How to control the speed of a three-phase induction motor? The speed control method includes: changing the number of poles, stator voltage control, stator frequency change, cascade speed control, double-feed speed control, hydraulic coupler, electromagnetic slip clutch, etc.
The actual speed of the three-phase asynchronous motor is given by n = ns (1 - s) = 120f / p (1-s). It can be seen from the formula that the speed of a three-phase induction motor can be changed by changing the number of poles “p” of the induction motor, the slip “s” and the frequency of the power supply “f”.
The actual speed of 3-phase induction motor
Pole-variable speed controller
As shown in the formula ns = 120f / p, you can change the synchronous speed of the motor by changing the number of poles of the stator coil, thereby changing the running speed. Pole-shift speed control is most commonly used in squirrel-wheel induction motors. The pole-changing speed controller has the following characteristics:
Heavier mechanical properties and good stability
No-slip loss and high efficiency
Easy wiring, convenient control and low price
However, due to the large degree of difference, speed control cannot be achieved smoothly with this method. Therefore, it can be used with voltage speed control and electromagnetic slip clutch to obtain a more efficient smooth speed control characteristic.
This method is for step-less speed control manufacturing machines such as metal cutting machines, hoists, cranes, fans, water pumps and so on.
three-phase induction motor
Variable sliding speed control
1. Change the stator voltage
The torque of the induction motor is proportional to the square of the stator voltage. That is, changing the stator voltage can change the mechanical properties and torque of the motor.
This method is not suitable for a conventional squirrel-wheel motor because the rotor resistance is very small and the current increases rapidly at low speed.
But for a wound-type induction motor, it can be used in the rotor circuit using a series resistor or a common varistor to reduce the heat of the motor.
2. Change the rotor resistance
This speed control method is only applicable to the winding motor. In the rotor circuit of an induction motor with a series of resistors when the load is fixed, the higher the resistance, the lower the motor speed. The lower the resistance, the higher the speed.
This method is simple, easy to manage, and has low initial investment. However, the sliding energy is heated by the resistance. It also has soft mechanical properties.
3. Cascade speed control
Currently, cascade speed control uses the SCR inverter cascade control circuit and has the following advantages: stronger mechanical characteristics, low rectifier voltage drop, small space, no rotating part, low noise, easy maintenance. This is one of the speed control methods of the wound-type motor.
This also has its drawbacks. That is, the rotor circuit is equipped with a filter reactor, so the power factor is low.
Variable frequency speed control
According to the induction motor speed formula, it can be seen that when the slip s is constant, the speed n of the motor is essentially proportional to the power frequency f. Therefore, changing the frequency f can control the speed of the induction motor smoothly. Changing the power supply frequency is an economical speed control method and is one of the most popular ways to control the speed of an induction motor.
Variable frequency control allows you to change the power frequency of the motor stator and then change its synchronous speed. The main equipment of the variable frequency speed control system is the frequency converter or the variable frequency converter (VFD), which provides a frequency change to the power supply. Variable frequency drives can be divided into two categories: AC-DC-AC VFD and AC-AC VFD.
Three-phase induction motor and VFD
Today, widely used VFDs use digital technology and tend to be miniaturized, highly reliable, and highly accurate. It not only provides significant energy-saving performance for applications but also has the following performance:
- High precision, smooth speed control.
- Full protection function that can diagnose the fault with self-diagnosis and easy maintenance.
- Starting on a direct line, with high starting torque and low starting current, which reduce the impact on the electrical network and equipment and are provided with lifting torque, thus saving the soft starting device.
- High power factor and save the capacitor compensation device.
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