# Transformers in series for AC phase measurement

Discussion in 'General Electronics Chat' started by ToonooT, Dec 31, 2015.

1. ### ToonooT Thread Starter New Member

Aug 8, 2013
11
0
Hi all,

I want to measure AC line voltage and frequency from a 3 phases ~400V generator (wind turbine application) and no neutral.
In order to measure this I need to get this signal to let's say 0-3.3V and isolated from high voltage.

The best solution for me is to measure between two phases with a transformer.
So, I was looking for a transformer which can directly handle such voltage but it seems hard to find. By doing research on compenents I found this one :
It can provide two 5V outputs output from 230V.
My questions are :
- Can I put 2 of this one in series to handle up to 460V ?
- Do I need to equilibrate load between this 2 transformers ? (link all outputs in parallel so 4 outputs because of 2 outputs per transformer)
- Do I need to install this solution between each phase to equilibrate the load ? For me, assuming the load will be very light, it's not necessary.

Hope someone can help me
Thanks and Happy new year !

2. ### RRITESH KAKKAR Senior Member

Jun 29, 2010
2,831
89
For this i think, you should use less VA transformer or Current transformer
by this convert to voltage divider 3.3V max.
by this it can work.

3. ### ToonooT Thread Starter New Member

Aug 8, 2013
11
0
Thanks for your answer but how can I measure Voltage with a Current transformer ?
Is it not only to measure current ?

4. ### RRITESH KAKKAR Senior Member

Jun 29, 2010
2,831
89
No current transformer are used to measure current only.
potential transformer are used to measure voltage/emf

Current transformer

5. ### ToonooT Thread Starter New Member

Aug 8, 2013
11
0
By the way, I do not need to measure current...
Can someone answer my 3 questions ?

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,841
962
A 100:1 voltage divider would not load down your generator, and would provide a 0 to 5 volt scaled reading of your generator voltage.
A 2 Meg resistor in series with 20k resistor would be my suggestion

7. ### ToonooT Thread Starter New Member

Aug 8, 2013
11
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Of course it will not load the generator but this solution is not isolated from power lines which can be dangerous.
Also, I don't have a common voltage reference between generator and my device so for me, it's not a good approach.

8. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,841
962
ANYTHING which generates 400 to 500 volts is dangerous, so having a current of 200 or so microamps flowing through a 2 Meg resistor to ground is not an increased risk in this respect.

Ground is a common electrical condition with both your generator and your meter. Use it.

9. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,434
1,625
Yes the series combo should work. Loading them the same is also a good idea to keep it all in balance.

I would not worry about the load on the 3phase power, it should not notice the slight imbalance.

Isolating the monitor from the HV as you are doing recommended.

ToonooT likes this.
10. ### BR-549 Well-Known Member

Sep 22, 2013
2,164
415
They make "control transformers" that go directly across two 480 legs. They usually output 120 or 24 vac. I use to find them at 12 v, but that's been awhile.

The output is proportional the the leg(line) voltage. Yours will be a little lower.

You can measure the frequency and voltage across the <24 vac. Or knock that down to where you need it.

One transformer per phase.

Aug 8, 2013
11
0