As electrical motor control centers get larger to handle additional controls (i.e. VFD), the added weight on the steel back plain can cause additional flexing or vibration from the movement of the concrete batch plant.

A clever way to reduce this vibration is to fill the void between the mounting plate and the junction box with an expandable foam.

The TAP X-30 Polyurethane Foam 2-gal kit is a great two-part expandable foam that can be mixed in the field and poured into the void.  With a 30X expansion rating, very little foam is needed to fill the void with a very dense, vibration reducing material. 

Within a few seconds the foam begins to expand so you will want to have the project prepared and ready prior to mixing with a high-speed drill agitator.  Take the time to seal all the edges and bottom with high strength tape and reinforced cardboard or plastic.  On a typical 6ft wide by 5ft high back plain, I would suggest pouring in the foam in 3 lifts.  The worst thing that can happen is pouring in too much foam and have it expand beyond the back plain area.  After sealing the bottom and edges, drill a couple of 1.5” holes to pour the foam into the void (black tape in picture) via a funnel.  You can calculate how much will be needed by determining the square inch area per lift.  If foam does come out through a pin hole or other area, you can trim with a knife.