My reference to PLCs was for the various forms of protection when switching particular loads. PLCs are just as sensitive to power dips if they drop below their minimum input voltage. You could try looking at Allen Bradley and Telemechanique websites for their app notes.

You've confirmed you have a power dip problem - how much does the power dip by? It could be the impedance of your mains circuit is inadequate to cope with four motors starting at the same time. It could also be that you are execeding the maximum current allowable on the circuit - you would want to check this with your local electrical standards. Nevertheless, you need to come up with a minumum input voltage that you want your system to work at and work your low voltage supply from these numbers. You may find the input voltage to your linear reg will be too high that power disipation becomes a problem so then you need to go to a switching type regulator either a mains switcher or a low voltage dc/dc type switcher. Recently I've used products like this:

http://www.cutter.com.au/proddetai...rod=cut508

Just up your transformer secondary voltage along with your filter capacitor voltage and pop one of these little things in.