Principle
Faradays law of electromagnetic induction
When an alternating current is passed thru the primary coil, an alternating flux is generated in the core. This flux travels thru the core and links with the secondary coil. This alternating flux cuts the secondary coil and an emf is induced in it depending on the no. of turns.
The amount of emf produced depends on the amount of flux linked with the sec winding. Air has high reluctance due to which the mag flux doesn’t link properly in full amount with the sec winding. To increase the amount of flux linkages, a (common) soft iron core of low reluctance used so that max amount of flux pass thru the core and links with the sec winding.
LOSSES
Copper loss which is usually heat loss due to the resistance offered to the load current. I2Rt.
CORE/IRON Loss – depends on mag prop of the material used in the core.
HYSTERISIS LOSS
The alternating mag flux in the core, the magnetic elements called as domains in the core tend to align themselves with the magnetic field. But it is alternating and the domains are not able to respond so fast and friction occurs…. This loss is called as hysteresis loss.
Sometimes core retains magnetization and becomes magnetized by the external field. This causes power loss and is called hysteresis.
Soft magnetic materials like silicon steel having low hysteresis are used.
Hysteresis loss increases with increase in temp and frequency.
EDDY CURRENT LOSS
Induction of eddy current within the core causes a resistive loss. Higher the resistance, lower the loss.
Laminations are used to reduce eddy current losses. Area decreases and resistance increases.
Why HUMMING NOISE
Magnetic flux in a ferro magnetic material causes it to physically expand and contract in each frequency cycle. This phenomenon known as magnetostriction causes the humming sound
The stray magnetic field causes the enclosure and eq to vibrate.
Factors affecting hysteresis leakage
High Permeability and low reluctance properties of the material
Hysteresis loss increases with increase in frequency and temp.
How to reduce flux leakage
Flux prop to NI and (A/L)
N-no of turns; I-current; A=area of the core and L- length of the flux path.
Prop of core material- high permeability relative to air (causes the mag field to be concentrated in the core) and magnetic properties.
Geometry of the core and Using shell type construction
Amount of air gap
Operating temp of core
Making transformer window long and narrow
Using soft core of lower reluctance and laminated
Faradays law of electromagnetic induction
When an alternating current is passed thru the primary coil, an alternating flux is generated in the core. This flux travels thru the core and links with the secondary coil. This alternating flux cuts the secondary coil and an emf is induced in it depending on the no. of turns.
The amount of emf produced depends on the amount of flux linked with the sec winding. Air has high reluctance due to which the mag flux doesn’t link properly in full amount with the sec winding. To increase the amount of flux linkages, a (common) soft iron core of low reluctance used so that max amount of flux pass thru the core and links with the sec winding.
LOSSES
Copper loss which is usually heat loss due to the resistance offered to the load current. I2Rt.
CORE/IRON Loss – depends on mag prop of the material used in the core.
HYSTERISIS LOSS
The alternating mag flux in the core, the magnetic elements called as domains in the core tend to align themselves with the magnetic field. But it is alternating and the domains are not able to respond so fast and friction occurs…. This loss is called as hysteresis loss.
Sometimes core retains magnetization and becomes magnetized by the external field. This causes power loss and is called hysteresis.
Soft magnetic materials like silicon steel having low hysteresis are used.
Hysteresis loss increases with increase in temp and frequency.
EDDY CURRENT LOSS
Induction of eddy current within the core causes a resistive loss. Higher the resistance, lower the loss.
Laminations are used to reduce eddy current losses. Area decreases and resistance increases.
Why HUMMING NOISE
Magnetic flux in a ferro magnetic material causes it to physically expand and contract in each frequency cycle. This phenomenon known as magnetostriction causes the humming sound
The stray magnetic field causes the enclosure and eq to vibrate.
Factors affecting hysteresis leakage
High Permeability and low reluctance properties of the material
Hysteresis loss increases with increase in frequency and temp.
How to reduce flux leakage
Flux prop to NI and (A/L)
N-no of turns; I-current; A=area of the core and L- length of the flux path.
Prop of core material- high permeability relative to air (causes the mag field to be concentrated in the core) and magnetic properties.
Geometry of the core and Using shell type construction
Amount of air gap
Operating temp of core
Making transformer window long and narrow
Using soft core of lower reluctance and laminated
Purpose of Instrument Transformers
Used to supply proportional current/voltage from a high power network for instruments and protection relays.
VT supplies voltmeter, protection relays and volt operated coils
CT supplies for ammeter, protection relays and current operated coils
CURRENT TRANSFORMER
348 mitra
Primary winding connected in series with the line carrying current and has only 1 winding or few turns. The secondary winding has more no of turns depending on how much primary winding has to be reduced to say 5A or 1A(by more winding) in the sec. The ammeter or wattmeter is connected across the secondary.
The sec terminals shud never be left open with the primary carrying current. If ther is no back emf in the sec, the mmf in the primary becomes high, an abnormal high flux in the core which will produce excessive core loss with heating and da,age to insulation and a high voltage across the sec terminals.
One of the terminals of sec is always earthed to protect the equipment and the personnel in the vicinity in the event of insulation breakdown
If the insulation btw the primary and secondary winding breakdown, the secondary circuit will be raised to full primary voltagewith a serious risk to person and also equipment. Hence one end of the CT is always earthed
Difference btw welding transformer and lighting transformer
WT is also a step down transformer to 15-45V with secondry winding has several taps to adjust the voltage. It has more cross section and less no of turns to ensure less voltage and more current(200-600A).
Impedance of WT is higher than normal which helps in establishing arc and controlling current
DC Welding transformer has a rectifier and smoothing choke filter and are highly reactive. Otherwise a separate reactor is used in series with the winding – Tapped reactor, Moving coil reactor, Moving shunt reactor, Continuously variable reactor.
VT,PT,CT comparision
PT are used in high power transmission lines above 33kv
Why 3 single phase transformers used instead of one single 3phase transformers
3phase 440/220V lighting transformers are usually composed of 3 separate single phase tranformers instead of one 3phase.
Enables easy replacement of a single phase unit if any fault.
Transformers for use on 3phase insulated systems are generally interconnected in delta-delta configuration using copper links between phase windings. If a fault occurs in one phase, the faulty unit can be disconnected using links, creating an open delta or V-configuration and a 3phase supply is still available.
Used to supply proportional current/voltage from a high power network for instruments and protection relays.
VT supplies voltmeter, protection relays and volt operated coils
CT supplies for ammeter, protection relays and current operated coils
CURRENT TRANSFORMER
348 mitra
Primary winding connected in series with the line carrying current and has only 1 winding or few turns. The secondary winding has more no of turns depending on how much primary winding has to be reduced to say 5A or 1A(by more winding) in the sec. The ammeter or wattmeter is connected across the secondary.
The sec terminals shud never be left open with the primary carrying current. If ther is no back emf in the sec, the mmf in the primary becomes high, an abnormal high flux in the core which will produce excessive core loss with heating and da,age to insulation and a high voltage across the sec terminals.
One of the terminals of sec is always earthed to protect the equipment and the personnel in the vicinity in the event of insulation breakdown
If the insulation btw the primary and secondary winding breakdown, the secondary circuit will be raised to full primary voltagewith a serious risk to person and also equipment. Hence one end of the CT is always earthed
Difference btw welding transformer and lighting transformer
WT is also a step down transformer to 15-45V with secondry winding has several taps to adjust the voltage. It has more cross section and less no of turns to ensure less voltage and more current(200-600A).
Impedance of WT is higher than normal which helps in establishing arc and controlling current
DC Welding transformer has a rectifier and smoothing choke filter and are highly reactive. Otherwise a separate reactor is used in series with the winding – Tapped reactor, Moving coil reactor, Moving shunt reactor, Continuously variable reactor.
VT,PT,CT comparision
PT are used in high power transmission lines above 33kv
Why 3 single phase transformers used instead of one single 3phase transformers
3phase 440/220V lighting transformers are usually composed of 3 separate single phase tranformers instead of one 3phase.
Enables easy replacement of a single phase unit if any fault.
Transformers for use on 3phase insulated systems are generally interconnected in delta-delta configuration using copper links between phase windings. If a fault occurs in one phase, the faulty unit can be disconnected using links, creating an open delta or V-configuration and a 3phase supply is still available.
Permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. Thus it is the degree of magnetization a material obtains in response of the applied magnetic field.
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