Unit 28 Single-Phase Transformers Objectives • After studying this unit, you should be able to: – Discuss the different types of transformers – Calculate values of voltage, current, and turns for single-phase transformers using formulas – Calculate values of voltage, current, and turns for single-phase transformers using the turns ratio Objectives (cont’d.) – Connect a transformer and test the voltage output of different windings – Discuss polarity markings on a schematic diagram – Test a transformer to determine the proper polarity marks Preview • Transformers – Among the most common devices found in the electrical field – Range in size and rating • Two main types of voltage transformers – Isolation transformers – Autotransformers Single-Phase Transformers • Transformers – Magnetically operated machines – Can change values of voltage, current, and impedance without a change of frequency • Transformer classifications – Isolation transformer – Autotransformer – Current transformer FIGURE 28–1 All values of a transformer are proportional to its turns ratio. Single-Phase Transformers (cont’d.) • Primary winding – Connected to the incoming power supply • Secondary winding – Connected to the driven load FIGURE 28–2 An isolation transformer has its primary and secondary windings electrically separated from each other. Isolation Transformers • Greatly reduce voltage spikes that originate on the supply side – Before they are transferred to the load side FIGURE 28–5 The isolation transformer greatly reduces the voltage spike. FIGURE 28–6 Basic construction of an isolation transformer. FIGURE 28–7 Magnetic field produced by AC. Isolation Transformers (cont’d.) • Important considerations – Basic operating principles • Self-induction: coil induces a voltage into itself – Excitation current • Amount of current required to magnetize the core of the transformer – Mutual induction • Ability of one coil to induce a voltage into another coil FIGURE 28–8 The magnetic field of the primary induces a voltage into the secondary. Transformer Calculations • Voltage of secondary winding – EP/ES = NP/NS • Current flow of the secondary – I = E/Z • Primary current – EP/ES = Is/Ip • Power in must equal power out Calculating Isolation Transformer Values Using the Turns Ratio • Ratio = 240 V/ 96 V FIGURE 28–9 Computing transformer values using the turns ratio. Multiple-Tapped Windings FIGURE 28–12 Transformer with multiple-tapped primary winding. Isolation Transformers with Multiple Secondaries FIGURE 28–15 Calculating values for a transformer with multiple secondary windings. Distribution Transformers FIGURE 28–17 Distribution transformer. Control Transformers FIGURE 28–23 Control transformer connected for 240-volt operation. Transformer Core Types • Several types of cores are used – Laminated cores – Core-type transformer – Shell-type transformer – H-type core – Tape-wound core or toroid core FIGURE 28–29 A core-type transformer. Transformer Inrush Current • Inrush current – Amount of current that flows when power is initially applied to the circuit • Transformers can produce extremely high inrush currents when power is first applied to the primary winding • Type of core used in construction – Important for inrush current prevention FIGURE 28–33 Reactors are used to help prevent inrush current from becoming excessive when power is first turned on. Autotransformers • One-winding transformers FIGURE 28–41 An autotransformer has only one winding used for both the primary and secondary. Transformer Polarities • Considerations – Relationship between different windings at the same point in time • Point in time: when the peak positive voltage is being produced – Polarity markings on schematics – Additive and subtractive polarities – Using arrows to place dots Voltage and Current Relationships in a Transformer FIGURE 28–54 At no load, the primary current lags the voltage by 90 degrees. FIGURE 28–55 The secondary voltage lags the primary current by 90º. FIGURE 28–56 Voltage and current relationships of the primary and secondary windings. Testing the Transformer FIGURE 28–57 Testing a transformer with an ohmmeter. Transformer Nameplates • NEC® 450.11 requires: – Manufacturer name – Rated kilovolt-ampere – Frequency – Primary and secondary voltage – Impedance of transformers rated 25 kilovoltampere and larger Transformer Nameplates (cont’d.) – Required clearances of transformers with ventilating openings – Amount and kind of insulating liquid where used – Temperature class for the insulating system of dry-type transformers Determining Maximum Current • Maximum current – IS = kVA / ES • Primary current – IP = kVA / EP Transformer Impedance FIGURE 28–58 Determining transformer impedance. Summary • Transformers – All values of voltage, current, and impedance are proportional to the turns ratio – Can change values of voltage, current, and impedance but cannot change the frequency – Primary winding connected to the powerline – Secondary winding is connected to the load – Step-down transformer: lower secondary voltage than primary voltage Summary (cont’d.) – Step-up transformer: higher secondary voltage than primary voltage – Isolation transformer: primary and secondary windings electrically and mechanically separated from each other – Self-induction: coil induces a voltage into itself – Mutual conduction: coil induces a voltage into another coil Summary (cont’d.) • Transformers can have very high inrush current when first connected to the powerline – Magnetic domains in the core material • Inductors – Provide an air gap in their core material • Causes magnetic domains to reset to a neutral position Summary (cont’d.) • Autotransformers: one winding – No line isolation • Isolation transformers – Help filter voltage and current spikes • Polarity dots – Added to schematic diagrams to indicate transformer polarity – Transformers can be connected as additive or subtractive polarity Summary (cont’d.) • Constant-current transformers – Also known as current regulators – Used to provide power to series connected loads • General rule: secondary windings should not be connected in parallel