SPECIAL MAINS TRANSFORMERS 



is easily seen by re-drawing the device as in Figure 22.3. Suppose the load 

 current is 10 amps, the power which the transformer has to handle if it is 

 of the conventional 'double wound' type is 110 X 10 == 1,100 watts, neg- 

 lecting losses. By regarding the autotransformer as a double wound com- 

 ponent as in Figure 22.3, it is easily seen that the power handled is only 

 550 watts. In general, the nearer the voltage transformation ratio is to unity 

 the greater is the advantage offered by autotransformation. 



The variac 



This {Figure 22.4) is an autotransformer of continuously variable turns-ratio 

 for interposition between a piece of electronic apparatus and the mains: 

 with its help, fluctuations in mains voltage may be manually corrected. 



In 



Out 



Figure 22.4 



Another important use is for subjecting apparatus possessing automatic 

 voltage regulating devices to dummy mains voltage changes, to check the 

 operation of the regulator. The output voltage is variable from zero to some 

 20 per cent above the input, and variacs can be had in a range of sizes enabling 

 them to handle maximum powers between 170 and 25,000 watts. 



EHT transformers 



Similar to mains transformers, but the HT winding is invariably intended 

 for half-wave rectification and delivers several thousand volts at a few 

 milliamps; one end is often internally earthed. Any LT windings present 

 have usually to be insulated to withstand approximately twice the peak EHT 

 voltage. 



Constant-voltage transformer 



These devices are capable of maintaining an output R.M.S. voltage constant 

 within 1 per cent, despite ±15 per cent changes in input voltage. They have 

 two secondary windings connected in series opposing, a main winding excited 

 by a core which is continuous and a subsidiary winding excited by a core 

 possessing an air gap. Matters are arranged so that over the working range 

 of input voltages the gapless core runs into saturation, whereas the gapped 

 one of course does not. The outputs have the form of Figure 22.5, and sum 

 to produce a rather constant output, as shown. Disadvantages of the 

 constant-voltage transformers are that, unless special filters are provided 



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