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Autotransformers

1. Autotransformers

It is attainable to get electrical device action by suggests that of one coil, provided that there’s a “tap connection” somewhere on the winding. Transformers having only 1 winding area unit known as autotransformers, shown schematically in figure. An autotransformer has the standard core however only 1 winding, that is common to each the first and secondary circuits. The primary is usually the portion of the winding connected to the AC power supply.

This transformer could also be wont to step voltage up or down. If the first is that the total winding and is connected to a offer, and the secondary circuit is connected across solely some of the winding (as shown), the secondary voltage is “stepped-down.”

 If only some of the winding is that the primary and is connected to the supply voltage and therefore the secondary includes all the winding, then the voltage are “stepped-up” in proportion to the quantitative relation of the whole turns to the quantity of connected turns within the coil.

When primary current I1 is within the direction of the arrow, secondary current, I2, is within the wrong way, as in figure 9b. Therefore, in the portion of the winding between point’s b and c, current is that the difference of I1 and I2. If the need is to step the voltage up (or down) solely a tiny low quantity, then the electrical device quantitative relation is small—E1 and E2 area unit nearly equal. Currents I1 and I2 also are nearly equal.

The portion of the winding between b and c, that carries the distinction of the currents, will be manufactured from a far smaller conductor, since the current is far lower. Under these circumstances, the autotransformer is far cheaper than the two-coil electrical device of a similar rating. However, the disadvantage of the autotransformer is that the first and secondary circuit’s area unit electrically connected and, therefore, couldn’t safely be used for stepping down from high voltage to a voltage appropriate for plant masses.

The autotransformer, however, is extensively used for reducing line voltage for step will increase in beginning larger induction motors. There area unit usually four or 5 faucets that area unit modified by timers in order that additional of the winding is supplementary in every step till the total voltage is applied across the motor. This avoids the massive inflow current needed once beginning motors at full line voltage. This transformer is additionally extensively used for “buck-boost” once the voltage needs to be stepped up or down only a tiny low share. One very common example is boosting 208 V up from one section of a 120/208-V three-phase system, to 220 V for single-phase masses.

2. Instrument Transformers

Instrument transformers (figure) area unit used for activity and management purposes. They supply currents and voltages proportional to the primary, however there’s less danger to instruments and personnel.

connection of instrument transformerhttp://electricalknowledgebank.com

Those transformers wont to step voltage down area unit called potential transformers (PTs) and people wont to step current down area unit called current transformers (CTs).

The operate of a noble metal is to accurately live voltage on the first, while a CT is employed to live current on the first

3. Potential Transformers

Potential transformers (figure) area unit used with voltmeters, wattmeter, energy unit meters, power-factor meters, frequency meters, synchroscopes and synchronizing equipment, protecting and regulation relays, and under voltage and overvoltage trip coils of circuit breakers. One potential electrical device will be used for variety of instruments if the total current needed by the instruments connected to the secondary winding doesn’t exceed the transformer rating.

Potential transformers area unit sometimes rated fifty to two hundred volt-amperes at 120 secondary volts. The secondary terminals ought to ne’er be short circuited as a result of a significant current can result, which might harm the windings.

potential transformer in switch yardInstrument Transformers

4. Current Transformers

The primary of a current electrical device usually has only 1 flip. This is not extremely a flip or wrap round the core however simply a conductor or bus going through the “window.” the first ne’er has over a really few turns, whereas the secondary could have a         good several turns,    depending      upon what proportion this should be stepped down.

 In most cases, the first of a current electrical device may be a single wire or bus bar, and the secondary is wound on a laminated core, placed around the conductor during which this must be measured, as illustrated in figure twelve. If primary current exists and therefore the secondary circuit of a CT is closed, the winding builds and maintains a counter or back EMF to the first magnetizing force. 

Should the secondary be opened with current within the primary, the counter voltage is removed; and the primary magnetizing force builds up an especially high voltage within the secondary, that is dangerous to personnel and can destroy this electrical device. Current transformers area unit used with ammeters, wattmeter, power issue meters, energy unit meters, compensators, protecting and regulating relays, and trip coils of circuit breakers.

One CT will be used to operate many instruments, provided the combined numerous the instruments don’t exceed that that the CT is rated. Secondary windings area unit sometimes rated at five amperes. a spread of current transformers area unit shown in figure thirteen. Many times, CTs have several faucets on the secondary to regulate the vary of current possible to live on the first. Current electrical device in

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