How We Select a QJ Submersible Well Pump for Generator-Powered Wells

Many agricultural, mining and remote water projects use a generator where grid power is unavailable. The main equipment is still the submersible well pump. The generator must be selected around the pump and motor.

At SLAPK, we begin with the water requirement and well conditions. After selecting the hydraulic pump, we confirm the motor, cable, starting method and generator requirements as one system.

Choosing a pump only because its motor matches an existing generator can result in insufficient flow, low pressure, motor overload or unstable starting.

Before discussing motor power, we confirm how much water the project needs and where it must be delivered.

The two main hydraulic values are:

• Required flow rate

• Total dynamic head

Total dynamic head is not the well depth. It includes lift from the pumping water level, discharge elevation, friction, fitting losses and required delivery pressure.

A pump installed 120 m below ground does not automatically need 120 m of head. The operating water level may be higher, while irrigation may require additional outlet pressure.

We select the pump from the point where the required flow and head occur together on the performance curve.

A suitable pump must fit the well and remain submerged during operation.

We ask for:

• Total well depth

• Minimum casing internal diameter

• Static water level

• Dynamic water level at a known flow

• Well yield

• Screen position

• Sand content

• Expected seasonal water-level change

If the dynamic level falls, the pump must lift water farther and flow may decrease as required head rises.

The pump should be installed below the lowest expected dynamic level with adequate submergence, but it should not be placed unnecessarily close to the well bottom or sediment zone.

Our QJ series is a multistage submersible well pump with cast hydraulic components. Different hydraulic models are designed for different flow ranges, while the number of stages is selected according to the required head.

We do not select a QJ pump from motor power or pump diameter alone. We compare:

• Required flow and head

• Casing diameter

• Number of stages

• Pump efficiency

• Motor power

• Water quality

• Material requirements

• Installation and maintenance conditions

The curve must pass through the duty within the approved range. Catalogue maximum flow and maximum head do not occur at the same point.

For clean, non-corrosive groundwater, a cast-iron QJ configuration may be suitable. Where water chemistry requires greater corrosion resistance, QJ pumps can be produced in stainless steel or other approved materials. Material selection should be based on water analysis rather than appearance or price alone.

The selected QJ pump outside diameter, cable guard, coupling and power cable must all fit through the minimum internal diameter of the well casing.

After hydraulic selection, we confirm absorbed power and choose the matching submersible motor.

The motor data should include:

• Rated power

• Rated current

• Voltage

• Frequency

• Single-phase or three-phase supply

• Number of leads

• Starting method

• Permitted voltage tolerance

An oversized motor does not correct a poor pump selection. An undersized motor may overload as water level or system resistance changes.

For generator-powered projects, the motor voltage and phase should be chosen only after confirming what the generator can supply reliably.

A submersible well pump motor requires more electrical power while starting than during normal running. A generator that can carry the running current may still fail to start the pump.

Possible symptoms include:

• The motor does not accelerate

• Generator speed falls sharply

• Voltage collapses during startup

• The contactor chatters

• The overload relay trips

• The pump starts only when other loads are disconnected

There is no single generator multiplier suitable for every well pump. We review the motor manufacturer's starting-current data and generator recommendations.

The generator supplier should confirm continuous and motor-starting capacity in kW and kVA. Peak power is not continuous output.

The starting method affects the generator, control panel and motor leads.

Direct starting is simple but normally produces the highest starting current. It may require a larger generator to keep voltage stable.

Star-delta starting can reduce starting current, but it also reduces starting torque. The motor winding and lead arrangement must support it.

A soft starter can reduce current and mechanical shock, but its ramp must provide enough torque without extending the start excessively.

A VFD can control acceleration and speed. The motor, minimum frequency, cooling, cable length and pump range must still be checked.

The generator must be sized for the VFD input characteristics. Installing a VFD does not automatically make every small generator suitable.

The cable route includes the surface section between the generator, panel and wellhead, plus the downhole cable.

An undersized cable causes voltage drop, which becomes greater during starting.

We size the cable from:

• Motor rated current

• Starting current

• Supply voltage and phase

• Complete cable length

• Conductor material

• Installation conditions

• Permitted running and starting voltage drop

Increasing generator capacity does not solve excessive voltage drop in a thin, damaged or poorly connected cable.

The underwater splice must be secure, insulated and permanently water-tight.

The generator may be operating correctly while the submerged motor is still at risk from poor cooling.

The motor depends on water moving past its outer surface. We check:

• Minimum operating water level

• Water velocity around the motor

• Well diameter

• Pump position relative to the well screen

• Water temperature

• Need for a cooling sleeve

In a large well, tank or installation where water enters above the intake, a flow sleeve may be required.

Dry-running protection is essential where well yield may be lower than pump flow.

The control panel should match the selected motor and starting method.

Depending on the project, protection may include:

• Short-circuit protection

• Motor overload protection

• Phase loss and phase sequence protection

• Under-voltage and over-voltage protection

• Current and voltage imbalance protection

• Dry-running protection

• Water-level control

• Temperature monitoring

Protection settings should follow motor data rather than being increased to prevent nuisance trips.

During initial operation, we record:

• Generator voltage before starting

• Lowest voltage during motor acceleration

• Running voltage and frequency

• Current on every phase

• Starting time

• Flow rate

• Discharge pressure

• Dynamic water level

• Vibration and abnormal noise

Compare these values with the pump curve, nameplate and approved operating point.

Incorrect phase sequence can reverse a three-phase pump. It may still deliver water with much lower flow and pressure, so rotation must be confirmed.

For a generator-powered well project, we work in this order:

1. Confirm the required water flow and delivery pressure.

2. Calculate total dynamic head.

3. Review the well yield, water levels and casing diameter.

4. Select the QJ pump from its performance curve.

5. Confirm motor power, voltage, frequency and current.

6. Select the starting method and control panel.

7. Calculate cable size and voltage drop.

8. Provide motor data for generator sizing.

9. Check cooling, submergence and dry-running protection.

10. Record the complete operating data during commissioning.

To select the pump, send:

• Required flow and outlet pressure

• Static and dynamic water levels

• Well depth and casing internal diameter

• Delivery elevation

• Pipe diameter, length and fittings

• Pump setting depth

• Water analysis and sand content

• Generator voltage, frequency, phase, kW and kVA ratings

• Total cable length

• Proposed starting method

• Other loads connected to the generator

• Daily operating hours

For a generator-powered well, the submersible pump should be selected from the real hydraulic duty and well conditions first. The motor, cable, starter, protection and generator must then be matched to that pump.

At SLAPK, we do not recommend a QJ well pump from motor kilowatts or generator size alone. We review the complete pumping system so the pump can start reliably and deliver the required water under actual operating conditions.

Send us your well data, required flow and head, cable length and generator information. We can prepare a suitable QJ submersible well pump recommendation for the project.

Contact SLAPK for a generator-powered QJ submersible well pump