VARIAC....MEX?

MrWizard wrote:
generator brushes are designed to be in contact with a moving surface they are on two 'slip' rings for the 'feild' current , not the output load

the 'load' of a generator output is applied to the output windings , not directly to the feild brushes

they are different surfaces with a different 'wear' factor

My generator produces the 120VAC output with the rotating windings, so all the power has to pass through the brushes. The field is produced by the non-rotating windings. This type of gen is called a "rotating armature" design. The type where the power comes off the non-rotating windings and the field is produced by the rotor windings is called a "rotating field" design.

The rotating armature design is the one in my RV - a 4.0KW Onan CCK. It's lasted 40+ years so far :) A rotating armature design is not unusual for smaller older gens.

I also have a newer Onan gen (NHD 6.5 KW) that is a rotating field design.

And when you start moving a Variac around under heavy load you can really feel it. The knob actually feels like it's resisting (no pun) movement of the wiper.

yes it is different
adjusting the variac/sliding the contact changes the voltage and hence the load

generator brushes are designed to be in contact with a moving surface
they are on two 'slip' rings for the 'feild' current , not the output load

the 'load' of a generator output is applied to the output windings , not directly to the feild brushes

they are different surfaces with a different 'wear' factor

Quite similar. Under heavy use a carbon smear wipes onto the turns. Clean it off and the contact will last a lot longer. This is for heavily used items. This is like comparing a car dome light to the needs of an aircraft warning beacon.

Is the carbon sliding contact much different than the commutator brushes in a motor? They seem to last when the motor is turning fast under load.

Like voting one way or the other. Personally with all things weighed, sacrificing sixty bucks every six to ten years seems to be not a great issue. If one were to wear out the knob like a stage technician monitoring lights for a Broadway Play then the carbon erosion factor becomes something to note. Speaking of noting, note how few times and little movement (in inches) a complete equalization cycle puts on the variac's carbon tipped swing arm. Little variacs have a larger carbon tip proportionately than large units.

When I was overhauling AC generator voltage reguators (Basler and Delco) I did wear contact tips out. Load the field, tweak, readjust the load, tweak, my fingers used to get tired. The last big equalization I did on a cruising sailboat (22 L-16 batteries), I had to move the ohmite rheostat eighteen times. This was to field the alternator. Wotta hoot! Full disconnect of the batteries and reconnect using 2/0 jumpers to series according to gravity. I was equalizing to 32 volts. The skipper was amazed his Balmar alternator could do that.

MEXICOWANDERER wrote:
Let's take it from the bottom. Mena is starting at the bottom. Least electrolysis. Then he is going to incrementally increase current until his charger is reading correct equalization potential. From there only slight and occasional tweaking during the equalization process will be needed. Bit by bit.

I took all the into account when I recommended he adjust the variac under load. My 3-amp, 5-amp and 7-amp variacs (Hwang Ho origin) all use the same carbon contact.

No, it isn't an Ohmite ceramic rheostat. So total contact lifespan may equal a few hundred feet rather than several miles of travel.

Moving the carbon very fast across the winding will definitely cause premature erosion. But the carbon can be replaced when necessary. After I'm dead.

Both ways work and have their advantages and disadvantages.

:B

ken white wrote:

MEXICOWANDERER wrote:
...As usual helpful folks have added so much additional information to the discussion it became murky :)

No way in hell will you have a problem twisting the knob under load. Remember the old fashioned light dimmers? The ones the grand kids played with like it was a video game? Meet its cousin. Just don't allow the knob to be thrashed back and forth while you have a load on the variac.

If we are talking about higher end, or better designed products, then I would partially agree but the fact remains the contacts will have their life reduced when moved under load.

The new low cost Chinese products are not manufactured to the same quality standards as older or higher end power equipment.

Just my murky opinion...

Let's take it from the bottom. Mena is starting at the bottom. Least electrolysis. Then he is going to incrementally increase current until his charger is reading correct equalization potential. From there only slight and occasional tweaking during the equalization process will be needed. Bit by bit.

I took all the into account when I recommended he adjust the variac under load. My 3-amp, 5-amp and 7-amp variacs (Hwang Ho origin) all use the same carbon contact.

No, it isn't an Ohmite ceramic rheostat. So total contact lifespan may equal a few hundred feet rather than several miles of travel.

Moving the carbon very fast across the winding will definitely cause premature erosion. But the carbon can be replaced when necessary. After I'm dead.

vermilye wrote:
There are (well were) versions of autotransformers used in the theatre industry for dimming lights in the 500 - 5000w load range. They were designed to operate under full load. If you can find one of them the contacts will hold up...

I'm pretty sure you are referring to a "magnetic amplifier" or saturable reactor device, a.k.a. a "mag amp." They were widely used in theatrical lighting for dimming control of high wattage lighting. They do look a lot like transformers and they have a control knob, like a variac, to control output, but they use a very different principle of operation.

A "magnetic amplifier" uses a DC control current fed into control windings to control the AC output. Since the control knob only controls low DC currents, it doesn't suffer from the problems of a Variac, where the control knob is moving a contact handling all the current.

The control current of the mag amp sets where the cores of a pair of AC windings will saturate. The AC winding of a saturated core will go from a high impedance state to a very low impedance state. Basically, the DC control current sets the voltage at which the magnetic amplifier turns on.

A mag amp allows a small DC current to control a large AC current, which is what was needed for stage lighting control. I've never been sure what the output waveform looks like in one of those stage lighting mag amp controls. It might not be suitable for feeding something that needs a sine wave - as a Variac provides.

MEXICOWANDERER wrote:
...As usual helpful folks have added so much additional information to the discussion it became murky :)

No way in hell will you have a problem twisting the knob under load. Remember the old fashioned light dimmers? The ones the grand kids played with like it was a video game? Meet its cousin. Just don't allow the knob to be thrashed back and forth while you have a load on the variac.

If we are talking about higher end, or better designed products, then I would partially agree but the fact remains the contacts will have their life reduced when moved under load.

The new low cost Chinese products are not manufactured to the same quality standards as older or higher end power equipment.

Just my murky opinion...