Shopping for a variac

[drussell]

drussell, are you concerned about some resistive dissipation in the windings causing higher temps?

Well, at Vout=Vin, the windings are dissipating their lowest possible amount, so, yeah. 

[tkamiya]

I did crank it few times up and down for a minute or so.  Nothing happened.

Do you guys want me to quickly test with 1000 watts bulb at 50V? 

[bdunham7]

OK, done. 

Summary:
Environment is outside in Florida 90 to 100 degrees F depending on if sun is out or not.
Total run time 10 minutes
50V 2.4amp for 8 minutes or so
120V 4.2amp for rest of the test
Highest 120V 4.2A = 504VA = 504watts
Core temperature stayed around 100 degrees F.
Hottest part is 137 F.  The big nut at bottom which holds the spindle.
Side and top is somewhere between the two.
No spark, nothing melting, no smoke.

At this time, I don't see any reason to continue so ended the test around 15 minutes.
Messing with video right now but do you guys really want to see this?  Absolutely nothing happens.

It's up to you, but variacs are often used in quasi-continuous service.  Can you do 100 volts for an hour?

[drussell]

Yeah, I would love to see what happens on some longer tests at closer to a full current rating for the wiper setting that are more than a few percent away from "straight through."

If, for example, you're troubleshooting an SMPS, it is going to TRY to pull more current as you lower the voltage, so will an induction motor load, etc., so that's what I'm most interested in. 

Right at 120v I'm sure it can do the 4.1A / 500VA just fine.

[engrguy42]

drussell, are you concerned about some resistive dissipation in the windings causing higher temps?

Well, at Vout=Vin, the windings are dissipating their lowest possible amount, so, yeah. 

I'm just ballparking that maybe 24AWG wire has say 5 ohms per 1,000 ft (just a guess), and say there's, I dunno, 200 ft wire in the variac, that's only like an ohm. At 5 amps that's dissipating only another 25 watts or so? Doesn't seem like it would do much, but maybe my numbers are off.

My basic problem with this is we're talking only 5 amps in this big hunk of copper. I can't imagine big damaging temperatures, unless I'm missing something.

[tkamiya]

Did a quick test with 1000 watt bulb.  I couldn't go up to 50V.  At 27V, it is already drawing 4.9A.  No appreciable temperature raise in 2 minutes or so.  What I did notice was I felt more HUM on the knob. 

[bdunham7]

I'm just ballparking that maybe 24AWG wire has say 5 ohms per 1,000 ft (just a guess), and say there's, I dunno, 200 ft wire in the variac, that's only like an ohm. At 5 amps that's dissipating only another 25 watts or so? Doesn't seem like it would do much, but maybe my numbers are off.

My basic problem with this is we're talking only 5 amps in this big hunk of copper. I can't imagine big damaging temperatures, unless I'm missing something.

More like 25 ohms/1000ft and I've no idea how long the wire is, but the way to test this would be to measure the voltage drop directly by comparing OC to loaded, which Tkamiya seems to be well equipped to do.  Based on how much adjustment it takes to keep my similarly-sized variac at the correct voltage when the load changes, I'm betting the dissipation is not negligible.

EDIT:  Or you could use an ohmmeter.  My particular unit has two inputs, both 120V but one labeled 50/60Hz, which is 14 ohms, the other 60Hz, which is 12 ohms.  This is the 2.25A/26awg model.

[Electro Fan]

snip

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

tkamiya,

Sorry to hear if you aren’t feeling well but it looks like you are having some more good finds.

Thanks for being a good referee and for volunteering some DUT gear and time to experiment.  Hope you have enough fire extinguishers and some good PPE - but I’d bet if needed somehow you could find some more.   

Seriously, keep us posted on your tests and stay safe.   

[tkamiya]

I really wish I had a better load.  All I have is 500 watt bulb and 1000 watt bulb.  Well, I also have 40 watt bulb but that's nothing. 

It was rather boring test, so I had to keep reminding myself of "potential danger."  500 watts can cause significant injury and destruction.

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

[engrguy42]

I'm just ballparking that maybe 24AWG wire has say 5 ohms per 1,000 ft (just a guess), and say there's, I dunno, 200 ft wire in the variac, that's only like an ohm. At 5 amps that's dissipating only another 25 watts or so? Doesn't seem like it would do much, but maybe my numbers are off.

My basic problem with this is we're talking only 5 amps in this big hunk of copper. I can't imagine big damaging temperatures, unless I'm missing something.

More like 25 ohms/1000ft and I've no idea how long the wire is, but the way to test this would be to measure the voltage drop directly by comparing OC to loaded, which Tkamiya seems to be well equipped to do.  Based on how much adjustment it takes to keep my similarly-sized variac at the correct voltage when the load changes, I'm betting the dissipation is not negligible.

I found an online copper wire resistance calculator and assuming a 200ft length of 0.5 mm wire it comes up with 5 ohms. If each turn is 1 foot long, then that's 200 turns? Seems reasonable? Maybe 1.2 volts per turn accuracy?

[tkamiya]

snip

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

tkamiya,

Sorry to hear if you aren’t feeling well but it looks like you are having some more good finds.

Thanks for being a good referee and for volunteering some DUT gear and time to experiment.  Hope you have enough fire extinguishers and some good PPE - but I’d bet if needed somehow you could find some more.   

Seriously, keep us posted on your tests and stay safe.   

Thanks.  I used to be in photography.  I was never in video.

During the test, I had a face shield on and fire extinguisher handy.  I also had a quick way to disconnect the power source. 

[engrguy42]

snip

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

tkamiya,

Sorry to hear if you aren’t feeling well but it looks like you are having some more good finds.

Thanks for being a good referee and for volunteering some DUT gear and time to experiment.  Hope you have enough fire extinguishers and some good PPE - but I’d bet if needed somehow you could find some more.   

Seriously, keep us posted on your tests and stay safe.   

Thanks.  I used to be in photography.  I was never in video.

During the test, I had a face shield on and fire extinguisher handy.  I also had a quick way to disconnect the power source.

Thanks much tkamiya. Greatly appreciated. Since I'm not really sure what specific issues people have with these devices in terms of poor/cheap/junky performance I'm not sure what else you can test.

Like I say I think it would be tough to under-design a 5 amp device like this. Now if we're talking say 50 amps that's a different story.

[drussell]

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

You don't happen to have any giant wirewound adjustable resistors or anything like that, eh? 

[bdunham7]

I really wish I had a better load.  All I have is 500 watt bulb and 1000 watt bulb.  Well, I also have 40 watt bulb but that's nothing. 

It was rather boring test, so I had to keep reminding myself of "potential danger."  500 watts can cause significant injury and destruction.

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

I think that the time periods are short as there is a significant thermal mass.  It sounds like you have better things to do and you don't want to leave it unattended, but I would do the test for several hours at a reasonable load--say 25V/4A with the 1000 watt bulb or 100V/3A with the 500--or find a small space heater or toaster to get a constant resistance.

[ArthurDent]

Well, at Vout=Vin, the windings are dissipating their lowest possible amount, so, yeah. 

That is very true but you still have to fuse the variac properly, and some people don't. Years ago I was given a nice bench Powerstat where someone hadn't fused the output. The input went to a tap to allow the output to be higher than the input and the wiper was just a couple of turns from the 100% tap when the output was shorted and burned the two turns between the tap and the wiper. I epoxied a small formed copper link in place of the burned turns and soldered the two good ends of the coil to the tabs I formed on the end of the piece. I ended up with an almost new bench variac with a very narrow band where the voltage didn't increase for a degree or two of rotation. As I said before these burned turns are generally at the low end of the winding but could be repaired the same way.

[bdunham7]

I found an online copper wire resistance calculator and assuming a 200ft length of 0.5 mm wire it comes up with 5 ohms. If each turn is 1 foot long, then that's 200 turns? Seems reasonable? Maybe 1.2 volts per turn accuracy?

Yes, 5 ohms for 200 feet, not 1000.  But I can't see a turn being a foot long on such a small device--but it likely has more than 200 turns.  But those are guesses, not facts.  An ohmmeter will actually answer the question.

[Electro Fan]

My moneys on the Fluke surviving.

+1 for the Fluke surviving but also...

My moneys on two additional things:  1) this debate will go on beyond the first test regardless of what comes from the first test, and 2) tkamiya will find some more good stuff if he decides to do further tests

[drussell]

That is very true but you still have to fuse the variac properly, and some people don't.

Indeed.

The red Chinese ones only have an input fuse, so anyone who uses one should really add an output fuse holder when they do their wiring check/repair/update.  This also makes it very easy to put in an appropriate output fuse for the device you're working on when needed.

[Electro Fan]

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

You don't happen to have any giant wirewound adjustable resistors or anything like that, eh? 

Challenging a champion finder to find something?

[BravoV]

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

Pssttt .... 

Too bad I don't have the time, as I have IR cam and curious too.

Don't feel guilty as I'm TEA god worshiper.   

[engrguy42]

I've had my eye on a 20A version of the cheap red Chinese variacs, and happened on a guy's video of the exact same model. If anyone is curious about what the internals look like in real good light, here it is:



He did what I would do and replace the high-side 20A fuse with a 15 amp since my bench circuit is on a 15 amp breaker. Otherwise the internals look reasonable.

My only concern is it's a big, 16 pound monster and I worry about shipping from whereever. Heck, I'm not even sure I need that many ampules. I can't imagine what I have that would use more than 20 amps on the low side. Or even 10. I dunno, maybe I'll just get a 10 amp one.

Oh, and here's what they give for specs on Amazon. Interesting they mention 8 working hours:

Max. 2000VA
Plug Type: US Plug
Max output : 20 Amps
Input Voltage: AC 110V/60Hz
Output Voltage: AC 0-130V/60Hz
Material: Iron (shell) & Copper Coil
Humidity : ≤ 5%
Working Hours: 8 hours
Condition Temperature:<40℃
Insulated Resistance: >10MΩ
Withstand Voltage: 2KV/1 minute

[Electro Detective]

This comment has been scanned and verified to ensure no 'personal attack' content or sneaky Auto-Troll browser addons I may be unaware of 

Let's have a brief gander at those specs above:

"Withstand Voltage: 2KV/1 minute"

what kind of 2KV are they specifying?

A hand held insulation tester at 2.5KV pumping out an awesome current of 2mA DC through the red shed paint powder coating? ? 


Another curious spec above states "Working Hours: 8 hours"

Is there a timer on board to indicate when the unit is at its 'end of 8 hour life' and time for another purchase?
Sounds expensive unless you only use it for 8 minutes every year, then sell it off at 4 hours and buy another new one 

 
Giggles aside     "Working Hours: 8 hours" tells us nothing about its true duty cycle with any load   

and "Condition Temperature: < 40℃"  and some/all of the other click buyer eye candy techie specs above, tell even less


If I just had no choice but to buy one of these  fire red  Very-Aches (ED 'Patented' term), I'd roll with a dissected, corrected and in/out tightly fused 20 amp job,
and use it under 5 amps max full load continuous, with crossed fingers confidence,

and not leave the bench unattended in case Nature calls
and stuck on the throne royale way longer than anticipated   

[tkamiya]

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

You don't happen to have any giant wirewound adjustable resistors or anything like that, eh? 

Challenging a champion finder to find something?

A second variac?  I found one!  On Amazon!!

[tkamiya]

I suppose, I could start with 500 watt bulb, and keep adding 100 watt bulbs or less until I reach the target?  You guys want 50V 4A, correct? 

I *could* do that.

What are we expecting to see?  At this point I'm fairly confident the thing will survive.  What's the "junk" criteria?

[engrguy42]

I suppose, I could start with 500 watt bulb, and keep adding 100 watt bulbs or less until I reach the target?  You guys want 50V 4A, correct? 

I *could* do that.

What are we expecting to see?  At this point I'm fairly confident the thing will survive.  What's the "junk" criteria?

Wow. Excellent question !!! 

Can't wait to hear the answer