resistance in DC. Impedance-matching is a means of creating a different impedance at the primary of a transformer than the actual impedance of the load. This is commonly required for speaker connections to amplifiers. Isolation Transformers A transformer with separate primary and secondary windings provides electrical isolation. The secondary is said to be floating. The primary may have a reference to ground. The secondary cannot be referenced to ground. This can be used to provide protection from shock hazards. TROUBLESHOOTING TRANSFORMER FAULTS Typical faults are open circuits or shorts. As with coils shorts between windings may be difficult to detect. |