Views: 0 Author: Site Editor Publish Time: 2026-01-27 Origin: Site
There are few things more frustrating than turning on a tap and getting no water, or watching a tank level drop while your pump sits silent. When a water system fails, the submersible pump motor is often the primary suspect. But before you go through the expensive and labor-intensive process of pulling the pump out of the well, you need to know exactly what is wrong.
Is the motor burned out? Is there a break in the cable? Or is it simply a blown capacitor in the control box?
Fortunately, you don’t need to be an electrical engineer to figure this out. With a standard digital multimeter and a systematic approach, you can diagnose the health of your submersible motor right from the surface. This guide will walk you through the essential steps to testing single-phase and three-phase motors, helping you decide if it’s time for a repair or a replacement from a trusted manufacturer like Ruirong Pump.
Electricity and water are a dangerous combination. Before you even open your toolbox, safety must be your priority.
Disconnect Power: Turn off the breaker to the pump.
Verify Zero Energy: Use a non-contact voltage tester or your multimeter to ensure no power is flowing to the control box or pressure switch.
Discharge Capacitors: If you are working with a single-phase control box, be aware that start capacitors can hold a charge even when power is off.
Once you have established a safe work environment, grab your digital multimeter. Set it to the Ohms ($\Omega$) setting. This measures resistance, which is the key indicator of your motor's internal health.
Before testing, you need to know if you are dealing with a submersible pump motor that is single-phase or three-phase, as the testing procedure differs slightly.
Single-Phase Motors (2-wire or 3-wire): These are common in residential deep wells. A 3-wire motor will have a control box (with a capacitor) and four wires coming up the well: Red (Start), Black (Run), Yellow (Common), and Green (Ground). A 2-wire motor has the starting components built inside the motor itself.
Three-Phase Motors: These are typically found in commercial or agricultural applications and often fall into the category of a high power submersible motor. They usually have three load wires (L1, L2, L3) plus a ground.
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Before blaming the motor, verify that the correct power is actually reaching the system. Set your multimeter to AC Voltage (~V).
Restore power briefly.
Measure the voltage at the load side of the pressure switch or contactor.
Single-phase: You should see roughly 230V (or 115V depending on the pump).
Three-phase: You should see consistent voltage across all three legs.
If voltage is present and within range (typically +/- 10% of the nameplate rating), but the pump won't start, the issue lies with the motor or the cable. Turn the power back off immediately before proceeding.
This test checks the condition of the copper windings deep inside the motor. You are looking for an open circuit (broken wire) or a short circuit (wires touching each other).
You will measure resistance between the Yellow (Common), Black (Run), and Red (Start) wires.
The Math of a Good Motor:
The resistance of the Start winding (Yellow to Red) plus the resistance of the Run winding (Yellow to Black) should equal the resistance between the two hot wires (Red to Black).
Yellow to Black (Main Winding): Lowest resistance reading.
Yellow to Red (Start Winding): Medium resistance reading.
Red to Black (Total): Highest resistance reading (Sum of the previous two).
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Interpret your results using this table:
Reading Condition | Diagnosis | Action Required |
|---|---|---|
Readings add up correctly | Motor Windings Good | The electrical path is intact. If the pump fails, check the pump end (mechanical) or capacitor. |
Infinite Resistance (OL) | Open Circuit | There is a broken wire in the cable or a burned-out winding. The motor must be pulled. |
Zero Resistance | Short Circuit | The windings have melted together. The motor is dead. |
Testing a three-phase submersible motor is simpler because the windings are symmetrical. Measure the resistance between each pair of wires: L1-L2, L2-L3, and L1-L3.
Result: All three readings should be almost identical. If one reading is significantly lower or higher than the others, you likely have a phase failure or a burned winding, and the motor requires replacement.
This is arguably the most critical test for a submerged device. You are checking to see if the insulation on the wires has cracked or if the motor windings are shorting out to the stainless steel casing or the water.
Set your multimeter to the highest Ohms range (often 200k or Megaohms).
Connect one probe to a clean ground point (the green ground wire or a metal pipe).
Touch the other probe to each of the motor wires (Black, Red, Yellow) one by one.
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Interpreting Ground Test Results:
Multimeter Reading | Condition | Diagnosis |
|---|---|---|
2.0 M$\Omega$ or higher (OL) | Excellent | The insulation is intact. New motors often read infinity. |
0.5 M$\Omega$ to 2.0 M$\Omega$ | Questionable | The insulation is degrading. The motor may run, but failure is imminent. |
0 - 0.5 M$\Omega$ | Failed (Short to Ground) | The motor has a "ground fault." It typically trips the breaker immediately. |
If you get a low reading here, the motor or the splice (where the drop cable meets the motor lead) has failed. The pump must be pulled from the well.
When dealing with a high power submersible motor, usually used in heavy-duty industrial or agricultural irrigation, a standard multimeter might not tell the whole story.
High-power motors operate under immense stress. Sometimes, a standard multimeter running on a 9V battery cannot detect microscopic insulation leaks that only appear under high voltage loads.
For these larger units, professionals often use a Megohmmeter (or "Megger"). A Megger injects 500V or 1000V DC into the winding to test the insulation under stress. However, if your standard multimeter shows a ground fault (zero resistance to ground), you don't need a Megger—you already know the motor is bad.
Companies like Ruirong Pump, which has over 30 years of experience manufacturing high-performance motors (including 6-inch and 8-inch encapsulated and water-cooling models), recommend strict adherence to these testing protocols to ensure longevity.
If your submersible pump motor passes the winding and ground tests but still won't run, the issue might not be underground. It could be the start capacitor or relay in the control box.
Look for visual signs: Bulging capacitors, burnt smells, or loose wires.
Test the capacitor: Many advanced multimeters have a capacitance setting. If the reading is outside the range printed on the capacitor casing, swap it out. This is a cheap fix that saves you from pulling the pump.
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If your testing confirms an "Open Circuit," a "Short Circuit," or a "Short to Ground," the motor has reached the end of its life.
When selecting a replacement, quality matters. Submersible motors are difficult to access, so you want to install a unit that won't require service for years. Look for manufacturers that use stainless steel components and high-grade insulation materials.
Ruirong Pump offers a wide range of replacement options, from 4-inch deep well motors for residential use to massive industrial units. Their motors are designed to withstand the harsh environment of deep wells, ensuring you won't have to repeat this testing process anytime soon.
Diagnosing a submersible motor doesn't have to be a guessing game. By logically checking the supply voltage, winding continuity, and insulation resistance, you can pinpoint the failure accurately.
If your multimeter confirms the worst, don't panic. Replacing the unit with a high-quality motor is an investment in your water security. Whether you need a standard residential replacement or a high power submersible motor, ensure you choose equipment engineered for durability.
