Water pump inverter sizing often begins with a message that is too short: “The motor is 7.5 kW. Please quote a VFD.” Nominal power does not reveal the real pump duty. Current, voltage, temperature, cable length, feedback and duty point can change the decision.
For contractors, a wrong selection can cause overload trips, unstable pressure and site visits. A reliable selection needs two checks: prove that the pump can perform the hydraulic job, then prove that the inverter can operate the motor under actual site conditions. The nine checks below apply mainly to booster and building water-supply projects using BEDFORD W713B or W713.
Table of Contents
- Check 1: define the hydraulic duty point
- Check 2: read the complete motor nameplate
- Check 3: size by output current, not kW alone
- Check 4: verify the site power supply
- Check 5: account for temperature and altitude
- Check 6: treat long motor cables as a design issue
- Check 7: match the feedback and control method
- Check 8: define single-pump or multi-pump operation
- Check 9: prove the selection during commissioning
- Contractor inquiry worksheet
- W713B, W713 or W191 selection direction
Check 1: Start Water Pump Inverter Sizing With the Hydraulic Duty Point
Before selecting a drive, record the required flow, total head and acceptable operating range. A useful first estimate is:
Total required head = static head + friction loss + required residual pressure
Static head comes from elevation or vessel pressure. Friction loss changes with flow and pipe conditions. Residual pressure is what the critical outlet still needs. The U.S. Department of Energy’s Variable Speed Pumping guide explains that the ratio of static head to friction head influences the benefit of speed control. A friction-dominated circulation loop and a high-static-head transfer system should not be evaluated in the same way.
Obtain the pump curve and mark peak, normal and minimum operating points. The DOE Pump Systems resource center treats the pump, motor, controls and pipe network as one system, preventing selection without a hydraulic check.
| Hydraulic input | What the contractor should record | Why it affects the inverter decision |
|---|---|---|
| Peak flow | m3/h or L/min | Shows the highest required pump output |
| Static head | Vertical lift or fixed pressure | Determines the minimum head that speed control cannot remove |
| Residual pressure | Required at the critical outlet | Prevents an unrealistically low pressure setpoint |
| Pump curve | Head, flow, efficiency and power | Confirms that the pump can reach the duty point |
Check 2: Read the Complete Motor Nameplate
Photograph the motor nameplate instead of copying only the kW value. Record:
- Rated power in kW or HP
- Rated voltage
- Rated current
- Number of phases
- Rated frequency
- Rated speed, motor type and duty
Motors with the same power can have different full-load current because of voltage, efficiency and design. The inverter’s continuous output current must cover the actual motor. The W713/W713B manual also identifies an incorrect motor-current setting as a possible cause of motor overload protection.
Check 3: Size by Output Current, Not kW Alone
Use rated output current as the pass-or-fail value. Nominal kW is only a catalog reference.
For example, the W713/W713B manual lists the 380 V W713(B)-4010 at 7.5 kW with a rated output current of 18.5 A. Consider two hypothetical 7.5 kW motors:
| Item | Motor A | Motor B |
|---|---|---|
| Motor power | 7.5 kW | 7.5 kW |
| Nameplate current | 16.2 A | 19.2 A |
| Candidate drive output current | 18.5 A | 18.5 A |
| Initial current check | Passes on paper | Does not pass |
Both motors say 7.5 kW, but only Motor A is below the candidate drive’s nominal current. Motor B needs a different verified selection. If the pump can overload, the grid is weak or derating applies, send the complete data to the supplier instead of adding an arbitrary margin.
IEC 61800-2 covers rating specifications for low-voltage adjustable-speed AC drive systems. Its practical meaning for buyers is that input rating, output rating and operating conditions are separate information; a kW label is not a complete specification.
Check 4: Verify the Site Power Supply
Compare the real supply with both motor and inverter. Record voltage, phase, frequency, measured voltage during pump operation, generator use and known phase imbalance. Do not confuse input and output: a single-phase-input model can supply a three-phase motor only when the exact specification supports it. The electrician must also verify breakers, cables, grounding and isolation against the latest manual and local rules. For generator projects, test voltage and frequency while the pump accelerates, not only at no load.
Check 5: Account for Temperature and Altitude
Catalog current is not automatically available in every enclosure and climate. According to the W713/W713B manual version V1.0.0, the stated ambient range is -10 to 40 degrees C. Above 40 degrees C, the controller is to be derated by 4% for each additional degree, with the maximum ambient kept below 50 degrees C. The same manual states a 1% derating for each 100 m above 1,000 m altitude.
At 45 degrees C, the manual’s rule represents a 20% derating. As an initial screening calculation, 18.5 A multiplied by 0.80 is 14.8 A, which no longer covers the hypothetical 16.2 A motor. BEDFORD should confirm the final choice, especially when temperature and altitude derating apply together.
Record:
- Maximum air temperature inside the final cabinet, not only outdoor weather
- Ventilation path and clearance around the drive
- Direct sunlight, dust, moisture and corrosive conditions
- Installation altitude
- Heat from adjacent equipment
A sealed, sun-heated cabinet can turn a correct catalog selection into an undersized site installation.
Check 6: Treat Long Motor Cables as a Design Issue
Long motor cables, common in deep-well projects, can increase reflected-wave voltage stress, interference and leakage current. ABB’s Technical Guide No. 102 shows why cable recommendations depend on motor voltage withstand, drive size and the use of an output reactor or dv/dt filter.
Provide the actual motor-cable length before BEDFORD confirms the model. Ask whether an output reactor, filter, shielded cable or revised carrier frequency is required. Keep transmitter and communication cables away from power cables and follow the manual’s grounding instructions.
Check 7: Match the Feedback and Control Method
A booster set may control discharge pressure, an HVAC loop differential pressure, and a tank-filling system level. Match the transmitter range, output signal, power supply and wiring to the controller input. An excessively wide pressure range can reduce useful feedback resolution.
Before tuning PID, verify pump rotation, valves, air removal, check valve, pressure vessel and sensor location. Then use the BEDFORD pump inverter PID settings guide for the response adjustment.
Check 8: Define Single-Pump or Multi-Pump Operation
For multiple pumps, describe the sequence, not just the quantity. The W713/W713B manual describes linkage for up to six pumps, automatic alternation and several modes, subject to model, wiring and cabinet design. Agree on:
- Number and power of pumps
- Lead, lag and standby sequence
- Add-pump and remove-pump conditions
- Alternation schedule
- Feedback arrangement, manual mode and fault transfer
The Hydraulic Institute’s variable-speed pumping overview reinforces that selection, installation, control and operation all affect the result.
Check 9: Prove the Selection During Commissioning
A spreadsheet selection is provisional until the pump runs with water.
| Test | What to record | Acceptance question |
|---|---|---|
| Motor direction | Rotation and phase sequence | Is pump rotation correct before full operation? |
| Normal duty | Frequency, current, flow and pressure | Does current remain within the approved limit? |
| Low demand | Sleep and wake behavior | Does the pump stop and restart without short cycling? |
| Pressure step | Response after a valve opens or closes | Is recovery stable without large overshoot? |
| Fault simulation | Sensor loss, low level or permitted test fault | Does the system respond according to the agreed logic? |
| Multi-pump stage | Lead/lag transitions | Are pressure and current stable during pump changes? |
Record final parameters and measured current. This baseline helps distinguish a setting problem from a hydraulic, electrical or mechanical change. Use the BEDFORD overvoltage guide when a trip appears during stopping or restart.
Booster Pump and Deep-Well Pump Sizing Are Not the Same
The worksheet is shared, but risk priorities differ.
| Design question | Booster system | Deep-well system |
|---|---|---|
| Main hydraulic concern | Variable demand and stable discharge pressure | Lift, well recovery and available water level |
| Feedback | Usually discharge pressure | Pressure, level or process signal depending on design |
| Cable issue | Often moderate | Frequently a major design input |
| Low-flow and dry-run risk | Sleep, leakage, inlet shortage and cycling | Motor cooling, falling water level and well recovery |
Do not copy one parameter set to both systems. Equal motor power does not mean equal hydraulic duty or protection logic.
Contractor Inquiry Worksheet
Send this information with the quotation request:
| Field | Buyer input |
|---|---|
| Application | Booster, building supply, deep well, irrigation or treatment |
| Pump and duty point | Model, flow, head and absorbed power |
| Motor nameplate | Photo plus kW, V, A, Hz and rpm |
| Site supply | Voltage, phase, frequency and generator use |
| Site conditions | Altitude, maximum cabinet temperature and cable length |
| Pump quantity and sequence | |
| Sensor type and range | |
| Pressure or process target | |
| Required functions | PID, sleep, level, restart, multi-pump, communication |
| Local safety restriction | Automatic restart permitted or prohibited |
W713B, W713 or W191: A Practical Direction
BEDFORD W713B and BEDFORD W713 are the main directions for this topic. Select from the exact voltage and output-current table, then check feedback, protection and pump-control functions. W713 also covers a broader higher-power range.
W191 is for small-pump applications and should not be substituted into a large building or heavy commercial booster project.
The decision is not “Which model has the same kW?” It is “Which model has sufficient current after site conditions are considered, supports the control logic, and passes commissioning?”
Final Decision Rule
Good water pump inverter sizing follows a fixed order: prove the duty, read the nameplate, pass the current check, verify power, apply derating, review cable length, define control logic, then test under water load. The worksheet gives the contractor a defensible selection and gives BEDFORD enough information to confirm the exact model before delivery.