The impact of variable frequency speed regulation on ordinary asynchronous motors
The design intention of variable speed motors is specifically designed for AC speed regulation, but the direct reason for the rise of variable frequency speed regulation is the simple structure, low cost, and convenient speed regulation of ordinary asynchronous motors. If frequency conversion speed regulation must be equipped with a dedicated frequency conversion motor, then a contradiction arises. Isn't the inherent simplicity, robustness, and durability of frequency conversion speed regulation gone?
The impact of variable frequency speed regulation on the motor and its efficiency. Regardless of the control method used, the voltage pulse output to the motor end is non sinusoidal. So the analysis of the operating characteristics of ordinary asynchronous motors under non sinusoidal waves is the impact of variable frequency speed regulation on the motor.
There are mainly the following aspects:
The loss and efficiency of motors operating on non sinusoidal power sources will result in many additional losses in addition to the normal losses generated by the fundamental wave. Mainly manifested in the increase of stator copper loss, rotor copper loss, and iron loss, thereby affecting the efficiency of the motor.
1. The harmonic current caused by stator copper loss in the stator winding increases I2R and. When the skin effect is ignored, the stator copper loss under non sinusoidal current is proportional to the square of the effective value of the total current. If the number of stator phases is m1 and the stator resistance of each phase is R1, then the total stator copper loss P1 is calculated by substituting the effective value Irms of the total stator current, including the fundamental current, into the above equation. The second term in the equation represents harmonic loss. Through experiments, it was found that due to the presence of harmonic currents and the corresponding leakage flux, the saturation degree of the leakage flux magnetic circuit increases, resulting in an increase in excitation current and an increase in the fundamental component of the current.
2. The copper loss of the rotor at harmonic frequencies can generally be considered as a constant resistance of the stator winding, but for the rotor of an asynchronous motor, its AC resistance greatly increases due to the skin effect. Especially for cage shaped rotors with deep grooves, it is particularly severe. A synchronous motor or reluctance motor under a sine wave power supply has a small harmonic magnetic potential in the stator space. The losses caused in the surface winding of the rotor can be ignored. When the synchronous motor operates on a non sinusoidal power source. The time harmonic magnetic potential induces the rotor harmonic current, just like an asynchronous motor operating at a synchronous speed close to its fundamental wave.
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