Un-grounded protection design

Hi Brandon, sounds like there's several different challenges to overcome there.

A varister for surge suppression does not require a ground so yes you can connect it between live and neutral as described, I've not used one for a while but that's how I think it's normally wired. That should protect from spikes.

I'm wondering if the best approach for handling brownouts / variable supply voltages is to use a switch mode power supply rather than a linear regulator. If you are not confident in designing your own then you could use an example circuit from one of the chip providers or even buy in a module to attach to your design.

If your supply is unreliable the other thing you might need to concider is if frequency variations will affect your design.

You should be able to find a lot of useful inform by looking UPS designs

Here's a few links to get you started:

Protecting the Protector: Adding Circuit Protection to UPSs | Passive Components content from Power Electronics

An Overview of Power Protection

Brownout Protector

Thanks for the information Andy! And as far as the brownouts go, we would like for our machine to shut off during periods of insufficient power to prevent damage to the machine. In our testing, we found that not enough power would cause damage, and the region in which our product is shipped is notorious for these conditions at sporadic times and of various length. We would like to prevent our customers the hassle of frequent returns and repairs.

Our current board design features minimal components, all passive, to control the device. There is no DC involved. Everything is wall power. Would a "brown out detector" need to use active components? I am not as familiar with these in AC electronics as I am in DC electronics, but I am certain there is a way to achieve this.   

Perhaps a transformer and relay could act as a brown out shut off. Once the voltage drops below a certain level then the relay would drop out. You'd need to match your transformer and relay well (or perhaps add a few other components)

Great answers Workshopshed

Another solution is what is called a line stabilizer. Basically it is a 1 to 1 isolation transformer that also senses when the line voltage drops below  certain thresholds

and uses a relays to kick windings in (or out ?) to change the primary to secondary winding ratio and bring the voltage on the secondary back up. Normally it takes

takes less than 1/2 cycle (8.33 mS @ 60 Hz) for this to occur so the power supply it is attached to needs to be able to maintain its output for at least this long with

no AC input voltage.

This is also used in Uninterruptable Power Supplies and is usually called Automatic Voltage Regulation.

Thank you all for the advice!

I should specify further. In short, we are controlling a Universal 2-stage Motor at one speed (full power) and a smaller universal motor, again at only 1 speed.  Our testing showed insufficient power damaged these motors. Will the 1:1 isolation transformer still be a suitable solution? This product will be running on 240v @ 50Hz.

An isolation transformer is different from a line conditioner/stabilizer in that they do not adjust the output voltage, they simply use a transformer to isolate the AC input

from the AC output and this will not stop a brownout. A line conditioner/stabilizer made for 240v @ 50Hz (they should be available) may help as long as the motors can

handle a brief (~ 1/2 to 1 cycle) voltage drop.