Wondering if there is a device that can be used with a PLC's analog signal, that can "vary" the voltage input between 0-120VAC. Goal is to slowly (set time frame) raise from 0 to 120VAC.

Input voltage of 120VAC > "variable voltage controller" > input voltage between 0-120VAC into step-up transformer (based PLC analog signal)

Can this be achieved with a motorized 15A (0-120VAC) variable transformer? What can I use as my voltage feedback at step-up transformer secondary (meter)? How do I make sure motor its always zeroed before each start?

 

What is the load?

 

Can this be achieved with a motorized 15A (0-120VAC) variable transformer?

Yes

What can I use as my voltage feedback at step-up transformer secondary (meter)?

I’d use a pot for positional feedback. It won’t be the actual secondary voltage, but with calibration, be very close

How do I make sure motor its always zeroed before each start?

You drive it there, typically onto a micro switch, as ‘home’.

Is it the best choice? Depends on your resources

 

A DIY sine wave inverter based on EG8010 might do the job.

 

Wondering if there is a device that can be used with a PLC's analog signal, that can "vary" the voltage input between 0-120VAC. Goal is to slowly (set time frame) raise from 0 to 120VAC.

Input voltage of 120VAC > "variable voltage controller" > input voltage between 0-120VAC into step-up transformer (based PLC analog signal)

Can this be achieved with a motorized 15A (0-120VAC) variable transformer? What can I use as my voltage feedback at step-up transformer secondary (meter)? How do I make sure motor its always zeroed before each start?

This is not at all clear. Which voltage do you want to vary? The input to the PLC analog input?? I have not seen a PLC with an AC analog input, EVER!
What I am guessing is that the PLC is intended to control the motor driven VVT that feeds a transformer delivering some higher voltage, and that the PLC is supposed to use it's analog input of a scaled, rectified, and filtered version of the transformer output to close the feedback loop controlling the voltage. This is my guess.
The simple part of this system is the returning the VVT to the zero position before starting. That can be done with a limit switch tied to the VVT shaft, and verified by the anaolg input voltage.

 

Do you have the motorized variac at the moment?
Otherwise a standard Variac made motorized with a suitable stepper motor, use a L.S. to set zero, the PLC would require a analogue input module and a Stepper motor controller or set simple pulse and dir outputs if the rotation speed is not required to be at all high.