I was reading in here last week an can't find the thread , anyhow someones niece had died in a pool accident and I feel so bad for the family , anyhow what we have been using on existing commercial pools with spas is called a Stingl unit.
The Link below has two types and they work , I think we have gone back an Installed at least 25 of these units and when motors are 3 phase or more than 2 HP single phase 240 v you need to set up a contactor after the motor starter using 120 v coil , either 2 pole or 3 pole , the wiring is pretty simple once you figure it out an Install one.
Click on the SR500 and you can read on how it operates with the motor an vacuum in a pool.
http://www.inyopools.com/safety_stingl_vacuum_release.aspx
The Link below has two types and they work , I think we have gone back an Installed at least 25 of these units and when motors are 3 phase or more than 2 HP single phase 240 v you need to set up a contactor after the motor starter using 120 v coil , either 2 pole or 3 pole , the wiring is pretty simple once you figure it out an Install one.
Click on the SR500 and you can read on how it operates with the motor an vacuum in a pool.
http://www.inyopools.com/safety_stingl_vacuum_release.aspx