ABB ACH400 Series Retrofit Manual page 13

 Mount the ACH550 into the mounting channel by hooking the
bottom mounting slots over the bolts and then tilting the top of
the drive back into the channel. Lift the drive slightly to slip the
top keyhole slots over the mounting screws, and then lower the
drive fully.
After the drive is in place, tighten all 4 mounting screws.

R4 Frame Size:
 Attach the top adapter plate to the back panel (using the two
mounting screws that you removed) as shown. The studs will be at
the bottom of the adapter plate.
 The two bottom mounting bolts should still be in place in the
existing bottom mounting holes.
1