Transformers




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TRANSFORMERS

Below are the commonly used formulas used in testing transformers.

1. Calculating transformer temperature rise.
To calculate the temperature rise of a power transformer us the following formula. The transformer sould be left at full load for at least 4 hours to reach and stabilise at its maximum temperature.


	H=


	R2 - R1

		R1


	(K + t1) - (t2 - t1)


		K = 234.5deg for copper and 225deg for aluminium H = Temerature rise R1 = Primary resistance before test. ie. cold R2 = Primary resistance after test. ie. hot t1 = Ambient temperature before test t2 = Ambient temperature after test
	2. Calculating Transformer Voltage Regulation All transformers have a different output voltage from NO LOAD to FULL LOAD. The % difference between the two levels is known as the regulation and is calculated as follows.


	NL - FL x 100

		NL 1

NL : No load voltage FL : Full load voltage

Transformer Insulation Temperature Class


	Winding type Temp Rise (Max)


	Class 105(A) Class 120(E) Class 130(B) Class 155(F) Class 180(H) Class 200 Class 220 Class 250


	75 90 95 115 140 160 180 210


	Power Suppy Technical Data A transformer's VA rating is calculated by multiplying the secondary AC voltage and the secondary AC current. For example, a 24V, 2A transformer would have a 48VA rating.Fig1 shows a full wave, centre tap, capacitor filtered, power rectifier arrangment, allowing more current at allowing more current at a lower voltage (for the same VA rating) than the standard full wave bridge arrangment shown in Fig.2, Fig. 3 shows a half wave rectifierwith a capacitor filter.






			Vac = Transformer secondary voltage Vdc = Suppy output voltage Aac = Transformer max secondary current Adc = Maximum supply output current