Hi there,
For a slightly inductive load the rise of current would be a bit slower, but geeze
for a somewhat capacitive load the current would shoot way up until the capacitive
part became charged. Lucky most loads are not capacitive but i guess power
supplies can look like that during turn on.
Also, turning on during the zero crossing means less switching losses assuming
some output current, because the switching loss is higher when the output
power has to be switched...at zero crossing the output power is zero and starts to
actually turn on at fairly low voltage like maybe 2v or 3v which means less power
dissipated during the switching time.
The IE curve during turn on for a device turning on with higher voltage can be
roughly approximated as a ramp, where the voltage across the device ramps down
and the current through the device ramps up. If you look at the power dissipated
during this time (I*E) it can be very high for that short time interval because of
the high E and high I somewhere within that switching interval and lasting for
some time. With E=0 (or very low like E=3v instead of E=170v) the switching
loss has to be much much lower meaning the device stays cooler.
Luckily here the switching frequency itself is usually low like 120Hz.
Also as many others have already mentioned, the radiated power is higher too
because of the steep and high turn on ramps (both E and I) which can actually
interfere with other devices. The steep rise or fall of the waveform contains high
harmonic frequencies (Fourier) and these frequencies propagate wherever they
feel like it
and sometimes end up being picked up by nearby appliances.
Even given the various reasons for zero cross turn on, many dimmer controls do
not use that technique because it would not allow them to be able to effectively
lower the total power getting to the appliance being controlled. The dimmer
has to be able to turn on mid cycle so that it can regulate or at least lower the
power getting to the appliance. Zero cross turn on is a good idea for things like
solid state relays where the appliance only has to turn on and turn off, and so
many of those relays have this feature already built in.