Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

three phase voltage measurement with out neutral

Status
Not open for further replies.

sivajoginaidu

Newbie level 4
Joined
Feb 11, 2007
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
hyderbad
Activity points
1,315
hai,
i want measure the three phase voltage without neutral wire. other way says three wire , three phase voltage measurement.
i was derived the 5 volts from three phase. i also done detect zero cross and phase seqence detection. but i need to measure phase to phase voltage.

present i done,measure the each phase and add two phase voltages. but it not accurately reading microntroller.

can any body suggest circuit, it will help tome.
 

hz,phase angle and then pd of course Some more things but what u wanna make it on high speed >noise< beware of it
post ur brief circuit for suggesting technique
 
Last edited:

If you measure 3 phase power from the mains, the phase voltages SHOULD be balanced about earth. But because of the possibility of different phase compensating capacitors on different phases and different current loading, there could be a significant voltage on the "neutral" wire. When I worked in a factory, we had a 6V bulb connected between the neutral and earth and it would flicker all the time and some times it would blow!!
3 phase from the mains is lethal, get 3 identical transformers (phase voltage to LV) and insulate all the connections so there is no possibility of you touching the incoming 3 phase. Make sure that its frame is securely earthed so if the insulation on the transformer fails, the supply fuse will blow.
Frank
 

thank you for suggetions. i was connected one phase to circuit ground. other phase was connected to ADC with dividing voltage to below 5volts. but my reading not linear. pls give sugget any helpful method.
 

"i was connected one phase to circuit ground." Wrong, you should connect your transformer and fuse (X3) between the phases. Then on the transformers secondaries earth the same pin of each transformer and and to earth then feed the remaining three wires to a diode and capacitor to give you DC ~ Vp-p. Or connect the primaries of the transformers via a fuse to each phase and connect the same pin of the primary of all the transformers to MAINS earth. Then you can use your diodes and caps on the secondary. Gives you DC~ Vp->earth. With three phase mains over here you get 440V phase to phase or 240V phase to earth. The neutral to earth is +- 10v (?). Your circuit ground might be convenient but it may not be connected to any MAINs earth at all.
In general, what range are you actually measuring cos' noise pick up might cause the readings to be high at low levels (ADC will measure any old rubbish).
Frank
 

Hey chuckey are those measurements accurate ?////////\\\\\\\\
what about non linearity and voltage swingsszzsszzzzzzz
 

Phase to earth X 1.732 = phase to phase voltage. So my figures should have been 240V and 415. Most people are after measuring the mains +_ 20% at the most, no point in measuring it down to 1V because by then the lights have gone out and you don't need measuring apparatus to detect that. So if your transformer is producing say 8V with 240V ac in, then you will have three versions, say 7.8, 8.1 and 8.3V, one for each phase. So you want to compare these voltages with (8v - 20%) 6.4V from a precision source. The highest voltage you would expect would be (8 + 20%) = 9.6V. So using 3 differential amplifiers with a range of 0 -> 3.2V would give outputs of 1.4, 1.5 and 1.7V, which the measuring DVM senses but it is scaled so 0.0V=198v, 1.6V = 240V and 3.2V = 288V. By this method non linearities are kept to a minimum but are dependent on the components you select. likewise the voltage swings are not excessive.
Frank
 

are those measurements accurate ?
Hard to decide without a real specification from your side. Reviewing the discussion up to now, I notice that all suggestions are based on assumptions about the purpose and the exact kind of voltage measurement. Also possible isolation requirements haven't been told.

As a first point, you may want to measure
- rms value
- rectified value
- instantaneous value (involving a sufficient sampling rate)
Secondly, an accuracy specification should be given

I hear from one of your posts, that isolation isn't required
i was connected one phase to circuit ground. other phase was connected to ADC with dividing voltage to below 5volts.
In this case, a directly connected measurement circuit would be an option, and I don't understand, why chuckey strictly claims it's "wrong". The circuit must of course be able to process the full AC amplitude without introducing systematic errors, e.g. clipping.

An isolated measurement has the advantage, that the processor part can be designed as SELV circuit and touched without danger. But cheap low power transformers are introducing a considerable linearity and phase error, so it depends on the accuracy specification, if they can be suggested.
 

sir,
i used three phase retifier in seirs capacitors. then regulated to 5volts. this 5volts for microcontroller. from each phase, i used resistr divider network to ground point. then applied to ADC PIN of microntroller. i was measureing rms voltage of each phase. this my idea to measure each phase voltage. but is it works or not. how to create virtual neutral.
 

You should better show a schematic, how you connected the measurement.

Generally, rectifier and filter capacitor doesn't give a RMS measurement. It's either rectified or peak value, or something in-between, depending on the circuit. Both measurements can be converted to RMS, but the calculation is only correct for sine waveforms. The error of rectified value will be acceptable in most cases, peak value has larger errors.

To get all three delta voltages correctly, you either need transformers (as suggested) or difference amplifiers. Only for instantaneous delta voltages, the third could be calculated as difference of the two others.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top