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SCRS are expensive as they MUST POSSESS LOW turn-on and turn-off times.

SCRs are expensive as they must possess low turn-on and turn-off times.

VSI:

1. Input voltage is maintained CONSTANT
2. The output voltage does not depend on the LOAD
3. The magnitude of the output current and its waveform depends on the NATURE of the load impedance
4. It requires feedback diodes 
5. Commutation circuit is complicated i.e. it contains capacitors and inductors.

CSI:

1. Input current is constant but adjustable
2. The output current does not depend on the load
3. The magnitude of the output voltage and its waveform depends on the nature of the load impedance
4. It does not requires feedback diodes
5. Commutation circuit is simple i.e. it contains only capacitors.

VSI:

CSI:

1. TRANSFORMER connections
2. SINUSOIDAL PWM
3. Multiple commutation in each CYCLE
4. Stepped wave inverters

1. Harmonic currents will lead to EXCESSIVE heating in the induction motors. This will reduce the LOAD carrying capacity of the motor.
2. If the control and the regulating circuits are not properly shielded, harmonics from power ride can affect their operation and malfunctioning can result.
3. Harmonic currents CAUSE losses in the ac system and can even some time produce RESONANCE in the system. Under resonant conditions, the instrumentation and metering can be affected.
4. On critical loads, torque PULSATION produced by the harmonic current can be useful.

1. The output voltage can be obtained without any additional components.
2. Lower order HARMONICS can be eliminated or MINIMIZED ALONG with its output voltage control. As the higher order harmonics can be FILTERED easily, the filtering requirements are minimized.

In this method, a fixed dc input voltage is given to the inverter and a controlled AC output voltage is obtained by adjusting the on and off periods of the inverter COMPONENTS. This is the most popular method of CONTROLLING the output voltage and this method is TERMED as PWM control.

In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as PWM control.

1. INDUCTION heating
2. Lagging VAR compensation
3. SPEED control of ac MOTORS
4. Synchronous MOTOR starting.

It is an IMPULSE commutated inverter which relies on LC circuit and an auxiliary thyristor for COMMUTATION in the load circuit.

It is an impulse commutated inverter which relies on LC circuit and an auxiliary thyristor for commutation in the load circuit.

1. LINE commutated INVERTERS.
2. LOAD commutated inverters.
3. SELF commutated inverters.
4. FORCED commutated inverters.

The thyristorised series inverter produces an approximately SINUSOIDAL waveform at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for fixed output applications such as

1. Ultrasonic generator.
2. Induction HEATING.
3. SONAR Transmitter
4. Fluorescent LIGHTING.

The thyristorised series inverter produces an approximately sinusoidal waveform at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for fixed output applications such as

An inverter in which the commutating ELEMENTS are CONNECTED in parallel with the LOAD is CALLED a parallel inverter.

An inverter in which the commutating elements are connected in parallel with the load is called a parallel inverter.

ANSWER : 4L R2 < ------ C

An INVERTER in which the commutating elements are CONNECTED in series with the load is CALLED a series inverter.

An inverter in which the commutating elements are connected in series with the load is called a series inverter.

VSI with RL LOAD.

VSI with RL load.

For RL loads, load CURRENT will not be in phase with load VOLTAGE and the diodes connected in anti parallel will allow the current to flow when the main THYRISTORS are TURNED off. These diodes are called FEEDBACK diodes.

For RL loads, load current will not be in phase with load voltage and the diodes connected in anti parallel will allow the current to flow when the main thyristors are turned off. These diodes are called feedback diodes.

It REQUIRE a 3-wire DC SUPPLY.

It require a 3-wire dc supply.

1. CSI does not require any FEEDBACK DIODES.
2. COMMUTATION circuit is simple as it INVOLVES only thyristors.

The output FREQUENCY is varied by VARYING the turn off TIME of the thyristors in the inverter circuit, i.e. the DELAY angle of the thyristors is varied.

The output frequency is varied by varying the turn off time of the thyristors in the inverter circuit, i.e. the delay angle of the thyristors is varied.

Thyristors require extra commutation CIRCUITS for turn off which results in increased COMPLEXITY of the CIRCUIT. For these reasons thyristors are not PREFERRED for INVERTERS.

Thyristors require extra commutation circuits for turn off which results in increased complexity of the circuit. For these reasons thyristors are not preferred for inverters.

1. VOLTAGE SOURCE INVERTER
2. CURRENT Source Inverter

1. Adjustable speed DRIVES
2. Induction HEATING
3. Stand-by AIRCRAFT power supplies
4. UPS
5. HVDC TRANSMISSION

A device that CONVERTS dc power into AC power at DESIRED output VOLTAGE and frequency is called an inverter.

A device that converts dc power into ac power at desired output voltage and frequency is called an inverter.

1. For high power applications, efficiency becomes very LOW because of high switching LOSSES at high operating frequencies.
2. Freewheeling diode is subjected to twice the supply voltage.
3. PEAK load voltage is equal to twice the supply voltage.
4. The commutating capacitor has to CARRY full load current at a FREQUENCY of half chopping frequency.
5. One thyristor pair should be turned-on only when the other pair is commutated. This can be realized by sensing the capacitor current that is alternating.

1. Commutating INDUCTOR is not REQUIRED.
2. It is CAPABLE of commutating any AMOUNT of load current.
3. It can work at high FREQUENCIES in the order of kHz.
4. Filtering requirements are minimal.

1. The capacitor always REMAINS CHARGED with the CORRECT polarity.
2. Commutation is RELIABLE as load current is less than the peak commutation current ICP.
3. The auxiliary thyristor TA is naturally commutated as its current passes through zero value.

In this process, the load current flowing through the THYRISTOR EITHER BECOMES zero or is transferred to ANOTHER device from the conducting thyristor.

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

In this process, a current pulse is MADE to FLOW in the REVERSE DIRECTION through the conducting thyristor and when the net thyristor current becomes zero, it is turned off.

In this process, a current pulse is made to flow in the reverse direction through the conducting thyristor and when the net thyristor current becomes zero, it is turned off.

In this process, a charged CAPACITOR MOMENTARILY REVERSE biases the CONDUCTING thyristor and TURN it off.

In this process, a charged capacitor momentarily reverse biases the conducting thyristor and turn it off.

1. VOLTAGE commutated CHOPPER.
2. CURRENT commutated chopper.
3. LOAD commutated chopper.

Average output voltage for step down CHOPPER is VO = α VS.

Average output voltage for step up chopper is VO = α VS x [1/ ( 1- α)].

Average output voltage for step down chopper is VO = α VS.

Average output voltage for step up chopper is VO = α VS x [1/ ( 1- α)].

In this CONTROL method, the on time Ton is VARIED but CHOPPING frequency is kept constant. The width of the pulse is varied and HENCE this TYPE of control is known as Pulse Width Modulation (PWM).

In this control method, the on time Ton is varied but chopping frequency is kept constant. The width of the pulse is varied and hence this type of control is known as Pulse Width Modulation (PWM).

In frequency modulation CONTROL, the CHOPPING frequency f (or the chopping PERIOD T) is varied. Here two controls are possible.

1. On-time TON is kept constant
2. Off period TOFF is kept constant.

In frequency modulation control, the chopping frequency f (or the chopping period T) is varied. Here two controls are possible.

1. CONSTANT FREQUENCY CONTROL
2. VARIABLE frequency control

In TRC, the VALUE of Ton / T is varied in order to change the AVERAGE output VOLTAGE.

In TRC, the value of Ton / T is varied in order to change the average output voltage.

1. TIME RATIO Control (TRC)
2. CURRENT LIMIT Control method (CLC)

Duty CYCLE is defined as the ratio of the on time of the CHOPPER to the total time PERIOD of the chopper. It is denoted by α.

Duty cycle is defined as the ratio of the on time of the chopper to the total time period of the chopper. It is denoted by α.

Average output VOLTAGE for STEP down CHOPPER

α? is the DUTY cycle.

Average output voltage for step down chopper

α? is the duty cycle.

AVERAGE OUTPUT voltage for STEP down chopper V0 = α? Vs, α? is the duty CYCLE.

Average output voltage for step down chopper V0 = α? Vs, α? is the duty cycle.

In a step- down CHOPPER or BUCK converter, the average output voltage is LESS than the input voltage. In a step- up chopper or BOOST converter, the average output voltage is more than the input voltage.

In a step- down chopper or Buck converter, the average output voltage is less than the input voltage. In a step- up chopper or Boost converter, the average output voltage is more than the input voltage.

1. BATTERY operated VEHICLES
2. Traction motor control in electric traction
3. TROLLEY cars
4. Marine HOISTS
5. Mine haulers
6. Electric BRAKING.

A dc chopper is a HIGH SPEED static switch used to obtain variable dc VOLTAGE from a CONSTANT dc voltage.

A dc chopper is a high speed static switch used to obtain variable dc voltage from a constant dc voltage.

the thyristor is FORCED to become ZERO by external circuitry.

the thyristor is forced to become zero by external circuitry.

Here the current flowing through the THYRISTOR goes through a NATURAL zero and ENABLE the thyristor to TURN off.

Here the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off.

1. Commutation is made simple.
2. DISTORTION on the ac side is reduced DUE to the reduction in lower order harmonics.
3. Inductance reduced in SERIES is considerably reduced.

The input power factor is DEFINED as the ratio of the total mean input power to the total RMS input volt-amperes.

PF = ( V1 I1 cos φ?1 ) / ( Vrms Irms) where V1 = phase voltage, I1 = FUNDAMENTAL

component of the supply current, ?φ1 = input displacement angle, Irms = supply rms current.

The input power factor is defined as the ratio of the total mean input power to the total RMS input volt-amperes.

PF = ( V1 I1 cos φ?1 ) / ( Vrms Irms) where V1 = phase voltage, I1 = fundamental

component of the supply current, ?φ1 = input displacement angle, Irms = supply rms current.

Average OUTPUT VOLTAGE Vdc = (VM /?Π) (1 + cosα).

Average output voltage Vdc = (Vm /?Π) (1 + cosα).

1. UJT firing circuit.
2. The COSINE wave CROSSING pulse TIMING control.
3. DIGITAL firing schemes.

The COMMUTATION period when outgoing and incoming THYRISTORS are conducting is known as overlap period. The angular period, when both devices SHARE CONDUCTION is known as the commutation ANGLE or overlap angle.

The commutation period when outgoing and incoming thyristors are conducting is known as overlap period. The angular period, when both devices share conduction is known as the commutation angle or overlap angle.

1. SCRS are subjected to a peak-inverse VOLTAGE of 2Vm in a FULLY controlled bridge rectifier. Hence for same voltage and current ratings of SCrs, power handled by mid-point configuration is about
2. In mid-point converter, each secondary WINDING should be able to supply the load power. As such, the TRANSFORMER rating in mid-point converter is double the load rating.

The delay angle is defined as the angle between the ZERO CROSSING of the input VOLTAGE and the instant the thyristor is fired.

The delay angle is defined as the angle between the zero crossing of the input voltage and the instant the thyristor is fired.