Some Questions about Polyphase Circuits

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In summary, the conversation revolves around a practice problem (12.6) involving Y-connected loads on a three-phase four-wire system. The question includes finding the total power drawn, combined power factor, total power lost, phase voltage at the source, and power factor of the source. There is also a discussion about the impact of neutral wire resistance on the analysis of the circuit and whether a mesh or nodal analysis should be used.
  • #1
xcvxcvvc
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I'm reading over my chapter and I encountered practice problem 12.6 below. I had some questions about this problem:

1.)how can the problem define the loads as balanced if the loads don't have the same power drawn.

3.) for a.) i thought total power meant total average power(real power). So I multiplied the given powers by the PF and added them. I got 21.55 KW instead of 22.6KW

work shown: 6 * 1 + 10 * .96 + 7 * .85 = 21.55KW

3.) The chapter says I cannot analyze the circuit on a per-phase basis if the neutral line has a resistance, but it never shows an example of doing one of those problems. Do I just run a mesh or nodal analysis?

12.6:
Three balanced Y-connected loads are installed on a balanced three-phase four-wire system. Load 1 draws a total power of 6kW at unity PF, load 2 requires 10kVA at PF = .96 lagging, and load 3 needs 7 kW at .85 lagging. If the phase voltage at the loads is 135 V, if each line has a resistance of .1 ohm, and if the neutral has a resistance of 1 ohm, find (a) the total power drawn by the loads; (B) the combined PF of the loads; (c) the total power lost in the four lines; (d) the phase voltage at the source; (e) the power factor at which the source is operating.

edit: the homework problems in the chapter are easier than this monster, so it's ok if I can't solve it. I just wanted to take some steps toward understanding it. What's worse is the practice problems rarely have diagrams drawn like with this one, which really puts my knowledge of terminology to the test.
 
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  • #2
xcvxcvvc said:
3.) The chapter says I cannot analyze the circuit on a per-phase basis if the neutral line has a resistance, but it never shows an example of doing one of those problems. Do I just run a mesh or nodal analysis?

Are you sure you didn't misread? If the loads are balanced (like they are in this case) the current in the neutral wire is 0, so the impedance in the neutral wire shouldn't affect anything. And that's the whole point of 3-phase systems - half the loss in transfer lines.
 
  • #3
Kruum said:
Are you sure you didn't misread? If the loads are balanced (like they are in this case) the current in the neutral wire is 0, so the impedance in the neutral wire shouldn't affect anything. And that's the whole point of 3-phase systems - half the loss in transfer lines.

well yeah, it has to be an unbalanced load with either no neutral wire or a neutral wire resistance. But how is that example problem balanced?
 
  • #4
xcvxcvvc said:
But how is that example problem balanced?

Nevermind that, I didn't read the question with thought...
 

1. What is a polyphase circuit?

A polyphase circuit is a type of electrical circuit that consists of multiple phases, or alternating current waveforms that are offset in time. These phases are used to power different types of electrical loads, such as motors, in a more efficient manner.

2. How many phases are typically used in a polyphase circuit?

Most polyphase circuits use either two or three phases, although some may use up to six phases. The most common polyphase circuits are single-phase (two phases) and three-phase circuits.

3. What are the advantages of using a polyphase circuit?

Polyphase circuits offer several advantages over single-phase circuits. These include higher power capacity, more efficient use of power, and the ability to power a wider range of electrical devices. Additionally, polyphase circuits are more reliable and have a longer lifespan compared to single-phase circuits.

4. What are the different types of polyphase circuits?

There are two main types of polyphase circuits: symmetrical and asymmetrical. Symmetrical circuits have equal voltage and current in each phase, while asymmetrical circuits have varying voltage and current levels in each phase.

5. What is the difference between a polyphase circuit and a single-phase circuit?

The main difference between a polyphase circuit and a single-phase circuit is the number of phases. Single-phase circuits have only one phase, while polyphase circuits have multiple phases. This allows polyphase circuits to handle larger power loads and be more efficient in powering different types of electrical devices.

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