In motor technology, the rotating magnetic field is a central concept that determines the operating characteristics and performance of a motor. When the rotor of a motor is removed and only a three-phase power supply is applied to the stator, a rotating magnetic field is generated inside the stator. The existence of this magnetic field is the basis for the operation of the motor, and its speed, direction and magnetic flux can be regulated by external conditions.
Formation and Regulation of Rotating Magnetic Field
The rotating magnetic field is formed by the three-phase windings on the stator of the motor by passing a three-phase alternating current through it. The rotational speed of this magnetic field, also known as the synchronous rotational speed (n0), is determined by the frequency of the power supply (f) and the number of pole pairs (p) of the stator windings. The formula for calculating the synchronous speed is n0 = 60f/p. Therefore, regulation of the rotational speed of the rotating magnetic field can be achieved by varying either the frequency of the power supply or the number of pole pairs of the stator.
Principle of variable frequency speed control
Frequency conversion speed control is realized by changing the frequency of power supply to regulate the speed of motor. In the frequency conversion speed control system, the frequency converter, as the core equipment, can convert the fixed industrial frequency power supply into frequency adjustable AC power supply. When the frequency of the power supply changes, the rotational speed of the rotating magnetic field also changes, thus driving the motor rotor to run at a new synchronous speed.
In the frequency conversion speed regulation process, special attention needs to be paid to the proportionality between voltage and frequency. In order to ensure that the magnetic flux (φm) inside the motor is constant, the voltage U and the power supply frequency f need to maintain a certain proportional relationship. This proportionality is usually represented by the V/F curve. In the fundamental frequency range, when the frequency increases, the voltage needs to increase accordingly to keep the magnetic flux stable.
Rotor Induced Potential and Rotation Rate
The rotor of a motor also cuts the rotating magnetic field generated by the stator during rotation, resulting in an induced potential (E2). The magnitude of this induced potential is related to the rotor speed (n) and the slew rate (s). The slew rate is defined as (n0 - n)/n0 and represents the difference between the rotor speed and the synchronous speed as a proportion of the synchronous speed. The slew rate is maximum when the motor is first started (s = 1), when the induced potential of the rotor is maximum. With the increase of the motor speed, the rate of rotational difference gradually decreases, and the induced potential of the rotor also decreases accordingly.
Frequency conversion over-voltage problem
In the process of frequency conversion speed regulation, if the motor suddenly reduces the frequency when it is running at high frequency and the motor speed is not controlled in time, the motor speed may exceed the synchronous speed. At this time, the motor will be in a power generation state, generating a reverse electromotive force to charge the inverter. If this reverse electromotive force exceeds the tolerance of the frequency converter, it will cause the frequency converter to report overvoltage fault. Therefore, in the frequency conversion speed control system, effective control measures need to be taken to prevent the occurrence of this overvoltage phenomenon.
In summary, the rotating magnetic field of the motor and frequency conversion speed regulation are important contents in motor technology. By analyzing the formation and regulation of the rotating magnetic field, the principle of frequency conversion speed control, the rotor induced potential and the rate of rotation, and the frequency conversion overvoltage problem, we can better understand the operating characteristics and performance of the motor, and provide strong technical support for the design, manufacture and application of the motor!