When motor fails due to insulation stress caused by high peak voltage and fast voltage rise times, failure occurs in the first turn as phase-to-phase short or phase-to-stator short. Under this condition, the motor insulation becomes stressed and can lead to a premature breakdown of a standard motor insulation. Motor line voltage can reach twice the DC link voltage with long cables.įast voltage rise times of 5000 V/μs can be measured at the motor terminals.

This is usually caused by voltage-doubling phenomena of a transmission line with unequal line and load impedance.

High peak voltages can be experienced at the motor terminals especially when long cable is employed (10–100 m depending on the size of motor). Yahya Shakweh, in Power Electronics Handbook (Fourth Edition), 2018 29.6.6.5 Motor Insulation If the motor is loaded beyond its rated capacity and service factor, the added heat from greater watt loss can exceed the capacity of the ventilation system, raise the temperature of the insulation beyond its rated limits, and rapidly destroy it. Air flow is affected by fans, shrouds, baffles, etc. The capacity of the ventilation system is determined by the thermal conductivity of the materials (aluminum versus cast iron and steel), degree of mechanical contact with radiation surfaces, available radiation surface area (stagger-stacked laminations, fins, etc.), and amount of air flow over the radiating surfaces. Motor ventilation systems are designed to achieve “watt loss equilibrium” and dissipate enough heat to keep the insulation within its temperature limits-as long as the motor remains within its rated load capacity (rated horsepower) and service factor. Hence, the motor runs progressively hotter as the load increases. When selecting a space heater, a simple calculation can give you a ball park estimate for power requirements to remain above the dew point.While the resistances of the windings and the rotor remain relatively constant, the amount of current increases along with the load. This drop in resistance to ground due to the surface moisture contamination can cause a low state of reliability for the next startup and increase the chance of a failure. Once the temperature falls below the dew point, air moisture will condensate on the windings and machined surfaces creating a conductive path for current to ground. However, very quickly after shutdown the temperature of the motor windings will start to drop. When a motor is secured (de-energized) there is an initial increase in temperature due to the loss of cooling air.

Depending on the environment and criticality of your electric motor, space heaters should be strongly considered if you want to ensure maximum reliability and expected motor life.

Motor winding heaters