Arc Voltage
- Thread starter jimmac49
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- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Arc Voltage
(Translation: Voltage equals inductance times the rate of change of current)
You need to know the speed at which the switch operates. This comes from the vendor. You also need to know the inductive reactance of the system. This means that you have to create a model of the circuit, and include the cables and all motors and other inductive loads. I don?t know if it can be easily performed without a power systems analysis software package.
What is it that you are trying to determine or to accomplish?
V = L x di/dt
(Translation: Voltage equals inductance times the rate of change of current)
You need to know the speed at which the switch operates. This comes from the vendor. You also need to know the inductive reactance of the system. This means that you have to create a model of the circuit, and include the cables and all motors and other inductive loads. I don?t know if it can be easily performed without a power systems analysis software package.
What is it that you are trying to determine or to accomplish?
Re: Arc Voltage
Charlie: I would approch that calculation slightly different using the conditions that the current just before opening the switch must equal the current just after opening the switch.
I (t=0-) = I (t=0+)
This implies that the voltage is going to have to be high enough to ionize the air (or other insulating medium) into conducting with enough voltage left over to push the load current through the ionized air. (This leaves out a whole bunch of other factors that would probably reduce the actual voltage obtained. Parallel capacitance is one that comes to mind.)
I think the equation V = L di/dt tells more about how long the arc will be maintained than it does about the voltage.
Steve
Charlie: I would approch that calculation slightly different using the conditions that the current just before opening the switch must equal the current just after opening the switch.
I (t=0-) = I (t=0+)
This implies that the voltage is going to have to be high enough to ionize the air (or other insulating medium) into conducting with enough voltage left over to push the load current through the ionized air. (This leaves out a whole bunch of other factors that would probably reduce the actual voltage obtained. Parallel capacitance is one that comes to mind.)
I think the equation V = L di/dt tells more about how long the arc will be maintained than it does about the voltage.
Steve
Re: Arc Voltage
Now this is a poser! I would think there are tables somewhere which would provide this info in terms of field strength, humidity, etc. If you switch off inductive current, the inductor acts somewhat like a current source and creates as much voltage as is necessary to maintain that current.
At t = 0+, the gap is small, and the voltage would be relatively small. As the gap widens the voltage would rise until the inductive energy can no longer supply the voltage and current to maintain the arc. Sounds like a good lab experiment.
In short, I think the voltage would be a matter of the distance between the switch contacts and could be more than the line voltage.
Now this is a poser! I would think there are tables somewhere which would provide this info in terms of field strength, humidity, etc. If you switch off inductive current, the inductor acts somewhat like a current source and creates as much voltage as is necessary to maintain that current.
At t = 0+, the gap is small, and the voltage would be relatively small. As the gap widens the voltage would rise until the inductive energy can no longer supply the voltage and current to maintain the arc. Sounds like a good lab experiment.
In short, I think the voltage would be a matter of the distance between the switch contacts and could be more than the line voltage.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Arc Voltage
Arc voltages can exceed system voltages. I pulled out my trusty Westinghouse T&D Reference Book, and there is a chapter on Power System Voltages and Currents During Abnormal Conditions . Because of energy stored in the reactance of their models, they are using generators and transmission lines more than distribution circuits, arc voltages can double and even triple normal levels.
I suspect that in a distribution system there is still some reactance for their model, but the resistance is significantly higher, so there would not be nearly the doubling or tripling.
Jim T
Arc voltages can exceed system voltages. I pulled out my trusty Westinghouse T&D Reference Book, and there is a chapter on Power System Voltages and Currents During Abnormal Conditions . Because of energy stored in the reactance of their models, they are using generators and transmission lines more than distribution circuits, arc voltages can double and even triple normal levels.
I suspect that in a distribution system there is still some reactance for their model, but the resistance is significantly higher, so there would not be nearly the doubling or tripling.
Jim T
physis
Senior Member
Re: Arc Voltage
The arc voltage can be many many times the supply voltage. The back EMF that results from the colapsing fields when the switch is open is used intentionally in DC to DC converters, switching power supplies and automobile ignition systems.
I had a problem once with a telephone system load bank. The unit we were building was the simplest thing ever. 5 resisters that could be switched on seperately in parallel. We used a relay to connect/disconnect the load to the system.
When the first unit was ready for testing we were mostly concerned with heat dissipation. So we're running this thing full blast when someone decided to disengage the relay (this relay was enclosed in transparent plastic) to our utter amazement this thing maintaned an arc across the points as long as the supply current remained.
The supply was 50 volts DC. You know that wont arc across an air gap.
We knew that it was inductance behind the arcing but all we had was resisters. So we broke one open and sure enough not only were they wire wound around a ceramic core (we knew that) but the resistive wire was also coiled to about 10 or 12 coils per inch at about 3/32 inch diameter.
I don't remember the current but I think it was in the 50 amp area.
I thought we should remove the inductance but it was decided to put a .1 uf capacitor across the points and that solved the problem. But I have a suspicion some of those units came back with punctured dialectrics in that capacitor.
Editted spelling
We didn't think to put an ammeter on it to see how much current the arc condudted.
[ December 17, 2004, 07:25 PM: Message edited by: physis ]
The arc voltage can be many many times the supply voltage. The back EMF that results from the colapsing fields when the switch is open is used intentionally in DC to DC converters, switching power supplies and automobile ignition systems.
I had a problem once with a telephone system load bank. The unit we were building was the simplest thing ever. 5 resisters that could be switched on seperately in parallel. We used a relay to connect/disconnect the load to the system.
When the first unit was ready for testing we were mostly concerned with heat dissipation. So we're running this thing full blast when someone decided to disengage the relay (this relay was enclosed in transparent plastic) to our utter amazement this thing maintaned an arc across the points as long as the supply current remained.
The supply was 50 volts DC. You know that wont arc across an air gap.
We knew that it was inductance behind the arcing but all we had was resisters. So we broke one open and sure enough not only were they wire wound around a ceramic core (we knew that) but the resistive wire was also coiled to about 10 or 12 coils per inch at about 3/32 inch diameter.
I don't remember the current but I think it was in the 50 amp area.
I thought we should remove the inductance but it was decided to put a .1 uf capacitor across the points and that solved the problem. But I have a suspicion some of those units came back with punctured dialectrics in that capacitor.
Editted spelling
We didn't think to put an ammeter on it to see how much current the arc condudted.
[ December 17, 2004, 07:25 PM: Message edited by: physis ]
iggy2
Senior Member
- Location
- NEw England
Re: Arc Voltage
jtester - if you're ever looking to get rid of that Westinghouse T&D reference book, like if you're going to move and don't want to pack it and unpack it, PM me.....
Iggy2
jtester - if you're ever looking to get rid of that Westinghouse T&D reference book, like if you're going to move and don't want to pack it and unpack it, PM me.....
Iggy2
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