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1. Input POWER factor is improved.
2. LOAD current waveform is improved and THUS the load PERFORMANCE is better.

It SERVES two process.

1. It prevents the output voltage from BECOMING negative.
2. The load current is transferred from the main thyristors to the freewheeling diode, thereby allowing all of its thyristors to REGAIN their BLOCKING states.

It serves two process.

1. STEEL ROLLING mills, printing press, textile mills and paper mills employing dc motor drives.
2. DC traction
3. Electro chemical and electro-metallurgical process
4. Portable hand tool drives
5. MAGNET power supplies
6. HVDC transmission system

It converts fixed AC VOLTAGE into VARIABLE DC voltage.

It converts fixed ac voltage into variable dc voltage.

1. Elimination of commutation of commutating components in forced commutation, resulting in reduction in cost, weight and VOLUME.
2. Reduction in acoustic noise and electromagnetic noise DUE to elimination of commutation chokes.
3. Faster turn-off, PERMITTING high switching frequencies.
4. IMPROVED efficiency of the converters.

Turn-off TIME for converter grade SCRS is 50 – 100 ms turn-off time for converter grade SCRs and INVERTER grade SCRs and for inverter grade SCRs is 3 – 50 ms.

Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.

Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the DEVICE MAY turn-on at an UNDESIRED instant, a process called COMMUTATION FAILURE.

Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.

It is defined as the TIME during which a reverse VOLTAGE is applied ACROSS the thyristor during its COMMUTATION PROCESS.

It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.

When GATE CURRENT is SEVERAL times higher than the minimum gate current required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor reduces its turn-on time and enhances its di/dt CAPABILITY.

When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor reduces its turn-on time and enhances its di/dt capability.

1. FORWARD CONDUCTION losses
2. LOSS DUE to leakage current during forward and REVERSE blocking.
3. Switching losses at turn-on and turn-off.
4. Gate triggering loss.

It consists of a series COMBINATION of a resistor and a capacitor in parallel with the thyristors. It is mainly USED for dv / DT protection.

It consists of a series combination of a resistor and a capacitor in parallel with the thyristors. It is mainly used for dv / dt protection.

The holding CURRENT is defined as the MINIMUM value of anode current below which it MUST fall to for turning off the THYRISTOR.

The holding current is defined as the minimum value of anode current below which it must fall to for turning off the thyristor.

The latching CURRENT is defined as the MINIMUM value of anode current which it must ATTAIN during turn on process to maintain conduction when GATE signal is removed.

The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.

A thyristor can be TURNED off by MAKING the CURRENT FLOWING through it to zero.

A thyristor can be turned off by making the current flowing through it to zero.

1. N-channel MOSFET
2. P-channel MOSFET

Because the OUTPUT (COLLECTOR) CURRENT can be CONTROLLED by BASE current.

Because the output (collector) current can be controlled by base current.

Because the OUTPUT (DRAIN) CURRENT can be CONTROLLED by gate-source VOLTAGE.

Because the output (drain) current can be controlled by gate-source voltage.

Because the CONTROLLING PARAMETER is gate-emitter VOLTAGE.

Because the controlling parameter is gate-emitter voltage.

<P>Power DIODE

1. Constructed with N-layer, called drift region between p+ layer and n+ layer.
2. The voltage, current and power ratings are higher.
3. Power diodes operate at HIGH speeds.

Signal diode

1. Drift region is not present.
2. The voltage, current and power ratings are Lower
3. OPERATES at higher switching speed.

Power diode

Signal diode

1. Forward VOLTAGE triggering
2. GATE triggering
3. dv/dt triggering
4. TEMPERATURE triggering
5. LIGHT triggering

1. LOWER hate requirements
2. Lower SWITCHING losses
3. SMALLER SNUBBER circuit requirements

IGBT has

• Lower turn on and turn off times than BJT
• Lower on state CONDUCTION losses than MOSFET
• Excellent safe operating area

IGBT has

In order to MAINTAIN these KIND of devices in on-state, we need to apply CONTINUOUS gate current /voltage.

Some of the level sensitive devices are: MOSFET, IGBT, MCT, IGCT

In order to maintain these kind of devices in on-state, we need to apply continuous gate current /voltage.

Some of the level sensitive devices are: MOSFET, IGBT, MCT, IGCT

To turn on these KIND of devices SINGLE pulse of short duration is sufficient. Continuous gate voltage of entire on time is not required. It will avoid the HARD triggering.

Example: Thyristor, GTO

To turn on these kind of devices single pulse of short duration is sufficient. Continuous gate voltage of entire on time is not required. It will avoid the hard triggering.

Example: Thyristor, GTO

IGBT, MCT, IGCT, SIT.

IGBT, MCT, IGCT, SIT.

SCR, GTO, GTR.

SCR, GTO, GTR.

In load COMMUTATION, the load current FLOWING through the thyristor either becomes ZERO or is transferred to ANOTHER DEVICE from the conducting SCR.

In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

The advantages of current commutated CHOPPER is;

1. Commutation is reliable as load current is LESS than the peak commutation current
2. The AUXILIARY SCR is naturally commutated as its current passes through zero VALUE.
3. The capacitor always remains charged with the correct polarity.

The advantages of current commutated chopper is;

The process of a CURRENT pulse is MADE to flow in the reverse direction through the conducting SCR and thus made the NET SCR current BECOMES zero, CONSEQUENTLY turn off the SCR is called as current commutation.

The process of a current pulse is made to flow in the reverse direction through the conducting SCR and thus made the net SCR current becomes zero, consequently turn off the SCR is called as current commutation.

The process of a CHARGED capacitor momentarily REVERSE biases the CONDUCTING SCR and turns it off is called as voltage commutation.

The process of a charged capacitor momentarily reverse biases the conducting SCR and turns it off is called as voltage commutation.

In STEP up CHOPPER, the AVERAGE OUTPUT voltage is more than the input SUPPLY voltage. It is also known as Boost converter.

In step up chopper, the average output voltage is more than the input supply voltage. It is also known as Boost converter.

In STEP down chopper, the average output voltage is less than the INPUT supply voltage. It is also known as BUCK CONVERTER.

In step down chopper, the average output voltage is less than the input supply voltage. It is also known as Buck converter.

The PROCESS of the CURRENT flowing through the thyristor is forced to become zero by EXTERNAL circuitry is CALLED as forced commutation.

The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation.

The PROCESS of the current flowing through the thyristor goes through a NATURAL zero and ENABLE the thyristor to TURN off is called as natural commutation.

The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.

NATURAL COMMUTATION
FORCED commutation

Natural commutation
Forced commutation

A dc CHOPPER is EQUIVALENT to the transformer in AC circuit. It is a static switch USED to GET the variable dc voltage from a constant dc voltage.

A dc Chopper is equivalent to the transformer in ac circuit. It is a static switch used to get the variable dc voltage from a constant dc voltage.

If the circuit turn off TIME is less than the thyristor turn off time the device may turn on at an UNDESIRED instant RESULTING in commutation failure.

If the circuit turn off time is less than the thyristor turn off time the device may turn on at an undesired instant resulting in commutation failure.

It is DEFINED as the TIME during which a reverse voltage is APPLIED across the thyristor during its commutation process.

It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.

It is also known as frequency changer. It CONVERTS INPUT power at one frequency to output power at ANOTHER frequency with one STAGE conversion.

It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.

The INPUT power factor is improved. It PREVENTS the output voltage from becoming negative. Load CURRENT waveform is also improved.

The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is also improved.

Forward conduction losses
Loss due to leakage current during forward and REVERSE BLOCKING
SWITCHING losses at turn on turn off
Gate triggering loss

Forward conduction losses
Loss due to leakage current during forward and reverse blocking
Switching losses at turn on turn off
Gate triggering loss

SCR or thyristor will have THREE regions of operations based on the mode in which the device is connected in the circuit.

Reverse blocking region: When the cathode of the thyristor is made positive with respect to the anode and no gate signal is applied. In this region SCR exhibits the reverse blocking characteristics SIMILAR to DIODE.

Forward blocking region: In this region the anode of the thyristor is made positive with respect to the cathode and no gate signal is applied to the thyristor. A small leakage current flow in this mode of operation of the thyristor.

Forward conduction region: when the forward voltage applied between the anode and cathode increases at particular break over voltage avalanche breakdown takes PLACE and thyristor starts conducting current in forward direction. By this type of triggering the device damages the scr. Hence a gate signal is applied before the forward break over voltage to trigger the scr.

SCR or thyristor will have three regions of operations based on the mode in which the device is connected in the circuit.

Reverse blocking region: When the cathode of the thyristor is made positive with respect to the anode and no gate signal is applied. In this region SCR exhibits the reverse blocking characteristics similar to diode.

Forward blocking region: In this region the anode of the thyristor is made positive with respect to the cathode and no gate signal is applied to the thyristor. A small leakage current flow in this mode of operation of the thyristor.

Forward conduction region: when the forward voltage applied between the anode and cathode increases at particular break over voltage avalanche breakdown takes place and thyristor starts conducting current in forward direction. By this type of triggering the device damages the scr. Hence a gate signal is applied before the forward break over voltage to trigger the scr.

The main task of POWER electronics is to control and convert electrical power from one form to another.

AC to DC conversion: Rectifier is used for converting an AC voltage to a DC voltage.

Rectifier applications: Variable SPEED dc drives, Battery chargers, DC power supplies and Power supply for a specific application like electroplating.

DC to AC conversion: Inverter circuit is used to convert DC voltage to an alternating voltage.

Inverter applications: Emergency lighting systems, AC variable speed drives, Un-interrupted power supplies and Frequency converters.

DC to DC conversion: A dc-to-dc CONVERTER circuit was called a chopper.

Chopper applications: DC drive, Battery CHARGER and DC power supply.

AC to AC conversion: A cycloconverter converts an AC voltage to another AC voltage.

Cycloconverter applications: It is rarely used. Can be used for controlling the speed of an AC traction motor

The main task of power electronics is to control and convert electrical power from one form to another.

AC to DC conversion: Rectifier is used for converting an AC voltage to a DC voltage.

Rectifier applications: Variable speed dc drives, Battery chargers, DC power supplies and Power supply for a specific application like electroplating.

DC to AC conversion: Inverter circuit is used to convert DC voltage to an alternating voltage.

Inverter applications: Emergency lighting systems, AC variable speed drives, Un-interrupted power supplies and Frequency converters.

DC to DC conversion: A dc-to-dc converter circuit was called a chopper.

Chopper applications: DC drive, Battery charger and DC power supply.

AC to AC conversion: A cycloconverter converts an AC voltage to another AC voltage.

Cycloconverter applications: It is rarely used. Can be used for controlling the speed of an AC traction motor

Power ELECTRONICS refers to CONTROL and CONVERSION of electrical power by power SEMICONDUCTOR devices wherein these devices OPERATE as switches.

Power electronics refers to control and conversion of electrical power by power semiconductor devices wherein these devices operate as switches.

We know there are three types of power in Electrical as Active, apparent &AMP; REACTIVE. So KVAR is STAND for ``Kilo Volt AMPS with Reactive component.

We know there are three types of power in Electrical as Active, apparent & reactive. So KVAR is stand for ``Kilo Volt Amps with Reactive component.

REVERSE Power FLOW relay are USED in generating station's protection. A generating stations is supposed to fed power to the grid and in case generating UNITS are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Reverse Power flow relay are used in generating station's protection. A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Knee point VOLTAGE is calculated for electrical Current transformers and is very IMPORTANT factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current TRANSFORMER).

Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).

Since two bulbs are in series they will GET equal amount of ELECTRICAL CURRENT but as the SUPPLY VOLTAGE is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

4-20 mA is a STANDARD range used to indicate measured values for any PROCESS. The reason that 4ma is chosen instead of 0 mA is for fail safe OPERATION .For example- a pressure instrument GIVES output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output REDUCES to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.