In - depth analysis of the rotation principles of various types of AC motors
In today's electrical field, AC motors are extremely widely used. Currently, the more commonly used AC motors are mainly divided into two categories, namely three - phase asynchronous motors and single - phase AC motors. Due to the differences in their own characteristics, these two types of motors play important roles in different fields. Three - phase asynchronous motors, with their powerful power output and stable performance, are mostly used in industrial production; while single - phase AC motors, characterized by their simple structure and convenient use, are widely used in various household appliances.
Rotation principle of three-phase asynchronous motor
The key prerequisite for a three - phase asynchronous motor to achieve rotation is the need for a rotating magnetic field. The stator winding plays a crucial role in this process, and its main function is to generate a rotating magnetic field. As we all know, there is a 120 - degree phase difference in voltage among the phases in a three - phase power supply. Meanwhile, the three windings inside the stator of a three - phase asynchronous motor are also spatially staggered by 120 degrees. When the three - phase power supply is connected to the stator winding, the stator winding will generate a rotating magnetic field.
To more clearly understand the generation process of a rotating magnetic field, we can conduct a detailed analysis with the help of Figure 1. Figure 1 divides the generation process of the rotating magnetic field into four moments for description. As the current completes one cycle of change, the rotating magnetic field rotates one full turn in space. This indicates that the rotation speed of the rotating magnetic field is synchronized with the change of the current. The rotational speed of the rotating magnetic field can be calculated using the formula \(n = 60f/P\), where \(f\) represents the power supply frequency, \(P\) is the number of pole - pairs of the magnetic field, and the unit of \(n\) is revolutions per minute. From this formula, we can know that the rotational speed of the motor is closely related to the number of magnetic poles and the frequency of the power supply used. Based on this, there are mainly two methods for controlling the rotational speed of an AC motor, namely the method of changing the number of magnetic poles and the variable - frequency method. In the past, the method of changing the number of magnetic poles was more commonly used; while nowadays, with the continuous development of technology, variable - frequency technology has gradually become the mainstream method for achieving stepless speed control of AC motors.
By observing Figure 1, we can also find that there is a close connection between the rotation direction of the rotating magnetic field and the phase sequence of the current in the windings. When the phase sequence is arranged clockwise as \(A\), \(B\), \(C\), the magnetic field will rotate in the clockwise direction. If we swap any two of the three power lines, for example, pass the phase - \(B\) current into the phase - \(C\) winding and the phase - \(C\) current into the phase - \(B\) winding, then the phase sequence will become \(C\), \(B\), \(A\), and at this time, the magnetic field will definitely rotate in the counter - clockwise direction. By using this characteristic, we can very conveniently change the rotation direction of the three - phase motor.
After the stator winding generates a rotating magnetic field, the rotor bars (squirrel-cage bars) will cut the magnetic field lines of the rotating magnetic field, thereby generating an induced current. The interaction between the current in the rotor bars and the rotating magnetic field will produce an electromagnetic force. The electromagnetic torque formed by this electromagnetic force will drive the rotor to rotate at a speed of \(n1\) in the direction of the rotating magnetic field. Generally, the actual speed \(n1\) of the motor will be lower than the speed \(n\) of the rotating magnetic field. This is because if \(n = n1\), there will be no relative motion between the rotor bars and the rotating magnetic field, and the magnetic field lines will not be cut, so naturally no electromagnetic torque can be generated. Therefore, the speed \(n1\) of the rotor must be less than \(n\). It is also for this reason that we call the three-phase motor an asynchronous motor.
Rotation principle of single-phase AC motor
The structure of a single - phase AC motor is relatively simple. It has only one winding, and the rotor adopts a squirrel - cage structure. When a single - phase sinusoidal current passes through the stator winding, the motor will generate an alternating magnetic field. The strength and direction of this magnetic field change with time according to the sine law, but its spatial orientation remains fixed. Therefore, we also call this magnetic field an alternating pulsating magnetic field.
This alternating pulsating magnetic field can be decomposed into two rotating magnetic fields. These two rotating magnetic fields have the same rotational speed but opposite rotation directions. When the rotor is in a stationary state, these two rotating magnetic fields will generate two torques of equal magnitude but opposite directions in the rotor, making the resultant torque zero. Therefore, the motor cannot rotate on its own. When we use an external force to make the motor rotate in a certain direction (for example, rotate clockwise), at this time, the cutting motion of magnetic field lines between the rotor and the rotating magnetic field in the clockwise direction will decrease; while the cutting motion of magnetic field lines between the rotor and the rotating magnetic field in the counter - clockwise direction will increase. As a result, the original balanced state is broken. The total electromagnetic torque generated by the rotor will no longer be zero, and the rotor will rotate in the direction of the push.
To enable a single-phase motor to rotate automatically, we can add a starting winding to the stator. The starting winding is spatially offset from the main winding by 90 degrees, and a suitable capacitor needs to be connected in series with the starting winding. This can make the currents in the starting winding and the main winding approximately 90 degrees out of phase in terms of phase. This is the so-called phase-splitting principle. When two currents that are 90 degrees out of phase in time are passed through two windings that are 90 degrees apart in space, a (two-phase) rotating magnetic field will be generated in space. Under the action of this rotating magnetic field, the rotor can start automatically. After starting, when the rotational speed rises to a certain level, the starting winding can be disconnected with the help of a centrifugal switch or other automatic control devices installed on the rotor. Only the main winding works during normal operation. Therefore, the starting winding can be designed for short-time operation. However, in many cases, the starting winding is not disconnected. We call this type of motor a capacitor-run single-phase motor. To change the rotation direction of this type of motor, it can be achieved by changing the connection position of the capacitor.
In single-phase motors, there is also another method for generating a rotating magnetic field, namely the shaded-pole method. Motors using this method are called single-phase shaded-pole motors. The stator of this type of motor is made in a salient-pole form, with two types: two-pole and four-pole. Each magnetic pole has a small slot at 1/3 - 1/4 of the full pole face, as shown in Figure 3. In this way, the magnetic pole is divided into two parts. A short-circuited copper ring is fitted on the smaller part, as if covering this part of the magnetic pole, so it is called a shaded-pole motor. The single-phase winding is fitted on the entire magnetic pole, and the coils of each pole are connected in series. When connecting, it must be ensured that the polarities they generate are arranged in sequence as N, S, N, S. When the stator winding is energized, a main magnetic flux is generated in the magnetic pole. According to Lenz's law, the main magnetic flux passing through the short-circuited copper ring will generate an induced current in the copper ring that lags behind by 90 degrees in phase. The magnetic flux generated by this current also lags behind the main magnetic flux in phase. Its function is equivalent to that of the starting winding of a capacitor motor, thus being able to generate a rotating magnetic field and make the motor rotate.