By the way, you can't build an inverter with SCRs. An SCR can only be
turned on, not off, on command. Once you turn it on, it will not shut
off until the current through it goes to zero. For building an
inverter, you want power devices that can switch on _and_ off in the
kHz range in order to PWM all the phases you are trying to drive. SCRs
are fine for dimmers and heaters, and if you look at designs for those
you will see that they rely on triggering the SCR at a variable phase
angle, but it is always variable moving back from the zero crossing.

So you want to be using IGBTs probably, although you can also do it
with FETs. As other people have mentioned, there are _lots_ of details
to take care of in this seemingly simple system. Try building a good
variable speed DC motor controller first, since that is sort of like
1/3 of what you are eventually going to get to.

-Holly