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Single phasing protection circuit

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Jul 31, 2006
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Hi everyone!
Please help me on this problem. I would like to design a circuit to protect three phase Ac motor from single-phasing condition. This circuit can detect one or two phase-loss then turn off motor immediately. Does anyone know this circuit? Please upload to this topic. Thanks.

Hi cameo_2007,

I only want to protect a inductance motor from phase unbalance or phase loss. There are no devices in control board. Could you give me some advice or give me some reference schematic. Thanks
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Look at the attached pdf schematic, if you have any question, feel free to ask.

Hi Laszlof,
This schematic seem to be simple, but i don't know how it works. Please explain details. I want to use an microcontroller to sense phase angle then detect phase loss. Do you know any schematics?

Yes you are right, that's quite a simple schematic, doesn't need any micro controller, just simple relay's, the phase loss is detected by the voltage drop between the phase and the NULL, if one relay(LS2) gets a voltage under his threshold(about 150V, depends from the relay), the motor is disconnected.

It works like this:
Start sequnece: SW2(ON) is pushed, it activates the main relay(LS3), each of the LS2 relay's are powered(if all 3 phase are present), the circuit closes, and the main relay state is latched(like a bistabil).

Stop sequence: SW3(OFF) is pushed, this interrupts the circuit if the main relay coil, and the motor is disconnected, if any of the 3 LS2 relay's coil get a voltage under there threshold, again the motor is disconnected(logical AND connection), as you can see, one of the LS2 relays seems redundant, but the main relay has a larger magnetic hysterezis, so this way the symetrical sensitivity of the circuit is kept.

This circuit can be modified to be used without the NULL line, but the majority of the relays got 220V coil (phase-NULL voltage).

My question is: Why you want to use a microcontroller based monitoring, isn't that like killing a bug with a riffle? I mean the phase angle is 120, it doesn't change, the worst case is when you have a low voltage on either of phase, or it's interrupted. For increased sensitivity you could use some electronics for the voltage monitoring/relay command, this way you can set the threshold voltage to 200V, or monitor the voltage between the phases, it is lower than 380(or higher) then it is out of phase, then a window comparator is needed, anyway the microcontroller is to complex for this simple task.

I hope my suggestions are helpful, but you can ask if not :)


Thanks for your suggestion, but i think this schematic is not completed.
If the three phase of motor is DELTA connected, it had not NULL phase. So you can not sense drop voltage between the phase and the NULL. One of problems when phase loss that volage between phase to phase did not decrease because of inductance between three coils in the motor. How do you resolve this problems?

The schematic is complete, every 3 phase plug has the NULL, the motor connection doesn't influence this, the star connection also doesn't need the null, because is a symetric load.

"One of problems when phase loss that volage between phase to phase did not decrease because of inductance between three coils in the motor. How do you resolve this problems?" This sentence i don't understand.

There are more complex methods to, like sensing the current in each phase, if there is an unbalance between the phases the motor is switched off, this way the overload protection is handled too.

To LaszloF,
"One of problems when phase loss that volage between phase to phase did not decrease because of inductance between three coils in the motor." That mean, when phase-loss occur the voltage phase to other did not decrease to zero. For eamples if phase X is opened, phase Y,X will induce a voltage in phase X therefor relay can not detect phase loss.

:) you are correct, to be more precise the voltage will be half as should be, this means 110V from the NULL, and 190 from phase-to-phase, at this voltage the relay will disconnect, by the way you can easily increase the sensitivity of the sensing relays by adding a series resistance to the coil, about 1/4 of the coil resistance should be a good starting point, you will need to experiment the optimal value.

Thanks, but this idea is used only in particular system. I want to design an electronic board which can be used in any three phase motor systems without any adjustment. I have known that there are many digitals relay can protect against phase-loss or unbalance phase. Do you know how it work? What parameters is sensed to detect when phase loss occur?


Ok, in this case you need to monitor the voltage phase-to-phase, using linearized opto couplers, or some other isolated voltage transducer(maybe transformers), the phase angle can be monitored from the 0 transitions, and this way over/under voltage can be sensed to, if only phase angle is needed than only the 0 transitions has to be monitored.

How to monitor angle phase then dectect unbalance phase? What is difference between angle phase when occur?

The unbalance is the voltage difference between the phases, this is the most common problem, the x,y,z phases are 120 degrees from each other, so the 0 transitions should be from 2.8ms from each other.

I understood the unbalance phase/voltage. I would like to know what do you do to detect angle phase when phase loss occur.


Hi everyone, do you know any schematic of digital relay to protect three phase motor against unbalance phase. Please help me !!!

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