Advantages of Asynchronous Motors
Asynchronous motors have clearly defined advantages, including simplicity of design, as they do not contain brushes and a collector. This makes them less demanding in operation and ensures their durability, provided that maintenance regulations are followed. However, the main disadvantage of these machines lies in the necessity for three-phase power supply. But this does not diminish their potential for use in a household 220 V network with single-phase supply.
Fundamentals of Three-Phase Power Theory
A three-phase network consists of three single-phase power lines that have a phase shift of 120 degrees. The windings of the motor are physically arranged relative to each other so that the current flows sequentially through the first, second, and third winding. This creates the torque that rotates the motor rotor.
When an asynchronous machine is connected to a single-phase network, it is necessary to simulate the presence of two additional phases. There are various ways to achieve this, including capacitor delay, which is still used. However, such methods often lead to significant (sometimes exceeding 30%) power losses and reduced efficiency of the motor. Currently, methods of connection through a frequency converter (FC) have become more acceptable, which is considered the optimal option.
Operating Modes of Asynchronous Motors
Asynchronous motors operate in two main modes: “star” and “delta,” which are named after the way the windings are connected. In star mode, despite the phase voltage of 380 V, each winding receives only 220 V, which results in smaller starting currents. However, this configuration limits the power of the motors, so it is usually used for low-power devices.
In delta mode, the ends of the windings are connected to each other, providing a voltage of 380 V between them. This allows for large currents to be supplied and for powerful motors exceeding 7.5 kW to be connected.
The combined star/delta mode allows for reduced starting currents and increased torque during startup (star), after which the delta is engaged, which is optimal for powerful motors. Switching between modes usually occurs automatically, for example, using a timer relay.
Functions of the Frequency Converter (FC)
A frequency converter (FC) is a device that converts a 50 Hz sine wave into a series of impulses with a frequency of up to 1 kHz. This allows for the adjustment of the rotor's rotation speed, facilitates smooth machine starting, and ensures stable operation without losing rated power. Modern FCs, such as the ESQ-A500-021, can automatically convert one phase into three full phases of 220 V/50 Hz, making it easier to connect three-phase loads to a household network.
- Protection against short circuits, overloads, and voltage spikes, ensuring emergency shutdown of the load;
- Indication of the main operating parameters of the motor, with the ability to save them;
- Optimization of energy consumption according to the load and implementation of high-torque starting.
Moreover, connection via an FC reduces noise and the temperature of the motor windings, which contributes to extending the lifespan of the device.
The only drawback of using an FC to power asynchronous motors is the power limit of up to 4 kW. However, for most needs that require connecting three-phase machines to 220 V, this is quite sufficient, as most such devices are equipped with power units of up to 1.5 kW.