I recently wired a parking garage with high pressure sodium lights. one of the circuits was overloaded. 28 amps on a 20 amp circuit. Needless to say the breaker would not hold. The engineers solution was to install capacitors in all of the fixtures. It lowered the amperage to about 16 amps. could someone please explain this theory to me.
capacitors
- Thread starter jonaslc
- Start date
- Status
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: capacitors
It has to do with power factor. AC pwer has two parts, true power and apparant power. You were seeing apparant power of 28 amps, but the true power was only 16 amps. Apparant power is like the foam on a beer, its not wanted. What we want is the beer, the true power. This relationship is often called beer factor, er power factor.
Information on ballast types is in the back of the mfg catalog. The less expensise HID ballasts have a higher current draw and low power factor.
I'm hoping someone else will jump in and help explain this.
It has to do with power factor. AC pwer has two parts, true power and apparant power. You were seeing apparant power of 28 amps, but the true power was only 16 amps. Apparant power is like the foam on a beer, its not wanted. What we want is the beer, the true power. This relationship is often called beer factor, er power factor.
Information on ballast types is in the back of the mfg catalog. The less expensise HID ballasts have a higher current draw and low power factor.
I'm hoping someone else will jump in and help explain this.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: capacitors
I'll give it a try.
Wrap a wire around a nail and connect it to a battery. What you get is not just a magnet, it is also a magnetic field. A generator and a motor have this same type of magnetic field. So does any load that is “inductive” in nature.
Place two metal plates close together and connect them to a battery. What you get is not just a capacitor, it is also an electric field.
When you connect a motor to a generator (or to a transformer), two things happen. The second thing is that the motor goes roundy-roundy. That is its job; that is the “real power” being put to use; that is the “beer.” But the first thing that happens is that the magnetic field of the generator exchanges energy with the magnetic field of the motor. It is in one sense a useless expenditure of energy, in that it doesn’t make the motor go any faster or drive a larger load. But the motor could not work at all, if it were not for that exchange of energy. So it is a necessary useless expenditure of energy. It is the “reactive Power,” the “foam.”
You get the “apparent power” by adding the real power to the reactive power. But you add them by treating each as one leg of a right triangle, with the apparent power being the hypotenuse (i.e., use the Pythagorean Theorem).
What happens when you add the capacitor to the load? Now the magnetic field of the motor will exchange energy with the electric field of the capacitor. It takes the magnetic field of the generator out of the picture. Note that this takes place close to the motor, and that the circuit breaker does not see the current associated with this energy exchange. Result: the current seen by the circuit breaker goes down.
This phenomenon is commonly described by saying that the capacitor is canceling out the inductor. More precisely, it is said that the “leading” reactive power added by the capacitor is opposite to the “lagging” reactive power of the motor. It can be pictured by taking one leg of a right triangle and shrinking its length to nearly zero. What you will see is that the hypotenuse (apparent power) starts looking very much like the other leg (real power).
How does that work for you?
Charlie B.
I'll give it a try.
Wrap a wire around a nail and connect it to a battery. What you get is not just a magnet, it is also a magnetic field. A generator and a motor have this same type of magnetic field. So does any load that is “inductive” in nature.
Place two metal plates close together and connect them to a battery. What you get is not just a capacitor, it is also an electric field.
When you connect a motor to a generator (or to a transformer), two things happen. The second thing is that the motor goes roundy-roundy. That is its job; that is the “real power” being put to use; that is the “beer.” But the first thing that happens is that the magnetic field of the generator exchanges energy with the magnetic field of the motor. It is in one sense a useless expenditure of energy, in that it doesn’t make the motor go any faster or drive a larger load. But the motor could not work at all, if it were not for that exchange of energy. So it is a necessary useless expenditure of energy. It is the “reactive Power,” the “foam.”
You get the “apparent power” by adding the real power to the reactive power. But you add them by treating each as one leg of a right triangle, with the apparent power being the hypotenuse (i.e., use the Pythagorean Theorem).
What happens when you add the capacitor to the load? Now the magnetic field of the motor will exchange energy with the electric field of the capacitor. It takes the magnetic field of the generator out of the picture. Note that this takes place close to the motor, and that the circuit breaker does not see the current associated with this energy exchange. Result: the current seen by the circuit breaker goes down.
This phenomenon is commonly described by saying that the capacitor is canceling out the inductor. More precisely, it is said that the “leading” reactive power added by the capacitor is opposite to the “lagging” reactive power of the motor. It can be pictured by taking one leg of a right triangle and shrinking its length to nearly zero. What you will see is that the hypotenuse (apparent power) starts looking very much like the other leg (real power).
How does that work for you?
Charlie B.
Ed MacLaren
Senior Member
Re: capacitors
This "sample" page from one of my tutorials may be of some help. The example is power factor correction of a single phase motor circuit, but the principles would apply to any inductive or low power factor load.
Note - Only the first sample on the page applies to this topic.
http://www.electric-ed.com/Samples.htm
Ed
[ March 03, 2003, 12:15 PM: Message edited by: Ed MacLaren ]
This "sample" page from one of my tutorials may be of some help. The example is power factor correction of a single phase motor circuit, but the principles would apply to any inductive or low power factor load.
Note - Only the first sample on the page applies to this topic.
http://www.electric-ed.com/Samples.htm
Ed
[ March 03, 2003, 12:15 PM: Message edited by: Ed MacLaren ]
BAHTAH
Senior Member
- Location
- United States
Re: capacitors
Inductive loads (ballast) require magnetizing current(KVAR). The ballast also uses power to do its job(KVA). Your original installation using low power factor ballast meant the power to magnetize the ballast had to be delivered through the branch circuit and resulted in a high current draw. When the capacitors were installed, they are a storage place for the reactive current (KVAR) needed to magnetize the ballast coil. By supplying the KVAR right at the load (Fixtures), the capacitor relieves the branch circuit of the burden of delivering the extra KVAR needed and thus reduces the amp draw on your branch circuit.
Inductive loads (ballast) require magnetizing current(KVAR). The ballast also uses power to do its job(KVA). Your original installation using low power factor ballast meant the power to magnetize the ballast had to be delivered through the branch circuit and resulted in a high current draw. When the capacitors were installed, they are a storage place for the reactive current (KVAR) needed to magnetize the ballast coil. By supplying the KVAR right at the load (Fixtures), the capacitor relieves the branch circuit of the burden of delivering the extra KVAR needed and thus reduces the amp draw on your branch circuit.
mikeames
Senior Member
- Location
- Gaithersburg MD
- Occupation
- Teacher - Master Electrician - 2017 NEC
Re: capacitors
Imagine the knocking sound you may get from turning the water off in a copper pipe. Imagine almost the same situation with a magnetic field. Instead of the water stopping the capicator absorbs the sudden stop and returns that power back to the circuit when the water or the voltage drops below the caps charge leve.
Imagine the knocking sound you may get from turning the water off in a copper pipe. Imagine almost the same situation with a magnetic field. Instead of the water stopping the capicator absorbs the sudden stop and returns that power back to the circuit when the water or the voltage drops below the caps charge leve.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: capacitors
Charlie and Electric Ed thanks!
Charlie why does the electric utility penalize a user for low power factor, this is commonly called a KVAR charge?
Charlie and Electric Ed thanks!
Charlie why does the electric utility penalize a user for low power factor, this is commonly called a KVAR charge?
Len_B
Member
- Location
- New Hampshire
Re: capacitors
Tom,
The typical kW meter only records real(in phase) power. The "apparent power" (the vector sum of the real and reactive power) must still be generated by the power company. The transmission lines, transformers, etc, must be sized to carry this total "apparent power" (the reactive power is never truly used, small amounts are lost to impedance/heat---but it must be initially generated and then carried by circuit conductors to maintain, for example, the magnetic field in a motor). Power factor correction capacitors at the motor or service localize this circulating reactive current, so that the poco distribution system doesn't constantly carry it. The poco charges users with poor power factor for the oversizing needed to produce, distribute, and carry this additional reactive current --- the KVAR charge. This can be metered via kVAR meter or can be assigned/estimated. A rather complex billing system ensues when combined with other utility charges for demand and load factors. This can amount to big dollars(I've heard of up to 10,000 percent penalties, mostly due to the demand factor!) when starting large AC motors "across the line", or running motors at certain "peak demand periods". Hence the use of power factor correction capacitors and also the attractiveness of AC drives and soft starts (and older reduced voltage starters, etc.) for large motors.
Len
[ March 04, 2003, 08:24 AM: Message edited by: Len_B ]
Tom,
The typical kW meter only records real(in phase) power. The "apparent power" (the vector sum of the real and reactive power) must still be generated by the power company. The transmission lines, transformers, etc, must be sized to carry this total "apparent power" (the reactive power is never truly used, small amounts are lost to impedance/heat---but it must be initially generated and then carried by circuit conductors to maintain, for example, the magnetic field in a motor). Power factor correction capacitors at the motor or service localize this circulating reactive current, so that the poco distribution system doesn't constantly carry it. The poco charges users with poor power factor for the oversizing needed to produce, distribute, and carry this additional reactive current --- the KVAR charge. This can be metered via kVAR meter or can be assigned/estimated. A rather complex billing system ensues when combined with other utility charges for demand and load factors. This can amount to big dollars(I've heard of up to 10,000 percent penalties, mostly due to the demand factor!) when starting large AC motors "across the line", or running motors at certain "peak demand periods". Hence the use of power factor correction capacitors and also the attractiveness of AC drives and soft starts (and older reduced voltage starters, etc.) for large motors.
Len
[ March 04, 2003, 08:24 AM: Message edited by: Len_B ]
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: capacitors
I like Len_B's answer, and could not offer anything better. Thanks, Len.
- Location
- Massachusetts
Re: capacitors
Thank you all for helping me understand what I thought was complicated, when all I had to do was think of a cold draft
Thank you all for helping me understand what I thought was complicated, when all I had to do was think of a cold draft
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