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Types of Transformers
Power
These operate at 50 to 400 Hz at a nominal line voltage from 105 to 130 V. They are made with single and multiple secondaries with varying step-up and step-down turns ratios.
Secondary
Secondaries can have a single tap, multiple tap or no tap. Some units are made with a tapped primary. Output voltages can range from three to several thousand volts with output currents from .01 to 1500 A.
The Cores
The cores are iron or steel laminations. They are packaged in a hermetically sealed case for military or space use or with an open frame or plastic enclosure for commercial, industrial or consumer use.
Isolation
These types operate with a one-to-one turns ratio between primary and secondary, while still isolating the line from the secondary load. Usually, an isolation transformer also includes a Faraday shield, which is a screen of nonmagnetic metal wound between the primary and secondary and connected to the transformer core.
The Shield
The shield acts to prevent capacitive coupling of spurious signals and noise between windings, and it also reduces transformer efficiency by increasing leakage current.
Ferroresonant (Constant Voltage)
These types operate from a varying ac power line (95 to 130 volts) to maintain a constant ac output (typically from 6 to 118 volts ac ± 1%). Line voltage frequency variations of more than 1 Hz above or below the nominal frequency cannot be tolerated without going out of regulation.
Control
These are small power transformers used for control components such as relays and low voltage ac control devices. Common output voltages are 12 and 24 Vac at current capabilities of 4 to 16 A.
Autotransformer
These are single winding types with either fixed or variable step-up or step-down turns ratios. They are smaller and less expensive that the equivalent two-winding types.
Audio
These transformers differ from the power types in that they are used to provide matching of the electrical characteristics of an output amplifier to that of a load speaker. In high-fidelity audio systems, they operate from 20 Hz to 20 KHz. In audio systems involved with voice communications only, they operate from 200 to 500 Hz.
Radio Frequency
These transformers operate at a fixed high frequency with a capacitor across either primary, secondary or both, to create a tuned or resonant circuit. Most types use an air core, however some are made with a ferrite slug to permit adjustment of the inductance of the windings over a given range. They are usually assembled in an aluminum shielded can to reduce pickup or radiation of magnetic fields.
Pulse
These types are used for the generation and transmission of square wave pulses with emphasis on fast rise and fall times of the pulse and high-frequency response. These transformers are packaged in a miniature enclosure, 1/4 inch to 1/2 inch in diameter, and use an air core.
