3.15 Transformers Flashcards
What is a transformer?
An electrical device that changes voltage and current in AC circuits without significant power loss.
What does a step-up transformer do?
Increases voltage and decreases current.
What does a step-down transformer do?
Decreases voltage and increases current.
What are the main components of a transformer?
Two coils and an iron core.
What is the primary winding of a transformer?
The input coil.
What is the secondary winding of a transformer?
The output coil.
What is the purpose of a laminated iron core?
To suppress eddy currents that waste power by heating the core.
How does a transformer operate?
It creates a changing magnetic field in the iron core, inducing voltage in the output coil.
What is mutual induction in a transformer?
The process of producing an EMF in the secondary coil due to the changing magnetic field from the primary coil.
Does frequency change in a transformer?
No, frequency does not change in a transformer.
What is the voltage phase relationship in a transformer?
It may be either 0° or 180°, depending on the winding of the secondary relative to the primary.
Why are transformer symbols marked with phasing dots?
To indicate the phase relationship between the primary and secondary coils.
Fill in the blank: A transformer transfers energy via a _______.
[magnetic circuit].
What is the efficiency range for small power transformers used in electrical equipment?
80-90%
Large commercial power line transformers can have efficiencies of over 98%.
What are the two types of total power loss in a transformer?
- Copper loss
- Iron loss
What causes Copper loss in transformers?
Direct current (DC) resistance in the primary and secondary windings.
What are the two subcategories of Iron loss in transformers?
- Hysteresis loss
- Eddy current loss
What do all transformer losses convert electrical energy into?
Undesirable heat energy.
What factors determine the amount of power dissipated as heat in Copper loss?
- Resistance of the wire
- Square of the current
How can Copper loss be minimized?
Using the proper diameter wire.
What type of material is commonly used for the core of a transformer?
Ferromagnetic material.
What is the effect of AC on the primary of an iron-core transformer?
Generates a fluctuating magnetic field.
What causes eddy currents to flow through the core of a transformer?
Induced voltage from the fluctuating magnetic field.
How are transformer coils designed to minimize eddy current loss?
They are laminated with thin, insulated layers.
What is hysteresis loss?
Energy dissipated as heat when magnetic domains realign with a changing magnetic field.
How often must magnetic domains realign in a transformer operating on 50 Hz AC?
100 times a second.
What can be done to minimize hysteresis loss in transformers?
Using appropriate core materials.
What happens when an electrical load is connected to the secondary winding of a transformer?
A current flows in the secondary winding and out to the load
What is the secondary current in a transformer influenced by?
The characteristics of the load
What does the secondary current create in the transformer core?
A self-induced secondary magnetic field
What direction does the self-induced secondary magnetic field flow in relation to the main primary field?
In the opposite direction
What effect does the opposition of the two magnetic fields have on the overall magnetic strength?
It reduces the overall magnetic strength
What happens to the back electromagnetic field (EMF) of the primary winding when the secondary current flows?
It reduces
What occurs to the primary current when the back EMF is reduced?
It slightly increases
What must a transformer maintain between the primary and secondary magnetic fields to operate correctly?
A balance
What is the relationship between the total induced voltage in each winding and the number of turns inside it?
Proportional
What is the formula for power output and input in a transformer?
Power = Volts x Amperes
What is the relationship between the voltage ratio and the turn ratio of a transformer?
The voltage ratio is equal to its turn ratio
Fill in the blank: Higher voltage results in _______.
Lower current
Fill in the blank: Lower voltage results in _______.
Higher current
What does a transformer ratio involve relationships between?
- The number of turns in the primary and secondary windings
- The voltage across each winding
- The current through the windings
What condition defines a transformer under no-load?
No electrical load connected to its secondary winding
What occurs when the secondary circuit of a transformer is open-circuited?
No secondary current flows
What limits the flow of primary current when a transformer is under no-load?
Back EMF and primary winding resistance
What must the no-load primary current maintain?
Enough magnetic field to produce the required back EMF
What two components make up the no-load primary current?
- An in-phase current that supplies core losses
- A small current at 90° to the voltage that sets up the magnetic flux
Is the no-load primary current large or small compared to the transformer’s normal full-load current?
Very small
What are the losses present in the transformer core that affect the no-load primary current?
- Iron losses
- A small amount of copper losses
What is the energy component and magnetising component of a single-phase transformer given in the example?
- Energy component: 2 Amps
- Magnetising component: 5 Amps
What is the power transfer characteristic of an ideal transformer?
In an ideal transformer, there is an equal amount of power in the primary and secondary windings, maintaining a power ratio of one (unity).
How does a transformer affect voltage and current?
A transformer can step up or step down voltage, but it does not change power; when voltage increases, current decreases and vice-versa.
What is the relationship between primary power and secondary power in a transformer?
Primary power equals secondary power.
What happens to power loss when voltage is increased in a transformer?
Increasing voltage decreases current, reducing losses by a factor of the square of the voltage increase.
What types of losses do transformers experience?
Transformers experience copper losses and iron losses.
What is the general efficiency range of real transformers at full load?
Typical efficiencies range from 94% to 96%.
What is the maximum efficiency that high-capacity transformers can achieve?
High-capacity transformers can achieve efficiencies as high as 98%.
What is the formula for the efficiency of a transformer?
Efficiency is equal to the ratio of power output of the secondary winding to the power input of the primary winding.
What does the primary power in transformers express as?
Volt-amps (VA).
What does the secondary power in transformers express as?
Watts (W).
How can the relationship between the transformer’s input, output, and efficiency be visualized?
Using a triangle that superimposes VA, W, and efficiency.
What is the purpose of polarity markings in transformers?
To denote phase relationships and ensure proper connections to maintain in-phase voltage and current.
What is the dot convention in transformers?
A marking system that indicates phase relationships by placing a dot next to each corresponding leg of a transformer winding.
What do matching dots on transformer windings indicate?
They indicate that the instantaneous voltage polarity across the primary winding is the same as that across the secondary winding.
What does it mean if the dots on each winding do not match up?
There is a 180° phase shift between primary and secondary.
How can one reverse the phase relationship in transformers?
By swapping the winding connections.
Fill in the blank: The efficiency of a transformer is equal to the ratio of the power output of the _______ winding to the power input of the primary winding.
secondary
What is the relationship between the effective length of a conductor and voltage in a transformer?
The effective length of the conductor influences the voltage induced in a coil.
How is output voltage calculated in a transformer?
Output voltage depends on the number of turns in the primary and secondary coils.
If an AC transformer converts 230 V to 24 V with a secondary coil of 100 turns, how many turns must the primary winding have?
Calculated using the transformer voltage formula.
What principle assumes power output equals power input in transformers?
Current transformation principle.
According to the power formula, what happens to current when voltage decreases in a transformer?
The current must increase to maintain the same power output.
If the output voltage is double the input voltage, how does the input current relate to the output current?
Input current must be double the output current.
How many amperes are consumed by a primary winding with 1000 turns if the secondary winding of a transformer with 60 turns consumes 20 A?
Calculated using the current transformation formula.
What is the purpose of using transformers in communications engineering?
To match resistances for maximum power transmission.
When does a transformer transmit the most power?
When the internal resistance of the generator equals that of the load.
What law is used to express the transformation of resistance in transformers?
Ohm’s Law.
What is the formula for transformation of resistance in a transformer?
R_primary = R_secondary * (N_primary / N_secondary)^2.
What is impedance matching in transformers?
It ensures maximum power transfer when load impedance equals source impedance.
What is required to calculate the necessary turns ratio for an impedance matching transformer?
The formula for turns ratio involving impedance.
True or False: Maximum power is transferred only when the load impedance is less than the internal impedance of the source.
False.
Fill in the blank: A transformer can match a pre-amplifier of 20,000 ohms input impedance to a moving coil microphone of ______ ohms.
200.
What happens if the internal resistance of the generator and load resistance differ?
The transformer functions as a transmitter to match them.
What is the common type of transformer used for three-phase a.c. systems?
A single, three limbed transformer
It is more common than using three interconnected single-phase transformers.
What is the efficiency percentage of Tight Coupling in a three-phase transformer?
99.9% efficient
Tight Coupling improves magnetic linkage efficiency.
What are the other types of coupling in a three-phase transformer?
- Loose Coupling
- Medium Coupling
In a three-phase transformer, how are the windings configured?
In a star or Y shape
The currents are out of phase by 120°.
What are the two types of connection configurations for a three-phase transformer?
- Delta (Δ)
- Star (Y)
What advantage do Star Y connections provide in a three-phase transformer?
Opportunity for multiple voltages
What is the primary advantage of DeltaΔ connections in a three-phase transformer?
Higher level of reliability
What is the power rating of a transformer measured in?
Volt-amperes (VA), kilo volt-amperes (kVA), or mega volt-amperes (MVA)
What is active power in a transformer?
Power used to generate real work, measured in kW
What is reactive power in a transformer?
Power consumed when an inductive load is connected, measured in kVAR
What does apparent power combine?
Active and reactive power
True or False: A transformer can step up power.
False
What is the formula for calculating the VA rating of a transformer?
VA = Voltage x Current
What is the maximum safe power loading for a transformer in terms of its VA/kVA rating?
Loading above 80% is not recommended
What is the formula for calculating the kVA rating of a transformer given voltage and current?
kVA = (Voltage x Current) / 1000
What is the formula for power factor in an AC circuit?
Power Factor = cos(Phase Difference)
What does a power factor of unity indicate?
There is no phase difference between voltage and current
Fill in the blank: A three-phase transformer has ______ windings.
six windings
How are the primary windings labeled in a three-phase transformer?
A, B, C
How are the secondary windings labeled in a three-phase transformer?
a, b, c
What happens to current when a transformer steps up voltage?
It steps down current
What type of insulation is used on the windings of a three-phase transformer?
Resin type such as Kapton
What is the purpose of the insulation between the primary and secondary windings?
To reduce the possibility of fire
What is the phase difference between the currents in a three-phase system?
120°
What is the principle behind a transformer?
A transformer operates on the principle of electromagnetic induction, where a fluctuating electric current in one coil generates a magnetic field that induces a current in a neighboring coil.
What are the primary and secondary currents in a transformer?
The current in the first coil is called the primary current, and the current in the second coil is called the secondary current.
What happens to voltage and current in a step-down transformer?
In a step-down transformer, the secondary voltage is smaller than the primary voltage, and the secondary current is larger than the primary current.
In a step-up transformer, how do the number of turns relate to voltage and current?
In a step-up transformer, the secondary coil has more turns than the primary coil, resulting in a higher secondary voltage and a lower secondary current.
What is the formula relating voltage, current, and number of turns in a transformer?
The formulas are:
* Vp / Vs = Np / Ns
* Ip / Is = Ns / Np
Fill in the blank: The power in an electric current is equal to the current times the _______.
[voltage]
What is the turns ratio of a transformer with 500 turns in the primary winding and 1000 turns in the secondary winding?
The turns ratio is 1:2.
True or False: A step-down transformer increases the voltage while decreasing the current.
False
What is the effect of wrapping coils around a soft iron bar in a transformer?
Wrapping coils around a soft iron bar increases the efficiency of electromagnetic induction between the coils.
What occurs if the voltage rating of a transformer coil is exceeded?
Exceeding the voltage rating can lead to insufficient current limits due to lower reactance, potentially damaging the transformer.
In a step-down transformer with 100 turns in the primary and 10 turns in the secondary, what is the relationship between primary and secondary voltage?
The secondary voltage is reduced by a factor of 10 compared to the primary voltage.
What do the terms ‘step-up’ and ‘step-down’ transformers refer to?
‘Step-up’ transformers increase voltage, while ‘step-down’ transformers decrease voltage.
What determines whether a transformer is classified as step-up or step-down?
The turns ratio between the two coils determines if the transformer is a step-up or step-down transformer.
What is the construction of an autotransformer?
An autotransformer has only one coil for the input and output sides, with primary and secondary sides sharing a part of the windings.
What is the primary function of an autotransformer?
To step up or step down voltages.
How do the primary and secondary windings of an autotransformer differ from conventional transformers?
In an autotransformer, the primary and secondary windings comprise different sections of the same coil, sharing a common section.
What is the major advantage of an autotransformer compared to a conventional transformer?
Its small design.
What is the throughput rating in an autotransformer?
The total power output that can be tapped by the wire of the input coil, which is less than the rated power.
What is the difference between rated power and throughput rating in transformers?
Rated power is the power transmitted by induction, while throughput rating is the total power output.
What is a significant disadvantage of an autotransformer?
It lacks primary to secondary winding isolation.
What happens if the secondary winding of an autotransformer becomes open-circuited?
Load current stops flowing through the primary winding, stopping transformer action and applying full primary voltage to the secondary terminals.
What is the consequence of a short-circuit condition in the secondary circuit of an autotransformer?
The resulting primary current would be much larger than in a double wound transformer, potentially causing damage.
What is the function of a variable transformer, also known as a Variac?
To vary the output voltage on the secondary side, either through switchable outputs or a slider arm.
What is a current transformer (CT)?
A special transformer that allows AC currents to be measured without breaking the current-carrying leads.
What must always be connected to current transformers to prevent high voltage?
A load (ammeter).
Fill in the blank: An autotransformer has a _______ for both the input and output sides.
single coil
True or False: An autotransformer can safely be used for stepping down higher voltages to much lower voltages.
False
