Views: 0 Author: Site Editor Publish Time: 2026-02-05 Origin: Site
There is nothing quite as frustrating as flipping the switch on your industrial or agricultural water system and hearing... absolutely nothing. When water stops flowing, operations grind to a halt, and immediate panic often sets in. Is the pump broken? Is the power out? Or has the motor finally given up the ghost?
Diagnosing a submersible motor is trickier than standard motors because, naturally, the unit is hundreds of feet underground or submerged deep in a tank. You cannot simply open the casing to look for burnt wires or smell the insulation melting. You have to rely on electrical diagnostics from the surface to understand what is happening deep down in the well.
For those using a 3-phase system—common in heavy-duty applications like those supported by manufacturers like Ruirong Pump—troubleshooting requires a specific set of tools and knowledge. A 3-phase system offers consistent power and high efficiency, but when it fails, you need to know if the issue lies in the power supply, the control panel, or the water pump motor itself.
This guide will walk you through the essential steps to test a 3-phase submersible motor, interpret the data, and decide whether it’s time for a repair or a replacement.
Before you touch a multimeter or open a control panel, you must prioritize safety. working with 3-phase power involves high voltages (often 230V, 380V, or 460V) that can be lethal.
Always follow these protocols:
Disconnect Power: Turn off the main disconnect switch.
Lockout/Tagout: Secure the switch so no one can accidentally turn it back on while you are working.
Discharge Capacitors: If your system uses capacitors (less common in pure 3-phase but possible in converters), ensure they are discharged.
Dry Environment: Do not stand in water while testing electrical equipment.
1
Before blaming the motor, you must rule out the easy stuff. Many "motor failures" are actually just blown fuses or loose connections in the control panel.
Open the control box and look for signs of overheating, such as melted wire insulation or scorch marks. Check for loose terminals. If everything looks clean, you need to verify that the motor is actually receiving power.
With the power on (be extremely careful), use a voltmeter to check the voltage across all three legs of the power supply at the starter terminals (L1, L2, L3).
Measure L1 to L2
Measure L2 to L3
Measure L3 to L1
The readings should be consistent and match the motor's rated voltage. If one phase is dead or significantly lower than the others, your problem is likely the power grid or the breaker, not the submersible motor.
If the supply power is good, it is time to check the motor windings. This test determines if the copper coils inside the motor are intact or if they have shorted out.
You will need an accurate ohmmeter for this. Ensure power is off and locked out before proceeding.
Disconnect the motor leads (T1, T2, T3) from the control panel to isolate the motor. You are now measuring the resistance of the drop cable plus the motor windings. Measure the resistance between each pair of wires:
Measure resistance between T1 and T2.
Measure resistance between T2 and T3.
Measure resistance between T3 and T1.
1
In a healthy 3-phase water pump motor, these three readings should be balanced. They should virtually be identical.
Balanced Readings: If all three readings are within 5% of each other, the windings are electrically balanced.
Unbalanced Readings: If one reading is significantly different, you likely have an open winding (broken wire) or a short circuit.
Zero Resistance: This indicates a dead short between phases.
Infinite Resistance: This indicates a broken wire (open circuit) somewhere in the cable or the motor.
1
This is arguably the most critical test for a submersible application. Because these units operate underwater—and many are a "water filled motor" design for cooling—the insulation around the copper wires is the only thing stopping the current from leaking into the water and grounding out.
For this, a standard multimeter often isn't enough because it doesn't use enough voltage to find weak spots in the insulation. You need a Megohmmeter (often called a "Megger").
Set your Megger to 500V or 1000V (depending on the motor rating).
Connect one lead to a solid ground point (the well casing or the green ground wire).
Connect the other lead to T1, then T2, then T3.
1
The resistance here should be very high (measured in Millions of Ohms or Megohms).
Reading (Megohms) | Condition |
|---|---|
200+ MΩ | Excellent (New Motor Condition) |
10 - 200 MΩ | Good (Normal aging) |
2 - 10 MΩ | Questionable (Monitor closely) |
< 1 MΩ | Failure Imminent or Failed |
If the resistance is low, the insulation has breached. Water has likely entered the motor windings, or the cable insulation has been scraped off against the well casing. In either case, the motor must be pulled.
If the motor passes the resistance and ground tests, but the pump is still tripping the overload, you need to check the running amps. This requires the system to be running, so re-connect the wires and turn the power on.
Using a clamp-on ammeter, measure the current on each leg (T1, T2, T3) while the pump is operating.
Calculate the current unbalance percentage. A 3-phase motor should not exceed a 5% current unbalance at full load.
Calculation: Take the average of the three readings. Find the difference between the average and the reading furthest from the average. Divide that difference by the average and multiply by 100.
Why does this matter? A current unbalance creates excess heat. For every 1% of current unbalance, the motor temperature rises significantly. Since submersible motors rely on water flow for cooling, overheating can destroy the internal thrust bearings and windings rapidly.
When troubleshooting, it helps to know what type of motor you are dealing with. Industry leaders like Ruirong Pump manufacture various configurations, including water-filled submersible motors.
In a water-filled motor, the internal stator is filled with a water/glycol mixture. This design is eco-friendly and excellent for heat dissipation. However, if the shaft seal fails, well water (which may contain sand or debris) can mix with the internal cooling fluid, leading to winding failure. If you pull a water-filled motor and the fluid inside is dark or gritty, the mechanical seal has failed.
Oil-filled motors use food-grade oil for cooling. If you see an oil sheen on the water surface of your well or tank, you have a leak, even if the electrical tests look okay initially.
If you are seeing specific symptoms, use this table to narrow down the potential culprit.
Symptom | Potential Cause | Test to Confirm |
|---|---|---|
Motor won't start | Blown fuse / No Power | Check Voltage at Panel |
Motor hums but won't start | Locked rotor / Seized Pump | Check Amps (High) & Resistance |
Breaker trips immediately | Dead short to ground | Insulation Test (Megger) |
Breaker trips after running | Overload / Heat / Unbalance | Check Running Amps |
Low water flow | Pump running backward | Swap any two leads (T1 & T2) |
Once you have completed these tests, you have a decision to make.
If the issue is in the control panel (bad contactor, blown fuse, loose wire), the repair is cheap and easy.
If the issue is in the drop cable (low megger reading, but resistance is balanced), you might only need to pull the pump and splice in a new section of cable.
However, if you find unbalanced winding resistance or a direct short to ground inside the motor, the unit is done. Rewinding a submersible motor is possible, but often cost-prohibitive compared to buying a new, high-efficiency unit.
When selecting a replacement, quality matters. Submersible pumps are expensive to pull out of the ground. You want to install a unit that will last for years without intervention. Companies like Ruirong Pump have over 30 years of experience producing high-tech stainless steel submersible motors and pumps designed for exactly this kind of longevity. Whether you need a 4-inch deep well pump or a heavy-duty 8-inch industrial unit, choosing a reputable manufacturer ensures you won't be repeating these tests anytime soon.
Check the data plate on the motor or the control box. A 3-phase motor will usually list a voltage like 230V 3PH, 380V, or 460V and will have three hot wires (plus a ground). A single-phase motor will usually say 1PH and may have a capacitor box required to start it.
3-phase motors rely on the sequence of the incoming power phases to determine rotation direction. If you hook up T1 and T2 swapped, the motor runs in reverse. This won't damage it instantly, but it will reduce water flow significantly. To fix it, simply turn off the power and swap any two lead wires at the starter.
Not directly. You would need a Variable Frequency Drive (VFD) or a phase converter to change your single-phase supply into a 3-phase output for the motor.
Troubleshooting a 3-phase submersible motor doesn't have to be a guessing game. By systematically checking voltage, winding resistance, insulation integrity, and amperage, you can pinpoint the failure with precision.
Regular checks can help you catch insulation degradation before it becomes a catastrophic failure. But when the time comes for a replacement, ensure you invest in technology built to withstand the harsh environment of a deep well.
If you need a replacement motor or technical advice on sizing a new pump, visit Ruirong Pump today to explore reliable solutions for your water systems.
