Variable Solid state relays?

[Tony]

Many simple low cost motor speed controllers and light dimmers have all the internal control electronics connected directly to the mains supply.
That means the control potentiometer and all the circuitry is "alive" inside, and must be enclosed in an insulated plastic box, and the potentiometer shaft must be insulated with a plastic knob for safety.
Quite often even the potentiometer shaft itself is plastic in these units.

No problem with that if it is used strictly as intended for controlling only one or two motors.

What you CANNOT do is disconnect the potentiometer and extend wires to the PTS automatic test pressure controller.
It would not work like that anyway, it would blow up the pressure controller for sure, and it would be extremely dangerous.

The PTS recommended random fire SCR power modules are internally optically isolated from the mains supply, and are completely mains isolated and safe.
The 24v transformer is to both supply power to the fully mains isolated control electronics, and provide a 50/60Hz timing reference.

This is a much more sophisticated and much safer way to go about it, and it will give much smoother motor control than the cheapy non isolated made in China controllers, as well as being much more robust and reliable.

It can also easily be sized upwards and rated powerful enough to control all the motors together in even the biggest and baddest flow bench.

[Hotz]

Tony great lesson, lots of information techniques difficult for me to understand, I'm not stupid only difficulty with some words English, his explanation was very good! thank you very much

[bubba2400]

I am going to pull this old thread back up. How would you wire this if you have 240 volt vacuum motors? With the relay only having two terminals for the load is it just varying the one leg of 120 volt power to the motor? I am just not seeing how to do this.

[Brucepts]

Correct one line in and out, the other leg of the 220 goes to the other side of the motors.