A Quick Technical Tip: Motor Control in Retrofit Jobs

On a recent project, we took control of motors that were not previously part of a PLC-based automation system. These sites generally have a pushbutton control panel. Typically, the control panel has pilot lights and relays on the back plate, which control the process equipment. As a result of automating the facilities, we learned some important lessons and best practices have been developed to help the design become standardized.

Full Voltage Non-Reversing (FVNR)

Full Voltage Non-Reversing (FVNR) is the most common type of starter. Figure 1 illustrates a common FVNR control setup in plants without an automated control system. The START/STOP button and RUN pilot light are located on the door of the control panel. While the RLY is located on the back plate inside the control panel. Figure 2 illustrates a standard FVNR controlled from a PLC. In this case, the relay/pushbutton combination is removed and replaced with the PLC run output. Additionally, the RUN pilot light wire is re-routed to a PLC input.

Figure 1. FVNR Pushbutton Control                           Figure 2. FVNR PLC Control


Control Power Transformer (CPT)

You may also encounter a control power transformer (CPT) installed in the motor starter to supply the 120VAC power to energize the contactor. This set up is beneficial because you don’t have to be concerned about over-stressing a 120VAC breaker or fuse in the control panel since the 120VAC motor control power is coming from the 480VAC feeder. Below, Figure 3 shows a general FVNR starter with a CPT installed. While Figure 4 shows how this type of starter would be automated with a PLC.

Figure 3. FVNR w/ CPT Pushbutton Control              Figure 4. FVNR w/ CPT PLC Control

Interposing relays, as shown in Figure 4, are necessary when using CPT’s and can be used with separate source power to allow the control circuit a higher starting current that cannot be supplied from a typical output. This is common for starters of NEMA size 3, 4, and 5. There are many other wiring variations of starters, including field START/STOP pushbuttons, HOA switches, interlocked lid/plug switches, reversing starters, and auto-transformer starters.

Each of these options can work for you with some minor adjustments according to the application.


Shane Heier, Interstates Project Engineer