Brochure

ESG
8.558.55
8.558.55
8.55
At the instant turn-on, transformer current is essentially
zero, with the highest peak usually occurring within a
half cycle, depending on the line phase angle, load
power factor, and magnetic state of the core. When the
SSR is energized at the ideal phase angle, as dictated by
power factor, a maximum back EMF is generated that
will tend to counter the magnetising current, thereby
reducing or eliminating the surge.
However, when switched on at, or near, zero voltage, the
back EMF is reduced, allowing an increase in magneti-
zing current that can be further enhanced by residual
magnetism in the core which almost always exists since
ferromagnetic core material has a natural tendency to
remain magnetized at turn-off.
If a random turn-on SSR is used to switch transformer
load, the likelihood of transformer core saturation is
greatly reduced.
Switching
Dynamic loads, such as motors and solenoids, etc., can
create special problems for SSRs, in addition to those
discussed for passive inductors. High initial surge current
is drawn because their stationary impedance is usually
very low. For example, after the initial surge, a solenoid
core will pull in and "seal" at a much lower steady-state
current, possibly by dropping to less than 25%. With
motors, the change in current from stall to run can be
even greater, possibly dropping to less than 20%,
depending on the type.
As a motor rotor rotates, it develops a back EMF that
reduces the flow of current. This same back EMF can
also add to the applied line voltage and create "overvol-
tage" conditions during turn-off. Mechanical loads with a
high starting torque or high inertia, such as fans and
flywheels, will, of course, prolong the start-up surge
period, which should be taken into account when selec-
ting the driving SSR. When the mechanical load is
unknown, as may be the case with a power tool, worst
case conditions should apply.
The inrush current characteristic of tungsten filament
(incandescent) lamps is somewhat similar to the surge
characteristic of the thyristors used in ac SSR outputs,
making them a good match. The typical ten times
steady-state ratings which apply to both parameters
from a cold start allow many SSRs to switch lamps with
current ratings close to their own steady-state ratings.
Some lamps have even higher instantaneous inrush
currents. This is rarely seen in practice, since line and
source impedances and filament inductance become
significant at higher currents, all of which tend to limit
the peak current. Generally the ten times steady-state
rating is considered a safe number for lamps.
Protective measures
Electromagnetic compatibility
Noise, or more properly defined as Electromagnetic
Interference (EMI), does not generally cause SSRs to fail
catastrophically. Some of the techniques used to reduce
noise in the coupler and drive circuits are also effective
against false triggering caused by voltage transients on
the input lines. When a capacitor is added, for example,
the response time which is not critical for ac SSRs may
be lengthened, possibly from a few microseconds to
tenths of milliseconds. Due to the induced delay, voltage
transients or bursts of shorter duration are rejected, thus
improving noise immunity.
Most ac SSRs use thyristors in their drive and output
circuits which, due to their regenerative nature, can latch
on for a whole half cycle when triggered by a brief
voltage transient, thus acting as a pulse stretcher. In
addition to responding to the amplitude of the transient,
a thyristor can also mistrigger when the rate of rise (dv/
dt) of a transient or applied voltage exceeds certain
limits. Transient suppressors are effective against the
former, and the RC snubber improves the tolerance of
an SSR to the latter.
du/dt (Rate effect)
The expression du/dt defines a rising voltage versus
time expressed in volts per microsecond (V/μS). When
applied to an ac SSR as "static" or "off state" du/dt, it is a
parameter that defines the minimum dv/dt withstand
capability of the SSR or, in other words, the maximum
allowable rate of rise of voltage across the output
terminals that will not turn on the SSR (typically 500 V/
μs).
Snubber
The internal RC network (snubber) used in ac SSRs is a
major factor in transient voltage and dv/dt suppression.
It deals effectively with two facets of a voltage transient.
Not only does the network slow down the rate of rise as
seen by the output thyristors and sensitive drive circuits,
but it also limits the amplitude to which it can rise. While
the typical internal snubber value and the typical dv/dt
specification are adequate for most applications, they
may not prevent what is commonly referred to as the
"blip" or "bleep" problem which occurs during start-up.
That is, when power is initially applied to the SSR/load
combination usually by means of a mechanical switch,
the resultant fast rising transient may mistrigger the SSR
and possibly "let through" a half cycle pulse, fortunately,
most loads are not troubled by this pulse.