What is a frequency inverter?The frequency inverter is the popular name we give to frequency inverters. It is an electrical device that converts a current with one frequency into a current with another frequency. The voltage is usually the same before and after the frequency conversion. Frequency converters are commonly used for speed regulation of motors used to drive pumps and fans.
The original name of the inverters in English is VFD (Variable Frequency Drive). Throughout the text we will refer to VFDs simply as inverters.
For example: a fan is supplied with a current of 400 VAC, 50 Hz. At this frequency (50 Hz), the fan can run at a certain speed. To make the fan run faster, a frequency converter is used to increase the frequency to (for example) 70 Hz. Alternatively, the frequency can be converted to 40 Hz if the fan runs more slowly.
The AC drive converts the input power (fixed voltage and fixed frequency) to a variable voltage and frequency to control AC induction motors.
It consists of power electronic devices (such as IGBT, MOSFET), high-speed central control unit (such as a microprocessor, DSP), and optional sensor devices, depending on the application used.
Most industrial applications require variable speeds under peak load conditions and constant speeds under normal operating conditions. The closed loop operation of the inverters keeps the motor speed at a constant level, even in case of input and load disturbances.
The two main features of the drive are adjustable speeds and soft start/stop capabilities. These two features make the inverter a powerful controller for controlling AC motors. The VFD consists mainly of four sections; these are rectifier, DC bus, inverter, and control circuit.
Inverters (variable frequency drives) not only offer adjustable speeds for precise control applications, but also have more benefits in terms of process control and energy conservation. Some of these are given below.
Scalar methods for inverter control work by optimizing the motor power flow and keeping the magnetic field strength constant, which ensures constant torque production. Often referred to as V/Hz or V/f control, scalar methods vary both the voltage (V) and frequency (f) of motor power to maintain a fixed, constant relationship between the two, so that the magnetic field strength is constant regardless of motor speed.
The appropriate V/Hz ratio is equal to the motor’s nominal voltage divided by its nominal frequency. V/Hz control is usually implemented without feedback (i.e. open loop), although closed loop V/Hz control – incorporating motor feedback – is possible.
V/Hz control is simple and low cost, while closed loop implementation increases cost and complexity. Closed loop control is not necessary, but it can improve the performance of the system.
The accuracy of speed regulation with scalar control is lower compared to vector control, so these methods are not suitable for applications where precise speed control is required. Open-loop V/Hz control is unique in its ability to allow one drive to control multiple motors and is arguably the most commonly implemented control method.
Vector control – also known as field-oriented control (FOC) – adjusts the speed or torque of an AC motor by controlling the spatial vectors of stator current, in a similar (but more complicated way than) DC control methods. Field-oriented control uses complex mathematics to transform a three-phase system that depends on time and velocity into a time invariant system of two coordinates (d and q).
The stator current in an AC motor consists of two components: the magnetizing component (d) of the current and the torque producing component (q). With FOC, these two current components are controlled independently (each with its own PI controller). This allows the torque-producing component, q, to be kept orthogonal to the rotor flow for maximum torque production and therefore optimal speed control.
Vector control, or field-oriented control, converts three-phase currents in a stationary referential into a two-phase system (consisting of a flow component (d), and a torque-producing component (q), with a rotating referential. Here, the torque production current (q) can be independently controlled to ensure maximum torque production. The system is then transformed back into a three-phase system on a stationary reference frame for output to the motor.
Like scalar methods, vector control methods for inverters can be open loop or closed loop. Open loop vector control (also known as sensorless vector control) uses a mathematical model of the motor’s operating parameters, rather than using a physical feedback device. The controller monitors the motor voltage and current and compares them to the mathematical model. It then corrects any errors by adjusting the current supplied to the motor, which adjusts the motor’s torque output accordingly. With nonsense vector control, it is important to have a very accurate mathematical model of the motor, and the controller must be tuned for proper operation.
Closed loop vector control uses an encoder to provide feedback of the shaft position and this information is sent to the controller, which adjusts the supplied voltage to increase or decrease the torque. This is the only method that allows direct torque control in all four quadrants of engine operation for dynamic braking or regeneration.
Vector control methods are more complex than scalar control methods, but offer significant benefits over scalar methods in some applications. For example, open loop vector control allows the motor to produce high torque at low speeds and closed loop vector control allows a motor to produce up to 200 percent of its rated torque at zero speed, useful for holding stationary loads. Closed loop vector control also provides very accurate speed and torque control for industrial applications.
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