Everyone knows that a variable frequency motor is a type of asynchronous motor, and that asynchronous motors can be controlled by a frequency converter. So the question is, if a regular asynchronous motor can be driven by a frequency converter, is it equivalent to a variable frequency motor?
The answers are definitely different. Let's take a look at the development history of asynchronous motors and variable frequency motors:
Electric motors have become deeply integrated into our lives; they are practically ubiquitous. The most common type we use is the asynchronous motor. With the development of technology, asynchronous motors operating at mains frequency have moved into the era of variable frequency drives, greatly expanding their development potential.
We all know that ordinary asynchronous motors can only operate under AC380V/50HZ conditions and can work under standard sine waves. Their basic characteristic is that the rotor windings do not need to be connected to other power sources, and the stator current is directly taken from the AC power system. Compared with other motors, asynchronous motors have a simple structure, are easy to manufacture, use, and maintain, have high operational reliability, are lightweight, and have low cost. Some people say that a variable frequency motor is just an asynchronous motor with a frequency converter added, and that both can work. While this seems correct based on their operating principles, it's not. The design specifications of variable frequency motors differ from those of ordinary asynchronous motors.
The development of variable frequency motors has been fraught with challenges. From the name itself, we can get a basic understanding: a motor whose frequency can be changed. Ordinary asynchronous motors are designed for constant frequency and voltage, making them unsuitable for variable frequency speed control. Variable frequency motors control their operating state by modulating the output waveform through a frequency converter. This is achieved through PWM control. Its carrier frequency is approximately several thousand to tens of kilohertz, breaking the limitation that motors can only operate on mains frequency circuits, significantly improving work efficiency.
The impact of frequency converters on motors:
1. Issues related to the efficiency and temperature rise of electric motors.
2. Electric motor insulation strength issues
3. Harmonic electromagnetic noise and vibration
4. The motor's ability to adapt to frequent starting and braking.
5. Cooling issues at low speeds
Only by overcoming these problems can the motor function properly. To achieve higher performance variable frequency motors, it's essential to optimize the motor's operation under the influence of the frequency converter. This requires overcoming numerous challenges and controlling the motor's operating state through the frequency converter's output. Due to its wide operating range, it can operate continuously within the 0.1Hz~130Hz range. However, the PWM (Pulse Width Modulation) waveform output by the frequency converter simulates sinusoidal AC power and contains numerous harmonics. Improper handling of these harmonics can severely impact the motor's lifespan.
In terms of structural design, due to the high impulse voltage of the windings, the insulation strength to ground and the insulation strength of the coils must be strengthened. Regarding motor vibration and noise, the rigidity of the motor components and the overall structure must be considered to avoid resonance with various force waves. For heat dissipation, a dedicated motor-driven cooling fan for the main motor is required. In electromagnetic design, the stator resistance must be reduced, which necessitates lowering the fundamental copper loss to compensate for the increased copper loss caused by higher harmonics. To suppress higher harmonics in the current, the motor inductance should be appropriately increased to ensure impedance matching throughout the entire speed range. Only by achieving these aspects can the rigidity requirements of the variable frequency motor be met.
Asynchronous motors and variable frequency motors each have their own characteristics. To select the right motor, a series of conditions must be considered, including the operating mode and the actual working environment. Because the mechanical structure and electrical parameters of variable frequency motors differ from those of asynchronous motors, they are more likely to meet the demands of more demanding operating conditions and requirements, but at a higher cost. To better test motor performance parameters, ZLG Zhiyuan Electronics' MPT motor testing system features a unique "free-loading engine" technology, which can meet the transient measurement needs of motors and drives, driving industry development.
