GGSTech ToolsThree common control architectures
Hardwired I/O
PLC DO → RUN; VFD relay/DO → RUN/FAULT feedback.
Analog speed reference
PLC AO → 4–20 mA or 0–10 V frequency/speed command.
Fieldbus
RUN, speed, current, fault codes and diagnostics over a supported industrial network.
Speed scaling example
If 4 mA = 0 Hz and 20 mA = 50 Hz, then 12 mA represents 25 Hz in a linear scale.
Command vs feedback
Monitor VFD ready, run, fault and actual speed/current where available. A RUN command should have a timeout; if run feedback does not arrive, raise a start-failure alarm rather than assuming the motor is operating.
Harmonics
A conventional 6-pulse VFD draws non-sinusoidal input current. Input THDi depends on drive loading, source impedance, DC-link design and any reactor/choke. THDv at the PCC also depends on system short-circuit strength and other loads; it cannot be calculated from drive kW alone.
| Mitigation | Purpose |
|---|---|
| AC line reactor / DC choke | Reduce current distortion and input current peaks; exact effect depends on system. |
| Passive harmonic filter | Target lower input current distortion over a defined load range. |
| AFE / low-harmonic drive | Actively controls input current waveform; typically higher cost/complexity. |
| AHF at PCC | Injects compensating harmonic current for a group of nonlinear loads; requires system study and rating. |
What you should understand first
The PLC decides when/how fast the process should run; the VFD performs motor voltage/frequency control. Keep the responsibility boundary clear and monitor the drive’s actual state.
Signal and energy boundaries
A PLC is a controller, not the power source for a motor. Separate the low-energy logic side from the field power circuit. Digital outputs often command an interposing relay, contactor coil, valve solenoid or VFD input. The motor power still passes through the MCC/MCCB/contactor/VFD power circuit.
Fail-safe thinking
For critical shutdowns, do not rely only on software logic. Emergency-stop, trip and safety functions may require dedicated safety-rated hardware and a risk assessment. A normal PLC output should not be assumed to provide a safety function unless the architecture is specifically designed and certified for it.
Common mistakes
- Switching contactors between VFD and motor while drive output is active unless specifically designed for it
- Ignoring minimum/maximum speed limits of the driven equipment
- Using analog reference without shield/grounding strategy
- Treating VFD output as a clean sine wave for long-cable insulation decisions
- Assuming fixed THDi/THDv without network data
Commissioning checklist
- Confirm VFD/motor ratings and grounding.
- Map every command and feedback bit/word.
- Scale analog or network speed reference with limits.
- Test communication-loss and PLC-loss behavior.
- Measure harmonics at the actual PCC if compliance/design requires it.
Frequently asked questions
Is 4–20 mA better than 0–10 V?
4–20 mA is generally more robust over distance and provides a live-zero concept, but the best choice depends on hardware and environment.
Can I place an overload relay after the VFD?
VFDs often provide electronic motor protection, but the required protection architecture depends on drive/motor/application and local requirements. Follow the drive manufacturer.
Does a VFD always eliminate starting current?
It can control acceleration and current much more effectively than DOL, but actual current depends on torque demand, ramp and control mode.