Submersible Motor Cooling, Flow Sleeves and Cable Sizing
Most submersible motor failures are not manufacturing faults. They are installation decisions: too little water moving past the motor, a cable that is too thin for the depth, or protection that was never fitted. This guide covers the three of them.
How a water-filled submersible motor cools itself
The pumped water is the coolant
A water-filled submersible motor has no fan and no air. The heat generated in the windings passes through the motor shell and is carried away by the water flowing upward past it on its way into the pump intake. If that flow is too slow, the motor overheats no matter how deep it is submerged.
Submergence is not cooling
A motor can be 200 metres under water and still overheat. Being surrounded by water prevents nothing; only water in motion removes heat.
Velocity is what matters
The relevant quantity is the velocity of the water in the annulus between the motor and the casing wall, expressed in metres per second.
The required minimum comes from the motor datasheet and differs between manufacturers and motor sizes.
Wide casings are the hidden risk
The wider the casing, the larger the annular area, and the slower the same flow moves past the motor. An oversized borehole is a cooling problem, not a convenience.
Position relative to the screen
If the pump is set below the well screen, water enters above the motor and may never flow past it at all. This is the classic case that requires a flow sleeve.
Calculating the cooling velocity
Diameters in metres, giving the annular area in m².
What that means in practice
The table below shows the flow that a 6″ motor of roughly 143 mm outside diameter needs in order to reach 0.15 m/s of annular velocity, in casings of different internal diameter. The effect of casing size is dramatic.
| Casing internal diameter | Annular area | Flow needed for 0.15 m/s |
|---|---|---|
| 150 mm | 0.0016 m² | 0.9 m³/h |
| 168 mm | 0.0061 m² | 3.3 m³/h |
| 200 mm | 0.0154 m² | 8.3 m³/h |
| 250 mm | 0.0330 m² | 17.8 m³/h |
| 300 mm | 0.0546 m² | 29.5 m³/h |
Worked from the formula above using a motor outside diameter of 143 mm and a target velocity of 0.15 m/s. Use the outside diameter and the minimum velocity stated on your own motor datasheet; both vary between motor sizes and manufacturers.
Read the number from the datasheet
Minimum cooling velocity is a motor property, not a universal constant. Never assume a figure taken from a different manufacturer applies to your motor.
Check it at the real operating flow
Cooling must be satisfied at the lowest flow the system will actually run at, including throttled or part-load operation, not at the rated flow.
Variable speed changes everything
A motor driven by a variable frequency drive at reduced speed pumps less water and therefore cools itself less, while still generating losses. Check the minimum permitted frequency.
When you need a flow sleeve
A flow sleeve forces the water to do its job
A flow sleeve, also called a flow inducer or shroud, is a tube fitted around the motor and the lower part of the pump. Water cannot reach the intake without first travelling along the motor surface, so the annular velocity is set by the sleeve rather than by the borehole.
The pump is set below the screen
Water enters the casing above the pump and flows straight into the intake, bypassing the motor completely. A sleeve is mandatory here.
The casing is much wider than the motor
In a wide borehole or an open pit, the calculated annular velocity falls below the motor minimum even at full flow.
The pump is installed in a tank or sump
Open water has no annulus at all. Without a sleeve there is no defined flow path past the motor.
The installation is horizontal
A horizontally mounted submersible motor needs a sleeve to guarantee that it stays full of moving water.
Flow is low relative to motor size
A high-head, low-flow duty with a large motor is a combination that frequently fails the cooling check.
The FANPO submersible motor range
Water-cooled motors from 5.5 to 150 HP
FANPO produces 6″, 7″ and 8″ water-cooled submersible motors, rewindable and designed for continuous duty in deep well service. Full performance data for every model is published on the product pages.
| Motor size | Models | Power range | Product page |
|---|---|---|---|
| 6″ | FM65 – FM650 | 5.5 – 50 HP | 6 inch submersible motor |
| 7″ | FM730 – FM790 | 30 – 90 HP | 7 inch submersible motor |
| 8″ | FM830 – FM8150 | 30 – 150 HP | 8 inch submersible motor |
Each product page carries the full table: current, power factor, speed and efficiency at 50 Hz / 380 V and 60 Hz / 460 V, plus length and weight for every model.
Match the motor to the absorbed power
Take the absorbed power at your duty point from the pump table and select the next motor rating above it. Sizing on the nominal pump rating instead of the duty point is a common error.
50 Hz and 60 Hz are different machines
A motor runs roughly 20 per cent faster on 60 Hz, which changes the pump curve and the power demand. Always state the supply frequency when ordering.
Check the thrust load
The axial thrust generated by the pump at maximum head must stay within the thrust bearing rating of the motor. High stage counts are the case to check.
Cable sizing and voltage drop
The cable is part of the motor, not an accessory
The drop cable runs the full depth of the installation. Every volt lost in it is a volt the motor never receives, and a motor running below its rated voltage draws more current and runs hotter to produce the same torque.
Size for the installed depth
Cable cross-section follows from the motor current, the cable length and the supply voltage. A cable sized only on current, ignoring a 200 m run, will be too thin.
Keep the drop small
Limiting voltage drop to a few per cent of nominal voltage is normal practice. The motor manufacturer states the permitted tolerance; staying well inside it costs one cable size and buys years of motor life.
Starting current is the hard case
Voltage drop is worst during starting, when current is several times the running value. A cable that is adequate in steady state can still cause stalling on start.
Include the surface run
The cable between the panel and the well head counts towards the total length. On large sites this section is often longer than the drop itself.
Splices are failure points
Every joint in the drop cable is a potential water ingress point. Use properly rated heat-shrink or resin kits, and keep the number of splices to a minimum.
Protect the cable in the borehole
Fix the cable to the rising main at regular intervals with cable guards or clips so it cannot chafe against the casing during starts and stops.
Protection and starting method
| Device | What it prevents | Notes |
|---|---|---|
| Overload relay | Winding burnout from sustained overcurrent | Set to the motor rated current, not to the breaker size |
| Dry-run protection | Running with no water, which removes all cooling at once | The most valuable single device on a deep well installation |
| Phase failure and phase sequence relay | Single-phasing, which quickly destroys a three-phase winding | Also prevents reversed rotation after maintenance |
| Undervoltage and overvoltage relay | Overheating from a weak or unstable supply | Common on rural networks and on generator supply |
| Soft starter or VFD | Mechanical shock, water hammer and high starting current | With a VFD, check the minimum speed against motor cooling |
| Non-return valve | Reverse rotation of the pump when it stops | Fit at the pump and at intervals on very deep installations |
Frequently asked questions
My motor keeps failing. Where should I look first?
Check the cooling velocity at the real operating flow, then the voltage at the motor terminals during starting. Those two account for the majority of avoidable failures.
Is a flow sleeve needed in every well?
No. In a narrow casing with the pump set above the screen, the natural flow path already passes the motor. Run the calculation; do not assume either way.
Can I use a bigger motor to be safe?
An oversized motor runs at a lower load factor, generates proportionally more loss relative to its output and is harder to cool. Match the motor to the duty point.
Does a deeper setting need a thicker cable?
Yes. Cable cross-section is a function of length as well as current, so setting depth directly affects the required size.
What frequency will my motor run at?
FANPO motors are supplied for 50 Hz / 380 V and 60 Hz / 460 V. The performance tables on each product page give both. Confirm the site supply before ordering.
Where do I find the matching pump?
Start with the pump selection guide, then the submersible pump range. Send your data through the contact page for a full selection.