Single Phase Controlled Rectifier Problem

In summary, the problem involves a heating system with a strongly inductive characteristic and electronic temperature control. The value of resistance needed to achieve a maximum power of 7500W at 220V is 6.543 ohm, assuming symmetrical conduction and using average voltage. However, there is some debate on whether effective voltage should be used instead. If a TRIAC is used for control, the power factor will be close to 0.9 and the inductive component of impedance will be about 10% of the resistance. The necessary switch angle for 50% duty cycle is approximately 95°.
  • #1
luiseduardo
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Question
Consider a heating system with strongly inductive characteristic and with an electronic control of temperature performed by varying the firing angle of a single-phase controlled bridge.
a) Determine the value of resistance to to ensure a maximum power of 7500W if this system is powered with 220V (effective).
b) Determine the fire angle to a power of 3000W.

My attempt of solution:
a)
If the system is strongly inductive so the current is constant, then we should use average voltage. So, maximum power is when the average voltage is maximum and this is possible when fire angle = 0 degree. Then:
quw8aKp.png

And the resistance is:
VcAStaF.png

b) Using the same idea:
QHUw6Ez.png

The problem is:

Many friends (and maybe the professor) solved this problem on a exam using 220 V as voltage to find the resistance when maximum power, but I don't think this is totally correct because the problem tells us that the system is strongly inductive, so Irms = Iaverage = I constant, then, to find the voltage we should use average voltage and not effective as many other did. So, What do you think ? If I'm incorrect, why ? I'm a bit upset because I studied a lot for this exam and I was confident that the correct was average voltage when constant current.
 

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  • #2
We assume that the 220V is AC RMS. We assume that conduction is symmetrical on both polarities of the AC supply.

If an SCR bridge is used the current through the heater will be DC. If it is DC it will be hard to phase control since there will be no zero current to turn-off with the SCRs.

If a TRIAC is used the current will reverse twice per cycle. So the control is with a TRIAC and the current through the load is AC. Now, just how big is the “strongly inductive characteristic”? The power factor will be close to 0.9, and so the inductive component of the impedance XL, will be about 10% of the resistance. you can compute R and L to get say a 0.9 phase angle.

If we ignore the unknown inductance, W = 7500 = V2 / R.
For 100% duty cycle, the resistance required will be R = V2 / 7500 = 6.543 ohm

Energy is the integral of v^2 / R, while the switch is conducting. Looking at the first half cycle we need less than 50% duty.
It will need to turn on at about 95° and conduct until 180°. I leave you to solve for R and XL.
 

Related to Single Phase Controlled Rectifier Problem

1. What is a Single Phase Controlled Rectifier?

A Single Phase Controlled Rectifier is an electronic circuit that converts alternating current (AC) to direct current (DC). It uses a switching device, such as a thyristor, to control the flow of current and produce a desired output voltage.

2. What is the purpose of a Single Phase Controlled Rectifier?

The purpose of a Single Phase Controlled Rectifier is to convert AC power to DC power, which is necessary for many electronic devices to function properly. It is commonly used in power supplies, battery chargers, and motor control circuits.

3. What are the advantages of a Single Phase Controlled Rectifier?

The main advantage of a Single Phase Controlled Rectifier is its ability to control the output voltage and current, allowing for a more efficient and accurate conversion of AC to DC power. It also has a simple design and is cost-effective compared to other types of rectifiers.

4. What are the potential problems with a Single Phase Controlled Rectifier?

Some potential problems with a Single Phase Controlled Rectifier include voltage and current ripple, which can cause fluctuations in the output voltage and affect the performance of connected devices. There may also be issues with switching losses and electromagnetic interference.

5. How can the problems with a Single Phase Controlled Rectifier be addressed?

The problems with a Single Phase Controlled Rectifier can be addressed by using filtering components, such as capacitors, to reduce voltage and current ripple. Proper circuit design and component selection can also help mitigate switching losses and reduce electromagnetic interference. Regular maintenance and testing can also help identify and address any potential issues before they become major problems.

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