1. Lecture 09Electro Mechanical System1 Chapter 11 Special Transformers

2. Lecture 09Electro Mechanical System2 Dual-voltage distribution transformer  Transformers that supply electric power to residential areas generally have two secondary windings, each rated at 120 V.  The windings are connected in series, and so the total voltage between the lines is 240 V while that between the lines and the center tap is 120 V. The center tap, called neutral, is always connected to ground.  These kind of transformers are still used in industry where two voltages are required.

3. Lecture 09Electro Mechanical System3 Autotransformer  On some occasions it is desirable to change voltage levels by small amount  For example, it may be necessary to increase a voltage from 110 to 120 V or from 13.2 to 13.8 kV.  These small rises may be made necessary by voltage drops that occur in power systems a long way from the generators.  In such circumstances, it is wasteful and excessively expensive to wind a transformer with two full windings, each rated at about the same voltage.  A special-purpose transformer, called an autotransformer, is used instead for such increase/decrease.

4. Lecture 09Electro Mechanical System4 Autotransformer  When we connect load at output CA  I 2 immediately causes I 1 to flow  BC will carry current I 1 and CA will carry current I 2 – I 1  The mmf due to I 1 must be equal to mmf due I 2 – I 1  Which can be further reduced to  Neglecting transformer losses and excitation current.Power drawn by the load must be equal to supplied power Home Work: Page 227 Example 11-1

5. Lecture 09Electro Mechanical System5 Conventional transformer as Autotransformer  Conventional two-winding transformer can be converted into autotransformer by connecting primary and secondary windings in series following rules apply: 1.Currents should not exceed nominal rating 2.Voltages should not exceed nominal rating 3.If rated current flows in one winding rated current automatically flows in other winding (ampere-turn ratio of the windings are always equal) 4.If rated voltages exists in one winding rated voltage automatically appears across other winding (ampere-turn ratio of the windings are always equal) 5.If current in one winding flows from H 1 to H 2, current in other winding should flow from X 2 to X 1 6.Voltages add with opposite polarity (H 1 & X 2, or H 2 & X 1 ) and subtracts with same polarity (H 1 & X 1, or H 2 & X 2 )

6. Lecture 09Electro Mechanical System6 Example A standard single phase transformer 15kVA, 600V/120V,60Hz Connect it as autotransformer to obtain following voltages a)600 V primary to 480 V secondary b)600 V primary to 720 V secondary c)120 V primary to 480 V secondary Also calculate maximum load in each case Nominal current of 600 V winding is I 1 = S/E 1 = 15000/600 = 25 A Nominal current of 600 V winding is I 2 = S/E 2 = 15000/120 = 125 A a) For 480 V, secondary 120 V must subtract from primary voltage 600. to obtain we must connect same polarity together.

7. Lecture 09Electro Mechanical System7 Example….cnt. current in 120 V winding is the same as that of load i.e nominal current rating of 125 A. Max. power drawn by the load is: S a = 125 A x 480 V = 60 kVA Current 125 A flows from X1 to X2. Current 25 A must flows from H2 to H1 Leaving 100 A to flow from the source Power supplied by the source is: S = 100 A x 600 V = 60 kVA

8. Lecture 09Electro Mechanical System8 Example….cnt. b) For 600 V/720 V, secondary 120 V must be added to the primary voltage 600. to obtain we must connect opposite polarity together (H 1 and X 2 ) Current in secondary is same 125 A Max. power drawn by the load is: S b = 125 A x 720 V = 90 kVA Autotransfarmer can supply a load of far greater capacity than original transformer c) For 120V/480V, same connections as solution In part a, source at X 1 & X 2 Load current cannot exceed 25 A Max. power drawn by the load is: S c = 25 A x 480 V = 12 kVA

9. Lecture 09Electro Mechanical System9 Voltage transformers  Measure/Monitor the voltage on a transmission line  Isolate metering equipment from the line  High-precision transformers in which the ratio of primary to secondary current is a known constant.  Secondary voltage is in phase with the primary voltage.  Rated secondary voltage is usually 115 irrespective of what the rated primary voltage may be  Similar to conventional transformers, the insulation must be to withstand full line voltage on HV side