* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Transformers
Electrical ballast wikipedia , lookup
Nominal impedance wikipedia , lookup
Electrification wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Stray voltage wikipedia , lookup
Mercury-arc valve wikipedia , lookup
Electric machine wikipedia , lookup
Ground (electricity) wikipedia , lookup
Current source wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Stepper motor wikipedia , lookup
Power inverter wikipedia , lookup
Electrical substation wikipedia , lookup
Power engineering wikipedia , lookup
Voltage optimisation wikipedia , lookup
Voltage regulator wikipedia , lookup
Mains electricity wikipedia , lookup
Power electronics wikipedia , lookup
Earthing system wikipedia , lookup
Distribution management system wikipedia , lookup
Magnetic core wikipedia , lookup
Single-wire earth return wikipedia , lookup
Buck converter wikipedia , lookup
History of electric power transmission wikipedia , lookup
Opto-isolator wikipedia , lookup
Three-phase electric power wikipedia , lookup
Alternating current wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Residential Cabling Technologies Transformers Copyright © 2005 Heathkit Company, Inc. All Rights Reserved Objectives At the end of this presentation, you will be able to: 2 Define: transformer, primary, secondary, step-up, step-down, signal inversion, excitation current, mutual inductance, impedance matching, center-tap, autotransformer, and phase-splitting. Name the parts of a transformer. Calculate turns ratio, power ratio, and impedance ratio. 3 Determine the relationship between voltage, current, and impedance ratios. State four uses for the transformer. State the difference between a transformer and an autotransformer. Explain why the transformer is considered a safety device. 4 Mutual Inductance Mutual Inductance occurs when the magnetic field of one inductor induces a voltage into the coils of a second inductor. Mutual Inductance is the operating principle that defines transformer action. 5 L1 L2 Mutual Inductance 6 Transformers are devices designed specifically to take advantage of mutual inductance. A transformer typically consists of a Primary and a Secondary winding. The Transformer Primary is the winding that connects to the input voltage source. 7 L1 L2 Primary Winding (Input) Secondary Winding (Output) Cores Transformer Components 8 The Transformer Primary carries the Primary Current in the circuit. The Transformer Secondary is the winding which connects to the output. The Transformer Secondary carries the Secondary Current in the circuit. The Primary and Secondary current are 180 out-of-phase. 9 By winding the primary and secondary in opposite directions we can cause Signal Inversion. Signal Inversion occurs when the polarity of the input signal is inverted in the output. The induced secondary EMF depends upon the mutual induction between the coils. 10 Mutual Inductance is determined by the amount of flux linkage between the coils. A more common term for flux linkage is “coefficient of coupling.” Transformer input power is always more than the output power. 11 Transformer Construction Transformer construction methods vary by transformer type and application. The size can vary from several tons to the size of an eraser. The iron-core transformer is the type you are most likely to encounter. 12 Iron-core transformers employ a solid or laminated core. The primary windings are wound around the iron core. The secondary windings are wound directly on top of the primary. The iron core increases flux density. 13 Schematic Symbol 14 Transformer Theory For a transformer’s output to be useful, it must be “loaded” or attached to a circuit. Transformer operation changes from the loaded to no-load condition. In a no-load condition, the transformer’s impedance and applied voltage determines the primary current. 15 The small amount of current flow in the primary in a “no-load” condition is called the Excitation Current. Excitation Current Primary Secondary “No-Load” Condition 16 When a load is attached, the operation of the transformer changes: Primary AC establishes a fluctuating magnetic field. Field changes induce CEMF into primary and EMF into secondary. Induced EMF causes current flow in secondary. 17 We can use this simple circuit as an example. Induced EMF Applied EMF Counter EMF 18 Secondary current creates a magnetic field. Secondary flux creates EMF in primary. Primary current increases. Counter EMF Induced EMF Increased Current 19 Transformer Ratios Several ratios exist which help describe the operation of the transformer. The Turns Ratio compares the number of turns in the primary to the number of turns in the secondary. The voltage, power, current, and impedance ratios can be determined with the turns ratio. 20 The turns ratio is related to the output voltage or current of the transformer. A Step-Up transformer has a higher output on the secondary. A Step-Down transformer has a lower output on the secondary. The amount of step-up or step-down is directly proportional to the turns ratio. 21 Here we have a simple Step-Up transformer. Primary Secondary 2000 Turns 1000 Turns To find the Turns Ratio, divide secondary turns by primary turns. The ratio is expressed as “2 to 1” or 2:1. The Voltage Ratio expressed as a formula is: NS 2000 or 2 N P 1000 ES EP NS NP 22 We can verify this by transposing the previous formula: In this case, if the input on the primary is 115V, the secondary output is 230V. We can adjust the turns ratio to obtain a specific output voltage. ES NS NP x EP ES 2 x 115 v 230 v 23 We can find the output of a step-down transformer in the same manner. Primary Secondary 1000 Turns 2000 Turns If we have 120V on the 1000 primary, solving gives ES x 120 60V 2000 us 60V on the secondary. When a transformer steps up voltage, it also steps down current. IP IS NS NP 24 We can transpose the formula to obtain: IS IP x NP NS If we have a step-up transformer with 100 turns on the primary, 400 on the secondary, and 1A of primary current. 1 x 100 IS 0.25A 400 If the situation were reversed for a stepdown transformer: 1 x 400 IS 4.0A 100 25 Similarly, we can derive a formula for the power ratio. Assume we have a stepup transformer with 500 turns on the primary, 1000 secondary turns, an input voltage of 100 VAC, and a secondary current of 25 mA. What is the power output? P NS NP x IS x E P 1000 P x .025 x 100 500 26 Impedance matching is one of the most important transformer applications. Maximum power can only be transferred when impedance between the source and load match. Impedance mismatches result in inefficient operation and power losses. 27 So far, we have seen that: Voltage is directly proportional to the turns ratio. Current is inversely proportional to the turns ratio. Power is unaffected by the turns ratio. The impedance ratio is handled a bit differently 28 NP ZS N 2 S The Impedance Ratio is equal to the turns ratio squared: ZP An alternate expression of the formula is: NP If we have an amplifier with a 100-ohm output and 4-ohm input speakers, what turns ratio is necessary to match the impedances? NS 2 ZP ZS 100 Ω 25 ZS 4Ω ZP 29 Transformer Losses All transformers have losses during normal operation. Transformer losses and efficiency are inversely related. Major power transformer loses occur in the iron core. 30 Transformer Applications Transformers are commonly used in power applications to provide specific voltage and current needs. Other applications include phaseshifting, phase-splitting, isolation, and autotransformers. 31 Transformers can phase shift the output 180 based upon the winding direction. Primary 1 3 Secondary Load 2 4 In-Phase Transformer 32 By crossing the output, we can create a 180 phase shift in the output. Primary 1 3 Secondary Load 2 4 Out-of-Phase Transformer 33 A “short hand” is used in schematic diagrams to show the phase relation. Phase Dots show the phase relation between the transformer input and output. Phase Dots In-Phase Out-of-Phase 34 A Center Tap allows out-of-phase signals to be taken from the same winding. This is referred to as Phase Splitting. 1 3 Center Tap 4 2 5 Center-Tapped Secondary 35 One of the key functions of a transformer is electrical isolation. Power transformers are used to isolate the equipment chassis from the AC input line. This isolation is a key safety factor which helps prevent electrical shocks. 36 An Autotransformer is a special transformer which does not isolate the primary and secondary windings. An autotransformer uses a single coil; part is the primary, and part is the secondary. Autotransformers offer advantages over conventional transformers. 37 Autotransformers may be configured as step-up or step-down transformers. 1 Primary Winding Step-Down Autotransformer 2 Secondary Winding Load 3 38 In a Step-up autotransformer, the configuration is reversed. 1 Step-Up Autotransformer Secondary Winding 2 Load Primary Winding 3 39 The autotransformer performs slightly different than conventional transformers. 1 Primary Winding (6000) 2 Secondary 5A Winding (1000) 1A Load 3 40 One unique type of autotransformer is the variable autotransformer. Variable Autotransformer Load 41 Residential Cabling Technologies End Copyright © 2005 Heathkit Company, Inc. All Rights Reserved