In the field of electrical equipment, conventional induction motors are typically designed for operation under constant frequency and voltage. However, this design presents limitations in meeting the high-performance demands of variable-frequency speed control applications.
1. Efficiency and Temperature Rise
• All types of frequency converters generate harmonic voltages and currents during operation, causing the motor to operate under non-sinusoidal power conditions.
• Taking the commonly used sinusoidal PWM (Pulse Width Modulation) converter as an example, its high-order harmonic components (approximately twice the carrier frequency) lead to increased losses in the motor, including stator and rotor copper/aluminum losses, core losses, and additional stray losses. Notably, rotor copper losses become more pronounced.
• When the induction motor operates near synchronous speed, high-frequency harmonic voltages induce significant losses in the rotor bars. Additionally, skin effect-induced extra copper losses further contribute to efficiency reduction.
• These losses result in additional heat generation, reduced efficiency, and decreased output power. Under non-sinusoidal power supply from frequency converters, the temperature rise of standard three-phase induction motors typically increases by 10% to 20%.
2. Insulation Stress
• Many small and medium-sized frequency converters utilize PWM control with carrier frequencies ranging from several kHz to tens of kHz. This subjects the motor windings to high dv/dt (voltage rise rate), equivalent to steep impulse voltages that challenge the turn-to-turn insulation.
• The rectangular chopping voltage generated by PWM converters superimposes on the motor’s operating voltage, posing a threat to the ground insulation. Repeated high-voltage impulses accelerate insulation aging.
To address these challenges, inverter-duty motors incorporate specialized electromagnetic and structural optimizations:
1. Electromagnetic Design
• The key focus is enhancing the motor’s compatibility with non-sinusoidal power supplies.
• Stator and rotor resistances are minimized to reduce fundamental copper losses, offsetting the additional losses caused by harmonics.
• Motor inductance is carefully increased to suppress high-frequency harmonic currents while ensuring proper impedance matching across the entire speed range.
2. Structural Design
• The motor’s construction accounts for the impact of non-sinusoidal power on insulation, vibration, noise, and cooling.
• Insulation System: Class F or higher insulation is adopted, with reinforced ground and turn-to-turn insulation, particularly emphasizing resistance to impulse voltages.
• Cooling System: Forced ventilation is employed, where an independently driven fan ensures efficient heat dissipation, counteracting the increased thermal stress under variable-frequency operation.
Inverter-duty motors are meticulously engineered to mitigate the adverse effects of frequency converters. Through optimized electromagnetic and structural designs, these motors achieve superior adaptability to non-sinusoidal power sources, making them the preferred choice for variable-speed applications. Their enhanced performance in efficiency, thermal management, and insulation reliability underscores their dominance in modern industrial drive systems.
