 # TRANSFORMERS Transformers are not semi conductor devices however

• Slides: 28 TRANSFORMERS Transformers are not semi conductor devices, however, they play an integral role in the operations of most of power supplies. l The basic schematic symbol for the transformer is as shown in figure, l TRANSFORMER TRANSFORMER The transformer basically consists of two inductors, which are in close proximity to each other. l However, they are not physically connected. l TRANSFORMER The device consist of two windings called the primary and secondary windings. l The input to the transformer is applied to the primary. l The output is taken from the secondary of the device. l TRANSFORMER An alternating voltage is applied to the primary which induces an alternating voltage in the secondary. l However, keep in mind that primary and secondary windings are physically isolated. l TRANSFORMER l 1. A transformer can be designed in three modes. The step up transformer provides a secondary voltage which is greater in amplitude as compared to primary. TRANSFORMER For example, a step up transformer may provide a 240 Vac out put against 120 Vac at the in put. A step up transformer is shown TRANSFORMER 2. The step down transformer provides a secondary voltage which is less than the primary voltage. For example, a transformer shown in the figure provides 30 Vac at the out put against 120 Vac input, as shown TRANSFORMER TRANSFORMER 3. An isolation transformer provides an out put voltage that is equal to the input voltage. These are used to electrically isolate power supplies from the power lines. TRANSFORMER TRANSFORMER Turn ratio • The turn ratio of a transformer is the ratio of number of turns in the primary to the number of turns in the secondary. • For example, the step down transformer shown has a turn ratio of 4: 1, which means there are four turns in the primary against each turn in the secondary. TRANSFORMER TRANSFORMER l The turn ratio of the transformer is equal to the voltage ratio of the two components, so that TRANSFORMER Where l N 2=Number of turns in the secondary winding. l N 1= Number of turns in the primary winding. l V 2=Voltage in the secondary. l V 1=Voltage to the primary. CLACULATING SECONDARY VOLTAGE When turn ratio and the primary voltage is known, the secondary voltage can be found as V 2 = (N 2/N 1)V 1 For example the step down transformer in the previous fig. has 120 Vac input. The secondary voltage can be found as CLACULATING SECONDARY VOLTAGE V 2 = (N 2/N 1) V 1 = 1/4(120 Vac) = 30 Vac CLACULATING SECONDARY CURRENT Ideally, transformers are 100 percent efficient. This means that the ideal transformer can transfer 100 percent of its input power to the secondary. So that by formula we may write P 2 = P 1 Since power is equal to the product of voltage and current CLACULATING SECONDARY CURRENT V 2 I 2 =V 1 I 1 and I 1/I 2 =V 2/V 1 The current ratio is inverse to the voltage ratio. This means that (1) For step down transformer, I 2>I 1 (2) For step up transformer, I 2<I 1 CLACULATING SECONDARY CURRENT In other words, current varies opposite to the variation in voltage. If voltage increases, current decreases and vice versa. Since the Voltage ratio of a transformer is equal to the turn ratio therefore, l CLACULATING SECONDARY CURRENT the previous equation can be written as I 1/I 2 =N 2/N 1 I 2 = (N 1/N 2)I 1 EXAMPLE l The fuse in the figure is used to limit the current in the primary of the transformer. Assuming that the fuse limits the value of I 1 to 1 A, what is the limit on the value of the secondary current. SOLUTION The maximum secondary current is found using the limit on I 1 and the turns ratio as follows: I 2 =N 1/N 2 I 1 = (1/4)1 A SOLUTION I 2 = 250 m. A If the secondary current tries to exceed the 250 m. A limit, the primary current will exceed its limit and blow off the fuse. TRANSFORMER INPUT/OUTPUT PHASE RELATIONSHIP l In the schematic symbol, there are two dots : one on the top side of the primary and the other on top of secondary. In this case we will be working with a transformer which does not produce any phase difference between the input and out put.  TRANSFORMER INPUT/OUTPUT PHASE RELATIONSHIP l In the schematic symbol, there are two dots : one on the top side of the primary and one on the bottom side of the secondary, in this case We are working with a transformer whose output voltage is 180 o out of phase with its input voltage as shown in fig. 