How to present VARS

[electro6]

I teach a Concepts of Electricity course and have always had a problem presenting the concepts of VARS (volt-amps reactive) to my students. Many of my students are working electricians and they are familiar with volts, amps, and watts. Most carry instruments that can measure those quantities; but vars just seem strange to most. I.ve been using the trig representation (the power triangle) for years, but I'm wondering what other instructors have tried, and with what success. Can you help.

 

[Hv&Lv]

Students huh? try the beer analogy...

 

[charlie b]

Wrap a wire around a nail, and connect it to a battery, and you get an electromagnet. This device can do some practical work, such as picking up paper clips. But consider what is going on after you connect the battery, but before you try to pick up a paper clip. There is current flowing through the coil, and there is energy being expended, but no ?practical work? is being done. There will be a certain minimum amount of energy that must be expended, just to establish and maintain the magnetic field within the coil.

What will then happen, when you bring this electromagnet close to a paper clip? Well, in order to do the work of moving the paper clip, against the force of gravity, from the table to the electromagnet hovering above the table, some additional current will have to flow within the coil. OK, so for this example, the additional amount of current will be small, perhaps too small to even measure, but it will not be zero additional current. That extra current is what is allowing this ?machine? to do its ?practical work.?

So now we have two types of energy being expended. One type is doing practical work. The other type is not doing practical work, but if it were not being used at all, then there would be no magnetic forces available to attract the paper clip. So this type of energy is necessary, even if it is not doing work.

A motor is essentially a coil of wire wrapped around a chunk of metal. When you push current through the coils, it creates a magnetic field. It is that magnetic field that forces the rotor of the motor to rotate. But here again, there are two types of energy being used. One type is doing the practical work of spinning the motor, so that it can turn the fan or pump or whatever else is connected to the motor?s shaft. The other type is merely creating the magnetic field, and is not doing any work by itself, but without the expenditure of this type of energy there would be no magnetic field to cause to rotor to rotate.

So essentially what are VARS? They represent the exchange of energy between the magnetic field of the power source (e.g., generator or transformer) and the magnetic field of the motor.

 

[electro6]

Wrap a wire around a nail, and connect it to a battery, and you get an electromagnet. This device can do some practical work, such as picking up paper clips. But consider what is going on after you connect the battery, but before you try to pick up a paper clip. There is current flowing through the coil, and there is energy being expended, but no “practical work” is being done. There will be a certain minimum amount of energy that must be expended, just to establish and maintain the magnetic field within the coil.

What will then happen, when you bring this electromagnet close to a paper clip? Well, in order to do the work of moving the paper clip, against the force of gravity, from the table to the electromagnet hovering above the table, some additional current will have to flow within the coil. OK, so for this example, the additional amount of current will be small, perhaps too small to even measure, but it will not be zero additional current. That extra current is what is allowing this “machine” to do its “practical work.”

So now we have two types of energy being expended. One type is doing practical work. The other type is not doing practical work, but if it were not being used at all, then there would be no magnetic forces available to attract the paper clip. So this type of energy is necessary, even if it is not doing work.

A motor is essentially a coil of wire wrapped around a chunk of metal. When you push current through the coils, it creates a magnetic field. It is that magnetic field that forces the rotor of the motor to rotate. But here again, there are two types of energy being used. One type is doing the practical work of spinning the motor, so that it can turn the fan or pump or whatever else is connected to the motor’s shaft. The other type is merely creating the magnetic field, and is not doing any work by itself, but without the expenditure of this type of energy there would be no magnetic field to cause to rotor to rotate.

So essentially what are VARS? They represent the exchange of energy between the magnetic field of the power source (e.g., generator or transformer) and the magnetic field of the motor.

I like your examples, they would be easily understood by my level students. There is one problem I foresee. Most AC students directly relate vars with inductive reactance and capacitive reactance. Our textbook states that these circuit characteristics do not exist in a pure (battery) DC circuit, so I may have to modify the first example. Thanks for your comments.

 

[electro6]

Students huh? try the beer analogy...

What is the beer analogy?

 

[Hv&Lv]

Students huh? try the beer analogy...

What is the beer analogy?

Here is a link to an excellent example. This is the example given to me by an engineer many years ago. It seems to be the easiest to understand for those that do not understand the triangle relationship.


http://www.emt-india.net/equipment_tips/electrical_system/pdf/Diane Power Factor.pdf

 

[Besoeker]

I teach a Concepts of Electricity course and have always had a problem presenting the concepts of VARS (volt-amps reactive) to my students. Many of my students are working electricians and they are familiar with volts, amps, and watts. Most carry instruments that can measure those quantities; but vars just seem strange to most. I.ve been using the trig representation (the power triangle) for years, but I'm wondering what other instructors have tried, and with what success. Can you help.

Interesting point.
I think it's a difficult concept to get across.
I've thought about how I might try to explain it to my fairly non-technical very pretty wife.
She knows engineering terms and does our buying. But I don't know that I could offer adequate explanation.
I just might run it by a few of my guys to see if I can get a good, not too technical, explanation.

 

[Hv&Lv]

Interesting point.
I think it's a difficult concept to get across.
I've thought about how I might try to explain it to my fairly non-technical very pretty wife.
She knows engineering terms and does our buying. But I don't know that I could offer adequate explanation.
I just might run it by a few of my guys to see if I can get a good, not too technical, explanation.

Try the one above. I have yet to meet someone that doesnt understand this analogy. (of course, it could also be used with pepsi or coke, but the suds dissapear too quick.)

 

[mivey]

Interesting point.
I think it's a difficult concept to get across.
I've thought about how I might try to explain it to my fairly non-technical very pretty wife.
She knows engineering terms and does our buying. But I don't know that I could offer adequate explanation.

In order to bread chicken for frying, we need extra flour.

Apparent power is the flour in the breading pan. Watts is the flour that sticks to the chicken. Vars is the flour than does not get used and is returned to the flour bin. You will never bread chicken without the extra flour in the pan.

 

[jumper]

Students huh? try the beer analogy...

In order to bread chicken for frying, we need extra flour.

Apparent power is the flour in the breading pan. Watts is the flour that sticks to the chicken. Vars is the flour than does not get used and is returned to the flour bin. You will never bread chicken without the extra flour in the pan.

So, the best way for me to understand VARS is with a 6 pack of Bud Lite and a bucket of KFC-cool, I'm in.:happyyes:

 

[mivey]

So, the best way for me to understand VARS is with a 6 pack of Bud Lite and a bucket of KFC-cool, I'm in.:happyyes:

Field trip!

 

[zog]

If you want to PM me you email address I can send you an animated PPT slide I made to explain this in my basic theory classes.

 

[jumper]

Field trip!

 

[charlie b]

What is the beer analogy?

It's one I never liked. But it is out there, and your students should be aware that it is out there. Just tell them not to take it too seriously. It has a couple drawbacks. For one, it is possible to pour a beer, without getting any foam. Secondly, if you let a beer stand for a few minutes, the foam will start to disappear, eventually leaving you with only the liquid. Neither of those characteristics is consistent with the concept of VARS. I think the emphasis should be on energy. You can't make a motor run without first creating a magnetic field, and that magnetic field will be present even if the motor is not turning (i.e., even if no useful work is being done).

 

[Hv&Lv]

It's one I never liked. But it is out there, and your students should be aware that it is out there. Just tell them not to take it too seriously. It has a couple drawbacks. For one, it is possible to pour a beer, without getting any foam. Secondly, if you let a beer stand for a few minutes, the foam will start to disappear, eventually leaving you with only the liquid. Neither of those characteristics is consistent with the concept of VARS. I think the emphasis should be on energy. You can't make a motor run without first creating a magnetic field, and that magnetic field will be present even if the motor is not turning (i.e., even if no useful work is being done).

I agree with you that it should not be taken to pictorally. The entire .pdf is interesting in that it also uses "mac" dragging a load, and tying all this in vectorally.
Sometimes just starting with a simple picture is the easiest way to start.
(I actually had one person ask me if "salt" would be the equivalent of capacitors :huh

 

[GeorgeB]

I explain it to MECHANICAL types as a die spring analogy. It takes force to compress or extend the spring, but you get all the work except losses back on the return. Voltage==force, displacement==amps, heating of spring and muscles==losses.

 

[electro6]

If you want to PM me you email address I can send you an animated PPT slide I made to explain this in my basic theory classes.[I would be happy to see your slide. My email address is etepper@q.com Thanks for your help./QUOTE]

 

[jumper]

Electro, after you quote someone, type your message outside the quote and it will come out correctly like this

If you want to PM me you email address I can send you an animated PPT slide I made to explain this in my basic theory classes.

I would be happy to see your slide. My email address is etepper@q.com Thanks for your help.

 

[Jraef]

I like the horse pulling the canal boat analogy. It actually visualizes the power triangle and it was something that actually existed at one time, so people probably saw it in history books.



If the horse could walk on water, ALL of the "horsepower" would be used to pull the boat. But the farther the horse is to the side, the steeper the angle is and the more horsepower is used dragging the boat sideways instead of pulling the boat forward.

There is a similar version with a horse and a railroad car, but since that never existed in real life I don't use it because I had someone get caught up in asking why anyone would do that. He missed the point of the exercise.



I only use the "beer analogy" when trying to explain power factor TARIFFS, because the bar tender has to pay for the beer used in making the foam, which the customer has no use for. So his cost of the foam going down the drain is included in his cost of doing business, which ultimately reflects the price of the beer, but not the VOLUME that he charges for at the point of sale.

 

[electro6]

Electro, after you quote someone, type your message outside the quote and it will come out correctly like this

Thanks for your help.