Harmonics and VFDs 22

[Marke]

One of the areas that I operate in, has a high density of VFDs on pumps on relatively weak supplies. The result is that the high levels of harmonics on the VFD inputs has accumulate in the supply and is causing a high THD of the supply voltage. While we do have supply regulations covering harmonics, in this instance, the harmonics are higher than they should be.

There is an option of using zig zag transformers and six phase rectifiers as a means of reducing the harmonics drawn by drives however in this case, the drives are already installed.

There is a transformer for each drive and sizes range up to 200KW.

One thought that I had, was that for future installations, and there are new installations going in all the time, that the new supply transformers be designed with a zig or a zag winding to give a phase shift, and install equal loading on the leading and lagging phase shifts. This should act like a twelve pulse input on one drive, only it will be across two drives.

Any thoughts on this??

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[waross]

Hi macmil;
Thanks for the comments. Well said and appropriate.
A little analogy to help understand things from the electrical side.
Consider a plant that must from time to time move rail cars.
It takes about 1000 lbs. of force to start the rail cars moving. The crew uses a truck on a road parallel to the track and a long rope with an ultimate strength of about 3000 lbs and have never had a problem. One day for some reason someone tries to pull the cars with a very short rope and instead of the angle between the tracks and the rope being almost zero degrees, the angle is about 85 degrees. The rope breaks before the cars move. Why? The rope is good for three times the force needed. The answer, as you well know, is that pulling at an angle to one side the force in the rope may be several times the force actually applied parallel to the tracks. At an angle of 85 degrees, the tension in the rope will be about 11 times the effective force moving the load.
That is power factor, the cosine of the angle between the volts and the amps. Also the power factor of a motor varies as the load changes. When we measure amperage, we know that we are not measuring power. We must make an adjustment for the power factor or the angle between the volts and the amps before we know the true power. Those of us who work with power are making these adjustments frequently. Some equipment is rated in watts and some is rated in amps. If we are concerned with energy usage we must convert the amps times volts (VA) to real power. If we are concerned with panel, conductor or transformer loading we must convert the watts to amps times volts (VA). Always using the cosine of the angle between the amps and the volts. When the electrical power professionals were told that the difference between amps (apparent power) and real power was not important a lot of them followed Elvis and left the building. Too bad. Someone who does know a lot about pumps shot himself in the foot with that one.
Thanks again for your comments, macmil. I felt that you deserved an explanation for the sudden chill that fell on this thread.
(Volts times Amps = Volt Amps (VA)
Volts times Amps times the cosine of the angle of displacement = Watts
There is a difference)

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

 

[Valvecrazy]

Volts times Amps times Power Factor equals watts. I didn't mean to make light of the power factor but,

"At low power levels, the differences between VA and watts are often slight."

Also Drive guys try to make a big deal out of the fact that Drives can keep the power factor up. However, the Volts times Amps part is also very important. You can say that a Drive maintains .97 power factor. Without a Drive the power factor will vary and the efficiency will vary. However, what really matters is the electric bill at the end of the month. No matter how you slice it or how you do the math, there is very little difference between the electric bill when using a Drive and when throttling a full speed pump, when constant pressure is the objective. I am used to the sudden chill when discussing this with Drive people, but then again, I always feel a chill when I am around Drive people.

 

[Skogsgurra]

Could someone tell this guy (you know who I mean) that the difference between W and VA is at its maximum when the motor is lightly loaded. That is when cos(phi) is low, often below .5 sometimes as low as .3

It is interesting that he has found someone electrical that he trusts that has told him about the relationship between V, A and W. It is a pity that he didn't get - or didn't bother to remember - the correct formula. It is three phase motors we are discussing? Isn't it?

Gunnar Englund

http://www.gke.org

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

 

[edison123]

Valvecrazy

While I found some interesting points of view about valves as a power saving device, your post "At low power levels, the differences between VA and watts are often slight." is wrong.

On the contrary, on the low power levels with rated voltage to the motor, the difference between VA and Watts is very high due to very low power factor. Also, electric bills, (which you rightly say is what matters), is based on power, which in turn is a product of voltage, current and power factor.



* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[Skogsgurra]

Thanks edison.

Gunnar Englund

http://www.gke.org

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

 

[Valvecrazy]

Many smaller pump motors are single phase. Can someone please give me a better link than this one?

http://www.powervar.com/Eng/ABCs/CalcVAWATTS.asp

 

[edison123]

VC

Your link clearly defines the relation of power, voltage, current and p.f. The ac example clearly shows the vast difference bewten VA and power when a low power factor is factored in.

I think you latched on to the last paragraph (which you posted above verbatim) and misunderstood it.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[waross]

The difference between the 240 VA apparent power and the 156 watts of true power is the reactive power or 84 VAR or volt-amps-reactive.

The reactive power in this example is actually 182 VAR
I think that by using this inaccurate website to prove your point you may have just shot your self in the other foot.
The simple difference between 240 and 156 is 84. Not a very big number when talking about watts, VAs or VARs.
It is about 56%. (156/240) That is a very big number when large amounts of power are involved.

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

 

[waross]

edison123 T
Take the watts and VA in the example and work out the real VARs. Then have a good laugh. He almost snuck that one past us.
I saw it on the internet!!! It must be true!!!

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

 

[edison123]

Bill

You are right. I just looked at VA & Watts. Mebbe, this company does not believe in "complex" math.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[Skogsgurra]

Waiting for next blamage. That guy, did he actually use single phase motors in his "demonstrator"? Or is he throwing them in now to get away with that incorrect formula?

Gunnar Englund

http://www.gke.org

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

 

[DickDV]

Please, guys, let's put this thread out of its misery!

Refer Mr. Valvecrazy to his nearest engineering university and arrange for an independent test of the two methods.

I'd settle for that! All this back and forth reasoning is like nailing Jello to the wall. You accomplish nothing, you make a mess, you spoil the Jello, and, for sure, frustration sets in about the same time logic floats away!

Please stop!

 

[gepman]

I would like to respect DickDV's wishes and put this thread out of existence but I feel like I have earned the privelege of making one last post on this. After the World Ag Expo Show I went sailing up the coast of California from Mexico on a small (45') boat and have been out of internet contact. I just got back and logged on and was surprised to see the number of replies.

I won't make any replies to Valvecrazy's statements in particular since using any engineering principles doesn't seem to go very far.

I would like to say that it is fairly obvious that using a valve does NOT create energy as has been intimated by Valvecrazy. The system was also about the best setup one could have to test a constant head system without actually pumping up a hill. Although the VFD was still better than the valve in the constant head system it probably wasn't better by enough in this circumstance to make a VFD worthwhile in a typical irrigation application. Valvecrazy should realize that contrary to his experience, constant head systems are NOT the norm in the vast majority of applications, mostly the flow is of concern, not the pressure and therefore VFDs would have greater advantages over valves in these cases. My Amprobe 41PQ indicated a 1% voltage THD and a 50% amperage THD. A line filter would not produce more than a few watts loss on this system. I don't think that I have seen any reputable evidence that VFDs cause birth defects like I believe I heard in an offhand comment at the booth with the test setup.

To answer Marke's question it was basically impossible to remove the valve without a lot of trouble so it was not done. Obviously this would have provided more savings for the VFD since even at wide open the valve will have some pressure drop (apparently around 7 psig per Valvecrazy).

To answer Waross's question, a pump curve with a rising pressure curve to zero flow would be good on a VFD application with a lot of static head. This way you could get higher pressure at lower flows than with a flat curve which would allow you to reduce the speed of the VFD more. A flat pressure curve to zero flow would be good for a VFD application with no static head or for a valve with constant head.

I did not think of this thread for the whole week that I was sailing and I don't plan on thinking about it again. If Valvecrazy makes a response then we should just let him have the last word.

 

[ePOWEReng]

Just an idea for the original post question....often the rural feeders such as this will have one or multiple sets of mid-line voltage regulators. If there is an existing mid-line voltage regulators the connection could be changed such that it introduced a 30 degree phase shift in the line. If this feeder has normally open ties to adjacent this may not be a possibility, for safety reasons.