A non-intrusive winding heating method for induction motor using soft-starter for preventing moisture condensation

Moisture condensation may cause degradation of key motor components when a motor is de-energized. To prevent moisture condensation, a motor´s temperature must be maintained higher than the ambient temperature when de-energized. This paper proposes a non-intrusive motor heating technique using soft-starters. By controlling the operation of the solid-state switches in soft-starters, adjustable ac current can be injected into the three phases of the stator windings without inducing any output torque. The motor temperature can therefore be maintained at a desired temperature due to the heat dissipation during current injection. In addition, two motor temperature control approaches are proposed in this paper to minimize the required energy consumption for preventing moisture condensation. The proposed technique is experimentally validated on a 7.5-hp ODP (open drip proof) induction motor. The experimental results show that the average stator winding temperature can reaches 9° above the ambient temperature with acceptable current magnitudes. In addition, it is shown that the motor temperature rise can be controlled by adjusting the operation of the solid-state switches in soft-starters. The importance of the proposed technique lies in its non-invasive nature: the technique only uses the existing hardware of soft-starters; the motor´s condition is not interrupted since no output torque is induced during the current injection. The proposed winding heating technique can be easily extended to other types of ac machines or inverter-fed applications.