Transcript 120 V + + + - McGraw Hill Higher Education

12 - 1
Electricity
Principles & Applications
Eighth Edition
Richard J. Fowler
Chapter 12
Transformers
(student version)
McGraw-Hill
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12 - 2
INTRODUCTION
•
•
•
•
•
McGraw-Hill
Transformer Fundamentals
Transformer Efficiency
Loaded Transformers
Three-phase Transformers
Impedance Matching
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
12 - 3
Dear Student:
This presentation is arranged in segments. Each segment
is preceded by a Concept Preview slide and is followed by a
Concept Review slide. When you reach a Concept Review
slide, you can return to the beginning of that segment by
clicking on the Repeat Segment button. This will allow you
to view that segment again, if you want to.
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12 - 4
Concept Preview
• The primary takes power from the source. (Page 310)
• The secondary provides power to the load. (Page 310)
• Primary and secondary voltages are 180
out-of-phase.
(Page 319)
• Any winding can be used as the primary. (Page 310)
• With unity coupling, the primary T/V equals
the secondary T/V.
McGraw-Hill
(Page 312)
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12 - 5
Facts About Transformers
• Transformers operate on mutual inductance.
• A transformer has a primary winding and a secondary winding.
• The coefficient of coupling is the portion of primary flux
that links the secondary.
• With 100% coupling, the turns-per-volt ratio is the same for
all windings.
• Transformers can have hysteresis, eddy current, and copper
(I2R) losses.
• Transformer losses can be reduced by using silicon steel cores,
laminated cores, and small gage wires.
• As the load on a transformer increases, angle theta decreases.
• Three-phase transformers use a three-legged core.
• Transformer windings can be connected© 2013
inTheseries
or parallel.
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12 - 6
Transformer Fundamentals (Page 309)
Primary
-
Secondary
+
+ Load -
A transformer has a primary coil and a secondary coil.
The primary is connected to a source. The secondary is connected to a load.
During the first half-cycle, the flux builds up and collapses.
This creates a half-cycle of induced voltage in the secondary.
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Transformer Fundamentals
Primary
+
Secondary
-
- Load +
During the next half-cycle, the flux again builds up and collapses.
This creates another half-cycle of induced voltage in the secondary.
Notice that the primary and secondary voltages are out-of-phase.
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12 - 8
Primary-Secondary Terminology (Page 310)
Transformers are bidirectional devices.
120 V
Source
120 V
Primary
90 V
Load
Secondary
This transformer was designed to step 120 V down to 90 V.
Load
120 V
Secondary
90 V
90 V
Source
Primary
However, either winding can be used as the primary.
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12 - 9
Turns-Per-Volt Ratio
(Page 306)
_
+
0.25 V
+
_
1.0 V
_
0.75 V
+
The 4-turn primary with a 1 volt source provides 4 turns-per- volt;
therefore, each turn has 0.25 V across it.
Each turn will produces a specific amount of flux in the core.
This same flux will, in turn, produce 0.25 V in each secondary turn.
Thus, a 1 turn secondary provides 0.25 V, and a 3 turn secondary
provides 0.75 V.
© 2013 The McGraw-Hill Companies Inc. All rights reserved.
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12 - 10
Concept Review
• The primary takes power from the source.
• The secondary provides power to the load.
• Primary and secondary voltages are 180
out-of-phase.
• Any winding can be used as the primary.
• With unity coupling, the primary T/V
equals the secondary T/V.
Repeat Segment
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12 - 11
Concept Preview
• Core and copper losses produce heat
and reduce transformer efficiency. (Page 316)
• Hysteresis, eddy current, and copper(I2R)
are the three losses in a transformer. (Page 315)
• Thinner laminations reduce eddy currents. (Page 316)
• Use laminations that have a narrower
hysteresis loop to reduce hysteresis loss. (Page 315)
• Use larger conductors to reduce copper loss. (Page 317)
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12 - 12
Transformer Losses (Page 315)
Copper (I2R) loss
B
H
Hysteresis loss (green area
of the hysteresis loop)
Eddy current loss
(I2R in the core)
These losses are minimized by a narrow hystersis loop,
thin laminations, and large diameter wire.
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12 - 13
Transformer Efficiency (Page 315)
Source
provides
1640 W
Transformer
90 W loss
(heat loss)
Load
1550 W
consumed
The transformer core and copper losses cause the transformer
to heat up as electric energy is converted to heat energy.
Efficiency = 1550 W / 1640 W = 0.945
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12 - 14
Transformer-Action Quiz
The ____ coil of a transformer is
connected to the source.
primary
Flux builds and collapses in a core ____
time(s) each cycle.
two
A transformer provides ____ degrees of
phase shift between its two windings.
180
Any winding of a transformer can be used
as the ____ winding.
primary
(or secondary)
Core losses can be reduced by using ____
laminations.
thinner
Transformer inefficiency is caused by core
and ____ losses.
McGraw-Hill
copper
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12 - 15
Concept Review
• Core and copper losses produce heat
and reduce transformer efficiency.
• Hysteresis, eddy current, and copper(I2R)
are the three losses in a transformer.
• Thinner laminations reduce eddy currents.
• Use laminations that have a narrower
hysteresis loop to reduce hysteresis loss.
• Use larger conductors to reduce copper loss.
Repeat Segment
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12 - 16
Concept Preview
•Energizing current is the no-load
primary current. (Page 318)
•Primary current is increased by the reflected
current when a load is applied. (Page 318)
•Loading a transformer decreases theta. (Page 318)
•At any instant, at least two phases are
creating flux in a 3- transformer. (Page 331)
•Flux density shifts from leg to leg in a
3- transformer. (Page 331)
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12 - 17
Primary Current (Page 320)
Load
Iref
Vpri
Ipri
With no load, Ipri is the
energizing current (Ien).
The transformer acts
like an inductor.
Theta is large;
power is low.
McGraw-Hill
Ien
Vpri
Ipri
With a load, Ipri
is composed of Ien
and Iref . Iref is the
resistive load current
reflected from the secondary to the primary.
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12 - 18
Flux In A Three-Phase Core
Core
Phase
1
coil
Phase
2
coil
Phase 3
(Page 331)
Phase 1
Phase 2
Phase
3
coil
Time 1
At time 1, flux created by phase 2 and phase 3 join together.
There is no phase-1 flux because phase-1 current is zero.
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12 - 19
Flux In A Three-Phase Core
( Time 2)
Core
Phase
1
coil
Phase
2
coil
Phase 3
Phase 1
Phase 2
Phase
3
coil
Time 2
At time 2, flux created by phase 2 and phase 3 still join together.
Also, flux created by phase 1 and phase 2 join together.
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12 - 20
Flux In A Three-Phase Core
(Time 3)
Core
Phase
1
coil
Phase
2
coil
Phase 3
Phase 1
Phase 2
Phase
3
coil
Time 3
At time 3, flux created by phase 1 and phase 2 still join together.
Also, flux created by phase 1 and phase 3 join together.
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12 - 21
Flux In A Three-Phase Core
(Time 4)
Core
Phase
1
coil
Phase
2
coil
Phase 3
Phase 1
Phase 2
Phase
3
coil
Time 4
At time 4, flux created by phase 1 and phase 3 still join together.
Also, flux created by phase 2 and phase 3 join together.
McGraw-Hill
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12 - 22
Primary-Current Quiz
The no-load primary current is called
the ____ current.
energizing
The additional primary current caused by loading reflected
the transformer is called the ____ current.
(or resistive)
Angle theta ____ when a load is
connected to a transformer.
decreases
A transformer with no load acts like a(n) ____.
inductor
In a three-phase transformer, at least ____
primary coils are creating flux at any instant.
two
McGraw-Hill
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12 - 23
Concept Review
• Energizing current is the no-load
primary current.
• Primary current is increased by the
reflected current when a load is applied.
• Loading a transformer decreases theta.
• At any instant, at least two phases are
creating flux in a 3- transformer.
• Flux density shifts from leg to leg
in a 3- transformer.
Repeat Segment
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12 - 24
Concept Preview
•Windings can be connected series-aiding
or series-opposing. (Page 328)
•For parallel connection, like instantaneous
polarities are connected together. (Page 327)
•The series winding with the smallest I rating
determines the available current. (Page 328)
•Transformers can provide impedance
matching between load and source. (Page 324)
McGraw-Hill
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12 - 25
Series-Opposing Windings (Page 328)
_
8V
2A
+
+
_
120 V
_
+
6V
3A
+
2V
2A
_
Connect two terminals with the same instantaneous polarities.
Take the output from the other two terminals.
The output voltage equals the difference between the two voltages.
The current is limited to the lesser of the two winding currents.
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12 - 26
Series-Aiding Windings (Page 328)
_
_
8V
2A
+
+
_
120 V
_
14 V
2A
6V
3A
+
+
Connect two terminals that have opposite instantaneous polarities.
Take the output from the other two terminals.
The output voltage equals the sum of the two winding voltages.
The current is limited to the lesser of the two winding currents.
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12 - 27
Parallel Windings
_
_
9V
4A
+
+
_
120 V
(Page 327)
_
9V
8A
9V
4A
+
+
The two windings must have equal voltage ratings.
The two windings should have equal current ratings.
Connect the negative terminals and the positive terminals.
Take the output from the negative and positive terminals.
Voutput = Vwinding and Ioutput = 2 x Iwinding
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12 - 28
Impedance Matching
100 W
10 V
0.1 A
10:1
ratio
0.1 A
10 V
1W
1V
20 V
Notice the source in both circuits
provides 0.1 A at 10 V. Thus, the
transformer makes the 1-W resistor
act like a 100-W resistor in terms
of the load on the source.
McGraw-Hill
(Page 324)
1W
1V
1A
1W
100 W
10 V
0.1 A
100 W
10 V
0.1 A
1W
20 V
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12 - 29
Series and Parallel-Windings Quiz
Series-opposing windings produce a voltage equal
to the ____ of the voltages of the two windings.
The current rating of ____ connected windings is
that of the winding with the smallest current rating.
A 12-V, 2-A secondary series aiding a 6-V, 3-A
secondary will provide ____ V and ____ A.
difference
series-
18, 2
For parallel windings, ____ instantaneous polarities
should be connected together.
like
Parallel windings must have equal ____ ratings.
voltage
A transformer used to make a load appear to be other impedancethan its true value is called a(n) ____ ____ transformer. matching
McGraw-Hill
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12 - 30
Concept Review
• Windings can be connected series-aiding
or series-opposing.
• For parallel connection, like instantaneous
polarities are connected together.
• The series winding with the smallest I
rating determines the available current.
• Transformers can provide impedance
matching between load and source.
Repeat Segment
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12 - 31
REVIEW
•
•
•
•
•
McGraw-Hill
Transformer Fundamentals
Transformer Efficiency
Loaded Transformers
Three-phase Transformers
Impedance Matching
© 2013 The McGraw-Hill Companies Inc. All rights reserved.