The frequency inverter is an electronic device used to control the speed of a three-phase motor. The frequency that arrives at the motor input determines the speed at which it will operate. Three-phase motors have the principle of operation based on the rotating electric field. The field that appears when an alternating power system is applied to the poles of an engine, 120 ° out of phase. The speed at which the motor works is provided by the rotating electric field, this speed is called the synchronous speed. It is determined as a function of the number of poles of the motor (constructive characteristic) and as a function of the frequency at which the motor enters.
Mathematically speaking, the synchronous speed (Ns) is the product of 120 times the frequency (f) in Hz, divided by the number of poles (p) of the motor. From this formula, it is clear that the higher the frequency that reaches the engine, the higher the working speed of the engine and the reverse also influences the speed and the engine’s lower speed. And it is this change that the frequency inverses make, it performs this intervention before the motor input.
Frequency is a quantity, measured in Hertz (Hz). It corresponds to the number of oscillations or cycles per second that occur in the electric current.
Using a frequency inverter has a number of advantages, such as: controlling the motor speed, without large torque losses; smooth acceleration through programming; direct braking on the engine, without the need for mechanical brakes; speed programming according to the need; automation; flexibility; safety; simple installation; greater precision; etc.
In order to understand how this change is made in the frequency supplied by the network to the motor input, it is first necessary to know the parts of a frequency inverter.
- Input circuit (bridge rectifier):
This block rectifies the alternating energy available for supplying the inverter. The most common configuration is a full-wave diode bridge and at the output a capacitor that filters the voltage obtained.
- Power inverter:
This part turns the DC voltage of the previous block into a three-phase voltage to supply the motor. Transistors (IGBTs) are used that switch the voltage from the PWM (Pulse Width Modulation) generator signals. When these signals generate an inductive load such as the three-phase motor, they take an almost sinusoidal shape, despite being generated as pulse trains.
In this circuit, waves are formed that determine the speed and power applied to the engine. The control block generates pulses that act on the switching transistors.
- Surge protection:
The voltage of the power grid is not perfect and may contain surges and transients, to protect the circuit, elements such as varistors, TVS and similar elements are used in the frequency inverses.
- Internal protection:
This block analyzes the voltages present at the inverter output so that if they present any disturbance, the control block is activated to take the necessary measures, such as interrupting the process.
- Driver’s board (IGBT trip, power supplies, etc.):
Signal generator block for excitation of output power transistors.
This block analyzes the conditions of the load, determining what voltage should be applied to it to generate the necessary torque.
The panel that presents general information and is also where the inverter is programmed.
- Interface (I / O):
Through this block, the inverter communicates with external devices, such as computers.
In this block, decisions are made according to schedules, and internal or external signals.
The frequency inverter is connected to the mains, and at its output, there is a load that will receive the frequency modified by the inverter. In the first stage, the inverter uses the rectifier circuit to transform the alternating voltage into continuous. After that, the second stage does the reverse, transforms voltage C into AC voltage (converter), and with the desired frequency. In the network, the frequency is fixed, usually, 60 Hz, and the voltage is transformed by the input rectifier into pulsed continuous (full-wave rectification). The capacitor (filter) transforms it and pure direct voltage. This continuous voltage is connected to the output terminals by the inverter’s semiconductor devices, the transistors, which act as a static switch. The control system controls the action of these semiconductors, to achieve a pulsed voltage, with fundamental frequencies out of phase 120º. The voltage is chosen so that the voltage/frequency ratio is constant, resulting in constant flow operation, and maintaining maximum motor overload capacity.
Scalar Inverter X Vector Inverter
Scalar inverters are used in simpler tasks such as controlling start and stop and maintaining speed at a constant value (regulation). The control logic used is the constant voltage/frequency ratio.
The vector inverter is more complex compared to the scalar inverter. Basically, it promotes the decoupling between flow control and speed control through the transformation of variables. By this control technique, these inverters are used in more complex tasks, which require great precision.
The biggest difference between these inverters and the operation mode of each one is the ability to invert the factorials. As can be seen, the scalar inverter changes the frequency according to the voltage/frequency ratio, while the vector inverter does this in a more complex way, making changes in the parameters that influence these quantities.