On 2017-06-08, wrote:

A variac can only accept AC power input, but with Rectifier installed
it can switch output from AC to DC.

Yes -- but if that is done, it is usually built into a power
supply.

Variac mainly used as a light dimmer, speed controller , machines for
motor test and repair ,experiment machines for teaching or lab test and
also industrial use.

Anywhere variable AC voltage is needed -- including for testing
AC-powered equipment to see how well it performs at extreme voltages.

Well, some people call it variac and others say variable transformer,

Variac was the brand name used by General Radio, who may have
been the originator of the device, and certainly the maker of the best
ones which I have used.

Superior Electric called their version a Powerstat.

But all are simply a variable autotransformer, wound on a
toroidal core which concentrates a lot of magnetic field in a small
space.

we can understand from the name that it basicly used to give out
constant adjustable voltage. For example single phase input 220v ,the
standard output is 0-250v ,

The typical General Radio Variac for house line voltage has four
fixed taps, 20V, 100V and another 20V all in series, not counting the
variable tap. This can be wired with the 120 VAC at the two end taps,
so the variable will go from 0-120VAC, or it can be wired with neutral
at one end, and at 20V from the other end, so you can vary the output
voltage from 0-140 VAC (useful for some testing.) There are the two 20V
taps at the two ends for one reason -- the Variac can be panel mounted,
with the shaft coming in the bottom mounting plate, or it can be a bench
use one sitting on the bottom mounting plate with the shaft coming in
from the top. This allows you to wire for 0-140VAC in either situation,
even though the knob is turning CW or CCW for higher voltage depending
on viewpoint.

I say "for house use" because there are also 240VAC industrial
versions, which have an additional (center) tap. You can feed in 120
VAC between one end and that center tap, and get up to 240 VAC output
(at the cost of lots more current from the 120 VAC line than you feed
out at 240 VAC, of course.

The Superior Electric "Powerstat" is pretty much the same,
including similar voltage taps.

I've seen one such variable autotransformer (actually a
Powerstat, not a Variac) set up in a recording room at a South
American US Embassy, used to adjust for sometimes rather low power line
voltages. (Unfortunately, it was a large one and had a knob which
looked like a steering wheel, and a small kid walked up to it and spun
it clockwise, burning out the rectifier in a recorder.)

three phase input 380v output 0-430v. Please
note that these are just standard output ,we can make the input and
output range as your needs. For frequency all good to use 50 or 60hz.

Well ... I have two sizes of Variacs (2A and 7A) made for 400
Hz. Minimum frequency is 350 Hz before it saturates. (And 400 Hz is
normally used to power aircraft instruments because at 400 Hz, you don't
need as much weight of core.) The core is shorter but the same diameter
as the same current rating for 50-60 Hz.

For three phase, there are three identical units, stacked up and
using a common shaft. This gets heavy quickly. :-)

Enjoy,
DoN.

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