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Phase rotation meter
4

Phase rotation meter

Phase rotation meter

(OP)
Hi all, I need to make a 3phase rotation meter (to measure rotation for 400V,50hz,incoming supply from diesel) since we don't have one (we will get one soon). The test equipments which we suppost to receive are missing and its a long story. To check and confirm the incoming phase rotation is a requirement for the client.

I have 1 capacitor 3uF (+-5%)/400Vac/40-60-Hz, 2 bulbs (filamen type)220volt, 50w. All these are cnnected in star with capacitor in L3, bulb no.2 in L2 and bulb no.1 in L1.
 
First question, what is the capative reactance at 50Hz, since the power frequency available is 50Hz? I am confused with the rating 40-60hz.

Say the cap. is same 3uF,so the -jXc is 1/(2*pi*50*3uF) = 1.06kohm. Each bulp is 220v*220v/50w = 0.968kohm.

Second question, the cap. reactance and bulp ohmic value are bit different is this acceptable?

3rd question, two bulps will have 400v accross their terminal and this should be allrite, correct?

4th question, Neutral Voltage formula is =Ya*Van+Yb*Vbn+Yc*Vcn/Ya+Yb+Yc

Guys I need your input before I make one. I shall draw and attach a file in the next post.

Thanks in advance.






 
 

RE: Phase rotation meter

Any small 3 phase motor will work as a phase rotation meter.

RE: Phase rotation meter

(OP)
Yes, correct. I have to seriously consider a small 3ph motor.

Thanks edison123. We connect the motor as per the name planet L1, L2 and L3. If the rotation is wrong we swap 2 phases. In this case we assume the L1,L2, and L2 on the motor connecting diagram is correct for the actual motor rotation as per motor data sheet?.

I have to look for correct motor now, this will be plan A.

How to tell the motor will rotate in particualar direction accrding to motor connecting diagram? I will lucky if I have rotation sign onthe motor somewhere



 

RE: Phase rotation meter

You can buy three phase rotation meters.

RE: Phase rotation meter

(OP)
one more problem at the site is there are few point (actually MCBs terminal) where these phase rotation test required for the documentation. Let we see. Any other input guys?..

RE: Phase rotation meter

What Edison says. We used small motors for years when phase rotation meters were something that was generally not available. Still, when I have to do work that involves possible phase rotation issues, I look for an installed motor that I may observe to verify phase rotation.
And a caveat on using phase rotation meters. I was called in to a site to trouble shoot a transfer switch that would not function after a transformer bank had been modified. I was told that the engineer on the transformer modification had a rotation meter and had used it to verify the phase rotation.
Moral, the rotation of an installed motor that may be energized by either the mains or the back-up supply is the final proof that the phases are correct, (or reversed as the case may be).
On the other hand, if you are going to use a phase rotation meter, and connect to first the mains connection and then the standby supply of the transfer switch it is well to be aware that many transfer switches are constructed with the phase relation ship of A-B-C, C-B-A. Depending on the design and construction of the switch this reversed arrangement may often facilitate the design of the jumpers on the load side of the switch.
Tip: Whenever possible when checking transfer switches, connect the meter on the load side and check the mains and the standby supply WITHOUT changing or moving the meter or test motor connections.
Good luck.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Phase rotation meter

I presume that you want to compare the phase sequences of your DG with the utility. As Bill says, it is important when comparing two different power sources, not to change the sequence of the meter (or motor). It is best to the leave the connections of the meter/motor as it is and check the DOR with both the supplies independently.

As davidbeach, phase rotation meters are dime a dozen... ok, may be under $ 100.

RE: Phase rotation meter

I have also checked rotation with a phase check relay. This is to compare two sources. Comparing two sources is one of the major reasons for phase rotation checks. The other major reason is to verify rotation after any changes to the supply system that may have reversed the phase rotation. In this case, look for a small installed motor and note rotation before and after changes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Phase rotation meter

I agree with waross, that if you are checking for same rotation of two sources, the best means is one instrument energized from both sources sequentially with no change in test leads.  That instrument could be a small motor, one that will cause no harm if spun backwards, a rotation meter, or a more sophisticate instrument such as a smart meter or a numeric relay that can provide rotation information and/or positive and negative sequence voltages.

RE: Phase rotation meter

2
I have done phase rotation check with an oscilloscope.  Just consider that the motor is a generator and select the orientation of leads that gives the same phase rotation of power output from your motor when spun as you intend to put into your motor.   

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RE: Phase rotation meter

Sorry - I was talking about phase rotation check of a motor, not a power supply (I didn't read the question closely enough).   I guess you could use O-scope for rotation check of power supply but would have to look at ratings of equipment and do you have the capability to safely install and energize the o-scope.

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RE: Phase rotation meter

(OP)
Thanks for the input guys. The problem is, this is a new installation. We tried to buy a ph rotation meter locally and its not availabe. We only could buy a multimeter. We don't anything else other than multimeter and a megger. We have site office power by a local utility and we have ask them a small DB box somewhere near site and it still not there yet.

We have some diesel generator at site and its the only supply available for the construction guys. They don't use any 3 ph motor.

We want to energise the first panel, for first time for some flushing. For this we have some doc to be filled out. We have transfer switch, to the next board but its way too soon.

In worse case maybe I shall energise the board and directly see the rotation of the real pump, If its wrong I shall tell to the client the rotation is wrong and we have to swap the cable. The rotation of the flushing motor is important than what ph rotation meter shows anyway.
   

RE: Phase rotation meter

The question is where will you swap the cables - at the incoming supply to the board or at the motor terminal ?

Google phase rotation meter. Both are cheap. :)

RE: Phase rotation meter

Quote:

In worse case maybe I shall energise the board and directly see the rotation of the real pump
I guess that falls along the lines of edison's original suggestion.  One important thing would be to uncoupled the pump from the motor before attempting this.  Motors are tolerant of rotating backwards, but pumps often are not.  

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RE: Phase rotation meter

(OP)
Hi edison123,

Powering up flushing motor without checking the imcomer from diesel phase rotation will be my last resort. If I have a ph rotation meter and my supply from diesel is in Ph 1,2,3 then I shall swap the cable at motor if the motor rotaiton is wrong. If I did this, the next time I don't have to touch the motor when the real supply comes in. I will just check the rotaiton of the permeneat supply. We will psycally see the actual rotation of the motor in both case anyway.

I agree ph rotation meter is cheap, the problem is getting one. My boss don't mind spending money, the problem is the delivery period and limited time in hand. Its not easy to get things fast in Africa. Luckly to have internet connection so I can get some idea here. :)
   

RE: Phase rotation meter

To answer the original question: How to make a simple rotation indicator out of scrap that you find in the shop.

Any pair of lamps having the right voltage will do. Then pick a capacitor to suit. I have used 15 W 230 V lamps and a 220 or 330 nF capacitor. Both work equally well. You even get a reliable result with 100 nF.

Connect the two lamps in series between phase A and C. Then connect capacitor fram phase B to midpoint between the lamps.

If rotation is A-B-C, the lamp connected to C will be a lot brighter than the lamp connected to A. If rotation is opposite, lamp connected to A will be the brightest.

The reason is that the two lamps form a resistive voltage divider between A and C, the voltage of which is perpendicular to voltage from B. Putting a capacitor from B to center of the voltage divider will add leading current to the voltage divider. Leading means adding to C current and subtracting from A current. The math is simple, if you use complex numbers. I'll do it for you if you think you need it. Or you do it yourself.

No 15 W lamps? Use any wattage and increase capacitor proportionally.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Phase rotation meter

(OP)
hi Skogsgurra,
Thanks for the post, I have only 220v, 50w bulps and a 3uF/400v cap. Will this items mach each other for a simple ph rotation meter? I will be a glad if you could show me the maths

RE: Phase rotation meter

Hello bdf!

50 W and 3 uF are just perfect. That combination makes the difference between the two lamps very big. One lamp will have almost all the voltage, so you need to be careful not to burn it out. Connect just a second or two. Using a lower value capacitor will make the difference smaller. You do not need much voltage difference to see what lamp is the brighter.

It is getting a bit late for the math. Will be back tomorrow - if work allows.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Phase rotation meter

(OP)
I was trying to get the neutral voltage from the formula I wrote in the question and I am getting funny numbers. If I am not wrong, we can get the neutral voltage and its phase angle we can compare with each pahse voltage and tell which lamb will glow brighter.  

RE: Phase rotation meter

Yes. Or you can use the Thevenin equivalent for the two lamps (no capacitor connected). That will give you a resistor equal to R/2 (if lamp resistance is R) connected to a voltage that is -200 V (imaginary voltages cancel). Then calculate current through C when connected to the Thevenin equivalent. Other side of C is, of course, connected to +400 V. This current will be superimposed on the current through the resistors and modify it so that one current increases and the other one decreases. That is what I shall try to get some time to do tomorrow - or anyone else that has some time on hand?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Phase rotation meter

Attached is a "brute force" numerical solution for the specific values mentioned, which confirms indeed the behavior will be as stated by skogsgurra.

I'm sure there are a number of more direct approaches.  One mentioned by skogsgurra.

Below is another attempt at a simpler approach that I started, but don't have the time to finish (have to do my taxes):
Simply consider that the system is a superposition of three voltage sources (A,B,C) applied between the phase terminal and the system neutral or ground (diffrerent than the floating ground we have created).

Solve each phase source by itself (with the other two phase sources shorted out to ground) to find the contribution to Vn from that source.
Add together the contributions of the three sources to deterimine the final Vn.  If you align Vb along the real axis as I have done in the attached, then it is very easy to see which source vector (Va or Vc) the Vn will be closer to by observing whether the imaginary portion is + or -.
 

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RE: Phase rotation meter

Here was an attempt to use the superposition approach.  The math is simpler, but the conclusion came out backwards from before.  Must have been some problem... have to look some more.

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RE: Phase rotation meter

There was just an error at the very end - forgot to negate I bringing it from the denominator to the numerator - will fix that and re-post.

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RE: Phase rotation meter

Here is the corrected version. I have also added some explanation and symbols.  I think it is a pretty straightforward proof of the conclusion stated by skogsgurra.  It relies on the assumtpion R*w*C <<1  (I don't think that assumption is required for the conclusion, but it makes the math easier and it should be easy to meet for practical values of C).

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RE: Phase rotation meter

One thing was not explicitly stated in the writeup but should be obvious. With angle B assumed 0, then B is a real number and the sum Va + Vc is also a real number. Those observations allows us to isolate the imaginary component and determine that it is positive.

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RE: Phase rotation meter

And of course Vb is not only real but also positive.

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RE: Phase rotation meter

Just as important as knowing what the phase rotation is is knowing what phase rotation the pump wants. I would bite the bullet and connect the pump motor and check the motor rotation. If need be, disconnect the motor from the pump before checking rotation.
Skogs, I like your cheap and simple rotation meter. It answers the original question. However, the original question didn't ask everything that needed to be asked.
Next question: I know the incoming phase rotation, how do I connect the pump so it will run the right way.
I still remember when it was common in a large plant for a mechanical guy with an electrical guy to check the rotation of every motor in the plant and correct it if needed. Later, a couple of electrical guys would take a phase rotation meter and a motor rotation meter and check every motor. If there was a special motor that could not be turned by hand for the motor rotation meter, the mechanical guy was called for assistance.
The point;
You have to check both the supply rotation and the motor rotation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Phase rotation meter

Looks like the sequence of execution within the worksheet resulted in some premature  results that might be confusing to the reader. Here is the real last version which may be less confusing.

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RE: Phase rotation meter

Thanks Pete!

You saved me some work. Still, I will try and do it "old fashion" with pencil and paper. But it will not be within the next hours. Stand by!

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Phase rotation meter

(OP)
Thanks for the post guys and I need some time to understand the formulas, at least to tell client and the principle behind it.

Thanks electricpete for the valuable attachement.  

RE: Phase rotation meter

Attached is the phasor drawing which shows the orientation I had in mind (angle B = 0)

If you compare my very first and very last of my previous attachments, I reached the same expression for Vn using two different approaches:  KCL (first attachment) and superposition (last attachment).

The KCL approach is simpler, but when I did it in my first attachment I substituted in the values of V too early  which made the expression complicated.

The bottom line quickest way to get to that same result is simply KCL as follows:

KCL at the midpoint (floating neutral):
(Va-Vn)/R + (Vc-Vn)/R + (Vb-Vn)*j * w * C=0

Move Vn to RHS
Va/R + Vc/R + Vb * j*w*C = Vn * (2/R + j*w*C)

Solve for Vn by dividing by (2/R + j*w*C)
Vn = [Va/R + Vc/R + Vb * j*w*C] /  (2/R + j*w*C)

Multiply by (-j*R)/-j*R)
Vn = [-j*Va -j* Vc + Vb w*C*R] /  (-2*j + R*w*C)

Above is the same result as the last attachment (from superposition) and the analysis can proceed as in the last attachment under the assumption R*w*C << 2.   

Even if we don't make that assumption, we can use this expression for Vn to compute
|Va-Vn| / |Vc - Vn|  for a given value R*w*C under assumption ABC phase sequence.  We know that swapping the phase sequence will simply swap the roles of A and C and the ratio would be inverted.

If I get a chance tonight I will try to make a plot of that function  ( |Va-Vn| / |Vc - Vn|  vs R*w*C )

Still maybe there is a simpler and more direct path to the conclusion.  I will be interested to see what Gunnar comes up with.

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RE: Phase rotation meter

Here is a plot of the function
|Va-Vn| / |Vc - Vn|  vs (R*w*C)
under assumption of ABC phase sequence.

(note it uses the complex functions in the excel "analysis toolpak" add-in)

If you swap phase sequence you can just relabel the plot as |Vc-Vn| / |Va - Vn|  (or else keep the same label and invert the ratio).

You can see the minimum ratio (max difference in brightness) occurs for R*w*C ~ 1 which is where resistance is approximately equal to capacitive reactance.  But there is still noticeable difference as we move away from that ideal point.

Of course as we all know there are temperature effects on resistance which would tend to push the two currents closer together, but that's a complication for another day.
 

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RE: Phase rotation meter

OK, Pete.

That's a challenge. Using Indian ink, paper and a Faber Castell 1/54 Slide Rule  winky smile , I got the attached result.

I have plotted the locus for the star point voltage in the phasor diagram (using the Real and Imaginary values for x and y) so one can see the distance between star point and phase voltages.

It is not at all necessary to use the optimal capacitor value (when Xc = R). Already a five percent difference in lamp voltage makes a huge difference in lamp brightness. Using larger capacitor values will burn out the brighter lamps in no time at all if lamp voltage is chosen to be equal to phase voltage, which is the natural thing to do.
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Phase rotation meter

(OP)
Thanks for the input guys, I really appreciate the post,this is very valuable for me.

From the understanding after all this posts, as advised by Skogsgurra and electricpete to prevent the light from burn I can connect 2 x 50w/220v bulbs in series on each leg A and C.

Looks like you guys have given me enough bulletproof vest to face the client.

 

RE: Phase rotation meter

Yes, two in series is, as you say, bullet proof. And so will you be, too.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Phase rotation meter

bdf5526, if you have two 3uf capacitors, putting them in parallel will give you a better voltage match between the two globes whilst still letting you tell the difference in brightness.

With a 3uf capacitor the voltages will be (approximately) 344 volts across one lamp and 92 volts across the other.

With a 1.5uf capacitor the voltages will be (approximately) 280 volts across one lamp and 119 volts across the other.

I like these types of threads as they force you to go back to the basics. I did a branch analysis to get the above figures. I used Gunnar's method of using a pen and paper to get the equations, but I admit I shamelessly and outrageously used a computer program to solve the complex simaltaneous equations.

See attached PDF.
 

RE: Phase rotation meter

sibeen, shall I send you a Slide Rule?

So you don't have to be ashamed using a computer?
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Phase rotation meter

(OP)
Hi guys, sorry for the late reply. It worked as it should. Thanks sibeen, for bring back those old memories with your method. This thread is my lesson of the week.

Thanks again guys. dazed

RE: Phase rotation meter

Glad it worked.

I liked Gunnar's idea to present the Vn in the complex plane.  I added it to my spreadsheet attached except changed to keep the parameter unitless (w*R*C) so it is more general.   I also changed the voltage ratio plot to make the horizontal axis (w*R*C) a log scale.

Some surprising (but unimportant from practical standpoint) symmetries are obvious looking at the graphical  solutions:
1 – The voltage ratio (on log scale) is symmetric about the vertical line w*R*C = 1.  So a factor of 2 above 1 or a factor of 2 below 1 gives the same ratio (even though different voltages).
2 – The trajectory of Vn in the complex plane is a semi-circle.  I understand the starting and endpoints and the qualtitative change... just surprised that it came out exactly as a semi-cirlce.

I don't see any obvious physical reason for these symmetries or any math reason (i.e. don't see any  equation of a circle buried in those equations).  It's not really important – just a curioisity.  

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RE: Phase rotation meter

It may be easier to just buy the phase rotation meter.  Knopp model K6 operates 25 to 480 volts and 50 to 400 HZ.  It is very simple and reliable unit; looks like a hockey puck.

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