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, 2015.
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
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Lecture 3 Motor Transformers and Distribution
Systems
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Chapter 3. Motor Transformers and Distribution Systems
• Part 1. Power Distribution
• Part 2. Transformer Principles
• Part 3. Transformer Connections and Systems
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Part 2. Transformer Principles
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Transformer Operation
• Change only voltage level: VP = 120V, Vs = 25V; IP = 1 A, IS = 4A
• PPrimary = Psecondary = 100 VA (neglect losses)
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Transformer is made up of:
• Core, Primary windings
• Secondary windings
• Enclosure
Figure 3-18 Simplified version of a single-phase (1Φ) transformer
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Part 2. Transformer Principles
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Transformer Voltage, Current, and Turn Ratio
•
•
•
•
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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
480V
120V
4:1
480V
24V
20:1
600V
120V
5:1
600V
208V
2.88:1
208V
120V
1.73:1
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2:1
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Part 2. Transformer Principles
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Figure 3-20 Step-up transformer
• Vp = 120V, Vs = 240V
• Ip = 10A, Is = 5A
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Figure 3-21 Step-down Transformer Operation
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Part 2. Transformer Principles
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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
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Part 2. Transformer Principles
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Transformer Power Rating (kVA)
• Single-phase loads: kVA = I * V/1000
• Three-phase loads: kVA =√3 * I * V /1000
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Transformer Current Rating
• Ip – full load current = kVA x1000/Vp = 52A
• Is – full load current = kVA x 1000/Vs = 104A
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Figure 3-22 Single-phase 25 kVA transformer, rated
480/240V
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Part 2. Transformer Principles
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Figure 3-23 Three-phase 37.5 kVA transformer, rated
480/240V
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Transformer Current
• Ip – full load current = kVA x1000/(√3 * Vp) = 45A
• Is – full load current = kVA x 1000/(√3 * Vs) = 104A
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Part 2. Transformer Principles
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Transformer Performance (efficiency)
• Efficiency = (Pout / Pin )* 100% - 94 to 98%
 Core losses (Iron losses)
 Coil losses (copper losses)
• Temperature rise
• Voltage regulation
• Inrush current
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Part 3. Transformer Connections and Systems
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
Transformer Polarity
Figure 3-25 Transformer polarity markings
• High-voltage windings: H1, H2
• Low-voltage windings: X1, X2
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Figure 3-26 Additive and subtractive transformer terminal
markings
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Single-Phase Transformers
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Figure 3-27 Motor control transformer wiring
• Step down: 480V => 120V
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Single-Phase Transformers
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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
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Single-Phase Transformers
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Figure 3-29 Fuse protection for both the primary and
secondary of the transformer and the correct ground
connection for a grounded control system
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Three-Phase Transformers
 Figure 3-30 Common wye and
delta transformer connections
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Three-Phase Transformers
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Figure 3-31 Three-phase, three-wire delta transformer
connections supplying power to a three-phase motor
load
• kVA (transformer) = ILine * VLine * √3/1000
• √3 = 1.732
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Figure 3-32 Wye-connected, three-phase, four-wire
distribution system
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
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VL = VP * √3 (Line voltage = 1.732 * Phase voltage)
IL = IP (Line current = Phase current)
kVA (transformer) = IL * VL *√3/1000
Common arrangement are 480Y/277V, and 208Y/120V
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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
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Three-phase loads: 208 V
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Single-phase loads: 208V or
120V
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Autotransformers
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Figure 3-34 Autotransformer
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Figure 3-35 Autotransformer motor starter
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Figure 3-36 Instrument Transformers
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Potential Transformers (PT)
Current Transformers (CT)
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Instrument Transformers
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Figure 3-37 Current transformers
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Figure 3-38 Current transformer polarity
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Summary & Conclusion
Questions?
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
Contact Prof. Lin through:
Email: lin@ipfw.edu
LINE Discussion Group
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