ECET 211 Electric Machines & Controls Lecture 3-2 (Part 2 of 2) Motor Transformers and Distribution Systems

Transcription

1 ECET 211 Electric Machines & Controls Lecture 3-2 (Part 2 of 2) Motor Transformers and Distribution Systems Text Book: Electric Motors and Control Systems, by Frank D. Petruzella, published by McGraw Hill, Paul I-Hai Lin, Professor of Electrical and Computer Engineering Technology P.E. States of Indiana & California Dept. of Computer, Electrical and Information Technology Purdue University Fort Wayne Campus Prof. Paul Lin 1 Lecture 3 Motor Transformers and Distribution Systems Chapter 3. Motor Transformers and Distribution Systems Part 1. Power Distribution Part 2. Transformer Principles Part 3. Transformer Connections and Systems Prof. Paul Lin 2 1

2 Part 2. Transformer Principles Transformer Operation Change only voltage level: V P = 120V, V s = 25V; I P = 1 A, I S = 4A P Primary = P secondary = 100 VA (neglect losses) Transformer is made up of: Core, Primary windings Secondary windings Enclosure Figure 3-18 Simplified version of a single-phase (1Φ) transformer Prof. Paul Lin 3 Part 2. Transformer Principles Transformer Voltage, Current, and Turn Ratio Pp = Vp*Ip = Ps = Vs* Is (Neglect losses) N = Turns in Primary/ Turns in Secondary = Np/Ns N = Primary Voltage / Secondary Voltage = Vp/Vs N = Secondary Current/ Primary Current = Is/Ip Table of turn ratio for single-phase transformers Primary V Secondary V Turn Ration N 480V 240V 2:1 480V 120V 4:1 480V 24V 20:1 600V 120V 5:1 600V 208V 2.88:1 208V 120V 1.73:1 Prof. Paul Lin 4 2

3 Part 2. Transformer Principles Figure 3-20 Step-up transformer Vp = 120V, Vs = 240V Ip = 10A, Is = 5A Figure 3-21 Step-down Transformer Operation Prof. Paul Lin 5 Part 2. Transformer Principles Figure 3-22 Step-down transformer connected to a resistive load Is = Vs/R = 24V/0.6Ω = 40A N = Vp /Vs = Is/Ip => Ip = Is/N = 2A Prof. Paul Lin 6 3

4 Part 2. Transformer Principles Transformer Power Rating (kva) Single-phase loads: kva = I * V/1000 Three-phase loads: kva = 3 * I * V /1000 Transformer Current Rating Ip full load current = kva x1000/vp = 52A Is full load current = kva x 1000/Vs = 104A Figure 3-22 Single-phase 25 kva transformer, rated 480/240V Prof. Paul Lin 7 Part 2. Transformer Principles Figure 3-23 Three-phase 37.5 kva transformer, rated 480/240V Transformer Current Ip full load current = kva x1000/( 3 * Vp) = 45A Is full load current = kva x 1000/( 3 * Vs) = 104A Prof. Paul Lin 8 4

5 Part 2. Transformer Principles Transformer Performance (efficiency) Efficiency = (Pout / Pin )* 100% - 94 to 98% Core losses (Iron losses) Coil losses (copper losses) Temperature rise Voltage regulation Inrush current Prof. Paul Lin 9 Part 3. Transformer Connections and Systems Transformer Polarity Figure 3-25 Transformer polarity markings High-voltage windings: H1, H2 Low-voltage windings: X1, X2 Prof. Paul Lin 10 5

6 Figure 3-26 Additive and subtractive transformer terminal markings Prof. Paul Lin 11 Single-Phase Transformers Figure 3-27 Motor control transformer wiring Step down: 480V => 120V Prof. Paul Lin 12 6

7 Single-Phase Transformers Figure 3-28 Typical dual-voltage 480V and 240V transformer connections 480V source High side windings in series 240V source High side windings in parallel Prof. Paul Lin 13 Single-Phase Transformers Figure 3-29 Fuse protection for both the primary and secondary of the transformer and the correct ground connection for a grounded control system Prof. Paul Lin 14 7

8 Three-Phase Transformers Figure 3-30 Common wye and delta transformer connections Prof. Paul Lin 15 Three-Phase Transformers Figure 3-31 Three-phase, three-wire delta transformer connections supplying power to a three-phase motor load kva (transformer) = I Line * V Line * 3/ = Prof. Paul Lin 16 8

9 Figure 3-32 Wye-connected, three-phase, four-wire distribution system VL = VP * 3 (Line voltage = * Phase voltage) IL = IP (Line current = Phase current) kva (transformer) = IL * VL * 3/1000 Common arrangement are 480Y/277V, and 208Y/120V Prof. Paul Lin 17 Figure 3-33 Typical delta-to-wye, three-phase, four-wire transformer system Neutral point at the secondary Side for supplying line-to-neutral power to single-phase loads Three-phase loads: 208 V Single-phase loads: 208V or 120V Prof. Paul Lin 18 9

10 Autotransformers Figure 3-34 Autotransformer Figure 3-35 Autotransformer motor starter Prof. Paul Lin 19 Figure 3-36 Instrument Transformers Potential Transformers (PT) Current Transformers (CT) Prof. Paul Lin 20 10

11 Instrument Transformers Figure 3-37 Current transformers Figure 3-38 Current transformer polarity Prof. Paul Lin 21 Summary & Conclusion Questions? Contact Prof. Lin through: LINE Discussion Group Prof. Paul Lin 22 11