Before the frequency converter technology became popular, air compressor speed regulation mainly relied on the following methods:

1.Pole-changing speed regulation: Speed regulation is achieved by changing the number of pole pairs of the motor. Although this method has a limited speed regulation range, it can achieve greater torque and stable operation. For example, the speed of a four-pole motor is 1500 rpm, while the speed of an eight-pole motor is reduced to 750 rpm. This speed regulation method is suitable for occasions where only two speeds are required.

2.Slip speed regulation: Use electromagnetic clutch (such as magnetic powder clutch) to generate resistance, and adjust the load speed by controlling the magnetic field current value to achieve speed regulation. However, this method is inefficient and will cause energy loss.

3.DC motor cascade system speed regulation: PID regulators for speed loop and current loop are used to achieve fast response and high-precision speed regulation with a wide speed regulation range. However, brushed DC motors are gradually withdrawing from the market due to maintenance difficulties and high costs.

4.Changing the excitation current to adjust the speed: By reducing the excitation current of the DC motor, the magnetic flux is reduced, constant power operation is achieved, and the speed adjustment range is expanded.

5.Loading and unloading control method: The air compressor adjusts the air intake through loading and unloading control to maintain the balance between air supply and air consumption, but there are problems such as high energy consumption, easy damage of the air intake valve, and unstable air supply pressure.

6.Non-intelligent control methods: including V/f control, slip frequency control, vector control, direct torque control, etc. These control methods are used in inverters, but before inverter technology is popularized, similar control effects may be achieved through other methods.

These methods provided a means of speed regulation for air compressors in industrial production before the frequency converter technology became popular. However, with the development of frequency converter technology, these traditional methods have gradually been replaced by more efficient and energy-saving frequency conversion speed regulation technology.