1. Pre-Commissioning Preparation (5 Checks)
Before power-on commissioning,5 checksmust be completed:
- Check wiring: Verify wire by wire against the wiring diagram. Focus on the VFD U/V/W output, the 4-20mA pressure sensor wiring, and the PLC communication cable.
- Measure insulation: Using a 500V megohmmeter, measure motor insulation to ground ≥0.5MΩ and VFD output terminal to ground ≥1MΩ.
- Set VFD parameters: Motor nameplate data (power, voltage, current, frequency, speed), control mode (V/F or vector), and command source (terminal/communication).
- Set pressure sensor range: Set the VFD AI channel to 4-20mA, corresponding to a range of 0-1.6MPa (per the sensor's rated value).
- Set target pressure: Set according to the piping network design pressure (e.g., 0.35MPa; floor height × 0.01 + margin 0.05~0.1MPa).
2. Basic VFD Parameter Settings (Motor Parameters + Control Mode)
Using a Siemens G120 as an example,parameters that must be set:
| Parameter No. | Name | Setting Value | What it should show |
|---|---|---|---|
| P0010 | Commissioning mode | 1 | Enter quick commissioning |
| P0304 | Motor rated voltage | 400V | Per motor nameplate |
| P0305 | Motor rated current | 42A | Per motor nameplate (22 kW) |
| P0307 | Motor rated power | 22kW | Per motor nameplate |
| P0310 | Motor rated frequency | 50Hz | Per motor nameplate |
| P0311 | Motor rated speed | 1480rpm | Per motor nameplate |
| P1300 | Control mode | 2 (V/F) | V/F mode for pumps and fans |
| P2200 | PID enable | 1 | Enable PID control |
3. PID Parameter Tuning (P first, then I, then D)
PID parameter tuning is the core of commissioning —a 3-step method:
- Step 1: Set P: with I=0 and D=0, start at P=0.5 and increase. Watch the VFD output frequency — after the pressure setpoint is entered, the VFD accelerates to chase the pressure. Once pressure reaches the setpoint, if itoscillates(frequency swings up and down), P is too high — back off 20%. If itnever catches up(frequency hits 50 Hz but pressure still does not reach the setpoint), P is too low — increase it. Typical value: P=2.0–5.0.
- Step 2: Set I: with P already tuned, start at I=30 s and decrease. The role of I is to eliminatesteady-state error— pressure settles slightly below the setpoint. Reduce I until the steady-state error is gone without overshoot. Typical value: I=5–15 s.
- Step 3: Set D: for constant pressure water supply, D=0 in most cases. Only if pump start/stop causes a shock, set D=0.5–1.0 s to suppress overshoot.
4. Pressure Setting and Feedback Calibration (target pressure + sensor calibration)
Target pressure formula:P_set = floor height (m) × 0.01 + pipe friction loss + margin. For example, a 6-story residential building: 6×3 m×0.01 + 0.05 + 0.05 = 0.28 MPa, so use 0.3 MPa.
① With the VFD stopped, read the AI value — it should be 4.00 mA (corresponding to 0 MPa); ② If it is not 4 mA → sensor zero offset; adjust the sensor zero potentiometer or the VFD AI offset parameter; ③ Close all water points, run the VFD until pressure is stable, read the sensor current value, and compare it with the pressure gauge reading — an error ≤1%FS is acceptable.
5. Common Commissioning Issues (5 typical symptoms)
① Pressure will not build→ Check pump selection (whether head/flow match), whether the piping has air leaks, and whether the VFD has reached 50 Hz; ② Pressure oscillation→ P value too high; reduce P or increase I; ③ Frequent pump start/stop→ Incorrect air tank precharge pressure (should be 0.7 times the set pressure), or leakage in the pipe network; ④ VFD reports overload→ Check for mechanical binding or bearing damage in the pump, or motor current exceeding rated value; ⑤ Pressure drops sharply when switching pumps→ The rotation switchover delay is too short; increase the switchover delay (5–10 s).
FAQ FAQ
How do I quickly set the PID parameters for a variable frequency constant pressure water supply system?
Rule of thumb: P = 2.0–5.0, I = 5–15 s, D = 0. Start with P = 3.0, I = 10 s, D = 0 and observe operation. Pressure oscillation → reduce P; pressure cannot reach setpoint → increase P or reduce I. Two to three rounds of fine-tuning on site are usually enough to stabilize.
Should I choose V/F mode or vector mode for the VFD?
For pumps and fans, we recommendV/F mode(P1300 = 2). V/F suits quadratic loads (variable torque loads) and automatically lowers voltage at low speed to save energy. Vector mode suits constant torque loads (such as conveyors and cranes); for pumps, vector mode actually reduces efficiency.
What target pressure should I set for constant pressure water supply?
Formula:P = floor height (m) × 0.01 MPa/m + piping resistance (0.05–0.1) + margin (0.05). A 6-story residential building is about 0.3 MPa, 12 stories about 0.5 MPa, and above 20 stories requires zoned water supply. In practice, set it according to the design drawing requirements.
Why is a delay needed when switching between variable frequency pumps?
At the moment of switchover, the VFD disconnects from one pump and connects to another, and needs time to stabilize. A 5–10 s delay lets the first pump come to a complete stop before the next pump starts, preventing both pumps from running at the same time and overpressurizing the pipe network. During the delay, the air tank maintains water supply.
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