What are the working principles of fixed speed and variable speed compressors?

Power frequency air compressors are also called fixed speed air compressors. In any case, as long as the air compressor is started, the speed will always be constant.

The motor uses a 3-phase asynchronous motor. The motor speed of different powers is generally in the range of 2930-2970 ( 380v50hz).

The variable speed(VSD) air compressor uses a 3-phase permanent magnet synchronous motor, which is used in conjunction with a frequency inverter to adjust the motor speed according to the pressure feedback signal to achieve the purpose of energy-saving.

What is the difference between fixed speed vs variable speed compressor?

Working Pressure:

1. The variable speed(VSD) air compressor uses the step-less speed regulation characteristics of the frequency converter, and can start smoothly through the PID regulator inside the frequency converter; it can quickly adjust the response to the working environment with relatively large fluctuations in air consumption;
2. Air pressure stability is much better than a fixed-speed air compressor.

Machine Start:

1. The frequency converter of the air compressor has the function of a soft starter. The maximum starting current is within 1.2 times the rated current, and the starting impact is small. The starting current of a power frequency air compressor is generally more than 6 times the rated current.
2. The start-up impact of variable frequency air compressors is much smaller than that of industrial frequency air compressors.

Air Flow Control：

1. The air compressor is driven by fixed speed can only work at one displacement. The variable speed air compressor can work in a wide range of displacement. The frequency converter adjusts the motor speed in real-time according to the actual air consumption to control the air displacement.
2. When the air consumption is low, the variable inverter can be automatically dormant, which greatly reduces the energy loss.
3. The optimized control strategy can further improve the energy-saving effect.

The Voltage Adaptability of AC Power:

1. Due to the over-modulation technology adopted by the inverter, sufficient torque can be output to drive the electric motor even when the AC power supply voltage is slightly lower. When the voltage is slightly higher, the voltage output to the motor will not be higher;
2. For the occasion of self-generation, the variable frequency drive can show its advantages better;
3. According to the characteristics of the motor’s VF (variable speed air compressors work below the rated voltage in an energy-saving state), the effect is obvious for sites with the low grid voltage.

Machine Noise：

Most of the working conditions of the frequency conversion system are lower than the rated speed, the noise and wear of the main engine are reduced, and the noise is lower than that of the fixed speed system.

Price:

Because the fixed speed air compressor does not have a frequency converter, the production cost and price are usually cheaper than the variable speed air compressor.

Let Us See More Simple and Clear:

Variable Speed Feature

• Energy-saving, reduce production energy consumption
• Extend the service life of the machine
• Stable airflow, suitable for intermittent operation
• Air compressor unit runs more quietly
• Automatically adjust the motor speed

Fixed Speed Feature

• Low machine cost
• Motor speed is not adjustable
• Airflow always maintains the same standard
• Under the same requirements, it is suitable for 7*24 hours of uninterrupted operation

Do I Need a Fixed Speed or Variable Speed Compressor?

Choose a Fixed Speed Drive for This Condition:

The fixed speed mainly relies on two points to control the start and stop (high-pressure stop, low-pressure start).

If there is no high requirement on the air pressure used, and the selected air compressor produces air volume close to the required air volume, it is the remaining amount. When the volume is not large, it is recommended to use the power frequency.

At this time, regardless of the power frequency conversion air compressor, in order to ensure the air volume demand, it will basically run at full load.

At this time, the power frequency conversion consumption is the same, and the frequency conversion air compressor is still more With the addition of a frequency converter, there is an additional component that can be easily damaged and repaired, and the power saved in long-term use is negligible.

In some cases, VSD is the correct solution. If your power supply costs are high, the compressor runs for many hours a day, and your needs vary greatly, then VSD makes sense. 

Finally, VSD needs a clean, dry installation area and a good power supply.

Therefore, the frequency conversion air compressor is not necessarily the best choice. You must choose the appropriate air compressor according to your own use situation and the characteristics of the air you use.