Strobing incandescents on classroom demo with open neutral.

[11bgrunt]

A classroom demo was built to represent power from the utility through the service, down to a QO 200 amp panel and out to loads. The board was built to demonstrate open, and resistive connections and the effect on the load that the customer can see. There are parallel circuits using bulbs for load and a series circuit using two 120 volt bulbs served in series with 240. When the loads are balanced and the resistor is cut into the neutral return there is no change in the load. Create an unbalance by bringing on another 120 volt load and the resulting voltage change can be seen with voltmeters and the bright/dimming of the bulbs. All of that was expected.
When the same situation is built with an open neutral the parallel 120 volt bulbs begin to strobe. The series circuit does not. I changed the amount of unbalance drastically and there was no change, the bulbs continued to strobe. This open neutral has no earth reference. When an earth reference is added allowing some neutral current to return, the strobing stops. I haven't seen this before. I can create the strobe every time with the unbalanced 120 volt load and the open neutral. Have a video but haven't converted to a file type yet that will upload.
Can someone offer an explanation for the strobing bulbs?

 

[Rampage_Rick]

Incandescent bulbs aren't a fixed resistance, i.e. a 100-watt bulb is 9.5 ohms cold, 144 ohms hot. Could there be some kind of hot/cold cycling taking place?

 

[gar]

150422-1045 EDT

11bgrunt:

You need to be more specific about the actual circuit when strobing occurs. This means bulb wattages, and the circuit of importance.

I believe the circuit is a 240 V 60 Hz sine wave source with the equavalent of two incandescent 120 V bulbs connected in series across the source.

I created a circuit with 1 60 W bulb in series with a 75 W bulb. I did not apply 240 V, but rather just 120 V. There is a very noticable lag in the brightness of the 75 W bulb following application of the input voltage, this is to be expected. But this a single shot transient, no oscillation. This is a result of the difference in the thermal time constants of the two bulbs.

Gradually increasing the voltage from 120 to 240 did not cause oscillation. increasing the disparity in wattage did not cause oscillation.

.

 

[mgookin]

What's the frequency of the strobe?

 

[11bgrunt]

The frequency of one bulb on/off and then the other bulb on/off is six times per second. All bulbs are 40 watt incandescent. The load to create unbalance on 120 volts is resistive or resistive with small fan. My choice and had no affect on strobing which one was used, 2 amps or 10 amps.
The common is the two 120 volt bulbs served by different 120 volt legs have a neutral wire on the neutral bussbar and the loads used for unbalance have their neutral on the same buss. The intent was to show that when the loads are balanced, the neutral could be opened and have no effect because the circuits turned into a 240 volt series circuit that required no neutral for return current. The actual built series circuit using two bulbs and 240 volts has no wire going to that neutral buss.

 

[rt66electric]

Are you sure its 240/120 supply??

Are you sure its 240/120 supply??

Are you sure its 240/120 midpoint grounded supply??
And not 2 legs of a 208/120 Y ???? Most large buildings have 208 3P

 

[gar]

150422-1940 EDT

11bgrunt:

I still don't know what your circuit is when the bulbs oscillate.

Leave out all the extraneous information.

Is the following your basic oscillating circuit?

You have a voltage source of either 240 or 208 V. This makes no difference other than the magnitude. The load consists of one 40 W bulb connected in series with at least two 40 W bulbs in parallel. The midpoint node of the bulbs is connected to the midpoint (neutral) of the voltage source. Upon opening the neutral the bulbs oscillate in intensity.

With my 60 and 75 W bulbs in series I can not generate oscillation with either a 208 or 240 V source, or any lower voltage, following the opening of the neutral connection.,

I don't have a fan of appropriate size to use in the experiment. Also I do not want to run an experiment with as much unbalance in voltage as would be equivalent to a 40 W in series with an 80 W and apply 240 V. Too likely to burnout the 40 W.

What other mysterious component is in your circuit, or special about the bulbs?

How do you get 2, let alone 10, amperes thru a 40 W bulb?

.

 

[11bgrunt]

Imagine the 120/240 panel serves five 120 volt circuits. All have their own hot leg leaving the 20 amp breaker and all have a neutral return that lands on the neutral bussbar.Two circuits consist of one 40 watt bulb each. These make up the balanced load. The other three circuits are a 250 watt bulb, 1000 watt heat gun and a 1200 watt bathroom heater. When only the bulbs are on, the loads are balanced and the neutral can be opened with no problem and nothing can be seen flicker wise in those two bulbs. Add any one of the three remaining 120 volt circuits to create the unbalance with the neutral open and the 40 watt bulbs begin the strobe.

 

[gar]

150422-2223 EDT

11bgrunt:

A 250 W incandescent in series with a 40 W incandescent connected across 240 V will put almost 240 V across the 40 W bulb and it won't last long.

A 250 W 120 V bulb is about 5 ohms at room temperature. A 40 W 120 V bulb at 120 V is about 360 ohms and will be even greater at near 240 V. I can expect about 230 V across the 40 W bulb for this series connection. I don't want to burnout a bulb so I won't try it.

How long does the 40 W bulb last in this unbalanced state?

What are you not telling us?

.

 

[11bgrunt]

The bulbs with the open neutral and no earth reference have not failed.
There is a 1 ohm resistor that can be added to the neutral circuit so that unbalance causes voltage to get on the 240 coil. I have pushed 190 volts on the lightly loaded side and caused bulbs on that side to fail. I had a UPS connected to sound alarm for over and under voltage caused by the resistive neutral. The 190 volts let the smoke out of the UPS.

 

[gar]

150422-2345 EDT

11bgrunt:

You are just not providing a clear description of your circuit. Earth has nothing to do with your circuit unless you are not providing us some details related to the earth.

To get any useful help you have to precisely describe the actual circuit that causes the light intensity oscillation. Leave out any information that has nothing to do with the problem. Precisely describe the simplest circuit that causes the oscillation. If the circuit oscillates when the 1 ohm resistor is not active, then don't mention the 1 ohm resistor.

Since you don't burnout a 40 W bulb it is clear that that bulb is not on the high voltage side of the unbalance. So what is on the high voltage side of unbalance?

.

 

[GoldDigger]

If the oscillation only happened when the fan was in the circuit, I could make a SWAG as to the cause. It would involve the combination of the lag time involved in heating the filament and increasing the resistance in response to a voltage surge coupled with the fact that the reverse EMF of the motor will depend on the speed (slip rate) of the fan motor and that also cannot change instantaneously.
If the fan is on the opposite leg from some of the bulbs, the initial low impedance would cause the motor to speed up in response to overvoltage, but not instantaneously. As the bulb heated and the motor sped up both the resistance and the series counter EFM would increase, lowering the voltage across the bulb and causing it to start cooling. Meanwhile the fan motor would be slowing down, lowering the counter EMF and allowing the current to increase again.

I am not in a position to actually try this experiment, but my mind considers such a scenario to be possible.

 

[ptonsparky]

If the oscillation only happened when the fan was in the circuit, I could make a SWAG as to the cause. It would involve the combination of the lag time involved in heating the filament and increasing the resistance in response to a voltage surge coupled with the fact that the reverse EMF of the motor will depend on the speed (slip rate) of the fan motor and that also cannot change instantaneously.
If the fan is on the opposite leg from some of the bulbs, the initial low impedance would cause the motor to speed up in response to overvoltage, but not instantaneously. As the bulb heated and the motor sped up both the resistance and the series counter EFM would increase, lowering the voltage across the bulb and causing it to start cooling. Meanwhile the fan motor would be slowing down, lowering the counter EMF and allowing the current to increase again.

I am not in a position to actually try this experiment, but my mind considers such a scenario to be possible.

In short, take the fans out of the circuit. Add them later to show what happens when inductive loads are added to the mix.

Lamps Only.
1. Start with balanced load with neutral.
2. Take current readings on all wires and VD across lamps.
3. Remove neutral connection and confirm current readings and VD across lamps.
4. Change circuit to include unbalanced load with neutral.
5. Take current readings on all wires and VD across lamps.
6. Remove neutral connection and check current on all wires and VD across lamps.

Now add the motor load(s). What happens when it is in parallel with the smaller lamp. Larger Lamp? Identical lamps? What happens when you add a 240v motor load? 240v resistive load? Combination of both?

Sounds like good classroom experiments.

 

[11bgrunt]

Six presentations later, the information is the same. When the neutral is open with no neutral return path of any kind, the 120 volt bulbs oscillate off/on. The two bulbs are never on at the same time. Turn one bulb off, the remaining bulb continues to oscillate/strobe.
The bulb/bulbs do not strobe when a neutral return path is provided through the earth through a ground rod. The soil at this location will allow 22 amps when the bare ground rod is energized 120 volts, so I calculate the resistance at 5.5 ohms. Much lower than required and much better than I usually see when doing these experiments at other locations. Part of this program is to demonstrate that the OCPD does not open when using the earth only as a ground fault return on 120 volts. The strobing light bulbs is a bunny trail I had not experienced before. The strobing happens when the controlled neutral unbalance was resistive only or resistive and very small motor load. More things to look at. I am sure there is some combination of things going on here. The 240 resistive load is on when the 120 is strobing. I have not connected a 240 volt motor load.
Gar, thanks for being involved. I don't know how to answer your questions.
I do appreciate everyone's responses. Our men work 120 volts to 138kV transmission. We try to talk about situations they will encounter in the field.

 

[ActionDave]

Six presentations later, the information is the same. When the neutral is open with no neutral return path of any kind, the 120 volt bulbs oscillate off/on. The two bulbs are never on at the same time. Turn one bulb off, the remaining bulb continues to oscillate/strobe.......

Something is up with your set up. Not sure what. There is no explanation without being there. Not saying you are wrong, just saying answer to your query needs a hands on second opinion.

 

[gar]

150516-0906 EDT

11bgrunt:

If you have a stiff (meaning low impedance) 240 source, then any additional loads that are only across the 240 source have nothing to do with the strobing.

I have put a 120 V 2 A fan in series with a 250 W heat lamp. I applied 120 V to this series combination. No strobing. I don't want to apply 240.

There is something in your circuit that we don't know about. Determine the minimum circuit that will cause pulsing, and accurately describe that circuit with a diagram. Obviously this boils down to a 240 V source with no neutral connection to the load plus some load components.

.

 

[JRW 70]

Fan Design

Fan Design

Could the fan motor have internal thermal protection
that is fairly sensitive and fast cooling? That doesn't
satisfy as a complete answer but could be a contributing
factor (maybe?)

JR

 

[Electric-Light]

Could the fan motor have internal thermal protection
that is fairly sensitive and fast cooling? That doesn't
satisfy as a complete answer but could be a contributing
factor (maybe?)

JR

Could have something to do with acceleration.

You have a variable resistor in series with a (variable resistor and motor in parallel).

You not only have a motor, but with a fan attached to it. As the fan accelerate, the load on motor increases exponentially (fan affinity) which disturbs the ratio split between the variable resistor in parallel. They go back and forth... hence oscillation. Does the fan have a run capacitor?

Try something less interactive, like an iron.

 

[11bgrunt]

There are three different loads mounted on the back of the display that are used at different times to create the neutral unbalance. The least was a 250 watt incandescent bulb. The oscillation occurred regardless of heat with very small fan (no capacitor) or the 250 watt bulb. I don't know if bathroom heaters have TP or not. I am sure the heat gun does not.
I will be back there in June and plan to do more experiments to determine cause.