1. The core of a pump VFD panel: VFD + pressure transmitter + PLC All 3 are essential
The 3 core components of a pump VFD panel:
- VFD: adjusts pump speed, changing frequency based on the pressure signal
- Pressure transmitter: installed at the end of the pipe network (or on the main header), 4–20 mA output, feeds back actual pressure
- PLC (or the VFD's built-in PID): receives the pressure signal, compares it with the setpoint, and outputs a frequency command to the VFD
For simple configurations, you canuse only the VFD's built-in PID(no external PLC needed), which costs less; complex configurations (multi-pump coordination / duty/standby) require an external PLC.
2. Single-pump VFD control schematic The simplest configuration
Single-pump VFD circuit:
- Power supply: three-phase 380V incoming line
- Main circuit: breaker → VFD → contactor (bypass, runs directly on line power if the VFD faults) → motor
- Control circuit: PLC/controller outputs 0–10V or 4–20mA as the frequency command to the VFD
- Feedback circuit: pressure transmitter 4–20mA wired back to the PLC
This is themost basic configurationSuitable for a single pump where pressure demand stays fairly stable.
3. How Constant Pressure Water Supply Works: VFD + PID Feedback Core formula + commissioning points
Constant pressure water supply PID control logic:
- P (proportional): pressure error × Kp = frequency adjustment; fast response but leaves steady-state error
- I (integral): accumulated pressure error × Ki; eliminates steady-state error
- D (derivative): rate of pressure change × Kd; suppresses overshoot
Typical VFD built-in PID parameter values from field experience:Kp = 0.5–1.0,Ki = 0.3–0.5 s,Kd = 0(Pumps generally do not use the D term — it tends to cause oscillation.)
When tuning PID on site, first turn off the integral term(Ki = 0) and use P only. Increase Kp from small to large until pressure holds near the setpoint without overshooting; then add Ki gradually until there is no steady-state error. If Kp is set too high from the start, pressure will oscillate repeatedly and the VFD will trip on overcurrent protection again and again — start low, raise it, and only add more once it is stable.
4. How Is Automatic Duty/Standby Rotation Implemented? Interlock + timer
Domestic water pumps and fire pumps commonly use a "duty/standby" scheme, where the standby pump starts automatically if the duty pump fails. A typical approach from Zhejiang Golden Unicorn:
- Hardware interlock: KM1 (duty pump) and KM2 (standby) are mechanically interlocked to prevent both from starting at the same time
- Control logic: a timer inside the PLC automatically switches duty/standby every 24 h or 1000 h; on a fault, KM1 trips and KM2 starts automatically
- Signal feedback: each pump is fitted with a flow switch / pressure switch that feeds back a "running normally" signal
Typical configuration example for a duty/standby cabinet (to be confirmed per project):
| Component | Model | Quantity |
|---|---|---|
| VFD | Schneider ATV320 / ABB ACS580 | 1 unit |
| Contactor | Schneider LC1-D series | 2 pcs |
| Circuit breaker | NSX series molded case | 2 pcs |
| Thermal overload relay | LRD series | 2 pcs |
| Pressure transmitter | 4–20 mA output, 0–1 MPa | 1 pc |
| PLC | Siemens S7-200 SMART | 1 set |
5. Multi-Pump Staging Control Logic (2 duty + 1 standby / 3 duty + 1 standby) Standard configuration for high-flow applications
Common in large water plants, high-rise residential water supply, and commercial complexesMulti-pump staging. Common configurations:
| Configuration | Pumps | Operating logic | Typical flow |
|---|---|---|---|
| 2 duty + 1 standby | 3 pumps | Pump 1 on VFD + Pump 2 on line power; high pressure → Pump 1 ramps down; low pressure → Pump 2 on line power; Pump 3 standby | 30–100 m³/h |
| 3 duty + 1 standby | 4 pumps | VFD rotation, 1 lead + 2 lag + 1 standby | 100–500 m³/h |
| All-VFD staging | N pumps, all on VFDs | Large pressure differential; multiple pumps run in parallel on VFDs, with automatic pump staging | 500+ m³/h |
Typical multi-pump staging panel configuration: N VFDs + N-1 soft starter / line-power bypass units + 1 PLC + 1 HMI touchscreen + 1 pressure transmitter + 1 flow meter (optional).
6. Factory Photos: 4 Shots of Pump Control Cabinet Assembly (shop floor photos)
FAQ FAQ
What causes large pressure fluctuations in constant pressure water supply?
Three common causes: ①PID parameters not tuned properly(Kp too high or Ki too low) ②Pressure transmitter in the wrong location(it should be installed at the far end of the pipe network, not at the pump outlet) ③Air in the pipe network(Exhaust valve not open). Check the signal first, then adjust parameters, and finally check for air lock.
Does a VFD pump panel always need a PLC?
A single pump can do without one(the VFD's built-in PID is sufficient);Multi-pump coordination / duty-standby setups must use a PLC. The main structural differences between the two are the number of contactors, the interlock logic, and the amount of programming — specific configuration and pricing are calculated based on the number of pumps and bypass requirements. Send us the drawings and we can quote.
How long is the auto-rotation cycle for duty-standby operation?
Rule of thumb: switch every 24h or 1,000 cumulative operating hours. For fire pumps, follow the code: inspect once a week, switch once a month (Code for Fire Protection Water Supply and Hydrant Systems, GB 50974-2014). The Golden Unicorn PLC program defaults to dual 24h + 1,000h logic.
How much energy can a VFD pump save?
20–50%, it depends on the actual operating conditions. The greater the variation in flow/pressure demand, the higher the energy savings.At a speed ratio of 0.7, actual power is only 0.34 of line-frequency power (cubic relationship). Golden Unicorn has run field measurements for multiple water plants and commercial complexes, with average energy savings of 30–40%.
What's the difference between a fire pump control panel and a standard pump panel?
Fire pumps must have dual power sources + mechanical emergency start(mandatory per GB 50974-2014). Specifically: ① Dual power source transfer (utility + generator) ② Mechanical emergency pump start (direct contactor, not dependent on PLC) ③ Inspection function (weekly self-test) ④ Start time ≤ 30s. Golden Unicorn fire pump panels include the full set and pass code inspection.
Related pages:Technical notes · Products · Industries · Resource center · FAQ · Get a quote