120/220 Transformer Wiring Question

[Ian.M]

Its transformer is certainly over 500VA  so use a 100W bulb, or 2x 60W in parallel.   

Reversing the Line and Neutral connections has *NOTHING* to do with phasing.

Incorrectly phase the parallel primaries by reversing one primary winding (i.e. red to black x2) , or placing them in 'series' joining two wires of the same color, with Line (via the bulb) and Neutral  to wires of the other color, or short one of the secondaries, and you will see the bulb light near full brightness, limiting the current and protecting the transformer from the miswiring.

[bostonman]

Reversing the Line and Neutral connections has *NOTHING* to do with phasing.

Hahahhah oh my. This entire time I was reversing hot/neutral thinking that changed the phase in the transformer. As you know, sometimes I over think, sometimes I'm just unfamiliar with such in depth portions of electronics, or a combination.

If that's the case, then I don't need to worry about anything. This entire time I questioned whether when the switch fell apart did the two tiny metal tabs touch more than the points I assumed they touch and/or did another metal tab fall on the floor without me knowing.

From what I can tell, the connections would have been HOT to red and red, and NEUTRAL to black and black, so I may be able to finally just assemble it based off that.

I can still try a light bulb test. There is only one other way to connect this to get them out of phase (now that I believe I'm understanding this better), and that would be HOT to red, and HOT to black on the second coil. Then NEUTRAL to black, and red on the second coil. That would make it one phase in coil one, and reverse phase on coil two.

[Ian.M]

Exactly.  That's why you do the dim bulb test if the primary wire colors or terminal numbers are ambiguous, or if you are replacing the voltage selector switch and need to confirm you wired the new one correctly - test with 115V supply and check neither selector switch position makes the bulb go bright.  It will also show most internal transformer faults like windings with shorted turns, though if the faulty secondary is low current and a small fraction of the total VA, the bulb may not go full brightness so the results may be hard to interpret.

[bostonman]

I got the expected results using a 100W light bulb.

With the assumed correctly connected phase (using an inline meter), the light bulb was off: 120V AC using a VARIAC, 203mA, 17.75W, 0.741 power factor.

With the assumed incorrectly connected phase (I was too afraid to go higher in voltage) the light bulb was on: 30V AC using a VARIAC, 413mA, 12.5W, 1.0 power factor.

Afterwards, knowing red to red and black to black was correct, I hardwired all the wires, reconnected the connectors, etc... and tested the power supply. As expected it turned on, but I didn't do a load test yet (unfortunately the only adjustable load I have is a single channel 180W variable load).

A few things baffle me about this experiment. Initially I used a 60W light bulb and it was on when the wires were wired correctly. Was this due to the resistance being higher in the filament?

Also, I don't comprehend how wiring the two primary coils out of phase causes a short whereas in phase they keep the bulb from turning on.

In phase and the two coils are in parallel. Initially I measured (rounding off) 1 ohm on both, so it would be 0.5ohms and would think the bulb would be on due to the low resistance in series with the bulb.

[wasedadoc]

In phase and the two coils are in parallel. Initially I measured (rounding off) 1 ohm on both, so it would be 0.5ohms and would think the bulb would be on due to the low resistance in series with the bulb.

Surely someone with your number of posts knows that a multimeter applies DC and measures resistance whereas the bulb test has an AC voltage source and the large inductance of the transformer primaries (when correctly phased) is dominant in determining what AC current flows.

[ejeffrey]

That's totally uncalled for, clearly he doesn't know about this in particular and thats why people ask questions. Nothing wrong with that.

When the two coils are wired in parallel, they generate magetic fields in the same direction.  This generates flux in the core and magnetizes it, which is why it has a large inductance that limits the current.

When the coils are anti parallel, the magnetic fields cancel.  There is no magnetic flux in the core and the inductance is very low.  Then the current is only limited by the DC resistance which is very low.

[Ian.M]

Thanks for the clear and concise explanation.  I'm sure it will help Bostonman grock transformers better. 

Incandescent bulbs typically have a cold filament resistance an order of magnitude lower than  their hot (at nominal power) filament resistance.   
A 100W 120V bulb will have a hot resistance of  144 ohms.   Its cold resistance will typically be between 13 and 21 ohms depending on how hot its filament normally runs (color temperature).  The hotter/brighter it normally runs the lower the cold resistance, so a halogen spotlight bulb will give you a significantly lower cold resistance than a rough service inspection lamp bulb of equal voltage and power rating.

To a first approximation the resistance increases linearly with absolute filament  temperature.  However (by Stefan–Boltzmann law) the emitted power (radiated heat and light) of the filament in vacuo^*, is proportional to the fourth power of the filament temperature.  In steady state, the filament must be in thermal equilibrium, which means that at low powers (relative to rated power), a small increase in current results in an increase in temperature and thus a large increase in resistance and power so the bulb acts as a current limiter. 

The trip current is hard to determine without a lot of math. Its easiest to get a feel for it with a small low voltage bulb and a bench CC/CV PSU (as most of us don't have good continuously variable mains AC loads of near unity power factor).  Set the voltage to the nom. bulb voltage and turn the current *way* down, then slowly bring it up plotting resistance (V/I) vs I on graph paper as you do so.   As low voltage bulbs typically run at lower color temperatures than most mains bulbs, unless you use something like a krypton torch bulb, the result wont be directly applicable to your 'dim bulb' but should give you a decent idea.

Once 'tripped' the bulb limit the current to a bit less than the bulb's normal operating current, so  around 0.8A for that 100W 120V bulb.   The estimated  500VA transformer must draw at least 4.17A total primary current in normal operation, so unless it relies on fan cooling, the 'tripped' current from the dim bulb tester is safe indefinitely.

* In practice, nearly all incandescent bulbs have a low pressure inert gas fill, so one also has to consider heat losses due to convection, but the fourth power term typically dominates.

[bostonman]

I appreciate you sticking up for me. I'm also sure sometimes the very active users who respond to questions can sometimes get frustrated with posts that are clearly from others who don't take time to conduct basic research before posting.

Sometimes I've been guilty of jumping the gun and posting about an anomaly in a measurement/test/etc only to realize I didn't look at the problem with both eyes open after getting a reply.

To his point, and your technical reply, I was thinking in terms of resistance and not magnetic fields. So basically when the magnetic fields cancel, I'm left with two 1ohm coils in series, and a light bulb that lights as a result due to higher current.

[bostonman]

ncandescent bulbs typically have a cold filament resistance an order of magnitude lower than  their hot (at nominal power) filament resistance. A 100W 120V bulb will have a hot resistance of  144 ohms.

On a side note, months ago I did a cold start 60W light bulb test with a current probe connected to my scope. I was actually shocked to not see a higher initial current draw.

Also, I didn't realize that a cold filament was so high in resistance until I measured one of the bulbs before applying power. Until then, and your explanation now, I thought they were well under 10ohms at initial turn on.