Hi, I have my LOGO controlling an HVAC system installed on a maxed out electrical system. I have current transducers installed on the main feeders, and I want the LOGO to monitor the power consumption, and turn off/on HVAC load to keep the system from overloading when other loads are turned on and off.

The 2 current transducers are 4-20mA wired to an analog module, and the HVAC is controlled via a thermostat wired to discrete inputs and relay outputs. I have the HVAC control working good, just struggling on how to set up a load shedding program.

Can anyone point me in the right direction in terms of logic?

Thanks!

Hello AutoGen_9135950,

What are two current transformers for ?

The maximum load of the HVAC must be known. This is subtracted from the maximum load of the power grid and then has the switching point of the maximum load from the other devices. So you only need a current transformer (for the other devices).

Ella_68

Thanks for the replies,

Heres some extra info. The HVAC system consists of 4 x 750W heaters (controlled via 4 outputs) and 1 x 2000W A/C installed in a trailer, with a 50A, 120/240V split phase supply. If I have access to a 50A 120/240V connection, there is no problem with overloading. Some locations, however, only offer a 30A 120V supply.

There are 2 relays installed to tell the LOGO what type of power source is connected.

The CTs are installed on each phase feeder. If connected to 120/240v supply, they will monitor each phase separately. If connected to a 120V supply, the CT current values will be added in the program.

Is there a way to shed the heating loads in stages to stay below the max power available but still maintain as much heating as possible?

Thanks again for the help!

It should work with some solid state thyristor controllers for stepless power control of heaters. They are also available with a 0-10V control.

But solid state thyristors never disconnect 100% from the mains. When working on the electrical devices, they must be completely disconnected from the mains.

Ella_68

The problem is that when the load is shed, the current drops, which in turn reconnects the overload when it is automatically switched on.The result would be a constant alternation between dropping out and turning on.

Ella_68