# how to detect phase unbalance in three phase power supply

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#### anan1975

##### Member level 3
hi,
Can any body suggest how detect phase unbalance, Means In three phase system if one of the phase has the 10% less volt than two others.

Anand

#### Mr.Cool

measure the 3 phases and compare the magnitudes.
for phase imbalance, perhaps a zero-crossing detector + timer.

Mr.Cool

V
Points: 2

#### IanP

Re: how to detect phase unbalance in three phase power suppl

The easiest way of monitoring the phase unbalance is to connect 3 resistors in star configuration and monitor the central point against the neutral .. see attached picture ..
If the central point is not 0 (the same as neutral) you will have to measure each phase magnitude to find out which one cases the unbalance to occur ..

Rgds,
IanP

Vipin Gupta

### Vipin Gupta

Points: 2

#### anan1975

##### Member level 3
Re: how to detect phase unbalance in three phase power suppl

Thanks for reply but there is no neutral please suggest a ckt without neutral line.

##### Full Member level 6
Re: how to detect phase unbalance in three phase power suppl

connect CT (Current Transformer) in series with each phase. there is no need for Neutral. if the voltage is constant then u can identify the unbalnced phase

ismu

### ismu

Points: 2

#### FvM

##### Super Moderator
Staff member
connect CT (Current Transformer) in series with each phase
Voltage unbalance and current unbalance are different things. No load has been mentioned with the said problem.
If you want to check for voltage unbalance, you have to measure voltages.

You have (at least) two measurement options:
- Determine the magnitude of all three interphase voltages and compare them
- Check magnitude difference and 60° angle between two interphase voltages. This method also verifies the correct phase sequence.

Both operations can be performed with basic analog circuits (or digital three phase energy meter chips).

embeddedlover

### embeddedlover

Points: 2

#### IanP

Re: how to detect phase unbalance in three phase power suppl

If you don’t have access to the neutral wire you can try to use the EARTH, as in many countries the Neutral and Ground (or "Earth") wires are tied or linked together at the building's incoming power main breaker panel ..

Rgds,
IanP

#### FvM

##### Super Moderator
Staff member
I prefer to understand the problem strictly as said. The voltages may be supplied from a transformer, without neutral earthing.

Either if it's an exercise or a real world problem, it can be answered exactly.

P.S.: As an example, see Omron K8AB-PA phase asymmetry relay:

#### anan1975

##### Member level 3
Re: how to detect phase unbalance in three phase power suppl

But the question is still there how to detect phase unbalance please suggest with the help of schematic .As it is 3-phase 3-wire 440 volt system no space to use neutral or earth. is it possible by microcontroller .

i think it is possible by microcontroller by measuring phase to phase voltages of each but how?
can anybody suggest schematic and from where the power supply will come to energize microcontroller?

#### Engr. Laeeq

##### Newbie level 3
There is an earth connection or ground connection with every power supply which must be connected to earth. u can get power for ur microcontroller from the phase & ground terminal and at the same time, you can check phase unbalance, by connecting 3 resistors in Y-connection & measuring the difference in voltage between the ground terminal & the 3 resistor's central terminal.

#### FvM

##### Super Moderator
Staff member
You are basically repeating the ideas mentioned in some previous posts: Measuring inbalance related to neutral or protective earth. But these "solutions" are based on assumptions that don't apply for all power supply systems. The OP has clarified this in post #4 and #9. I referred to the state-of-the-art methods in post#6 and #8, they work with just three phase terminals without additional prerequisites. It may be however required to additionally monitor the balance against neutral, w.g. to know if single phase loads can be safely operated in a 230/400 V supply. The monitoring relay linked in post #8 has both options.

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