Due to the rapid development of AC power systems, AC motors have become the most commonly used motors. This article first introduces the classification and principles of AC motors, then explains the advantages and disadvantages of AC motors and the different types of AC motors, and finally introduces the power supply and frequency conversion speed regulation of AC motors. Let's take a closer look together.
Overview of AC Motors
Due to the rapid development of AC power systems, AC motors have become the most commonly used type of motor. Compared to DC motors, AC motors, lacking a commutator (see Commutation of DC motors), have a simpler structure, are easier to manufacture, are more robust, and are readily made into high-speed, high-voltage, high-current, and high-capacity motors. The power range of AC motors is vast, from a few watts to hundreds of thousands, or even millions, of kilowatts. In the early 1980s, the largest steam turbine generators reached 1.5 million kilowatts.
AC motor principle
AC motors are divided into synchronous and asynchronous motors. Synchronous motors are mainly used as generators, while asynchronous motors are mainly used as electric motors. This section mainly introduces electric motors. Due to their simple structure, low price, easy maintenance, and reliable operation, asynchronous motors are widely used. Although AC motors have a simple structure, their working principle is actually more complex than that of DC motors.
When a three-phase symmetrical alternating current is applied to the stator of an AC motor, and the stator remains stationary, a rotating composite magnetic field is generated simply by the change in current. This magnetic field acts like a magnet rotating around the stator. With this rotating magnet, any closed coil placed inside the stator will induce an electromotive force and current, generating an electromagnetic force, causing the coil to rotate. Alternatively, the stator can be understood as having a selective magnet, and the rotor's closed coil, due to induction, also becomes an electromagnet. The rotation of the external electromagnet causes the internal electromagnet to rotate, thus making the AC motor's rotor rotate.
The rotational speed of the stator magnetic field is also called the synchronous speed. The rotor inside is actually pulled by the stator magnetic field to rotate, so its speed is slower than the speed of the stator magnetic field, hence the name asynchronous speed.
Advantages and disadvantages of AC motors
Advantages of AC motors
1. Simple structure.
2. Low manufacturing cost.
3. Maintenance and repair are simple and economical.
Disadvantages of AC motors
If the speed cannot be adjusted by itself, a frequency converter is needed to change the speed.
What are the different types of AC motors? What are the classifications of AC motors? What are the power supplies and frequency converters for AC motors?
What are the classifications of AC motors?
According to statistics, AC motors can be classified in the following two ways:
1. By function
AC motors are generally classified into several categories according to their function, including AC generators, AC motors, and synchronous condensers. Due to the reversibility of the motor's operating state, the same motor can function as both a generator and a motor.
It's not entirely accurate to divide motors into generators and electric motors; some motors primarily function as generators, while others primarily function as electric motors.
2. By variety
AC motors are broadly classified into two categories: synchronous motors and asynchronous motors. In a synchronous motor, the rotor speed ns is the same as the rotational speed of the rotating magnetic field; this speed is called the synchronous speed. ns has a strict relationship with the frequency (f) of the AC current and the number of pole pairs (P) of the motor.
ns=60f/P
In China, the power frequency is 50 Hz, so the synchronous speed of a single-pole motor in a three-phase AC motor is 3000 rpm, the synchronous speed of a two-pole motor is 1500 rpm, and so on. The rotor speed of an asynchronous motor is always lower or higher than the speed of its rotating magnetic field, hence the name "asynchronous." The difference between the rotor speed and the rotating magnetic field speed of an asynchronous motor (called slip) is usually within 10%.
What are the different types of AC motors?
1. AC synchronous motor
The rotor is made of permanent magnet material, so after it rotates, as the rotating magnetic field of the stator of the motor changes, the rotor also responds with a change in frequency and speed. Moreover, the rotor speed equals the stator speed, hence the term "synchronization".
2. AC asynchronous motor
The rotor is composed of induction coils and materials. After rotation, the stator generates a rotating magnetic field. This magnetic field cuts the rotor's induction coils, inducing a current in the rotor coils. This, in turn, generates an induced magnetic field in the rotor. This induced magnetic field follows the changes in the stator's rotating magnetic field, but the change in the rotor's magnetic field is always less than the change in the stator's. Once they equal, there is no longer a changing magnetic field cutting the rotor's induction coils, and no induced current appears in the rotor coils. The rotor's magnetic field disappears, the rotor stalls, and a speed difference with the stator is created, allowing it to regain induced current. Therefore, a key parameter in AC asynchronous motors is slip, which is the ratio of the speed difference between the rotor and stator.
3. Corresponding to AC synchronous and asynchronous motor frequency converters, there are corresponding synchronous frequency converters and asynchronous frequency converters. Servo motors also have AC synchronous servo and AC asynchronous servo. Of course, AC asynchronous frequency converters are more common in frequency converters, while AC synchronous servo is more common in servo motors.
AC motor power supply and frequency converter speed control
1. AC motor power supply
AC motors generally use a three-phase system because, compared to single-phase motors, three-phase AC motors have significant advantages in performance indicators, raw material utilization, and price. A three-phase motor of the same power is smaller, lighter, and cheaper than a single-phase motor. Three-phase motors have self-starting capability. Single-phase motors lack starting torque, requiring special measures to address starting issues. Single-phase motors have pulsating torque and are relatively noisy, but require simpler power supplies, making them particularly convenient for home use. Therefore, small household motors and instrument motors often use single-phase motors.
2. AC motor frequency conversion speed regulation
A frequency converter is a power transmission component that uses frequency conversion technology and microelectronics technology to control an AC motor by changing the frequency and amplitude of the motor's power supply.
Features of AC motor speed control frequency converter
■ Low-frequency torque output of 180%, excellent low-frequency operating characteristics
■ Maximum output frequency of 600Hz, capable of controlling high-speed motors
■Comprehensive detection and protection functions (overvoltage, undervoltage, overload) and instantaneous power outage restart
■ Protection functions such as acceleration, deceleration, and stall prevention during operation.
■ Automatic motor dynamic parameter identification function ensures system stability and accuracy.
■ Fast response during high-speed shutdown
■ Abundant and flexible input/output interfaces and control methods, with strong versatility
■ Adopting SMT full-mount production and conformal coating process, the product has high stability.
■ The entire series uses the latest Siemens IGBT power devices to ensure high quality.
