How the frequency inverter works

In general, a frequency inverter is an electronic device capable of varying the turning speed of a three-phase induction motor. It is a device that transforms fixed alternating electric current (current and voltage) into variable AC electric current, controlling the power consumed by the load by varying the frequency delivered by the network. This device has this name because of the way it makes this three-phase motor rotation variation.

Three-phase induction motors are highly employed for having high efficiency, low cost, robustness, and also for the configuration of our energy distribution system, which is made in alternating current (AC). Due to these characteristics, the three-phase motor is ideal for almost any type of operation, widely found in the industry. With regard to speed, this motor has a constant speed, depending on the loads attached to it and the use of a frequency inverter. Its operating principle is based on the rotating magnetic field, which arises when an alternating current supply system is applied to 120 ° out of phase poles. In this way the magnetic field appears, through this lag.

The speed of rotation of the three-phase motor is linked to the speed provided by the rotating magnetic field, this speed is called synchronous speed, depending on the number of poles of the motor (constructive characteristic) and depending on the frequency of the network to which it is connected. Therefore, we conclude that the speed of the three-phase electric motor is directly proportional to the network frequency. Mathematically: Synchronous speed (Ns) in RPM is the product of 120 times the frequency in Hz (f), divided by the number of poles of the motor (p).

Frequency inverter.

How the Frequency Inverter works

The frequency inverter, as previously mentioned, is an electronic device whose main function is to vary the rotation of a three-phase motor, by changing the frequency it provides in its output contacts. This way we can easily switch the speed at which the engine will work. The formula presented earlier shows us how this works. The frequency supplied by the network (motor input frequency) determines the synchronous speed of the electric field through which the motor works. The inverter acts by changing this frequency at the motor input, if the frequency is higher, consequently the motor speed will be higher, and if the frequency is lower, the speed is also lower. The use of a frequency inverter has a number of advantages, such as, for example, explore the operation of the engine and conditions not described in its construction characteristics. Follow in the video below the illustration of this explanation and the speed reduction test of a three-phase motor through a frequency inverter.

The use of the frequency inverter provides speed flexibility with safety and precision. It is possible, for example, to control the engine speed without major torque losses, smooth acceleration through programming, direct braking on the engine, without the need for mechanical brakes, in addition to various forms of speed programming according to the need of the occasion. Other advantages of using the frequency inverter are:

  • Replacement of mechanical and electromagnetic variations;
  • Automation, security and flexibility in industrial processes;
  • Simple installation;
  • Reduction of mechanical shocks at engine start;
  • Precision and processes;
  • Less human intervention;

In addition to these advantages, the frequency inverter has great cost-benefit, as it provides electricity savings, a longer durability of gears, pulleys and other mechanical components.

Acceleration ramps

The frequency inverter is widely used in this type of function, being one of its biggest advantages. When a motor is energized, most of the time, it starts from inertia to its maximum capacity in a few seconds. This is detrimental to the engine components, as this sudden start can cause wear of the belts, gears, and damage to other parts. This way, the maintenance number is longer and the engine life is shorter.

The acceleration ramp is used to solve this problem. By configuring a frequency inverter to act as an acceleration ramp, it is able to determine the time that the motor will go out of inertia and reach its maximum capacity, without “jerks”. This type of configuration increases the useful life of the engine and its components, in addition to reducing maintenance and replacement costs. In the same way that there are acceleration ramps, there are deceleration ramps, which consist of a smooth stop of the engine, in order to avoid sudden braking.

Configuring the acceleration ramps in inverters is very simple. By default, the inverters leave the factory with the acceleration ramp set to 5 seconds, but it is possible to adjust this value from 0.1 seconds up to 245 seconds.