An induction motor is an AC electric machine. It uses a rotating magnetic field that is generated in the stator to create torque. Three-phase asynchronous electric motors are used in industry, construction, and are used in household appliances. They have a simple and reliable design, require little maintenance, are easy to start and withstand high overloads.
Content
- Device and principle of operation
- Connection methods
- Advantages and disadvantages of three-phase motors
- Motor protection
Device and principle of operation
An asynchronous three-phase motor has a fixed part - a stator and a rotating part - a rotor. There is formed between the rotor and the stator air gap about two millimeters. The motor stator has three windings at an angle of 120 °. There is a magnetic circuit inside. When AC voltage is applied to the windings, a rotating magnetic field is created in them, which induces an induction current in the rotor.
Torque appears when the rotating magnetic fields of the stator and rotor interact. In operating mode, the rotor speed is always less than the speed of the magnetic field. Because of this lag, the motor is called asynchronous. The conductors of the three-phase stator winding are laid in the grooves. The stator magnetic core is assembled from plates.
Motors are available with squirrel-cage and phase rotor. A squirrel-cage rotor is sometimes called a "squirrel wheel". It is composed of rods closed at the ends by two rings. The material of the rods is aluminum and less often copper or brass. Phase rotors have three windings located in the same way as on the stator.
The winding leads are connected to slip rings fixed to the shaft. The rings are isolated from each other and from the shaft. With the help of brushes, a rheostat is connected to them, which serves to adjust the speed of the electric motor.
Connection methods
The ends of the windings are brought out to the terminal block and switched according to standard "star" or "delta" circuits. The difference between the circuits is that the "star" has a higher line voltage than the "delta". In practice, a combined connection is often used. During start-up and acceleration, the "star" is commutated, and the "triangle" is used in the operating mode. Methods for connecting the motor to the network:
- Direct connection.
- Through a soft starter. It is used in the case when adjustment is required only at the moment of start-up.
- By means of an inverter that regulates the frequency of the voltage applied to the input. It regulates the smooth start and stop of the engine, changes the rotor speed.
Advantages and disadvantages of three-phase motors
Asynchronous electric motors are used in drives of machine tools, cranes, elevators, winches, agricultural machines, rolling mills. Their benefits include the following factors:
-
simple device; - reliability and durability;
- low noise level;
- work directly from a three-phase network.
The low cost of electrical machines and their ease of use make them frequently used in industrial installations. But along with the undoubted advantages, these machines also have disadvantages:
- Lack of easy ways to regulate speed.
- Dependence of the rotor speed on the load on the shaft. As the load increases, the rotation speed decreases.
- High starting current.
- Sensitivity to changes in mains voltage.
- Large consumption of reactive power.
Induction motors are inductive loads and consume reactive power that does not produce mechanical work. It heats up the cables, lowers the voltage, increases the current.
The inductive power consumption can be compensated by installing capacitor banks in parallel with the motor. This will reduce losses and energy costs.
Motor protection
Three-phase motor protection circuit breakers protect against short-circuit current, against prolonged overloads, against phase imbalance in the power supply or inside the electric motor. This leads to overheating of the engine and to malfunctions. A protective device will automatically shut off the engine when emergency situation.
Protection of the electric motor is often used with the help of universal motor-automatic devices. These devices have a modular design and control the operation of power contactors, and some automatic machines allow you to accurately adjust the parameters of the protective shutdown.
When choosing asynchronous machines and during their operation, the characteristics of the asynchronous electric motor should be taken into account. Only under this condition can the most efficient use of the installation be achieved.
