This article mainly introduces the AC motor speed control system, first introduced the composition of the AC motor speed control system, followed by the classification of the AC motor speed control system, and finally introduced the AC motor speed control methods, the specific follow small series Get up and learn about it.
Composition of AC motor speed control systemAC motor speed control system generally consists of power converters, controllers and detectors, as shown in Figure 1:
1, pole changing speed
By changing the number of windings of the asynchronous motor to change its synchronous speed. This method is graded speed regulation, and the speed of asynchronous motor cannot be changed continuously. It can only be applied to some special occasions and can only achieve the purpose of coarse adjustment in a large range.
Pole-changing speed control is only used for specially produced variable-speed multi-speed squirrel-cage asynchronous motors. By changing the stator winding connection or separately setting the winding method, D/YY, Y/YY two-speed motors, three-speed and four-speed can be obtained. Etc.
2, rotor resistance resistance
By changing the resistance of the resistor connected in series in the rotor circuit to change the motor slip, and then achieve the purpose of speed control. Since the resistance of the series resistance can be changed in multiple stages, it is possible to achieve speed regulation of various speeds.
This method is simple in structure, cheap and easy to operate; however, due to the slip power loss in the resistance, the efficiency decreases with the increase of the slip rate, and at the same time its mechanical characteristics are relatively soft, and it is only suitable for occasions where the speed regulation performance is not high. .
This speed control method is only applicable to wound rotor asynchronous motors.
3, cascade speed
By connecting a diode or thyristor rectifying bridge to the rotor side of the asynchronous motor, the slip frequency alternating current is converted into direct current, and the slip power is returned to the power supply by a direct current motor rotary converter or inverter, or the slip power is directly changed. Used for mechanical energy. Cascade speed control is widely used in drive systems such as fans and pumps.
The common structural schemes of this speed regulation method are: electrical cascade method, motor cascade method, low synchronous synchronous cascade speed regulation, super synchronous synchronous cascade speed regulation.
This method is suitable for wound asynchronous motors.
4, pressure regulator
This is the thyristor anti-parallel connection, constitute the AC speed control circuit, by adjusting the trigger angle of the thyristor, change the asynchronous motor terminal voltage (see Figure 6-5).
This method also changes the slip rate S. The slip power is consumed in the rotor circuit and has low efficiency. It is only applicable to small-capacity motors such as special cage motors (such as deep-slot motors and high-slip motors) and wound rotors. .
Usually this kind of speed regulation method should constitute the rotational speed or the voltage closed loop, can actually apply.
5, electromagnetic speed adjustable asynchronous motor
This kind of system transfers the mechanical power through the electromagnetic coupling between the asynchronous motor and the load, regulates the excitation of the electromagnetic coupler, and can adjust the magnitude of the slip ratio S so as to achieve the purpose of speed regulation.
The speed control system has a simple structure and a low price. However, in the speed regulation process, the energy of the slip loss is on the coupling. The efficiency is low, and it is suitable for a small-capacity transmission control with a low speed control performance requirement.
6, frequency control
Adjust the speed by changing the frequency of the power supply. The use of a rotating frequency conversion unit as a variable-frequency power supply has rarely been adopted. A large number of static frequency converter power supplies using semiconductor devices are currently used in large quantities. Control programs include:
Adopt "AC-DC-AC" frequency converter, pulse width modulation type frequency converter, "Cross-AC" frequency converter, vector control, etc.
7, no commutator motor speed
No commutator motor is a frequency-controlled synchronous motor controlled by a thyristor. Fig. 6-10 is the basic structure diagram of the motor without commutator. Its feature is to detect the position of the rotor of the synchronous motor as a trigger signal of the adjustable frequency inverter, that is, the output frequency of the frequency converter is synchronized with the rotation frequency of the synchronous motor. The closed-loop system determines that the phase relationship between the magnetic field and the armature winding is given by the position detector. Therefore, the non-commutator motor is a mechanical commutation device consisting of a brush and a commutator of a DC motor, which is replaced with a position detector and a thyristor.
The non-commutator motor has the advantages of both the performance of the DC motor and the ease of maintenance of the synchronous motor, and is therefore widely used for speed regulation of fans, pumps, extruders, etc., and as a gearless transmission for use in mine hoists, rolling mills, and cements. Tube mill speed control.
First, change pole logarithm speed control method
Change the red connection of the stator winding to change the stator pole pair of the cage motor to achieve speed regulation.
Second, frequency control method
Use the frequency changer to change the frequency of the motor stator power, thereby changing the speed control method of its synchronous speed.
Third, cascade speed control method
Cascade speed regulation refers to an adjustable additional potential that can be used to change the motor slip to achieve speed control. Most of the slip power is absorbed by the additional electrical potential in series, and re-use creates additional devices that utilize the absorbed slip power back to the grid or convert energy. According to the slip power absorption and utilization method, the cascade speed regulation can be divided into the motor cascade speed regulation, the mechanical cascade speed regulation and the thyristor cascade speed regulation form, and the thyristor cascade speed regulation is mostly used.
Fourth, winding motor rotor resistance control method
The rotor of the linear induction motor is connected in series with an additional resistor to increase the slip of the motor and the motor operates at a lower speed. The greater the resistance in series, the lower the motor speed. This method is simple in equipment and convenient to control, but the slip power is consumed on the resistor in the form of heat. A graded speed control, mechanical properties are relatively soft.
Fifth, stator speed regulation method
When the stator voltage of the motor is changed, different rotation speeds are obtained. Since the torque of the motor is proportional to the square of the voltage, the maximum torque drops a lot, and its speed range is small, making it difficult to use a general cage motor. In order to expand the range of speed regulation, a cage type motor with a large rotor resistance value should be adopted for pressure regulation and speed control, such as a torque motor for regulating the pressure and regulating the speed, or a series frequency sensitive resistor on a wound motor.
Six, electromagnetic speed motor speed control method
Electromagnetic speed control motor from the cage motor, electromagnetic slip clutch and DC excitation power supply. The DC excitation power is small, usually consisting of a single-phase half-wave or full-wave thyristor rectifier. The thyristor conduction angle can be changed and the magnitude of the excitation current can be changed.
Seven, hydraulic coupling speed control method
The hydraulic coupler is a hydraulic transmission device. Generally, it is composed of a pump wheel and a turbine. When the pump wheel is driven by the prime mover, the liquid in the impeller is rotated by the blade and moves along the outer ring of the pump wheel under centrifugal force. When entering the turbine, the turbine blades are thrust on the same steering wheel to drive the production machine. Speed ​​is achieved through different hydraulic forces (oil and turbine).
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