Motor Winding Failure in Submersible Pumps — Causes and Prevention

Motor winding failure is the most common cause of total submersible pump failure — and in the majority of cases it is preventable. The winding is the heart of the motor: copper conductors insulated and impregnated to withstand heat, vibration, and — in the case of submersible motors — the risk of fluid ingress. Understanding the four primary causes of winding failure and the conditions that trigger each provides the basis for an effective prevention strategy.

How Submersible Motor Windings Are Constructed

Flygt submersible motor windings use copper wire wound into stators and insulated with a resin impregnation system designed for the specific thermal class of the motor (typically class F or H, rated to 155°C or 180°C respectively). The impregnation fills voids between conductors and between the winding and the stator core, providing both electrical insulation and mechanical protection against vibration. The quality of this impregnation — its viscosity, penetration depth, and cure — is critical to long-term insulation resistance and thermal conductivity out of the winding and into the stator core.

The Four Primary Causes

1. Thermal Overload

Submersible motors are cooled by the surrounding fluid. When the pump operates dry — even briefly — the motor temperature rises rapidly. A motor rated for 150°C winding temperature can reach that limit within minutes of running dry at full load. Repeated thermal cycling — even below the rated limit — fatigues the insulation through differential thermal expansion between the copper conductors and the impregnation resin. Over time, micro-cracks develop in the insulation, reducing dielectric strength and increasing moisture absorption.

Thermal protection devices — thermistors or thermostats embedded in the winding — protect against sustained overtemperature but not necessarily against rapid dry-running events. Motor protection relays must be set correctly for the motor's thermal characteristics, and dry-run protection devices are essential for pump installations where pump-out to dry conditions is possible.

2. Moisture Ingress

Moisture in the stator housing causes two failure mechanisms: direct track formation across insulation surfaces (particularly when the motor is energised), and progressive insulation resistance degradation as moisture is absorbed by the resin and winding materials. The source of moisture ingress is almost always a degraded mechanical seal — which is why the condition of the sealing system must be inspected and serviced before it fails completely, not after.

Flygt motors include a moisture detection sensor (float switch or conductivity probe) in the oil-filled seal housing that should trigger an alarm — not a trip — before the motor itself is at risk. This early warning system is only effective if the alarm is monitored and acted upon. Many failures traced to moisture ingress involved a moisture alarm that had been disabled or ignored.

3. Voltage Supply Issues

Voltage imbalance between phases in three-phase systems causes unequal current distribution in the winding, resulting in one or more phases running at elevated temperature. Even a 2% voltage imbalance can cause a 6–10% increase in winding temperature in the highest-loaded phase. Voltage imbalance can be caused by unbalanced loading on the distribution system, single-phase faults, or poor connection quality at the motor terminal box.

Undervoltage causes higher-than-rated current draw for a given load — increasing winding temperature. Overvoltage increases core losses. Both conditions accelerate insulation ageing and reduce winding life.

4. Excessive Start Frequency

Motor starting generates a current surge typically 5–7× the rated running current. This surge heats the winding rapidly. Motors are rated for a maximum number of starts per hour — for submersible pumps, typically 10–20 starts per hour depending on motor size. Installations with incorrectly sized pressure vessels, poorly calibrated level controls, or no minimum on-time relay can generate start frequencies far above the rated limit. Over time, the cumulative thermal cycling from excessive starts degrades the insulation significantly.

Prevention Strategy

• Install and maintain seal moisture detection — monitor the alarm and act on it before the motor is at risk
• Implement dry-run protection — float switches or pressure switches that prevent the motor starting or continuing to run without fluid
• Measure voltage balance at the motor terminal box — not just at the panel — and correct imbalance above 1%
• Set motor protection correctly — verify thermal relay or overload settings against the nameplate current and the motor's thermal characteristics
• Count starts per hour — many VSD and control systems can log this; if excessive, review the system control logic and vessel sizing