Speed-regulating motors were originally designed for AC speed regulation, but the rise of variable frequency speed control (VFDs) is directly due to the simple structure, low cost, and convenient speed adjustment of ordinary asynchronous motors. If VFDs require dedicated VFD motors, a contradiction arises: wouldn't the inherent simplicity, robustness, and durability of VFDs be lost?
The impact of frequency converter speed regulation on motors and their performance: Regardless of the control method used, the voltage pulses output to the motor terminals in frequency converter speed regulation are non-sinusoidal. Therefore, the analysis of the operating characteristics of ordinary asynchronous motors under non-sinusoidal conditions is essentially an analysis of the impact of frequency converter speed regulation on the motor.
The main aspects are as follows:
Motor losses and efficiency: Motors operating under non-sinusoidal power sources will experience many additional losses in addition to the normal losses generated by the fundamental wave. These losses are mainly manifested in increased stator copper losses, rotor copper losses, and iron losses, thus affecting the motor's efficiency.
1. Stator copper losses are caused by harmonic currents in the stator windings, which increase I²R. When the skin effect is ignored, the stator copper loss under non-sinusoidal current is proportional to the square of the total effective value of the current. If the number of stator phases is m1, and the stator resistance per phase is R1, then the total stator copper loss P1 is calculated as follows: Substituting the total effective value of the stator current, including the fundamental current, into the above equation, we obtain the second term, which represents harmonic losses. Experiments show that the presence of harmonic currents and the corresponding leakage flux increases the magnetic circuit saturation of the leakage flux, thus increasing the excitation current and consequently increasing the fundamental component of the current.
2. Rotor copper losses: At harmonic frequencies, the stator winding resistance can generally be considered constant. However, for asynchronous motors, the AC resistance of the rotor increases significantly due to the skin effect, especially in deep-slot squirrel-cage rotors. In synchronous motors or reluctance motors operating under sinusoidal power, the stator space harmonic magnetomotive force is very small, and the losses in the rotor surface windings are negligible. When a synchronous motor operates under a non-sinusoidal power supply, the time harmonic magnetomotive force induces rotor harmonic currents, similar to those of an asynchronous motor operating close to its fundamental synchronous speed.
Both the counter-rotating 5th harmonic magnetomotive force and the forward-rotating 7th harmonic magnetomotive force induce rotor currents six times the fundamental frequency. At a fundamental frequency of 50Hz, the rotor current frequency is 300Hz. Similarly, the 11th and 13th harmonics induce rotor currents twelve times the fundamental frequency, or 600Hz. At these frequencies, the actual AC resistance of the rotor is much greater than its DC resistance. The actual increase in rotor resistance depends on the conductor cross-section and the geometry of the rotor slots. For a typical copper conductor with an aspect ratio of around 4, the AC resistance to DC resistance ratio is 1.56 at 50Hz, approximately 2.6 at 300Hz, and approximately 3.7 at 600Hz. At higher frequencies, this ratio increases proportionally to the square root of the frequency.
3. Harmonic Iron Loss: The core loss in the motor also increases due to the presence of harmonics in the power supply voltage. The various harmonics of the stator current establish time-harmonic magnetomotive force (MTMF) in the air gap. The total MMF at any point in the air gap is the synthesis of the fundamental and time-harmonic MMFs. For a three-phase 6-step voltage waveform, the peak magnetic flux density in the air gap is about 10% larger than the fundamental value, but the increase in iron loss caused by time-harmonic flux is very small. Stray losses caused by end leakage flux and skewed slot leakage flux will increase under the influence of harmonic frequencies. This must be considered when using non-sinusoidal power supply. End leakage flux effects exist in both the stator and rotor windings, mainly due to eddy current losses caused by leakage flux entering the end plates. Due to the phase difference between the stator and rotor MMFs, skewed slot leakage flux is generated in the skewed slot structure, with the maximum MMF at the ends, resulting in losses in the stator and rotor cores and teeth.
