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Automatic line voltage switch?

Chances are that you might have to roll your own. Earlier this year I was looking for a 120/240 switch. Like the ones on old tube radios. Pretty much all gone and one mfg said because of liability and approval reasons.

Just as an idea: Little transformer (or a tap on the big one), a relay that is in the 240V position when its coil is not energized, and some electronics. The electronics would determine whether or not the line voltage is under a certain limit and then turn on the relay, which in turn would swing the transformer to step-up mode. There'd have to be some hysteresis, of course, to avoid relay chatter in case of a browning-out 240V line.

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Regards, Joerg

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Joerg
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Yep, before I fixed it.

The original used inrush current limiters (thermistors), which don't "reset" immediately when the power switch is toggled.

"New and improved" used a real resistor bypassed by a TRIAC. Turn power off, TRIAC goes off, can't turn back on for a delay time equal to 10*R*C ;-)

I've never been that terribly troubled by failures... that's how you learn.

Which is why I now (in simulation) ramp up power in a (CMOS) chip design while watching for latch-ups... got burned a few years ago by one of those. Chip didn't die, it just drew enough current to crow-bar the supply :-( H-bridge ) ...Jim Thompson

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Reply to
Jim Thompson

Yep, you're stuck if the motor is "native" to 230VAC. Other way around is easy... naturally :-(

I believe an auto-transformer may offer some weight advantages. I know it does in the step-down direction, and I'd expect that it's bilateral. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
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Reply to
Jim Thompson

Then take a look at variable frequency drives (VFDs). The can do that, and you don't have to use the "variable" part, the front panel control on many of them can be disables so nobdoy can muck around with it. And some come in black :-)

Not sure what your motor does but VFD can also gain you an enviro-sticker if it's multi-speed, because of better efficiency. But, expensive.

[...] 
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Joerg

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This is what I had sketched:

circuit breaker .--. L O---+--| |-----. || .-------O | '--' ) || ( | ) || ( .---. ) || ( |RLY| .----' || ( | | +----. || ( '---. | ) || ( | . o NO ) || ( | . ) || ( N o---+--o-->o----' || '-------O NC T1

RLY runs on 24VDC, pulls in when voltage is

Reply to
dagmargoodboat

a
C

That's an excellent, excellent idea. I'll jump on that, pronto.

Thanks Joerg.

James Arthur

Reply to
dagmargoodboat

Sorry for all the typos, I got tired. Anyhow, when looking for deals they are often just listed under "drives". Wide input range versions can be a bit frustrating to find.

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Joerg

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tic

For failsafeness, add a self resetting poly fuse thing. Relay contact have a failure rate.

You'll also need a series impedance for the relay primary, as pull-in voltage is somewhere vaguely in the region of half rated voltage.

NT

Reply to
Tabby

A thermo-pill in each transformer primary with the good.

That should have some sort of electronic threshold, with hysteresis. You don't want it go to 120V mode when the 240 line is half browning out and hovering at 160V or so.

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Regards, Joerg

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Joerg

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Good idea. Do poly switches work at 220VAC? Hmmm. I found one 240V/

2A at Digikey, nothing bigger. Still, interesting concept.

That's not a problem--RLY is a solid-state gizmo with a stable settable threshold. The 'relay' electronics are powered from rock- solid 24VDC. Sorry for the crappy ASCII art--those details weren't as clear as they could've been.

James Arthur

Reply to
dagmargoodboat

,

Typos? What typos? (some posts improve with age.)

A greater concern is that VFDs put out 3-phase--in my enthusiasm I forgot about that. Could I use just one phase to drive a single phase motor? I guess so, if it's not a fancy-pants BEMF-sensing thing, but that's not quite as elegant as at first glance.

Hmmm.

I'm gonna search for single-phase VFDs.

-- Cheers, James Arthur

Reply to
dagmargoodboat
[...]

Should have a hysteresis, too.

[...] 
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Reply to
Joerg

There are some, but they are almost useless in terms of interrupting capacity.

Interrupting capacity should be higher than the ability of the source to deliver current, otherwise there may be 'issues'. Even at 60V things might not be so nice.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Thanks for the suggestion Rich, but that's not quite the problem I'm trying to solve. (You've quoted my original post above.)

I've got a piece of equipment with a 230VAC pump, yet it needs to operate seamlessly off whatever voltage it's plugged into,

110VAC-230VAC (nominal).

Best, James Arthur

Reply to
dagmargoodboat
[...]

Usually all that EMF and torque sensing can be turned off, most are quite programmable. But: The electrolytics in the 3-phase ones are most likely too small for single-phase use.

There are many single-phase VFDs but tough to find a wide input range version. Keep in mind that the electrolytics in those VFDs have a finite lifetime and also do not like toasty mounting locations. So if this is a place-and-forget-about-it kind of installation the xfmr plus relay method may be better in the long run.

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Joerg

I once worked at a battery charger manufacturer, and he had a 230V relay with some kind of weird circuit such that it would pull in on

230, but not on 115; it switched the primaries of the transformer to series of parallel.

The only way I know of to get 230 from 115 is a transformer, but with a relay you could have it automatically switch the transformer in or out.

The guy was a wizard when it came to designing transformers, but the voltage select thing kind of scared me. I know it used a diode and a cap, and a couple of resistors, and the guy sold a lot of those battery chargers, so evidently it worked.

All you need to worry about is that your relay pulls in reliably at

230V, taking the tranny out of the circuit, and reliably _doesn't_ pull in at 115, leaving the transformer connected.

I know, not much help, but it _is_ doable; look for a 230VAC relay that is guaranteed NOT to pull in at 115.

Good Luck! Rich

Reply to
Rich Grise

Maybe this, using a 3P 240VAC relay (or 120VAC, with suitable dropping resistor in series), for a 120:240 transformer?

no 0----------0 no no 0-----+ | L O--+----0 0---Motor---0 | | \ / / | | nc 0 ||.---0 nc 0 nc | | | ||( | | .---. | ||( | | |RLY| | ||( | | | | | ||( | | '---' '---.||( | | | )||( | | | )||( | | | )||( | | | +---'|| '--------------+ | | | T1 | N o--+------+-----------------------------+

You lose isolation with that when the line is 240, but you didn't mention that as a requirement. Maybe it is or should be?

If you use a 240:240 with a center tap, you can just short out

1/2 of the primary with the NC contacts of a 1P 240VAC relay:

L O--+----0-----+ | \ | | nc 0 +-.||.------Motor----+ | | )||( | .---. | )||( | |RLY| | )||( | | | | )||( | '---' +-----)||( | | )||( | | )||( | | )||( | | )||( | N o--+------------'|| '--------------+ T1

I haven't looked at the cost of the above transformers, but I'm guessing the 240:240 with center tap is more expensive.

Ed

Reply to
ehsjr

As in any voltage in that range (like good switched mode PSU will) which is harder or for 110 +/- 15% and 230 +/- 15%. The latter could be solved by a changeover relay that is only driven when v>200.

ISTR that at airports you get moron proof overseas transformers that do exactly this. Depends on the power you need to deliver. I have a nice toroidal one good for a few hundred watts somewhere from when I lived in Japan. Actually it does roughly the opposite of what you want 100Vac out whatever goes in.

Regards, Martin Brown

Reply to
Martin Brown

To protect the manufacturer from spurious warranty claims.

Sylvia.

Reply to
Sylvia Else

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Seems to me some folks want to make this a more complex problem than it is. A mechanical relay is all that's needed, plus the transformer. If the pump were 110v one could probably get rid of the transformer too.

I dont remember what quantity you were looking at, you'd need to test a few samples for switching threshold and pick a dropping capacitor accordingly.

NT

Reply to
Tabby

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