28 Questions
What is the primary power source for three-phase controlled rectifiers?
Three-phase AC supply
What is an advantage of three-phase controlled rectifiers compared to single-phase rectifiers?
Higher DC output voltage and higher DC output power
What is a common application of three-phase controlled rectifiers?
Electrochemical processes
What is a characteristic of the voltage sources in a three-phase AC supply?
They are 120° displaced from each other in time
What is a benefit of using three-phase controlled rectifiers in high-power applications?
They provide lower ripples in the output voltage and current
What type of equipment uses three-phase controlled rectifiers?
Traction equipment and motor drives
What is the condition for the load current to flow through the supply phase winding 'a - n' and through thyristor T1?
When thyristor T1 is triggered at 𝑤𝑡 = (30 + 𝛼)
What happens to thyristor T1 when thyristor T2 is triggered?
It becomes reverse biased
What is the condition for the phase voltage 𝑉𝑐𝑛 to appear across the load?
When thyristor T3 is triggered at 𝑤𝑡 = 270 + 𝛼
Why is it necessary to know that L>>R in a three-phase controlled rectifier?
To derive an expression for the average output voltage
What is a characteristic of a three-phase controlled rectifier with RL load when L>>R?
It never has discontinuous conduction mode
What is the purpose of triggering thyristor T1 at 𝑤𝑡 = (30 + 𝛼)?
To allow the phase voltage 𝑉an to appear across the load
What is the term often used to describe the voltage between two lines?
Line voltage
What is the phase difference between voltage A and voltage B in a three-phase supply?
120 degrees
What is the purpose of the firing current pulse in a three-phase half wave converter?
To turn on the thyristor
What is the common point that connects the supply and the load in a three-phase half wave converter?
Common neutral point
What is the term used to describe the three-phase half wave converter circuit?
3-pulse converter
What is the effect of delaying the firing pulse by an angle α in a three-phase half wave converter?
Controls the load voltage
What is the configuration of the supply transformer in a three-phase half wave converter?
Star connected
What is the condition for a thyristor to conduct in a three-phase half wave converter?
Anode-to-cathode voltage is positive and a firing current pulse is applied
What is the formula to calculate the average output voltage in circuits with RL load?
Vdc = 3Vm / 2p * (-cos(wt) + sin(wt))
What is the purpose of the formula for average output voltage in circuits with RL load?
To calculate the average output voltage
How is the average output voltage and current calculated in circuits with RL load?
Using the trigonometric relationship
What is the symbol 𝜔 in the formula for average output voltage?
Angular frequency
What is the relationship between the average output voltage and current in circuits with RL load?
They are proportional to the cosine of the angle 𝛼
What is the purpose of simplifying the formula for average output voltage in circuits with RL load?
To make the calculation more simple
What is the significance of the angle 𝛼 in the formula for average output voltage in circuits with RL load?
It is the phase angle between the output voltage and current
What is the RMS component of the output voltage and current waveforms determined from?
The average output voltage and current
Study Notes
Three-Phase Controlled Rectifiers
- Three-phase controlled rectifiers have a wide range of applications, including electrochemical processes, motor drives, traction equipment, and controlled power supplies.
- Features of three-phase controlled rectifiers:
- Operate from 3-phase AC supply voltage
- Provide higher DC output voltage and power
- Provide lower ripples in the output voltage and current
- Require lower values of capacitance for filtering
- Are extensively used in high-power variable-speed industrial DC drives
Review of 3-Phase Supplies
- Three-phase supplies usually represent the voltages generated by three coils (physically displaced by 120° from each other) in an AC rotating machine (alternator).
- The three-phase supply waveforms are 120° displaced from each other in time.
- Assuming a maximum phase voltage of Vm, the phase sequence is A-B-C, and B lags A by 120°, C lags B by 120°, and A lags C by 120°.
Controlled Three-Phase Half-Wave Rectifiers
- A 3-phase half-wave converter combines three single-phase half-wave controlled rectifiers in one single circuit feeding a common load.
- The 3-phase input supply is applied through a star-connected supply transformer.
- The thyristor will conduct (ON state) when the anode-to-cathode voltage is positive, and a firing current pulse is applied to the gate terminal.
- Delaying the firing pulse by an angle α controls the load voltage.
Operation of Controlled Three-Phase Half-Wave Rectifiers
- When thyristor T1 is triggered, the phase voltage Van appears across the load when T1 conducts.
- When thyristor T2 is triggered, T1 becomes reverse-biased and turns off, and the load current flows through thyristor T2 and the supply phase winding 'b − n'.
- When thyristor T3 is triggered, T2 becomes reverse-biased and turns off, and the load current flows through thyristor T3 and the supply phase winding 'c − n'.
Circuits with RL Load
-
There is no discontinuous conduction mode for three-phase controlled rectifiers if L >> R.
-
The average output voltage can be calculated using the following equation:
Vdc = (3Vm / 2p) * [sin(wt + α) + sin(wt - α)]
-
The average output voltage and current can be calculated as follows:
Vdc = (3Vm / 2) * cos α Idc = (3Vm / 2) * cos α
Quiz on three-phase controlled rectifiers, their applications, and uses in electrochemical processes and motor drives.
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