hde_electric
Member
I am studying transformers I want to know what is the difference between a kva and kw. Does this affect me when iam calculating residential loads?
What is the difference between KVA and KW?
- Thread starter hde_electric
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- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: What is the difference between KVA and KW?
The difference comes from the fact that rotating machines (generators and motors, but let?s just talk about motors) have to first store energy in a magnetic field (i.e., in their coils), before then can do their thing (i.e., spin the motor). The power taken up in this energy storage process is measured as Volt-Amps Reactive (VAR), or Kilo Volt-Amps Reactive (KVAR). The ?useful power? (i.e., used to turn the motor) is measured as Watts or Kilowatts (KW). These two can be drawn as the two sides of a right triangle. The hypotenuse (third side ? the longest side) is the result of KW added to KVAR, but added using the Pythagorean Formula: (KVA)**2 = (KW)**2 + (KVAR)**2. If you take measurements with an ammeter and a voltmeter, and multiply the two, what you get is KVA.
For most residential loads, the power factor is very close to 1. That means that if there are any motors, they are so small that KW and KVA are almost the same number. In this case, you don?t have to pay attention to which is which. The only household component that has a large motor is generally the air conditioner. In my neck of the woods, most households don?t have one.
The difference comes from the fact that rotating machines (generators and motors, but let?s just talk about motors) have to first store energy in a magnetic field (i.e., in their coils), before then can do their thing (i.e., spin the motor). The power taken up in this energy storage process is measured as Volt-Amps Reactive (VAR), or Kilo Volt-Amps Reactive (KVAR). The ?useful power? (i.e., used to turn the motor) is measured as Watts or Kilowatts (KW). These two can be drawn as the two sides of a right triangle. The hypotenuse (third side ? the longest side) is the result of KW added to KVAR, but added using the Pythagorean Formula: (KVA)**2 = (KW)**2 + (KVAR)**2. If you take measurements with an ammeter and a voltmeter, and multiply the two, what you get is KVA.
For most residential loads, the power factor is very close to 1. That means that if there are any motors, they are so small that KW and KVA are almost the same number. In this case, you don?t have to pay attention to which is which. The only household component that has a large motor is generally the air conditioner. In my neck of the woods, most households don?t have one.
russ
Senior Member
- Location
- Burbank IL
Re: What is the difference between KVA and KW?
hde
http://www.mikeholt.com/technical.php?id=calculations/technicalcalculationformulas
Scroll down to Electrical formulas based on 60HZ
hde
http://www.mikeholt.com/technical.php?id=calculations/technicalcalculationformulas
Scroll down to Electrical formulas based on 60HZ
Re: What is the difference between KVA and KW?
Just a thought..
I was just reading this and decided to look up my old circuits book ...Old is right, the first page I came to was on 'Complex Numbers' the sub heading was, Use of the Slide Rule with Complex Numbers. copywrite 1966, guess I ought to update.
WmColt
Just a thought..
I was just reading this and decided to look up my old circuits book ...Old is right, the first page I came to was on 'Complex Numbers' the sub heading was, Use of the Slide Rule with Complex Numbers. copywrite 1966, guess I ought to update.
WmColt
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: What is the difference between KVA and KW?
here is how I explain it, its like a glass of beer with foam on top. KW or true power is the beer. The foam is the apparent power or KVA. It doens't do any usefull work, and is not what you want when you sip a cool one.
here is how I explain it, its like a glass of beer with foam on top. KW or true power is the beer. The foam is the apparent power or KVA. It doens't do any usefull work, and is not what you want when you sip a cool one.
Ed MacLaren
Senior Member
Re: What is the difference between KVA and KW?
This is how I used to explain it to my student apprentices.
Compare the delivery of electrical energy to a motor, and the delivery of milk to a home, in the days when refillable glass bottles were used.
The bottles were esential to the transaction, but were not paid for or consumed by the customer. They were washed and returned to the dairy each day. The load the milkman had to carry to the house consisted of both milk and glass.
Milk = Watts, (True Power) the product consumed and paid for by the customer.
Glass = Vars, (Reactive power) required to supply the magnetic field, returned to the power generator each cycle, not consumed or paid for by the customer.
Milk + Glass = VA, (Apparent Power) the vector sum of the Watts and Vars, which is what the transformers and conductors have to carry.
Ed
[ September 18, 2003, 11:13 PM: Message edited by: Ed MacLaren ]
This is how I used to explain it to my student apprentices.
Compare the delivery of electrical energy to a motor, and the delivery of milk to a home, in the days when refillable glass bottles were used.
The bottles were esential to the transaction, but were not paid for or consumed by the customer. They were washed and returned to the dairy each day. The load the milkman had to carry to the house consisted of both milk and glass.
Milk = Watts, (True Power) the product consumed and paid for by the customer.
Glass = Vars, (Reactive power) required to supply the magnetic field, returned to the power generator each cycle, not consumed or paid for by the customer.
Milk + Glass = VA, (Apparent Power) the vector sum of the Watts and Vars, which is what the transformers and conductors have to carry.
Ed
[ September 18, 2003, 11:13 PM: Message edited by: Ed MacLaren ]
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: What is the difference between KVA and KW?
Great analogy, Ed! Showing that the milkman has to carry both milk and glass, and that the milk could not get delivered without the bottle, corresponds very well with the physical reality (i.e., interactions of magnetic fields). I love it!
Great analogy, Ed! Showing that the milkman has to carry both milk and glass, and that the milk could not get delivered without the bottle, corresponds very well with the physical reality (i.e., interactions of magnetic fields). I love it!
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