3-phase line phase imbalance detector

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Hello guys:

 

This time I'm designing a 3-phase signal generator for 50HZ 3 phase tests.

Most of the tests are for amplitude and phase imbalances, and are to be used

in signal stages of protection designs (no 380V involved)

 

The generator design is discrete, fixed frequency and has provisions for master and independent phase amplitude,  phase

shift (with pots) and phase reversal.

 

I need the phase imbalance detector in order to "zero" every phase channel, and to signal with a led when

any particular phase is outside it´s 120 deg angle.

 

I though of a method using a 3p full wave rectifier and a comparator, so when a phase is outside it's 120 deg point, a

spike will trip the comparator and probably a flip flop also, signaling the phase imbalance.

 

The problem that I see with this approach are the characteristics of the rectifier diodes, that may make the circuit trip falsely (or at a

phase imbalance too high to be useful) , the thermal performance of the opamp/comparator, and the impossibility to know which phase was altered.

 

I 'm going for a discrete approach if possible, that is,  no uC or programmables, since I'd like to do the programming on 

the "product under test" side.

 

What do you suggest for this?

Best regards

 

nachus

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I don't know much about power engineering and 3-Phase stuff, but...

 

You might want to read up on Direct Digital Synthesis, DDS, as a signal generator technique.

 

It is easy to shift the phase by adding an extra offset to the accumulator.

 

It is easy to generate synchronized outputs, as the micro just adds 120 or 240 degrees to the 2nd and 3rd output channels, with respect to the primary phase.

 

JC

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Second DocJC's suggestion of DDS.

 

You can very precisely set the phase differences of 3 sines and you won't need to calibrate source. Zilch. They will be in absolute sync, and absolute phase relationship.

 

But, what is not clear is whether you are looking at phase errors in the source signal, or in the load. If load, its a different ballgame.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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If you hang a neon lamp between each phase on the line and load, the lamp will light up when there is voltage. So the idea is speed up the motor to match the speed of the line and the lamps flash slower and slower, and you hit the switch to close the contactor right at the instant the lamps are out, and the motor is locked in synch to the line. Don't flip the switch when the lamps are on, or the motor will hop off the floor and probably land on your foot.

 

Imagecraft compiler user

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Hey Jim:

 

the phase error is to be detected at the source, in order to tell the operator that the phases are not aligned,

but as i'm looking for solutions, a discrete one if very very far from what I can realize since is too much circuitry involves

 

maybe it's better to start with a dds like design like the AVR314, but using 3PWM out in a chip like ATMEGA16. I'll try

 

thank you all

 

nachus