Phase rotation meter 4

[bdf5526]

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.

 

[edison123]

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

 

[bdf5526]

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

 

[davidbeach]

You can buy three phase rotation meters.

 

[bdf5526]

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?..

 

[waross]

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

 

[edison123]

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.

 

[waross]

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

 

[davidbeach]

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.

 

[electricpete]

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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[electricpete]

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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[bdf5526]

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.

 

[edison123]

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.

 

[electricpete]

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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[bdf5526]

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.

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[bdf5526]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[bdf5526]

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.

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...