Transformer anatomy

Among the most common passive electromagnetic devices in use, transformers enable very efficient change of alternating current (ac) voltage from one value to another at constant frequency. Providing different input voltages to multiple power supplies in industrial equipment is a frequent application.

12/01/2003


Among the most common passive electromagnetic devices in use, transformers enable very efficient change of alternating current (ac) voltage from one value to another at constant frequency. Providing different input voltages to multiple power supplies in industrial equipment is a frequent application.

Voltage level is raised or lowered by electromagnetic inductance between the device's input (power source) and output (load side), which in a basic transformer consists of a primary coil and a secondary coil of insulated copper wires wound around an insulated steel core . This mutual induction feature is unique to ac circuits and not available to direct current (dc) devices. Magnetic linking between the two windings is provided by the core, which often is made of thin steel laminations —rather than solid material—to limit electric losses due to heating.

The relative number of windings in the primary and secondary coils, or turns ratio, determines a transformer's main characteristics. In a step-up transformer, more secondary windings than primary windings result in higher output-to-input voltage (in proportion to number of turns). An opposite turns ratio produces a step-down transformer . Voltage per turn is a constant in a specific transformer. Current has an inverse relationship, such that a typical step-up transformer has fewer primary turns of larger wire (higher- current) and many secondary turns of smaller wire (lower current). Input-to-output impedance changes as the square of the turns ratio. So transformers can be used to change current and impedance magnitudes as well.

An isolation transformer adds electrostatic shielding between input and output windings to reduce harmful electrical noise. It's used in medical devices and other safety-critical equipment. As the term implies, this transformer type can provide circuit isolation even with a 1:1 turns ratio (no voltage transformation).

Another device type is the autotransformer, consisting of a single tapped coil. The number of turns between the tap and one end of the primary winding defines one coil of the transformer; and the entire primary winding serves as the other coil. This device is smaller, lighter, and often less costly than a transformer with standard core.

Size matters

Output rating of transformers is based on the product of rated voltage and rated current (VA), also known as apparent power or simply "volt-ampere" value. Transformers range in size from gigantic units used in electric power distribution and liquid-filled power transformers with ratings in hundreds of MVA down to devices small enough to be surface mounted on circuit boards (see graphic). Surface-mount transformers accommodate pick-and-place assembly methods. Coverage here leans toward transformers with lower VA capacities.

Many transformers operate at 50/60 Hz, but models are available for higher ac frequencies, for example 400 Hz, at reduced power output. Low power, surface-mount models operate at 100 kHz or higher. Even smaller RF transformers for changing impedance values work at frequencies up to 1,500 MHz.

Core shapes

Low-cost transformers have square or rectangular-shaped cores and a center cutout of the same shape that allows for winding of coils. Toroidal or ring-shaped cores are also common; they're more compact and help enhance symmetry in the magnetic circuit. However, the core's cross section remains typically square or rectangular. A further shape enhancement is a toroidal core with circular cross section formed by steel strip gradually pointed towards the ends—rather than constant-width strips. Such "o-ring" transformers are smaller, lighter, and work with shorter-length windings versus standard toroidal cores. However, higher production cost is a tradeoff.

For three-phase voltage transformation, design requirements for each phase must be met. Either three, single-phase devices or one three-phase transformer can do the job. The latter choice offers smaller size, higher efficiency, and less cost—a downside is less flexibility in case of failure.

Two product examples illustrate the gamut of transformers available. For more information on this industry sector, visit The Transformer Association's Web site ( www.transformer-assn.org ).

fbartos@reedbusiness.com





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