Voltage regulation
As we saw in a few SPICE analyses earlier
in this chapter, the output voltage of a transformer varies some with
varying load resistances, even with a constant voltage input. The degree
of variance is affected by the primary and secondary winding
inductances, among other factors, not the least of which includes
winding resistance and the degree of mutual inductance (magnetic
coupling) between the primary and secondary windings. For power
transformer applications, where the transformer is seen by the load
(ideally) as a constant source of voltage, it is good to have the
secondary voltage vary as little as possible for wide variances in load
current.
The measure of how well a power
transformer maintains constant secondary voltage over a range of load
currents is called the transformer's voltage regulation. It can
be calculated from the following formula:
"Full-load" means the point at which the
transformer is operating at maximum permissible secondary current. This
operating point will be determined primarily by the winding wire size (ampacity)
and the method of transformer cooling. Taking our first SPICE
transformer simulation as an example, let's compare the output voltage
with a 1 kΩ load versus a 200 Ω load (assuming that the 200 Ω load will
be our "full load" condition). Recall if you will that our constant
primary voltage was 10.00 volts AC:
freq v(3,5) i(vi1)
6.000E+01 9.962E+00 9.962E-03 Output with 1k ohm load
freq v(3,5) i(vi1)
6.000E+01 9.348E+00 4.674E-02 Output with 200 ohm load
Notice how the output voltage decreases
as the load gets heavier (more current). Now let's take that same
transformer circuit and place a load resistance of extremely high
magnitude across the secondary winding to simulate a "no-load"
condition:
transformer
v1 1 0 ac 10 sin
rbogus1 1 2 1e-12
rbogus2 5 0 9e12
l1 2 0 100
l2 3 5 100
k l1 l2 0.999
vi1 3 4 ac 0
rload 4 5 9e12
.ac lin 1 60 60
.print ac v(2,0) i(v1)
.print ac v(3,5) i(vi1)
.end
freq v(2) i(v1)
6.000E+01 1.000E+01 2.653E-04
freq v(3,5) i(vi1)
6.000E+01 9.990E+00 1.110E-12 Output with (almost) no load
So, we see that our output (secondary)
voltage spans a range of 9.990 volts at (virtually) no load and 9.348
volts at the point we decided to call "full load." Calculating voltage
regulation with these figures, we get:
Incidentally, this would be considered
rather poor (or "loose") regulation for a power transformer. Powering a
simple resistive load like this, a good power transformer should exhibit
a regulation percentage of less than 3%. Inductive loads tend to create
a condition of worse voltage regulation, so this analysis with purely
resistive loads was a "best-case" condition.
There are some applications, however,
where poor regulation is actually desired. One such case is in discharge
lighting, where a step-up transformer is required to initially generate
a high voltage (necessary to "ignite" the lamps), then the voltage is
expected to drop off once the lamp begins to draw current. This is
because discharge lamps' voltage requirements tend to be much lower
after a current has been established through the arc path. In this case,
a step-up transformer with poor voltage regulation suffices nicely for
the task of conditioning power to the lamp.
Another application is in current control
for AC arc welders, which are nothing more than step-down transformers
supplying low-voltage, high-current power for the welding process. A
high voltage is desired to assist in "striking" the arc (getting it
started), but like the discharge lamp, an arc doesn't require as much
voltage to sustain itself once the air has been heated to the point of
ionization. Thus, a decrease of secondary voltage under high load
current would be a good thing. Some arc welder designs provide arc
current adjustment by means of a movable iron core in the transformer,
cranked in or out of the winding assembly by the operator. Moving the
iron slug away from the windings reduces the strength of magnetic
coupling between the windings, which diminishes no-load secondary
voltage and makes for poorer voltage regulation.
No exposition on transformer regulation
could be called complete without mention of an unusual device called a
ferroresonant transformer. "Ferroresonance" is a phenomenon
associated with the behavior of iron cores while operating near a point
of magnetic saturation (where the core is so strongly magnetized that
further increases in winding current results in little or no increase in
magnetic flux).
While being somewhat difficult to
describe without going deep into electromagnetic theory, the
ferroresonant transformer is a power transformer engineered to operate
in a condition of persistent core saturation. That is, its iron core is
"stuffed full" of magnetic lines of flux for a large portion of the AC
cycle so that variations in supply voltage (primary winding current)
have little effect on the core's magnetic flux density, which means the
secondary winding outputs a nearly constant voltage despite significant
variations in supply (primary winding) voltage. Normally, core
saturation in a transformer results in distortion of the sinewave shape,
and the ferroresonant transformer is no exception. To combat this side
effect, ferroresonant transformers have an auxiliary secondary winding
paralleled with one or more capacitors, forming a resonant circuit tuned
to the power supply frequency. This "tank circuit" serves as a filter to
reject harmonics created by the core saturation, and provides the added
benefit of storing energy in the form of AC oscillations, which is
available for sustaining output winding voltage for brief periods of
input voltage loss (milliseconds' worth of time, but certainly better
than nothing).
In addition to blocking harmonics created
by the saturated core, this resonant circuit also "filters out" harmonic
frequencies generated by nonlinear (switching) loads in the secondary
winding circuit and any harmonics present in the source voltage,
providing "clean" power to the load.
Ferroresonant transformers offer several
features useful in AC power conditioning: constant output voltage given
substantial variations in input voltage, harmonic filtering between the
power source and the load, and the ability to "ride through" brief
losses in power by keeping a reserve of energy in its resonant tank
circuit. These transformers are also highly tolerant of excessive
loading and transient (momentary) voltage surges. They are so tolerant,
in fact, that some may be briefly paralleled with unsynchronized AC
power sources, allowing a load to be switched from one source of power
to another in a "make-before-break" fashion with no interruption of
power on the secondary side!
Unfortunately, these devices have equally
noteworthy disadvantages: they waste a lot of energy (due to hysteresis
losses in the saturated core), generating significant heat in the
process, and are intolerant of frequency variations, which means they
don't work very well when powered by small engine-driven generators
having poor speed regulation. Voltages produced in the resonant
winding/capacitor circuit tend to be very high, necessitating expensive
capacitors and presenting the service technician with very dangerous
working voltages. Some applications, though, may prioritize the
ferroresonant transformer's advantages over its disadvantages.
Semiconductor circuits exist to "condition" AC power as an alternative
to ferroresonant devices, but none can compete with this transformer in
terms of sheer simplicity.
- REVIEW:
- Voltage regulation
is the measure of how well a power transformer can maintain constant
secondary voltage given a constant primary voltage and wide variance
in load current. The lower the percentage (closer to zero), the more
stable the secondary voltage and the better the regulation it will
provide.
- A ferroresonant transformer is
a special transformer designed to regulate voltage at a stable level
despite wide variation in input voltage.
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