View Full Version : Another troubleshooting challenge

steveparrott

08-25-2010, 12:09 PM

Have you ever wondered how changes in transformer input voltage affect the total amperage draw? This is good to know so you can evaluate changes in amperage readings.

Say, for example, you install a system and make the following intial measurements:

Voltage at the GFCI (under full load): 115V

Primary transformer amperage: 8.0 amps

You return to the site a year later and take new measurements:

Voltage at the GFCI (under full load): 126V (about 10% higher)

Primary transformer amperage: ???? amps

What would you predict (if only the voltage has changed)? Would the amperage be higher, lower, or stay the same? If it does change, by how much?

Tips: Amps=Watts/Volts but this will not give the correct answer, unless you consider what's happening with the lamps.

Pro-Scapes

08-25-2010, 08:09 PM

you say only the voltage has changed. Why has it changed. Was there an AC unit on when you took it the first time and now there isnt ? Some other draw ?

I had a situation at a recent job where I had 103v coming in. As soon as the AC cut off it jumped to 123. Cause... corrosion due to a lousy job at the sub panel.

Did you check the system for proper lamps ? Perhaps it was lamped with smaller wattages reducing the draw and increasing the voltage. More info is needed for a proper diagnosis

Have you ever wondered how changes in transformer input voltage affect the total amperage draw? This is good to know so you can evaluate changes in amperage readings.

Say, for example, you install a system and make the following intial measurements:

Voltage at the GFCI (under full load): 115V

Primary transformer amperage: 8.0 amps

You return to the site a year later and take new measurements:

Voltage at the GFCI (under full load): 126V (about 10% higher)

Primary transformer amperage: ???? amps

What would you predict (if only the voltage has changed)? Would the amperage be higher, lower, or stay the same? If it does change, by how much?

Tips: Amps=Watts/Volts but this will not give the correct answer, unless you consider what's happening with the lamps.

Faraday's Law states that the EMF generated is proportional to the rate of change of the magnetic flux.

Our 12 volt transformers are a 10 to 1 ratio in windings. 120 windings of copper on the primary and 12 windings on the secondary.

If the primary voltage is 115 volts then the secondary voltage will be 11.5 volts at the 12 volt tap.

Using OHM's law we know that the combined wattage at initial start up was 920 watts. (115v x 8 amps)

If the primary voltage increases to 126 volts then our 12 volt tap would now be 12.6 volts.

The first question I have is why did the primary jump up 11%? Less load or resistance on the circuit? Some lamps are burned out, if so then we started with 920 watts and we have lost 11% of the load. 920 x 11%=101.2

920-101.2 = 818.8 watts. This is now the total watts. Using OHM's law we know the voltage is 126v and we now know the wattage so the ampacity will be P/V=I 818.8/126= 6.5A

Just my guess if that is the reason the voltage went up. If the primary voltage is taken without a load on it and it is still 126v at the GFCI then the ampacity would be 7.3 roughly.

Just thinking here but good question.

The nutty professor :)

Just another guess. But here it is. The ampacity will be 7.12 if you have 11% difference in primary voltage.

If the voltage is increased then the current is decreased by the same factor (11%).

I have to stop drinking coffee after 11pm. :)

Ken

steveparrott

08-26-2010, 08:22 AM

Let's step back a bit. I stated in this case scenario that the only thing that's changed is the primary voltage - everything else about the system has stayed the same. Let's assume that, for some unknown reason, the house voltage has increased - maybe a seasonal difference, or time of day, air conditioners turned off, or something else.

I should say that this is not the actual project troubleshooting scenario. Rather, it came from a contractor's question about why he's seeing a change in the primary amperage draw when nothing in the system has changed. I got to thinking about what kind of amperage change you would see if the primary voltage increased.

I did some calculations and came up with an answer, then set up a lab experiment and got a different result. I did some more digging into technical data sheets and found out why. It was surprising, interesting, and potentially useful so I created this simplified scenario to challenge our group.

steveparrott

08-26-2010, 08:32 AM

Just another guess. But here it is. The ampacity will be 7.12 if you have 11% difference in primary voltage.

If the voltage is increased then the current is decreased by the same factor (11%).

I have to stop drinking coffee after 11pm. :)

Ken

This was essentially what I predicted would happen, but my lab test proved it wrong. I saw my mistake as I ramped up voltage and saw the lamp brightness increase. Aha, lamp wattage increases also! That changes the equation.

Pro-Scapes

08-26-2010, 09:21 AM

well of course lamp wattage increases. A lamp is esentially a variable resistance isnt it ? The harder you drive it the more it will consume.

You would think amperage would drop yet it increased due to as you said the wattage increasing. My quick guess is your now drawing closer to 8.75 amps because your driving the system harder.

This is another reason you need to check primary and seconday amperages and voltages and record them at the time of installation. I want to find a mobile laminating device so I can make a simple card with all my specs on it and laminate it and stick it in the transformers. Including colorcodes...amperages...voltages and date the system was initialized.

steveparrott

08-26-2010, 09:42 AM

well of course lamp wattage increases. A lamp is esentially a variable resistance isnt it ? The harder you drive it the more it will consume.

You would think amperage would drop yet it increased due to as you said the wattage increasing. My quick guess is your now drawing closer to 8.75 amps because your driving the system harder.

This is another reason you need to check primary and seconday amperages and voltages and record them at the time of installation. I want to find a mobile laminating device so I can make a simple card with all my specs on it and laminate it and stick it in the transformers. Including colorcodes...amperages...voltages and date the system was initialized.

Your guess is on the right track, but. . .

Let me ask you this, if voltage increases by 10% and wattage increases by 10%, wouldn't the amperage stay the same? (Amps = Watts/Volts x 0.1/0.1)

The answer is no - amperage doesn't stay the same. Why not?

steveparrott

08-26-2010, 10:43 AM

Correction: the above equation should be: Amps = Watts/Volts x 1.1/1.1

Ok wow I was out to lunch on that one.

I am a little confused..ok well maybe really confused on this one. We are talking OHM's law correct?

If you increase the voltage on the primary and the resistance remains the same on the secondary than your ampacity will rise porportionally. If you raise the voltage up by 10% than your ampacity will raise 10% only if the resistance remains the same.

Now lets talk about the bulbs here. If you have installed a 20 watt light bulb are you telling me that by placing 12.6 volts to that bulb it will use more than 20 watts of power? The filament is resistance measured in ohms and that is a constant piece of tungsten so the resistance would not change for that particular bulb.

I am way confused on this one lets here the answer.....

Your guess is on the right track, but. . .

Let me ask you this, if voltage increases by 10% and wattage increases by 10%, wouldn't the amperage stay the same? (Amps = Watts/Volts x 0.1/0.1)

The answer is no - amperage doesn't stay the same. Why not?

Ok this is what I did...Your initial readings were 115 volts and 8amps.

To find P=VxI which equals 920 watts

To find R=Vsquared/P which equals 14.375

Add 10% to initial voltage = 126.5

Add 10% to initial Resistance = 15.8125

To find I=V/R which is 8

Just to test the theory with bulbs...

11.5 volts on a 20 watt bulb

To find R= Vsquared/P which is 6.6125

To find I=V/R which is 1.74

An increase of 10% on voltage which would be 12.65

To find R on new voltage R=Vsquared/P which is 7.89

To find I on new voltage I=V/R which is 1.6

So what am I missing here?

v3100

08-26-2010, 03:35 PM

> The answer is no - amperage doesn't stay the same. Why not?

Applying a higher voltage across the lamp results in greater current through the lamp but not in a linear manner. The resistance of a lamp filament goes higher with higher applied voltage.

Pro-Scapes

08-26-2010, 06:54 PM

Your guess is on the right track, but. . .

Let me ask you this, if voltage increases by 10% and wattage increases by 10%, wouldn't the amperage stay the same? (Amps = Watts/Volts x 0.1/0.1)

The answer is no - amperage doesn't stay the same. Why not?

You just answered your own question. The wattage increased by 10% It is like putting a bigger hole in a sinking boat. You increase flow when you increase wattage.

Pro-Scapes

08-26-2010, 06:57 PM

Ok wow I was out to lunch on that one.

I am a little confused..ok well maybe really confused on this one. We are talking OHM's law correct?

If you increase the voltage on the primary and the resistance remains the same on the secondary than your ampacity will rise porportionally. If you raise the voltage up by 10% than your ampacity will raise 10% only if the resistance remains the same.

Now lets talk about the bulbs here. If you have installed a 20 watt light bulb are you telling me that by placing 12.6 volts to that bulb it will use more than 20 watts of power? The filament is resistance measured in ohms and that is a constant piece of tungsten so the resistance would not change for that particular bulb.

I am way confused on this one lets here the answer.....

You cannot modify ohms law. What is in question here is to WHY the increase in amperage. Its simple. When you drove the system 10% harder in consumed 10% more power. You get the opposite effect with dimmers. Some UPB devices like controlled recepticals cut the amperage to the outlet and not the voltage. I had this issue 2 years ago when i was using LED string lights on a UPB system. The outlet simply altered the wave.

steveparrott

08-27-2010, 09:37 AM

> The answer is no - amperage doesn't stay the same. Why not?

Applying a higher voltage across the lamp results in greater current through the lamp but not in a linear manner. The resistance of a lamp filament goes higher with higher applied voltage.

V3100 hits the nail on the head (except that the change is fairly linear).

If the lamp responded in a linear 1:1 manner with an increase in voltage, then resistance and amperage would stay the same.

Instead, incandescent lamps respond in approximately a 1:1.5 manner. In other words, a 10% increase in voltage results in an increase in lamp resistance of about 15%. This greater resistance leads to more watts consumed and a higher amperage.

So, to finish off this thread, the final answer is:

A 10% increase in primary voltage results in a 5% increase in amperage. In this example, the inital 8.0 amps would increase to 8.4 amps.

Pro-Scapes

08-27-2010, 11:43 AM

V3100 hits the nail on the head (except that the change is fairly linear).

If the lamp responded in a linear 1:1 manner with an increase in voltage, then resistance and amperage would stay the same.

Instead, incandescent lamps respond in approximately a 1:1.5 manner. In other words, a 10% increase in voltage results in an increase in lamp resistance of about 15%. This greater resistance leads to more watts consumed and a higher amperage.

So, to finish off this thread, the final answer is:

A 10% increase in primary voltage results in a 5% increase in amperage. In this example, the inital 8.0 amps would increase to 8.4 amps.

That is pretty much what I said but I did not have the ratio. It is just like hitting the throttle in your car. Your going to have more power but your going to eat more fuel.

David Gretzmier

08-27-2010, 10:50 PM

I would say this ratio is only true of the certain transformer tested. I recently upgraded a transformer from a 900 watt to a 1200 watt, and also went to a 12-22 rather than the 12-15 that was installed. we used the same exact taps, both were torrodial core , both from the same manufacturer. same outlet. same wire, same bulbs, same number of fixures. same imput voltage, 113V. the reason for the upgrade was after adding fixtures to the existing trans, it's amp reading was now at 7.1. too close to max for me. so on the new trans, once all wires were hooked up, the reading was...7.9.

the taps were now putting out more voltage, about .2 to .3 higher than the previous trans that was too close to max. the new trans is now putting out more voltage, and my lamps are getting a bit more. also, there is an issue of efficiency. It is my opinion that different trans are going to have different efficiencies at different wattages. a 900w trans may be 90% efficient at producing 750 watts, but that may drop to 70-80% at either 500 watts or 850 watts.

simply put, at a 500 watt bulb load, even using same wire, identical taps and the same manu and same type, either magnetic or torroidial, with the same voltage imput, a 600, 900, 1200 watt trans can all show different amp use at the primary. further, using different manu's, my experience tells me that you can see different primary amp readings for identical loads on various manu's trans all rated at the same wattage.

Pro-Scapes

08-28-2010, 07:23 AM

I would say this ratio is only true of the certain transformer tested. I recently upgraded a transformer from a 900 watt to a 1200 watt, and also went to a 12-22 rather than the 12-15 that was installed. we used the same exact taps, both were torrodial core , both from the same manufacturer. same outlet. same wire, same bulbs, same number of fixures. same imput voltage, 113V. the reason for the upgrade was after adding fixtures to the existing trans, it's amp reading was now at 7.1. too close to max for me. so on the new trans, once all wires were hooked up, the reading was...7.9.

the taps were now putting out more voltage, about .2 to .3 higher than the previous trans that was too close to max. the new trans is now putting out more voltage, and my lamps are getting a bit more. also, there is an issue of efficiency. It is my opinion that different trans are going to have different efficiencies at different wattages. a 900w trans may be 90% efficient at producing 750 watts, but that may drop to 70-80% at either 500 watts or 850 watts.

simply put, at a 500 watt bulb load, even using same wire, identical taps and the same manu and same type, either magnetic or torroidial, with the same voltage imput, a 600, 900, 1200 watt trans can all show different amp use at the primary. further, using different manu's, my experience tells me that you can see different primary amp readings for identical loads on various manu's trans all rated at the same wattage.

just about any transformer will vary in output. Even tho it is the same manu it may be a diff core. I have opened 900w transformers to find an extra common lead behind there.

Transformers will vary from winding to winding also that why it is so critical to do all of your tests regardless.

steveparrott

08-28-2010, 11:50 AM

I would say this ratio is only true of the certain transformer tested.

While different transformers do have different efficiencies and will react differently to voltage changes, the ratios mentioned here are specific for lamps.

Note the following charts from GE. You'll see how (red box), when voltage is set to 110%, amperage increases by 105%. Also, when voltage is decreased by 10% (yellow highlit area) the amperage decreases by about 5%.

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