variac question?

I'm nearly ready to call it random chance that the breaker tripped at the low setting but then not when I turned the setting up to 100. I'm planning to buy one of these:

GWE

I think by "must" he means it must be, for it to behave that way = not that it is the right way.

For a while I couldn't plug the variac in at all without popping the breaker. No load on the variac, nothing else plugged into the 20A circuit, no difference caused by the position of the variac lever. After letting the breaker be on for about 20 minutes, however, now I can plug it in again.

It looks like this unit (which, by the way is a Superior Electric Powerstat) just barely sneaks past my 20A panel breaker and then only sometimes.

I only have a little more derusting to do, then I'll pull apart the unit and check all its wiring. At a minimum I'll fuse the hot lead and not the neutral.

GWE

Superior Electric is a quality unit. Nothing wrong with the design. The magnetizing force might be just high enough to break over the breaker.

The breaker might be aging as well. It was an odd thought to me when a HVAC guy stated it. If your house is 10 or more years old it might be the case. They might be fast acting and not slow blow or delayed type for inductive loads.

A 20 amp breaker in a shop is normally a pain! Looks like you found one.

Martin

Mart> For a while I couldn't plug the variac in at all without popping the

Whatta ya mean?

Somebody suggested putting a light bulb in series with the power input to the Variac. They had them setup that way as a way of protecting against a bad device being hooked up to the output. If you set yours up that way and added a bypassing switch across the light bulb it would most likely stop popping the breaker. ie open the bypass switch, turn on/plug in the Variac, apply bypass switch. A bit round-a-bout, but simple enough and a lot better than beating up your breaker. Even the Square D line of breakers go bad if you pop them enough.

Variacs are usually designed to run at pretty high flux density because core temperature rise is not normally a problem. This means that the magnetising (i.e.no-load) current can be a lot higher than in comparable fixed ratio transformers.

The toroidal core used is a completely closed iron circuit with no residual air gap. The the reluctance of this iron circuit is so low that, when power is switched off, it remains magnetised at almost the full value of the flux density that existed at the instant of disconnection of the supply.

Steady state, this is no problem but it can result in possible large current peaks for the first few cycles after power is re-applied.

If power is re-applied at the same phase instant as the previous disconnection phase instant there is no transient disturbance and it simply resumes normal no-load current.

If power is removed at the peak positive flux density instant in the supply wave and power later re-applied at the instant that requires peak negative flux density the core saturates because it cannot support the doubled flux swing. Very large peak currents then occur that decay in the first few cycles of the supply waveform.

This means that the switch on current surge appears to be random because the actual value depends on the precise instants when power is removed and when it is re-applied.

Jim

Well, that is certainly the information I was looking for! Thank you!

Grant Erwin

Also be absolutely certain that the neutral connects to the zero output end of the main winding. If reversed, both output leads are 120 volts to ground when the variac is set for zero out. Very hazardous.

Don Young

According to :

Interesting.

OlK.

O.K. Then this suggests to me that the best way to control a large one like his (the rating of his breaker) is to replace the switch with a solid-state relay, which maintains the "on" state until both the control voltage is off, *and* the current through the load drops to zero. This should minimize the spikes when switching on to half of worst-case. (Even better would be if the turn-on control could be zero-crossing as well.

Glad to see an explanation of the phenomenon which makes sense, and this at least shows that the 30% setting of the wiper when it tripped the breaker was just a coincidence.

Enjoy, DoN.

Somehow I cant imagine that the material in the "iron core" has that large a hysteresis in the B H curve to retain that much field. But it would account for the effect. ...lew...

lew sez:

"Somehow I cant imagine that the material in the "iron core" has that large a hysteresis in the B H curve to retain that much field. But it would account for the effect."

If "Pentagrid" says it -- you can take it to the bank! Jim Pentagrid (not his real name) is a highly respected engineer and author of several books. RCM is sometimes fortunate enough to get Jim's informed comments; usu. toward the end of some overlong thread like this one. It seems he may grow tired of the endless litany of half-assed posts and finally comes out with some meaningful information. Keep up the good work Jim, RCM needs you.

Bob (respecter of knowledge, hater of BS) Swinney

...lew...

Think about it a bit more. The flux is at a maximum at the zero crossing of the current. So switching at the current zero crossing would insure that the core is magnetized as much as possible. Turning on at the voltage zero crossing would insure that the breaker would trip half of the time.

Dan

I have seen the same kind of thing with aorospace transformers designed for minimum weight.. The secondary was feeding a full wave rectifier and charging a capacitor. Sometimes this would cause the capacitor to charge to a much higher voltage than was intended.

Dan