Three phase supply need help.......

[malik_zohaib]

hello guys me back again......
Can anyone help me???????

I want to build the circuit that will monitor the voltage sequence of 3 phase supply and i want it will only give me the signal when the sequence is incorrect. How can i implement this without using zero crossing?

Second how to calculate the turn ratio, and current values to design the current transformer (assume my load is of 6A)?????

Thanks!!!!

 

[Prototyp_V1.0]

I want to build the circuit that will monitor the voltage sequence of 3 phase supply and i want it will only give me the signal when the sequence is incorrect. How can i implement this without using zero crossing?

You can use a voltage transformer, not a regular 3 phase, but three independent and equal ones. Connect their outputs in series. If the phases and voltage for 3 phase is correct, the sum of voltages should be zeero.

Second how to calculate the turn ratio, and current values to design the current transformer (assume my load is of 6A)?????

Depends of the device/circuit that is meassuring the current value.

 

[malik_zohaib]

Can you tell me how to design a CT if i know the load current ????

 

[Prototyp_V1.0]

Can you tell me how to design a CT if i know the load current ????

The CT is very simple to design. You just a iron core (for transformers) and wind it woth two set of windings so you get a turn ratio. Or is it the CT itself you are not sure how to design?

Anyway, you have to know something of the measuring device on the other side of the transformer before you can go and shop (or make) transformers.

 

[mtwieg]

For the CT the only two parameters you really care about are the turns ratio and the magnetizing inductance. You need to make sure the magnetizing inductance is high enough so that it doesn't load down your burden resistor at your frequency of interest.

 

[malik_zohaib]

I only wanna know how to calculate the turn ratio.
At primary there are 2-5 turns, i guess, and for secondary i dont know...........

If i design the CT like PT then it means current will step down and voltage ll step up.....how to handle that much voltage???????
Am i right or wrong??????????

 

[Prototyp_V1.0]

I only wanna know how to calculate the turn ratio.
At primary there are 2-5 turns, i guess, and for secondary i dont know...........

Well, it's hopeless when you don't know what current you want on secondary side. It's just like an equation with two unknown, cannot solve until you make up your mind and decide what current you want on the output. Unless you have more info I really cannot help you?
If you have technical question, feel free to ask. I beleive you don't really know how to use a ct.

If i design the CT like PT then it means current will step down and voltage ll step up.....how to handle that much voltage???????
Am i right or wrong??????????

Wrong. Because a ct isn't supposed to output any voltage - exept for a very small voltage. And it's also supposed to be connected to some equipment that can sense when current is too high so any connected components (usually in power grid) can be disconnected from the rest of the power grid.

Unlike a regular transformer, a ct is connected in SERIES to the load.

 

[mtwieg]

I only wanna know how to calculate the turn ratio.
At primary there are 2-5 turns, i guess, and for secondary i dont know...........

If i design the CT like PT then it means current will step down and voltage ll step up.....how to handle that much voltage???????
Am i right or wrong??????????

You generally put a burden resistor on the secondary so that the transformed current creates a voltage which is proportional to primary current. So if your primary has 1 turn and your secondary has 10, and you have a 1 ohm burden resistor then you will get 0.1V per amp. That is assuming your magnetizing impedance is much greater than 1 ohm. If you want to lower the secondary voltage, then you lower the burden resistance.

 

[FvM]

You can use a voltage transformer, not a regular 3 phase, but three independent and equal ones. Connect their outputs in series. If the phases and voltage for 3 phase is correct, the sum of voltages should be zero.

Assuming the transformers are star connected (each one between phase and neutral), then zero output will indicate phase symmetry, but not correct sequence. The sequence may be reversed as well. A phase sequence detector always involves a phase shifter or similar circuit as a reference for the observed sequence.

 

[malik_zohaib]

Well, it's hopeless when you don't know what current you want on secondary side. It's just like an equation with two unknown, cannot solve until you make up your mind and decide what current you want on the output. Unless you have more info I really cannot help you?
If you have technical question, feel free to ask. I beleive you don't really know how to use a ct.

Basically i want to interface the ct with ucontroller, so the secondary current should be within safe range (5ma-10ma).
2nd i have to design ct all by myself that is the real problem........for me.......

---------- Post added at 11:42 ---------- Previous post was at 11:41 ----------

Assuming the transformers are star connected (each one between phase and neutral), then zero output will indicate phase symmetry, but not correct sequence. The sequence may be reversed as well. A phase sequence detector always involves a phase shifter or similar circuit as a reference for the observed sequence.

Means i have to use zero detector????

 

[malik_zohaib]

You generally put a burden resistor on the secondary so that the transformed current creates a voltage which is proportional to primary current. So if your primary has 1 turn and your secondary has 10, and you have a 1 ohm burden resistor then you will get 0.1V per amp. That is assuming your magnetizing impedance is much greater than 1 ohm. If you want to lower the secondary voltage, then you lower the burden resistance.

Is the secondary burden resistor depends on primary turns???Please explain it.

 

[Prototyp_V1.0]

Basically i want to interface the ct with ucontroller, so the secondary current should be within safe range (5ma-10ma).

Ok. Let's assume that 10mA is the maximum current that you want to read (given you have choosen a resistor so voltage can be dropped over). Then the turns ratio must be 6A/10mA = 600.

Then just use ohms law to adjust the resistor value to get the voltage drop so you can read it.

The drawing i added shows how to use a CT.

To adjust the voltage output to meet get within a spesific voltage range, you can either adjust the turn ratio of the CT, OR you can adjust the value of the resistor, wich obvious is the easiest method.

 

[mtwieg]

Normally when I use CTs I just have one primary "turn" (it's just the wire threaded through the CT once). Putting more turns on the primary basically just increases sensitivity (which you may not want).

To figure out how to choose your Np, Ns, and burden resistance, you need to know what sensitivity you want (in volts per amp) and what bandwidth you need. Also you need to pick a core shape and material. Until you choose those specifications, you can't do anything.

 

[malik_zohaib]

Thnx guys now i know how to handle ct.

Now my next problem is the correction of phase sequence. I have able to design the circuit that monitors the 3 phase voltage sequence. The problem is,how do i know which two phases are reversed?? and how to correct them??? I have tried it by using relays but they only change the yellow and blue phases.
Can anyone help me......????

 

[Prototyp_V1.0]

Do you have a regular 3 phase transformer with known identity of the input phases?

 

[DXNewcastle]

how to correct them??? I have tried it by using relays but they only change the yellow and blue phases.

How many phases do you need to swap to achieve a change in the sequence?

 

[malik_zohaib]

Do you have a regular 3 phase transformer with known identity of the input phases?

No i only have the 3 phase supply (with known identity of the input phases).

---------- Post added at 11:05 ---------- Previous post was at 11:03 ----------

How many phases do you need to swap to achieve a change in the sequence?

Basically i want to correct the sequence; if fault occurs. Only two of them need to swap. But the problem is which ones??

 

[Prototyp_V1.0]

Basically i want to correct the sequence; if fault occurs. Only two of them need to swap. But the problem is which ones??

Why do you want to swap it? To get a motor run in reverse? Then it doesn't matter wich two phases you chose to swap. It will have the same effect.

 

[malik_zohaib]

Why do you want to swap it? To get a motor run in reverse? Then it doesn't matter wich two phases you chose to swap. It will have the same effect.

no no no ..........
Let me explain it first.
I want to protect motor from reverse rotation; that is due to the any two phase changes. I want to design the circuit that will correct the fault in phase changes.

I just want to use motor in only one direction whether the phase sequence is correct or not.

I have already design the circuit that monitors the fault in phases (phase change), i just want another circuit that will correct these faults.
That is it.

 

[Prototyp_V1.0]

I was just about to suggest something. But then I remembered that you want to avoid using zero crossing. Why is that?

I would have used a voltage transformer connected to three set of opams configured as schmitt triggers and then using some logic to assure the phases is correct. By using schmitt-triggers you'll avoid high frewuensy noise that can fool a normal zero crossing circuit so the timing gets wrong.
And apart from that, you can use a three phase voltage transformer that outputs is connected in series so you can also check that the phase angles is right if that is important for you.
This could be the same transformer if it's equiped with a secondary AND tertiary (third) winding.
Obvious, the first problem (phase sequenze) is the most important to detect it it's right. If phase angle isn't exactly 120 deg between each will cause the motor to not run properly. But it's a rare situation and will most likely not happens unless the center is grounded and there is a ground short somewhere in the net.