Chap6_bekg1113
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CHAPTER 6 Transformer & Electrical Machines
TRANSFORMERS
A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts.
When two loops with or without contacts between them affect each other through the magnetic field generated by one of them they are said to be magnetically coupled.
TRANSFORMERS
Transformers uses the magnetically coupled coils to transfer energy from one circuit to another.
Transformer is used for stepping up or stepping down ac voltages or current, for isolation between two portion of a circuit and for impedance matching.
Transformer consists of two coils. The coils that is directly connected to the voltage source is called the primary winding.
Coil that is connected to the load is called secondary winding.
TRANSFORMERS
TRANSFORMERS
The iron core of a transformer is made up of sheets of rolled iron.
This iron is treated such that it has high magnetic conducting quality (high permeability) throughout the length of the core.
Permeability is the ability of a material to allow the magnetic flux, φ to flow through it.
TRANSFORMERS
From the figure, N1 and N2 is the number of turns for primary winding and secondary winding respectively.
V1 and V2 is the voltage across the primary winding and secondary winding respectively.
TRANSFORMERS
When a sinusoidal voltage is applied to the primary winding, the same magnetic flux φ goes through both windings.
According to Faraday’s law, the voltage across the primary winding and secondary winding are
d V1 N1 , dt or
d V2 N 2 dt
V1 = 4.44 φm f N1,
where φm is the maximum flux f is the supply frequency
V2 = 4.44 φm f N2
TRANSFORMERS
Dividing both of the above equation gives
V2 N 2 n V1 N1 where n is the turns ratio or transformer ratio.
For the reason of power conservation, the energy supplied to the primary must equal the energy absorbed by the secondary, since there are no losses in an ideal transformer. Or the apparent power for both winding are the same. Thus,
S1 S 2
V1 I1 V2 I 2 I1 V2 n I 2 V1
When n = 1, we generally call the transformer an isolation transformer.
TRANSFORMERS
If n > 1, we have a step-up transformer, as the voltage is increased from primary to secondary (V2 > V1).
If n < 1, the transformer is a step-down transformer, since the voltage is decreased from primary to secondary (V2 < V1).
The rating of a transformer are usually specified as V1/V2.
A transformer with rating 2400/120 V should have 2400V on the primary and 120V on the secondary (a step-down transformer)
Keep in mind that the voltage ratings are in rms.
TRANSFORMERS
EXAMPLE: An ideal transformer is rated at 2400/120 V, 9.6kVA and has 50 turns on the secondary. Calculate d) the turns ratio e) the number of turns on the primary side f) the current ratings for the primary and secondary windings
TRANSFORMERS
Solution:
This is a step-down transformer, since V1 = 2400V > V2 = 120V thus the turns ratio is
V2 120 n 0.05 V1 2400
N2 n N1
50 0.05 N1
50 thus N1 1000 turns 0.05
TRANSFORMERS
S = V1I1 = V2I2 = 9.6kVA. Hence,
9600 9600 I1 4A V1 2400 9600 9600 I2 80A or V2 120
I1 4 I2 80A n 0.05
POWER SUPPLY
A power supply is a device or system that supplies electrical or other types of energy to an output load or group of loads.
Figure below show the block diagram of a power supply system.
POWER SUPPLY
The power supply consists of rectifier, filter and voltage regulators.
Starting with an ac voltage, a steady dc voltage is obtained by rectifying the ac voltage, then filtering to a dc level and finally, regulating to obtain a desired fixed dc voltage.
The regulation is usually obtained from an IC voltage regulator unit, which takes a dc voltage and provides a somewhat lower dc voltage, which remains the same even if the input dc voltage varies or the output load connected to the dc voltage changes.
POWER SUPPLY
The ac voltage, typically 240rms, is connected to a transformer, which steps down that ac voltage to a level for the desired dc output.
A diode rectifier then provides a full-wave rectified voltage/convert ac to dc voltage that is initially filtered by a simple capacitor filter produce a dc voltage. This resulting dc voltage usually has some ripple or ac voltage variation.
A regulator circuit can use this dc input to provide a dc voltage that not only has much less ripple voltage but also remains the same dc value.
This voltage regulation is usually obtained using one of a number of popular voltage regulator IC units.
POWER SUPPLY
RECTIFIER
In order to convert the ac voltage into dc voltage, a rectifier circuit is needed.
The rectifier can be divided into two types, half-wave rectifier and full-wave rectifier.
POWER SUPPLY
The half-wave rectifier uses the transformer during only one-half of the cycle.
Thus, for any given size transformer, less power is developed than if the transformer were used on both halves of the cycle.
Therefore a full-wave rectifier is used.
POWER SUPPLY
Full-wave rectifier 1) Bridge network:
Based on the figure above, during the period t = 0 to T/2 the polarity of the input and output are as shown in the next figure.
POWER SUPPLY
D2 and D3 are conducting while D1 and D4 are in “off” state.
The load voltage is vo = vi since the diodes are ideal.
POWER SUPPLY
During the period t = T/2 to T the polarity of the input and output are as shown in figure below.
POWER SUPPLY
D1 and D4 are conducting while D2 and D3 are in “off” state.
The polarity of the output is the inverse of the input, thus the full-wave waveform is produced.
This produced waveform is then send to the filter.
POWER SUPPLY
2) Center-tapped transformer:
A second popular full-wave rectifier which use only two diodes but require a center-tapped transformer.
During the positive portion of vi applied to the primary of the transformer, D1 is assumed to be short-circuit while D2 is open-circuit.
POWER SUPPLY
During the negative portion of vi applied to the primary of the transformer, D2 is assumed to be short-circuit while D1 is open-circuit.
POWER SUPPLY
From the figure it can be seen that the obtained waveform is a full-wave rectifier waveform.
POWER SUPPLY
FILTER A filter is used to remove unwanted signal components. A very popular filter circuit is the capacitor-filter circuit shown below.
A capacitor is connected at the rectifier output and a dc voltage is obtained across the capacitor.
POWER SUPPLY
Figure 19.4(a) shows the output voltage of a full-wave rectifier before and after the signal is filtered. Notice that the filtered waveform is essentially a dc voltage with some ripple.
POWER SUPPLY
To further reduce the amount of ripple across a capacitor filter, one can add an additional RC filter as shown below.
POWER SUPPLY
IC REGULATOR Regulator IC units contain the circuitry for reference source, comparator amplifier, control devices and overload protection all in a single IC.
The IC regulator will provide an output dc voltage that is regulated/maintained at a set value even if the input voltage varies or if the load connected to the output changes.
The series 78 regulators provide fixed regulated voltages from 5V to 24V. Example: A 7805 provide a +5V dc output, a 7812 provide a +12V dc output.
POWER SUPPLY
POWER SUPPLY
An unregulated input voltage Vi is filtered by capacitor C1 and connected to the IC’s IN terminal. The IC’s OUT terminal provides a regulated +12V, which is filtered by capacitor C2 (mostly for any high-frequency noise). The third IC terminal is connected to ground (GND).
The ac line voltage (120Vrms) is stepped down to 18Vrms.
A full-wave rectifier and capacitor filter then provides an unregulated dc voltage, shown as a dc voltage of about 22V, with ac ripple of a few volts as input to the voltage regulator.
The 7812 IC then provides an output that is a regulated +12V dc.
MOTOR
TRANSFORMERS
A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts.
When two loops with or without contacts between them affect each other through the magnetic field generated by one of them they are said to be magnetically coupled.
TRANSFORMERS
Transformers uses the magnetically coupled coils to transfer energy from one circuit to another.
Transformer is used for stepping up or stepping down ac voltages or current, for isolation between two portion of a circuit and for impedance matching.
Transformer consists of two coils. The coils that is directly connected to the voltage source is called the primary winding.
Coil that is connected to the load is called secondary winding.
TRANSFORMERS
TRANSFORMERS
The iron core of a transformer is made up of sheets of rolled iron.
This iron is treated such that it has high magnetic conducting quality (high permeability) throughout the length of the core.
Permeability is the ability of a material to allow the magnetic flux, φ to flow through it.
TRANSFORMERS
From the figure, N1 and N2 is the number of turns for primary winding and secondary winding respectively.
V1 and V2 is the voltage across the primary winding and secondary winding respectively.
TRANSFORMERS
When a sinusoidal voltage is applied to the primary winding, the same magnetic flux φ goes through both windings.
According to Faraday’s law, the voltage across the primary winding and secondary winding are
d V1 N1 , dt or
d V2 N 2 dt
V1 = 4.44 φm f N1,
where φm is the maximum flux f is the supply frequency
V2 = 4.44 φm f N2
TRANSFORMERS
Dividing both of the above equation gives
V2 N 2 n V1 N1 where n is the turns ratio or transformer ratio.
For the reason of power conservation, the energy supplied to the primary must equal the energy absorbed by the secondary, since there are no losses in an ideal transformer. Or the apparent power for both winding are the same. Thus,
S1 S 2
V1 I1 V2 I 2 I1 V2 n I 2 V1
When n = 1, we generally call the transformer an isolation transformer.
TRANSFORMERS
If n > 1, we have a step-up transformer, as the voltage is increased from primary to secondary (V2 > V1).
If n < 1, the transformer is a step-down transformer, since the voltage is decreased from primary to secondary (V2 < V1).
The rating of a transformer are usually specified as V1/V2.
A transformer with rating 2400/120 V should have 2400V on the primary and 120V on the secondary (a step-down transformer)
Keep in mind that the voltage ratings are in rms.
TRANSFORMERS
EXAMPLE: An ideal transformer is rated at 2400/120 V, 9.6kVA and has 50 turns on the secondary. Calculate d) the turns ratio e) the number of turns on the primary side f) the current ratings for the primary and secondary windings
TRANSFORMERS
Solution:
This is a step-down transformer, since V1 = 2400V > V2 = 120V thus the turns ratio is
V2 120 n 0.05 V1 2400
N2 n N1
50 0.05 N1
50 thus N1 1000 turns 0.05
TRANSFORMERS
S = V1I1 = V2I2 = 9.6kVA. Hence,
9600 9600 I1 4A V1 2400 9600 9600 I2 80A or V2 120
I1 4 I2 80A n 0.05
POWER SUPPLY
A power supply is a device or system that supplies electrical or other types of energy to an output load or group of loads.
Figure below show the block diagram of a power supply system.
POWER SUPPLY
The power supply consists of rectifier, filter and voltage regulators.
Starting with an ac voltage, a steady dc voltage is obtained by rectifying the ac voltage, then filtering to a dc level and finally, regulating to obtain a desired fixed dc voltage.
The regulation is usually obtained from an IC voltage regulator unit, which takes a dc voltage and provides a somewhat lower dc voltage, which remains the same even if the input dc voltage varies or the output load connected to the dc voltage changes.
POWER SUPPLY
The ac voltage, typically 240rms, is connected to a transformer, which steps down that ac voltage to a level for the desired dc output.
A diode rectifier then provides a full-wave rectified voltage/convert ac to dc voltage that is initially filtered by a simple capacitor filter produce a dc voltage. This resulting dc voltage usually has some ripple or ac voltage variation.
A regulator circuit can use this dc input to provide a dc voltage that not only has much less ripple voltage but also remains the same dc value.
This voltage regulation is usually obtained using one of a number of popular voltage regulator IC units.
POWER SUPPLY
RECTIFIER
In order to convert the ac voltage into dc voltage, a rectifier circuit is needed.
The rectifier can be divided into two types, half-wave rectifier and full-wave rectifier.
POWER SUPPLY
The half-wave rectifier uses the transformer during only one-half of the cycle.
Thus, for any given size transformer, less power is developed than if the transformer were used on both halves of the cycle.
Therefore a full-wave rectifier is used.
POWER SUPPLY
Full-wave rectifier 1) Bridge network:
Based on the figure above, during the period t = 0 to T/2 the polarity of the input and output are as shown in the next figure.
POWER SUPPLY
D2 and D3 are conducting while D1 and D4 are in “off” state.
The load voltage is vo = vi since the diodes are ideal.
POWER SUPPLY
During the period t = T/2 to T the polarity of the input and output are as shown in figure below.
POWER SUPPLY
D1 and D4 are conducting while D2 and D3 are in “off” state.
The polarity of the output is the inverse of the input, thus the full-wave waveform is produced.
This produced waveform is then send to the filter.
POWER SUPPLY
2) Center-tapped transformer:
A second popular full-wave rectifier which use only two diodes but require a center-tapped transformer.
During the positive portion of vi applied to the primary of the transformer, D1 is assumed to be short-circuit while D2 is open-circuit.
POWER SUPPLY
During the negative portion of vi applied to the primary of the transformer, D2 is assumed to be short-circuit while D1 is open-circuit.
POWER SUPPLY
From the figure it can be seen that the obtained waveform is a full-wave rectifier waveform.
POWER SUPPLY
FILTER A filter is used to remove unwanted signal components. A very popular filter circuit is the capacitor-filter circuit shown below.
A capacitor is connected at the rectifier output and a dc voltage is obtained across the capacitor.
POWER SUPPLY
Figure 19.4(a) shows the output voltage of a full-wave rectifier before and after the signal is filtered. Notice that the filtered waveform is essentially a dc voltage with some ripple.
POWER SUPPLY
To further reduce the amount of ripple across a capacitor filter, one can add an additional RC filter as shown below.
POWER SUPPLY
IC REGULATOR Regulator IC units contain the circuitry for reference source, comparator amplifier, control devices and overload protection all in a single IC.
The IC regulator will provide an output dc voltage that is regulated/maintained at a set value even if the input voltage varies or if the load connected to the output changes.
The series 78 regulators provide fixed regulated voltages from 5V to 24V. Example: A 7805 provide a +5V dc output, a 7812 provide a +12V dc output.
POWER SUPPLY
POWER SUPPLY
An unregulated input voltage Vi is filtered by capacitor C1 and connected to the IC’s IN terminal. The IC’s OUT terminal provides a regulated +12V, which is filtered by capacitor C2 (mostly for any high-frequency noise). The third IC terminal is connected to ground (GND).
The ac line voltage (120Vrms) is stepped down to 18Vrms.
A full-wave rectifier and capacitor filter then provides an unregulated dc voltage, shown as a dc voltage of about 22V, with ac ripple of a few volts as input to the voltage regulator.
The 7812 IC then provides an output that is a regulated +12V dc.
MOTOR
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