I. Characteristics of Single-Phase Asynchronous Motors
Characteristics of asynchronous motors:
1. Small and lightweight.
2. It can easily achieve high-speed rotation exceeding 10,000 r/min.
3. High operating efficiency at high speed and low torque.
4. It has high torque at low speeds and a wide speed control range.
5. High reliability (robust).
6. Low manufacturing cost.
7. Simplification of control devices.
The basic characteristics of an asynchronous motor are that its rotor winding does not need to be connected to any other power source, and its stator current is directly drawn from the AC power system. Compared with other motors, asynchronous motors have a simpler structure, are easier to manufacture, use, and maintain, have higher operational reliability, are lighter, and have lower costs. Taking a three-phase asynchronous motor as an example, compared with a DC motor of the same power and speed, the former weighs only half as much and costs only one-third as much. Asynchronous motors are also easy to derive into various series of products according to different environmental requirements. They also have near-constant speed load characteristics, which can meet the requirements of most industrial and agricultural production machinery. Their limitation is that their speed has a fixed slip ratio with the synchronous speed of their rotating magnetic field (see asynchronous motor), resulting in poor speed regulation performance. In applications requiring a wide range of smooth speed regulation (such as driving rolling mills, winches, and large machine tools), they are less economical and convenient than DC motors. Furthermore, when an asynchronous motor is running, it draws reactive power from the power system for excitation, which can worsen the power factor of the power system. Therefore, in high-power, low-speed applications (such as driving ball mills and compressors), synchronous motors are more suitable.
Due to the large production volume and wide application of asynchronous motors, a wide variety of types and specifications are required to match various machines. Therefore, the design and production of asynchronous motors must pay special attention to standardization, serialization, and generalization. Among the various series of products, the three-phase asynchronous motor series, which has the largest production volume and the widest application, is the basic series; in addition, there are several derived series (series derived from the basic series with partial modifications) and special-purpose series (series with special structures designed for special needs).
A single-phase asynchronous motor is a small-power motor powered by a single-phase AC power supply, also known as a fractional horsepower motor. Structurally, it is similar to a three-phase asynchronous motor; the stator core is also made of laminated silicon steel sheets, the stator winding has only one phase, embedded in the stator slots, and its rotor is also a squirrel-cage type. Due to its simple structure, low price, and ease of maintenance and use, it is widely used in automatic control, medical machinery, washing machines, refrigerators, vacuum cleaners, and small blowers. Compared to a three-phase asynchronous motor of the same capacity, a single-phase asynchronous motor is larger, less efficient, and has poorer operating performance. Therefore, it is generally only manufactured in small and miniature series.
II. Installation of Single-Phase Asynchronous Motors
Before installing the motor, the insulation resistance between the stator winding and the casing, as well as between the main winding and the auxiliary winding, should be measured. The resistance at room temperature should not be less than 10MΩ. Otherwise, the winding should be dried. The bulb heating method can be used.
The shaft extension diameter of the motor is ground to standard tolerance dimensions at the factory. Therefore, users are required to select accessories with national standard inner diameters for the pulleys or other matching parts. During installation, simply push it in by hand or gently tap the shaft extension platform. It is strictly forbidden to strike it with a hammer, otherwise it may easily break the centrifugal switch, causing the motor to fail to start, damaging the bearings, and increasing the operating noise of the motor.
Before installing the motor into the machinery, carefully inspect the motor's base for cracks or other issues that could affect its structural strength. If any problems are found, installation and use are prohibited. The motor should be mounted on a flat plate with mounting holes and secured with bolts that match the mounting holes.
To ensure safety, before running the motor, be sure to connect the grounding wire to the grounding screw of the motor and ensure reliable grounding. The grounding wire should be a copper wire with a cross-sectional area of not less than 1 mm².
The centrifugal switch used in single-phase asynchronous motors is a mechanical switch. When the motor speed reaches 70% or more of the rated speed, the contacts open, disconnecting the auxiliary winding (starting winding) or the starting capacitor, preventing it from operating. When the centrifugal switch is damaged or the starting capacitor is frequently burned out due to low voltage in rural areas, a time-delay relay (220V type) can be used instead. The method involves connecting the two wires of the centrifugal switch inside the motor together, and then connecting the normally closed contact of the time-delay relay in series outside the motor (to ensure contact durability, multiple sets of contacts need to be connected in parallel or an intermediate relay needs to be added). The power supply to the time relay coil can be connected in parallel with the main winding, and the operating time can be adjusted to 2-6 seconds. After numerous trials, the results have been excellent, preventing the starting capacitor from burning out even in rural areas with low voltage, and users are very satisfied.
Time delay relays are available in both mechanical and digital types; either type can be chosen. Most commonly, mechanical time delay relays are used because they are cheaper and more readily accepted by users.
