for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. The term is often used to describe similar variable autotransformers made by other makers. To provide very small increments of adjustment, the secondary connection is made through a brush that slides across the winding coils. A variable autotransformer is an efficient and quiet method for adjusting the voltage to incandescent lamps. While lightweight and compact semiconductor light dimmers have replaced variacs in many applications such as theatrical lighting, variable autotransformers are still used when an undistorted variable voltage sine wave is required.
Polyphase transformers Edit
For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Δ, Δ-Y, Δ-Δ and Y-Y. If a winding is connected to earth (grounded) the earth connection point is usually the center point of a Y winding. There are many possible configurations that may involve more or fewer than six windings and various tap connections.
Resonant transformers Edit
A resonant transformer is one that operates at the resonant frequency of one or more of its coils. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. If the primary coil is driven by a periodic source of alternating current, such as a square or sawtooth wave, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are therefore used to generate high alternating voltages. The current available from this type of coil can be much larger than that from electrostatic machines such as the Van de Graaff generator and Wimshurst machine. They also run at a higher operating temperature than standard units.
- Tesla coil
- Oudin coil (or Oudin resonator; named after Paul Marie Oudin, 1851-1923)
- D'Arsonval apparatus
- ignition coil or induction coil used in the ignition system of a petrol engine
- Flyback transformer of a CRT television set or video monitor.
Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where a large measure of the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.
A voltage regulating transformer uses a resonant winding and allows part of the core to go into saturation on each cycle of the alternating current. This effect stabilizes the output of the regulating transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an ac power supply. However, due to the hysteresis losses accompanying this type of operation, efficiency is low.
Current transformersEditA current transformer is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.
Current transformers are commonly used in electricity meters to facilitate the measurement of large currents which would be difficult to measure more directly.
Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary as in this circumstance a very high voltage would be produced across the secondary.
Current transformers are often constructed with a single primary turn either as an insulated cable passing through a toroidal core, or else as a bar to which circuit conductors are connected.
Voltage transformers (also known as potential transformers) are used in the electricity supply industry to measure accurately the voltage being supplied. They are designed to present negligible load to the voltage being measured.
Pulse transformers Edit
A pulse