Household Type 240/120 Volt Power (240 Volt Center Tapped transformer)

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ruko

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Question about current in the neutral circuit. I think I understand if the load is perfectly balanced between the two 120 volt sides there is no current flowing in the neutral. If this is true then how does one calculate the current in the neutral if one side has for example a 100 ohm load and the other a 10 ohm load?

Thanks
 

GoldDigger

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Question about current in the neutral circuit. I think I understand if the load is perfectly balanced between the two 120 volt sides there is no current flowing in the neutral. If this is true then how does one calculate the current in the neutral if one side has for example a 100 ohm load and the other a 10 ohm load?

Thanks

Knowing the voltage (120) and the resistance on each side you can calculate the current.
One side will be +1.2A and we will call the current in the other -12A since it flows in the opposite direction at every moment in time.
Now the sum of all three currents must be zero, since there is no other place for it to go, so Il1 + Il2 + In = 0.
So the neutral current will be +10.8A.
 

iwire

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The assumption is made that you are using constant resistance type loads and not an incandescent lamp.

I would think that for the purposes of this discussion we could treat an incandescent lamp as a constant resistance. No need to jump ahead to chapter 3. :)











(BTW, I looked at your profile. I love your interests.:D)
 

iwire

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I would just like to point out that the discussion so far has been only about 120/240 volt single phase systems. 208Y/120 systems have different calculations for neutral current.
 
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infinity

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If you didn't make it to engineering school (good answer Digger :)), since it's 1?, 120/240 you could find the current for each load and subtract the smaller from the larger.

I=E/R
I=120/100=1.2
I=120/10=12
12-1.2=10.8
 

ruko

Member
Location
Mid USA
Household Type 240/120 Volt Power (240 Volt Center Tapped transformer)

I would think that for the purposes of this discussion we could treat an incandescent lamp as a constant resistance. No need to jump ahead to chapter 3. :)

(BTW, I looked at your profile. I love your interests.:D)

Strictly resistance and single phase. Any reactive component I assume would muddy up the water. So is 120/10=12 amps the highest current the neutral will see in this configuration?

Thanks for all the answers.
 

ronaldrc

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Knowing the voltage (120) and the resistance on each side you can calculate the current.
One side will be +1.2A and we will call the current in the other -12A since it flows in the opposite direction at every moment in time.
Now the sum of all three currents must be zero, since there is no other place for it to go, so Il1 + Il2 + In = 0.
So the neutral current will be +10.8A.


The neutral carries the difference between line 1 and line 2.

And I would like the people to no that the current only tries to
flow in opposite directions only on the neutral conductor.
 

GoldDigger

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Strictly resistance and single phase. Any reactive component I assume would muddy up the water. So is 120/10=12 amps the highest current the neutral will see in this configuration?

Thanks for all the answers.
Exactly, if only that load is turned on.
 

ruko

Member
Location
Mid USA
Household Type 240/120 Volt Power (240 Volt Center Tapped transformer)

The assumption is made that you are using constant resistance type loads and not an incandescent lamp.

I have always thought incandescent lamps are a constant resistance load. Am I wrong about this?
 

iceworm

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I have always thought incandescent lamps are a constant resistance load. Am I wrong about this?

Tungsten lamps have a positive temperature coeficient. As the fillament gets hotter, the resistance goes up. Here is an example: These are nominal numbers for a 120V, 100W bulb
Cold 20C resistance 19 ohms, current = 6.3A
Hot 1400C resistance 140ohms, current = .86A

When the switch is turned on, there is a high inrush current until the fillament heats up - maybe 15milliseconds (~ 1 cycle - to 90% final Temp)

That's why the lamps seem to always burn out when they are turned on. As the bulbs are used, the fillament gets thinner. Eventually, the cold inrush high current jerks the fillament apart.

ice
 

gar

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ruko:

The resistance of a tungsten filament incandescent bulb has a very large change in its measured resistance as you change its applied voltage. I ran an experiment last night for the thread "lights flickering or dimming" and gave several values in post # 8.

A 60 W bulb was about 11 ohms when the filament was at room temperature using a Fluke 27. With 1.57 V from a D cell about 18 ohms. With 1 V applied the resistance was 17 ohms. When the bulb is supplied by 120 V the resistance is about 120*120/60 = 240 ohms. This is because of the very high temperature of the filament when at its operating voltage of 120 V, and the high positive temperature coefficient of resistance.

An interesting experiment:
Connect a 250 W reflector flood heat lamp to an ohmmeter. Go outdoors and point the bulb in different directions, and directly at the sun. My test heat bulb reads 4.9 ohms at room temperature, 70 F. A 100 W bulb in an aluminum reflector in the basement as a radiant heat source (no other major heat sources heating test bulb) 1 foot away raises the resistance to 6.0 ohms. And a 25 W bulb at the same distance produces a change to about 5.2 ohms.

I can not run a sun test today.

.
 

Strathead

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Tungsten lamps have a positive temperature coeficient. As the fillament gets hotter, the resistance goes up. Here is an example: These are nominal numbers for a 120V, 100W bulb
Cold 20C resistance 19 ohms, current = 6.3A
Hot 1400C resistance 140ohms, current = .86A

When the switch is turned on, there is a high inrush current until the fillament heats up - maybe 15milliseconds (~ 1 cycle - to 90% final Temp)

That's why the lamps seem to always burn out when they are turned on. As the bulbs are used, the fillament gets thinner. Eventually, the cold inrush high current jerks the fillament apart.

ice

Which IMO makes the statement that it isn't a constant resistance sort of a "See how smart I am statement." (not your explanation the other post) We might as well say, it depends on whether the sun hits the wiring during the day or if the switch is new enough so the contacts don't vary in resistance. Most ceramic resistors share the same properties as a lamp in regards to their resistance varying with the ambient temperature. There is always a but. The earth revolves around the sun, but since one theory is that the Sun is moving, in a few trillion years it might revolve around in a circle, so technically that would b revolving around the earth once every few trillion years.
 

gar

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iceworm:

On a 100 W 120 V bulb I read about 9.4 ohms with a Fluke 27. This applies about 0.004 V at this resistance level.

With 1.56 V applied the current is 0.0813 A and a calculated resistance of 19.2 ohms.

What voltage is applied for your 19 ohms cold measurement?

.
 

Strathead

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Measure the resistance of an incandescent lamp. Use Ohms law and calculate the current.

So a constant speed motor is not, in fact a constant speed motor either. All these years we have been lied to. For all practical purposes and surely for the purposes of the OP's question, an incandescent light bulb is a constant resistance. Why confuse the issue?
 

gar

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When the AC excitation frequency to the bulb is high compared to the thermal time constant of the filament, then there is little instantaneous variation in temperature, and the bulb under those steady state conditions can be considered a constant resistance. At 60 Hz I can see modulation of the light intensity from a 100 W bulb. Lower the frequency to 1 Hz for a 100 W bulb and you will see a large variation in the instantaneous resistance and intensity.

.
 
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iceworm

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I have always thought incandescent lamps are a constant resistance load. Am I wrong about this?
(edit to add) Following Strat's post - Let me restate this:

No, for this conversation, you are not wrong.

The circuits in this post are 120V line to neutral and the lamps are connected line to neutral. So, if a 100W lamp is connected L1-N, it will draw .86A. If another lamp is connected to L2-N, that does not change current draw of the first lamp. As long as the lamps are connected across the 120V the loading is constant. And that is really what this post is about.

Yes, tungsten lamps have a positive temperture coeficient and the cold resistance changes from room temperature to blazing hot. That doesn't matter for this post. All your circuits have the lamps connected. Considering them constant loads is a good model.

ice
 
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infinity

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So a constant speed motor is not, in fact a constant speed motor either. All these years we have been lied to. For all practical purposes and surely for the purposes of the OP's question, an incandescent light bulb is a constant resistance. Why confuse the issue?

:)
 
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