# 3 Phase Unbalanced System

Discussion in 'Homework Help' started by ngkt722, May 11, 2015.

1. ### ngkt722 Thread Starter New Member

May 10, 2015
3
0
Can anyone teach me how to draw the circuit diagram and solve the problem?? Thanks

The following loads are connected in star across a 3-phase, 380 Volts, 50 Hz supply as follows:

-> A 380 Volt, three phase 10kW input power induction motor, with 0.8 lagging power factor
is connected to the Red, Yellow and Blue phase via a 3-phase, 3-wire cable;
-> A 2kW single phase electrical heater (assume pure resistive load) is connected between the
Yellow phase and neutral;
-> A 4kW single phase electrical load, with 0.7 leading power factor is connected between the
Blue phase and neutral.

Calculate:
(a) ALL the phase currents that flow to the induction motor;
(b) the phase current that flows to the 2kW electrical heater;
(c) the phase current that flows to the 4kW electrical load;
(d) ALL the line and neutral currents taken from the supply.

Thanks~~~

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,402
1,227
What book is the origin of that question or the book you are using in your class?

There should be examples from whatever material the instructor has given you.

3. ### ngkt722 Thread Starter New Member

May 10, 2015
3
0
So

My class use this book for reference:

Allan H. Robbins, Wilhelm C. Miller, "Circuit Analysis: Theory and Practice, 5 edition"

There are no similar question in the reference book. I learned how to solve some basic balanced and unbalanced 3-phase problem. BUT for this question, It is difficult for me to figure out the circuit diagram.

Thanks

4. ### WBahn Moderator

Mar 31, 2012
18,088
4,917
So take your best shot at it and post it so that we have something to start from.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
@ngkt722
If you have done both balanced and unbalanced 3 phase calculations in class I would be surprised if you cannot at least offer solutions for each of the three individual loads.
The final step of how one combines individual load currents together to give the composite load currents, can obviously be left as the final task.
You should be able to show us the individual per-phase line & neutral load currents for each load case. The most 'complex' load is the induction motor, but I presume you realise this would be considered of itself, as a balanced load. This should inform you as to what the absence of the neutral wire connection to the motor means in this context.

6. ### MrAl Distinguished Member

Jun 17, 2014
2,562
519
Hi,

And at the very least try to solve (b) first.

7. ### ngkt722 Thread Starter New Member

May 10, 2015
3
0
Sorry to reply late, because I am a part time student@@.

How can I know from the context about the motor (star)connection.

And I really don't know how to draw the diagram in star or delta connect from my problem@@.

Thanks

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