Submersible Sewage Pump Motor Overload, Overheating and Tripping: 5 Hydraulic and Mechanical Problems to Check

Submersible Sewage Pump Motor Overload, Overheating and Tripping: 5 Hydraulic and Mechanical Problems to Check

Submersible sewage pumps operate in demanding environments such as wastewater pits, municipal sewage stations, industrial wastewater systems, drainage sumps and treatment plants.

A common field problem is:

The pump starts normally, runs for several minutes, becomes unusually hot, and then trips on overload or thermal protection. After cooling down, it can restart—but the same problem happens again.

When this occurs, many operators immediately suspect a damaged motor.

Others simply increase the overload protection setting.

Neither should be the first step.

For a submersible sewage pump, overheating and tripping can be caused not only by electrical problems but also by excessive hydraulic or mechanical load.

Shanghai Shangcheng Pump & Valve Manufacturing Co., Ltd. recommends starting with the actual operating current, then checking the hydraulic conditions, pump components and motor cooling conditions.

Here are five common causes worth investigating.

1. Impeller Blockage or Fibrous Material Wrapped Around the Impeller

Sewage pumps rarely handle perfectly clean water.

Wastewater may contain:

Fibers

Plastic bags

Hair

Cloth

Wet wipes

String

Sludge

Other suspended solids

These materials do not always completely lock the impeller.

Sometimes the impeller continues rotating while fibers or debris gradually increase its resistance.

The pump may still discharge water, but the motor has to work harder.

Typical symptoms include:

higher operating current → reduced flow → increased vibration → rising motor temperature → overload trip.

This is why:

“The pump is still running” does not mean the impeller is clean.

After safely isolating the electrical supply, inspect the impeller passages, pump chamber and areas around the impeller for accumulated or wrapped material.

If the problem repeatedly occurs, the application may also require reconsideration of the impeller type and solids-handling capability.

2. Excessive Flow Can Push the Motor Into an Overload Condition

This is one of the more easily overlooked hydraulic causes.

Operators often assume:

More discharge flow means the pump is working better.

Not necessarily.

Every centrifugal sewage pump has its own performance curve and recommended operating range.

Suppose a pump was selected for:

20 m head

but the actual system resistance is much lower.

The real operating point can move toward the high-flow side of the pump curve.

Depending on the pump's hydraulic characteristics, the required shaft power may increase as flow increases.

The result can be:

excessive flow → higher power demand → current above rated value → motor overheating → overload protection trip.

Therefore, when a sewage pump repeatedly trips, do not check only for blockage.

Measure:

actual flow, actual head and motor current.

Then compare the operating point with the manufacturer's pump curve.

A pump can overload even when the pipeline appears to be delivering plenty of water.

3. Higher Liquid Density or Viscosity Increases Pump Load

Consider a pump that operated normally for months and then gradually began overheating.

The pump itself may not have changed.

But has the wastewater changed?

This is particularly important in:

Industrial wastewater

Sludge pits

Sedimentation tanks

Process wastewater

High-solids sewage

For example, the original liquid may have been relatively dilute wastewater.

Over time, sediment accumulates and the concentration of sludge, sand or suspended solids increases.

The pump is now handling a different hydraulic load.

Higher liquid density can increase the power required by the pump.

Higher viscosity can also change pump performance and increase system losses.

Therefore, when a previously stable pump suddenly starts drawing excessive current, ask:

Has the liquid concentration, density or viscosity changed?

Do not evaluate concentrated sludge using clean-water performance alone.

For difficult wastewater applications, Shanghai Shangcheng Pump & Valve recommends providing actual liquid properties during pump selection rather than simply specifying “sewage.”

4. Bearing Wear or Impeller Rubbing Creates Additional Mechanical Resistance

If the impeller is not obviously blocked but operating current remains high, mechanical resistance should be investigated.

Possible causes include:

Worn bearings

Damaged bearings

Bent shaft

Impeller rubbing against the casing

Rotor resistance

Internal mechanical damage

Excessive wear of rotating components

These problems increase the torque required from the motor.

Typical symptoms may include:

high current + abnormal noise + increased vibration + rising temperature.

This combination is particularly important.

A motor that becomes hot is not necessarily failing electrically.

The motor may simply be working against an increasingly difficult mechanical load.

For older sewage pumps with long operating hours, bearing condition and rotating clearances deserve particular attention.

If abnormal metallic rubbing, grinding or strong vibration appears, continuing to restart the pump may increase the damage.

5. Low Water Level Can Cause Insufficient Motor Cooling

This is an important difference between many submersible pumps and conventional dry-installed centrifugal pumps.

A submersible motor must operate under the cooling conditions specified for that particular pump design.

In some wastewater pits, operators try to pump the liquid level as low as possible.

If the stop level is set too low, part of the motor may remain exposed for too long or the required cooling flow around the motor may no longer be available.

The pump may continue running, but heat dissipation deteriorates.

Eventually:

motor temperature rises → thermal protection activates → pump stops.

This leads to an important diagnostic point:

Overheating does not always mean excessive current.

If the three-phase current is reasonably normal but motor temperature rises rapidly, check:

Actual water level

Stop-level setting

Required minimum submergence

Cooling conditions around the motor

Sediment accumulation around the pump

Whether the specific pump design allows continuous operation at that installation level

Do not assume that because a pump is called “submersible,” it can run indefinitely with most of the motor exposed.

Another Important Check: Is the Pump Rotating in the Correct Direction?

For a three-phase submersible sewage pump, incorrect phase sequence can cause reverse rotation.

The pump may still move some water, which makes the problem easy to overlook.

Possible symptoms include:

Low flow

Insufficient head

Abnormal vibration

Unusual noise

Poor hydraulic performance

If a new pump begins behaving abnormally immediately after installation—or after electrical maintenance, cable replacement or control-panel work—verify the direction of rotation.

Electrical work and phase correction should be performed according to the pump manufacturer's instructions and applicable electrical safety requirements.

Why You Should Not Simply Increase the Overload Setting

If a pump trips repeatedly, increasing the overload setting may appear to solve the problem.

In reality, it may only remove the protection.

For example, if the motor rated current is being exceeded because the impeller is partially blocked, raising the protection threshold does nothing to remove the blockage.

Instead, the motor may continue operating under excessive load until the winding insulation or other components are damaged.

The correct approach is:

Measure the current first. Find the reason for the overload second.

Protection settings should follow the motor nameplate data and the requirements of the specific pump and control system.

A Practical Troubleshooting Sequence

When a submersible sewage pump overheats and trips after running for a period of time, Shanghai Shangcheng Pump & Valve recommends avoiding repeated forced restarts.

Start by measuring the three-phase operating current.

If current is clearly too high:

Check impeller blockage, excessive flow, liquid properties and mechanical resistance.

If the three-phase currents are significantly unbalanced:

Check the electrical supply, phase condition, voltage, cable and control system.

If current is relatively normal but temperature rises quickly:

Check water level, submergence and motor cooling conditions.

If high current occurs together with strong vibration or mechanical noise:

Prioritize impeller blockage, bearing damage, rubbing and rotating-component problems.

This sequence helps separate hydraulic, mechanical and cooling problems before unnecessarily dismantling the motor.

Six Pieces of Information Worth Recording

If you need technical support for a sewage pump that repeatedly trips, record these six items before contacting the supplier:

Motor rated current

Actual current on all three phases

Three-phase voltage

How long the pump runs before tripping

Actual operating water level

Liquid condition, including sludge, fibers and solids

If available, also provide:

actual head, pipeline arrangement, pump model and photographs of the installation.

These measurements are far more useful than simply reporting:

“The motor gets very hot.”

Conclusion

A submersible sewage pump that overheats, overloads and repeatedly trips does not automatically have a defective motor.

From a hydraulic and mechanical perspective, five problems deserve priority:

1. Impeller blockage or fibrous material wrapping

2. Excessive flow outside the intended operating range

3. Increased liquid density or viscosity

4. Bearing wear or impeller rubbing

5. Insufficient motor cooling caused by low water level

Rotation direction, electrical current balance, voltage and overload-protection settings should also be checked.

For troubleshooting, Shanghai Shangcheng Pump & Valve Manufacturing Co., Ltd. recommends a simple sequence:

Measure current → check water level → verify operating conditions → inspect for blockage → investigate mechanical and motor conditions.

The key point is simple:

Tripping is usually the result of a protection system responding to an abnormal condition. Finding what is increasing the motor load or temperature is more important than simply trying to prevent the trip.