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