Why is it called an asynchronous motor?
A three-phase asynchronous motor is used as an electric motor. The rotor speed of a three-phase asynchronous motor is lower than the speed of the rotating magnetic field. Due to the relative motion between the rotor winding and the magnetic field, an electromotive force and current are induced, and electromagnetic torque is generated through the interaction with the magnetic field, thus realizing energy conversion.
The "asynchronous" in a three-phase asynchronous motor means that the rotor speed is always lower than the synchronous speed during motor operation. Three-phase alternating current is applied to the stator windings, generating three magnetic fields that combine to form a rotating magnetic field in the stator air gap. The speed of this rotating magnetic field is called the synchronous speed. The stationary rotor windings then move relative to this magnetic field, cutting magnetic lines of force and inducing an electromotive force.
Since the rotor windings are closed, a rotor current is generated. This current interacts with the rotating magnetic field, producing an electromagnetic torque in the rotor windings, driving the rotor to rotate in the direction of the rotating magnetic field. However, the rotor's speed is never synchronized with the rotational speed of the magnetic field. In fact, if they were synchronized, the rotor windings and the magnetic field would not have relative motion to cut the magnetic lines of force, and no induced electromotive force or induced current would be generated, preventing the rotor from continuing to rotate. Therefore, the rotor's speed is always slightly lower than the synchronous speed.
An asynchronous motor is an AC motor that converts energy by generating electromagnetic torque through the interaction between the rotating magnetic field in the air gap and the induced current in the rotor windings. Because the rotor winding current is induced, its speed differs from the synchronous speed.
Asynchronous motors differ from other motors in the following ways:
1. Simple structure and easy maintenance
2. Power is drawn from the power grid, making it convenient to use.
1. Poor speed regulation performance
2. Lower efficiency than DC motors
Working principle of asynchronous motor
When the stator of an asynchronous motor is connected to a three-phase power supply, three-phase currents flow through the stator, generating a series of air-gap rotating magnetic flux densities. The primary component is the fundamental air-gap rotating magnetic flux density, which rotates at synchronous speed along the winding phase sequence. The magnitude of the synchronous speed determines the frequency of the sub-grid and the number of winding pole pairs.
Figure 5-1(a) is a schematic diagram of a two-pole asynchronous motor. The arrow n1 indicates the rotation direction of the air gap magnetic flux. The innermost large circle represents the rotor, and the two smaller circles represent the conductors of the rotor windings. We first consider the case where the rotor has not yet started rotating. The rotating air gap magnetic flux is figuratively represented by the N and S poles. At the instant shown in the figure, the N pole is on top and the S pole is on the bottom.
Therefore, the rotor conductors cut through the air gap and rotate to induce an electromotive force, the direction of which is shown in the [image] and ⊙ in Figure 5-1. Because the rotor windings are short-circuited, there will be a current in the rotor windings. At the instant shown in Figure 5-1, the direction of the current in the conductors is assumed to be in phase with the induced electromotive force.
Based on the polarity of the rotating magnetic flux density in the air gap and the direction of the current, the left-hand rule shows that an electromagnetic torque acting on the rotor will be generated in the same direction as the rotating magnetic flux density in the air gap. If this electromagnetic torque can overcome the load torque applied to the rotor, the rotor will start to rotate and accelerate. As long as the rotor speed is lower than the synchronous speed, the induced electromotive force and current direction in the rotor conductors remain unchanged, and the direction of the electromagnetic torque also remains unchanged; it is a driving torque.
(a) Electric motor; (b) Generator; (c) Electromagnetic brake
Figure 5-1 Three operating states of an asynchronous motor
If the rotor speed accelerates to equal the synchronous speed n1, there will be no relative motion between the rotor windings and the rotating magnetic flux density in the air gap. Consequently, the rotor windings will no longer induce an electromotive force, and both the current and electromagnetic torque will be zero. This means that this situation cannot be sustained.
However, as long as n < n1, there is relative motion between the rotor windings and the rotating magnetic flux density in the air gap, resulting in current in the rotor windings and electromagnetic torque acting on the rotor. When the electromagnetic torque equals the load torque, the rotor runs at a constant speed. In this case, the stator absorbs active power from the power source. This is the simple operating principle of an asynchronous motor. It is evident that the rotor speed n of an asynchronous motor cannot reach the synchronous speed n1; it is generally always slightly less than n1, hence the term "asynchronous."
The ratio of the difference between the synchronous speed and the motor rotor speed to the synchronous speed is usually called slip (also called slip ratio or slip), denoted by s. The definition of slip ratio is:
s is a dimensionless quantity that reflects the rotational speed of the motor rotor. For example, when n = 0, s = 1; when n = n1, s = 0; when n > n1, s is negative; when the direction of rotation of the motor rotor is opposite to the rotational magnetic flux density in the air gap, s > 1.
If another prime mover drives the motor at a speed higher than the synchronous speed n1 (i.e., n > n1), the directions of the electric potential and current in the conductor, as well as the direction of the generated electromagnetic torque, are reversed, as shown in Figure 5-1(b). In this case, the electromagnetic torque acts as a braking torque for the prime mover. To keep the motor rotor rotating, the prime mover must input mechanical power into the motor.
Therefore, the stator of the asynchronous motor changes from absorbing power from the grid to generating power to the grid, i.e., it is in generator operation mode.
If the motor rotor is driven by other machinery to rotate in the opposite direction to the air gap magnetic flux density, i.e., s > 1, as shown in Figure 5-1(c), the directions of the electromotive force and current in the rotor remain the same as when the motor is operating. The direction of the electromagnetic torque acting on the rotor is still consistent with the direction of the air gap magnetic flux density, but it is opposite to the actual direction of rotation of the rotor. It can be seen that the electromagnetic torque at this time is opposite in direction to the torque applied to the motor rotor by the driving machinery, and they are in balance. The electromagnetic torque is the braking torque. We call this situation the motor being in electromagnetic braking operation.
In addition to absorbing the mechanical power from the machinery it drives, the electric motor also absorbs electrical power from the power grid. Both of these power sources are ultimately converted into heat energy and dissipated within the motor through losses.
