# Three-phase circuits problems

Discussion in 'Homework Help' started by PsySc0rpi0n, Jun 14, 2014.

1. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Anyway to check my math for these kind of circuits with LTSpice?

2. ### ericgibbs Senior Member

Jan 29, 2010
2,418
369
Yes, lets see your asc circuit.

E

3. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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I don't know if what I've done is even close to be correct.

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4. ### ericgibbs Senior Member

Jan 29, 2010
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369
Your 3 Phase Sources are OK, whats the tutors question .?

E

5. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Well, the original problem has no C and L values. It only has Xc and Xl.
I've calculated C and L with 50Hz which is our usual electric frequency.

He asks us to calculate a few things that I haven't still calculated.

I have attached the image we need to analyse.

1st thing to calculate is U between O' and O.

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6. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
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Well, I've calculated all phases currents and then I was able to calculate that Voltage using that phase impedance and current!

How can I check with LTSpice my calcs????

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
You could also post your results & we can confirm their validity.

ln the meantime I'll look at the Spice matter.

8. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
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I've used the "mesh" laws to do it.

Considering RST, the 3 phases, and N, the neutral:

Ir = 12.52|50.63
Is = 14.36|223.6 (the angle could also be -136.4º, deppending on what calculator i do the maths).
It = 7.03|113.9
In = 8.16|130.54

Then with Zn and In I calculated Vn = 10 x 8.46|130.54 = 81.6|130.54 [V]

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782

The problems with using a simulation to check your results include:

1. Accuracy errors.
2. Difficulty of resolving phase relationships.

Why don't you simply trust your answers & confirm them by mathematical means - such as plugging values back into the circuit to confirm agreement with Kirchoff's laws. Do you use applications such as Matlab which can make the mathematical manipulations more robust?

10. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
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I have used MatLab but in a very light way. I also have Octave installed in my Linux OS.

But let me know how to do it in matlab or Octave!

11. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
While I could provide that information, I think in the interests of adhering to the homework forum rules as well as for your own benefit, that you need to put in some effort.

The answers you gave for the problem were very accurate and presumably machine (rather than hand + calculator) generated - so you must have a good idea of how to attack problems requiring the efficient & accurate solution of simultaneous equations with three or more unknowns.

Matlab and the Matlab clones are great for the aforementioned task where matrix operations can be effectively applied to problems such as the one at hand.

I understand your rationale for wanting to use simulation to verify your problem solutions. Personally I'm not sure LTSpice is the greatest option, notwithstanding the fact that it is free for personal use. I use an application called PSIM for 3-phase & other power electronic simulation work. A demo version is available for free here:
http://powersimtech.com/products/psim/

If you are looking to a career in electronics or electrical engineering then you may need to develop familiarity with a broad range of simulation software tools which target particular areas of electronics or electrical engineering.

A cautionary note: Simulation is no substitute for practical work experience, disciplined critical reasoning & hard work.

12. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
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I understand and accept your advices. However, I do not have the time to satisfy all those advices.
I need now how to check my math results and move forward to the next problem, understand it and solve it!

My exam is next Friday but I have another exam next Wednesday, so my time is really tight!

I would love to have more knowledge at MatLab/Octave, LTSpice, Kikad or gEDA software that I know they are powerful and handy, but I just don't have the time.
I would also love to have more knowledge in programming languages like C, java or perl and VHDL but I only know the basics of C.

These are the dilemma's of a family man that wants to graduate after his daily job!

13. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Understood.
A final comment. Under exam conditions you probably won't have much time to check your answers. One can probably only look for answers to be reasonable - such as order of magnitude.
Hope all goes well for your exam.

14. ### ericgibbs Senior Member

Jan 29, 2010
2,418
369
hi Psy,
I would agree with 'tnk' its your method and way of calculating in order to solve the exam questions that the examiner will be looking for, not the precise numerical answers.

In engineering its often not possible or necessary to work to a very high degree of precision as most components we use have tolerances in their values.

E

15. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
3
Sure, I agree. But checking if results are correct is the only way I have to see If my thoughts are correct.

That's why I was looking to check my results.
It's worthless, I think, that I solve the problems if they are not correctly solved. And this subject, 3 phased circuits, are quite subjective! We need to know a lot of theoretical concepts so that we can solve them correctly! Like phase angles, phase sequence, RMS or not RMS values, phase values and line values, etc, etc, etc...

16. ### ericgibbs Senior Member

Jan 29, 2010
2,418
369
Hi,
I would say that LTSpice will give results that are good enough to check your calculations.

I cannot help with MatLab.
E

17. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
3
The attached picture represents a 3-phased circuit feeding 2 loads with a voltage of 380V/50Hz.

Load 1 - inductive engine/motor that can be in a "star" or a "triangle" setup and each phase's impedance is a resistor of 10Ω and an inductor of 0.1H.

Load 2 - 60 lamps, 20 for each phase, of 50W each lamp!

Teacher asks currents at A1 and A2 lines of the attached picture... A1 and A2 are Amp meters!

First of all, let me ask if the problem is understandable, please!

Thanks
Psy

Edited;

I've replaced the original picture by the following one to make things easier to me... I'm considering the engine is at a "triangle" setup!

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• ###### 20140617_095047.jpg
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18. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
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=======================Stage 1====================
First thing, I think I can say is:
(Ia = current at line A)
(Ib = current at line B)
(Ic = current at line C)

(Ial = current from Line A for the lamps)
(Ibl = current from Line B for the lamps)
(Icl = current from Line C for the lamps)

(Iam = current from Line A for the motor/engine)
(Ibm = current from Line B for the motor/engine)
(Icm = current from Line C for the motor/engine)

Ia = Ial + Iam
Ib = Ibl + Ibm
Ic = Icl + Icm

Then I need to set up Voltage angles.

If I consider the counter-clockwise rotation phase, "I think I get" the following:

ACB

My first question comes now. Is it the Simple Voltages that has the 0º, -120º and 120º or is it the Line Voltages or is it the same no matter which ones I choose to have the 0º, -120º and 120º angles?

=======================Stage 2====================
Calculate currents inside the engine triangle setup.

With 10Ω and the 0.1H I can calculate each phase impedance.

So, each phase's impedance will be
Now I can calculate phase current by:

Now, as we know all phases are separated by 120º, so I can say that the other 2 currents are:

With these currents I can calculate Iam, Ibm and Icm by the Nodes Law.

If I consider currents inside the triangle setup flowing counter-clockwise I'll get the following:

=======================Stage 3====================
Calculate currents in the "star" lamp setup.

As we are given the total Power/Potency we can calculate lamps resistance.

As we are said that the system is "balanced", all 3 resistances are equal, so

I have another question here:

Which angles should these currents have? 0º, -120º and 120º or 30º, -90º and 150º????

Ok, continuing...

=======================Stage 4====================
Now that I have all currents I can find the requested currents by Nodes Law stated at the beginning:

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19. ### ericgibbs Senior Member

Jan 29, 2010
2,418
369
hi,
The Line voltages with respect to a 'neutral' would be 380/sqrt(3) ~ 220Vac and its these 'Line' voltages that have the 120deg relative phase shift.

In the 'old' UK 3 phase supply the 240Vac 50Hz, had a phase difference of 120deg for the 3 phases.

Voltages between phases was 240V * sqrt(3) = 415Vac between any two phase conductors.

Is this what you are asking.?
E

20. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,184
3
I think so. I'm not sure what would be the correct terms for these voltages.

So, let me check if my knowledge is correct:
Uline=380V
Uphase=380/sqrt 3V.

Uline => 0°, - 120° and 120°
Uphase => +-30 for each phase.

Is this correct?