Autotransformer

Consider an autotransformer with a tapping at 40%. Then by equating the powers of the primary and the secondary sides of the autotransformer (Figure 4.4)... 

Generalizing the above equation, autotransformer tapping for a particular starting current, /at >s... 

A squirrel cage motor has its 4, on DOL as six times its rated current. Find the required tapping on an autotransformer to limit the starting current to 1.5 times. 

Since an autotransformer is in the circuit for only a short period (during the start only), it can be short-time rated. The rating of the autotransformer can be calculated from... 

An autotransformer of nearly 100 kVA continuously-rated should be sufficient for this application. 

Figure 4.9 illustrates transient-free switching through such electrolyte starters. This is a definite advantage of electrolytic switching over conventional Y/A or autotransformer switching. 

For oil-filled control equipment such as autotransformer starters or oil circuit breakers (BOCBs or MOCBs, Chapter 19), insulating oil should be checked periodically for its insulating properties. Leading manufacturers of this equipment indicate the number of switching operations under different conditions of load and fault, after which the oil must be replaced and these must be followed. 

The next step is to deeide how the seeondary windings should be arranged. That is, whether it is desired to have isolated seeondaries, eenter-tapped or non-eenter-tapped, or have autotransformer style seeondaries that share windings of lower voltage outputs (refer to Figure 3-18). 

I chose an autotransformer style secondary which means that the higher voltage windings will include the lower voltage windings. The turns associated with this and the wire sizes are... 

If the transformer has multiple seeondaries, the ZCS resonant eapaeitor ean be split and plaeed on eaeh seeondary or ean be plaeed aeross the entire seeondary winding if an autotransformer seeondary arrangement is used as shown in Figure 4-17. 

Figure 4-17 Second-side resonance arrangements for complex secondary designs in ZCS QR supplies (a) autotransformer secondaries (b) isolated secondaries.

The voltage from an autotransformer has both a lower bound (0 V a.c.) and an upper bound (usually about 130 V a.c.) and is an example of a naturally constrained discrete factor. The upper constraint could be changed if the autotransformer were... 

Figure 2.14 General system theory view showing how the use of an autotransformer imposes artificial constraints on the factor temperature.

If the voltage from an autotransformer is to be used to adjust the temperature of a chemical reactor (see Figure 2.14), then the natural boundaries of the autotransformer voltage will impose artificial constraints on the temperature. The lower boundary of the autotransformer (0 V a.c.) would result in no heating of the chemical reactor. Its temperature would then be approximately ambient, say 25°C. The upper boundary of the autotransformer voltage would produce a constant amount of heat energy and might result in a reactor temperature of, say, 300°C. Thus, the use of an autotransformer to adjust temperature imposes artificial lower and upper boundaries on the factor of interest. 

The natural constraint on temperature, the natural constraints on autotransformer voltage, and the artificial constraints on temperature are all examples of inequality constraints. If T is used to represent temperature and E represents voltage, these inequality constraints can be expressed, in order, as... 

Autotransformers are also widely used in laboratories. They allow the temperature to he controlled within a narrower interval. Let us consider a type JIATP-1 autotransformer (Fig. 12). Its front panel has six terminals. It is connected to 220-V mains via terminals 1 and 3,... 

The upper panel of the autotransformer has a scale graduated from 0 to 250 V, and a handle provided with a pointer. Turning of the handle changes the voltage of the current in the furnace winding and thus sets the required temperature. Connect a furnace to the mains via an autotransformer as follows place the pointer on the upper autotransformer scale opposite the zero graduation, next connect the furnace to it and connect the autotransformer to the mains in accordance with their voltage. 

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