PLC + VFD + HMI bundle: how it works
Most industrial automation systems consist of three components: PLC (logic), variable frequency drive (power section), and HMI panel (operator interface). These three devices, connected via Modbus or Ethernet, form a complete control system for motors, pumps, fans — anything with an electric drive.
We assemble such systems weekly. Typical configuration: Siemens S7-1200 or Delta DVP as the brain, Delta VFD or Sinamics for motor control, and an HMI panel for the operator.
System architecture: who does what
| Component | Role | Example |
|---|---|---|
| PLC | Logic, interlocks, PID, communication | S7-1200, Delta DVP-SA2, FX5U |
| VFD | Motor speed control | Delta VFD-E, Sinamics V20 |
| HMI | Display, setpoints, alarms | Delta DOP-107, Siemens KTP700 |
The PLC is the system center. It reads sensors, calculates logic, and sends commands to the VFD. HMI is the operator window.
PLC-VFD communication: Modbus RTU standard
The most common PLC-to-VFD link is Modbus RTU over RS-485. One PLC can poll 5-20 VFDs on a single bus.
- Physical connection: RS-485 two-wire line (A+, B-) from PLC to all VFDs
- VFD setup: set Modbus address, speed (9600/19200), parity
- PLC programming: write frequency to VFD register, read current and status
More on protocols: Modbus RTU and TCP for PLCs.
PLC-HMI communication: Ethernet or RS-232
- Ethernet (recommended): fast, reliable. Siemens S7-1200 + KTP700 out of the box
- RS-232: for Delta DVP + DOP standard link
- RS-485: HMI as Modbus device, slower
Practical example: pump station
Task: control three pumps maintaining 3.5 bar. Equipment:
- PLC: Delta DVP-SA2
- VFDs: 3 x Delta VFD-E 2.2 kW
- HMI: Delta DOP-107WV
- Pressure sensor: 0-10 bar, 4-20 mA
Logic: PLC reads the pressure sensor, calculates PID, outputs frequency to the first VFD. If one pump cannot keep up — second one starts. Rotation — daily.
Common mistakes
- Mismatched Modbus speed: check settings on EVERY device
- Missing terminator: 120 ohm on last RS-485 device
- Cable too long: issues start after 500 m
- One COM port for everything: HMI on RS-232, VFDs on RS-485 — different ports
Frequently Asked Questions
How many VFDs can connect to one PLC?
Via Modbus RTU — up to 247 on one RS-485 bus. In practice, stable operation up to 16-20 devices.
Is an HMI panel mandatory?
No, but highly recommended. Without HMI, the operator cannot see parameters or change setpoints without a laptop.
What cable for RS-485?
Shielded twisted pair (STP), 0.5-0.75 mm2 cross-section. Ground shield on one end only.
Can a VFD be controlled without a PLC?
Yes, via built-in keypad or potentiometer. But then there is no automation: PID, interlocks, pump rotation.
How much does a PLC+VFD+HMI system cost?
For one 2.2 kW motor: $800-1,200. For three 5.5 kW pumps: $3,000-5,000.
Summary
PLC + VFD + HMI is the building block of industrial automation. Modbus RTU over RS-485 connects everything simply and reliably.
Select components: Delta PLCs, Siemens PLCs, HMI panels.
