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How did the motor speed control when there was no inverter?

Nov 26, 2020

How did the motor speed control when there was no inverter?

When there was no frequency converter, it was basically a DC motor speed control system. Apollo also used an accurate speed control system when landing on the moon, and even required position positioning control. At that time, DC servo was used to complete it. Before the 1960s, when the thyristor was not born, the DC motor was driven by the generator to adjust the speed. Adjusting the excitation current of the generator can control the output voltage of the generator, thereby controlling the speed of the DC motor. The speed control system can be seen in the early textbook "Motor Drive", but it is just a stupid point, but the speed control range is very wide, the torque is large, and it is stable and reliable. In addition, the theory of DC speed control has been very mature. This kind of speed control system is used in cars.

丨DC Motor Speed Control System


The DC motor mentioned here, of course, is a brushed DC motor, because the magnetic field of the motor and the armature coil are controlled separately and orthogonally at 90°. There is no coupling problem. When the excitation current remains constant, the stator The field winding flux is constant

Motor torque = field winding flux * armature current


Therefore, as long as the armature current is adjusted, the precise torque control of the motor can be directly realized, and the control requirement of constant torque can be easily met. This is the fundamental reason why the low-speed torque of the DC motor speed control system is very good.


For the speed regulation of DC motors, the speed n=(armature voltage U-armature current I*armature internal resistance R)/constant Kφ,


Because the internal resistance R of the DC motor is very small, the speed n≈the armature voltage U/constant

The armature voltage U is almost proportional to the speed n, which is also an important reason why the speed regulation of the DC motor can satisfy the control through the voltage regulation of the generator.


Later, thyristor and other devices were invented. Through a full control bridge or a half control bridge, the alternating current can be directly turned into a controllable direct current. The voltage can be adjusted quickly and arbitrarily, so as to control the armature voltage of the DC motor. , Thereby changing the speed of the motor.

After the development of control theory, a cascade system was used to adjust the speed of the DC motor, that is, the speed loop is outside, the speed deviation is used as the setting of the current loop, and the current loop is used as the inner loop. Both loops use PID regulators. To complete the control, the response is fast, the accuracy is high, the torque is large, and the speed range is wide.

In addition to constant torque speed regulation, the DC motor can also be operated in a constant power region by reducing the excitation current to reduce the excitation flux. In this way, the torque decreases with the increase of the speed, and the power remains the same, but it can be widened. Speed range.


In fact, today's vector control mode of inverter speed regulation is carried out by imitating the speed regulation method of DC motor, and the effect is not as ideal as a DC speed regulation system. It is only because of the severe wear of the carbon brushes of the brushed DC motor, troublesome maintenance, and the high cost of motor manufacturing that the brushed DC motor speed control system has gradually withdrawn from the market. Even so, many low-power motors still use the DC speed control system, after all, the price has an advantage, and the performance is better.


丨Pole-changing speed


In addition to variable frequency speed regulation, there is actually another debugging method for asynchronous motors, which is to achieve variable speed by changing the number of pole pairs. For example, the speed of a four-pole motor is 1500 rpm, and an 8-pole motor only has 750 rpm. This kind of speed regulation The method has a great limit. It is generally called a two-speed motor. It often has only two speed stages, but the torque is relatively large and relatively stable. In some occasions where only two speeds are required, it is very important to use this speed regulation method. Ideally, for example, some mixing systems have such a speed control system. After running at low speed for a period of time, switch to high speed mode. This control system is very simple, a bit similar to the star-delta switching, so the cost is low. Today, even though frequency conversion is very popular, this kind of speed regulation is still used in many occasions.


In addition to constant torque speed regulation, the DC motor can also be operated in a constant power region by reducing the excitation current to reduce the excitation flux. In this way, the torque decreases with the increase of the speed, and the power remains the same, but it can be widened. Speed range.


In fact, the vector control mode of today's frequency converter speed regulation is carried out by imitating the speed regulation method of the DC motor, and the effect is not as ideal as the DC speed regulation system. Only because of the severe wear of the carbon brushes of the brushed DC motor, troublesome maintenance, and the high cost of motor manufacturing, have the brushed DC motor speed control system gradually withdrawn from the market. Even so, many low-power motors still use the DC speed control system, after all, the price has an advantage, and the performance is better.


丨Pole-changing speed


In addition to variable frequency speed regulation, there is actually another debugging method for asynchronous motors, which is to achieve variable speed by changing the number of pole pairs. For example, the speed of a four-pole motor is 1500 rpm, and an 8-pole motor is only 750 rpm. This kind of speed regulation The method has a great limit. It is generally called a two-speed motor. It usually has only two speed stages, but the torque is relatively large and relatively stable. In some occasions where only two speeds are required, it is very important to use this speed regulation method. Ideally, for example, some mixing systems have such a speed control system. After running at low speed for a period of time, switch to high speed mode. This control system is very simple, a bit similar to the star-delta switching, so the cost is low. Today, even though frequency conversion is very popular, this kind of speed regulation is still used in many occasions.

For example, in the clutch, there are coils and a lot of magnetic powder. When the magnetic powder is energized, it will stick together due to the magnetic field of the coil. The larger the current, the stronger the magnetic field, and the tighter the combination of magnetic powder will reach a certain level. It can become a certain rigid thing, and directly connect the output and output shaft to maintain a consistent speed output, so that the load can be as fast as the motor speed.


When there is no current at all and the magnetic field disappears, the magnetic powder will become a disc of loose sand, and there is no magnetic powder connection between the output and input shafts. Although the motor is still rotating, the load speed can become zero.

If the magnetic field current is at a certain value, the magnetic powder has a certain adhesion, but the stiffness is not enough, it will slip inside, which will cause a certain speed difference between the input and output shafts, and control the magnetic field current value to control the speed difference Size, so that the speed of the load can be changed.


Because it is slipping, it will definitely generate heat due to friction, so that a part of the electric energy will be wasted, and the efficiency of speed regulation will be low. Of course, it also has its advantages. It can be made into a speed closed-loop control, and the torque at low speed is even more ideal than the inverter speed regulation.