The frequency inverter is an electronic device specially used to control the speed of the AC induction motor and controls the speed and torque of the AC motor by changing the input frequency and voltage of the motor. The frequency inverter adjusts the voltage and frequency of the output power supply by turning on and off the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy-saving and speed regulation. Also, the frequency converter has many protection functions, Such as overcurrent, overvoltage, overload protection, etc.
The frequency converter can convert the "current" electrical variable and the alternating current with a specific (fixed) frequency into a voltage with variable amplitude and frequency. Does not affect the electric consumption, torque, impedance, magnetic flux, etc. of the motor. It is integrated into the operator interface for receiving the required speed control commands (using the keyboard). The frequency converter is driven by a constant speed motor that drives two variable speed sheaves positioned by the pneumatic operator.
The main circuit of the frequency converter can be roughly divided into two categories: voltage type is the frequency converter that converts the DC of the voltage source to AC, and the filter of the DC loop is the capacitor. The current type is an inverter that converts the DC of the current source into AC, and the DC loop filter is an inductor. The electronic frequency converter consists of a rectifier and an inverter acting on it. The rectifier provides the so-called "intermediate DC". This can convert the supplied current to a specified voltage. Frequency converters are used in applications ranging from small equipment to large mine drives and compressors. Electric motors in industrial applications consume about 25% of the world's electrical energy, which is particularly beneficial for energy saving using frequency converters in centrifugal load services.
The first job of the frequency converter is to convert the AC sine wave to DC. The frequency converter will change the frequency and amplitude of the output voltage to change the speed, power, and torque of the connected induction motor to meet the load conditions. The first component of all frequency converters is a device called a rectifier or frequency converter. The electronic circuits in the frequency converter unit are divided into an input converter (bridge rectifier stage), a DC bus (filter stage), and an output inverter ( Use microcontroller and IGBT) these three main stages.
Input converter (bridge rectifier stage): This stage consists of high-power diodes arranged in a conventional bridge configuration. The AC power applied here is converted to DC after rectification. This direct current has no residual alternating current components and harmonics. It requires further filtering. The rectifier part consists of a series of fast-acting switches that convert the input AC voltage power supply into a pulsating DC voltage. The intermediate circuit consists of a DC bus and related circuits to stabilize and smooth the output of a pulsating rectifier.
IGBT is usually not used in the front end of the inverter rectifier. Inverter rectifiers usually use SCR or similar slower switching components. The advantage of SCR is that, given the variable input voltage quality, its simpler design is more rugged, and the cost is relatively low.
DC bus: Use inductors and capacitors to remove and filter the rectified DC power from the remaining harmonics and AC residues. This stage helps to make the motor output completely free of fluctuations, which is very suitable for AC motors.
Inverter(https://www.scientekpower.com): This stage converts the DC in the DC bus back to AC, but in a very special way it forms the heart or brain of the circuit. It consists of complex microcontroller ICs, which are specially designed and programmed to change the output frequency and voltage proportionally and to produce three-phase output from a single-phase input. In particular, this stage makes the inverter very unique, and it is the most ideal choice for controlling the speed of AC motors.
Output: The command from the previous stage (microcontroller IC) is sent to the output IGBT (insulated gate bipolar transistor), which switches the voltage received from the DC bus to a narrow chopping order (much like a dimmer switch Principles used in). To this end, the IC uses PWM technology and converts DC to a quasi-sine wave(Pure sine wave inverter). The longer the switching time of these waves, the higher the voltage on the motor output.
The program is actually responsible for two important functions-changing the output voltage without wasting any electrical energy, and it is very critical to simultaneously change its frequency at a specific given rate to keep the torque and magnetic flux constant.
Frequency converter acceleration and deceleration time:
Usually, the rise and fall of the frequency setting signal are used to determine the acceleration and deceleration time. In order to prevent overcurrent, it is necessary to limit the rate of increase in frequency during acceleration during motor start and to prevent overvoltage during the period of deceleration, it is necessary to limit the rate of decrease in frequency.
Acceleration time setting requirements: Limit the acceleration current below the inverter's overcurrent capacity, so that the overcurrent speed loss does not cause the inverter to trip; the deceleration time set point is to prevent the smoothing circuit voltage from being too high so that the regenerative overvoltage will not cause frequency conversion Tripping. The acceleration and deceleration time can be calculated according to the load. It is best to set a longer acceleration and deceleration time during commissioning to see if there is an overcurrent or overvoltage alarm during motor start/stop; then gradually reduce the acceleration and deceleration time according to the principle of no-alarm operation. Repeat several times to determine the best acceleration and deceleration time.
Frequency limit
The frequency limit is to prevent malfunctions caused by improper operation or the output frequency of the external frequency setting signal source is too high or too low and has a protection function to prevent equipment damage. The frequency converter can be used to reduce the wear of the machine and belt by setting the maximum frequency of the frequency converter to a certain value, which can make the conveyor belt run at a fixed low speed.
Electronic thermal overload protection
In order to prevent the motor from overheating, the internal CPU of the inverter calculates the temperature rise of the motor according to the operating current and frequency, thereby enabling overheat protection. This function is only applicable to "one inverter drives one motor". If one inverter drives multiple motors, a thermal relay should be installed on each motor.
Frequency deviation
When the frequency is set by an external analog signal (voltage or current), when the frequency setting signal is the minimum value, use this function to adjust the output frequency value. When the frequency setting signal is 0%, the deviation value of some inverters can be provided from 0 to the maximum value, and some inverters can even set the bias polarity. In commissioning, when the frequency setting signal is 0%, the output frequency of the inverter is not 0 Hz, but x Hz, then the deviation frequency can be set to negative x Hz so that the output frequency of the inverter is 0 Hz.
The frequency converter is specially used to control the speed of the AC motor by strictly following the above parameters. By changing the magnitude and frequency of the input voltage at a constant rate to change the speed of the motor, the motor can maintain a constant torque even at lower speeds. The drive isolation transformer is designed to protect the inverter from the interference of the upstream power supply, and can hardly reduce the amplitude of the harmonic current reflected by the inverter back to the power system.
The common frequency setting methods of frequency converter are operator keyboard setting, contact signal setting, analog signal setting, pulse signal setting, and communication method setting. These frequency reference methods have their own advantages and disadvantages. They must be selected and set according to actual needs. At the same time, different frequency reference methods can be selected for superposition and switching according to functional needs.
The basic speed of any AC motor is inversely proportional to the number of stator poles and directly proportional to the frequency of the supply voltage. The basic characteristics of AC motors make the voltage and frequency that must be applied always maintain a specific constant ratio. To change the speed of an AC motor, you need to change the frequency or the number of stator poles. Since the number of stator poles of each motor is fixed, obviously we cannot change them. The speed of the motor can be changed by changing the frequency of the power supply voltage through some simple methods.
Changing the frequency only at a constant voltage (120 or 230) will cause the equivalent impedance of the motor to decrease, resulting in greater magnetic flux and causing the motor to start drawing dangerously large currents. Therefore, the supply voltage must be reduced in proportion to the frequency at a certain fixed ratio. Otherwise, the magnetic flux of the motor will be saturated and the motor will be damaged. Since the magnetic field in the air gap is constant, changing the frequency and voltage proportionally can also ensure a constant torque.
If the rest of the system changes, the advantages of the inverter are energy-saving, maintenance, information (feedback), and future control flexibility. Constant torque ensures a wider speed control range, enabling energy-saving control over the entire range. For medium-sized 480 V motors, the installation cost of the frequency converter is usually lower than the cost of control valves when stainless steel valves are required. The inverter has a built-in filter and supports PWM output signals. It has a good anti-interference ability and greatly improves stability. Has the strong anti-interference ability, safe to use.
The frequency converter is intelligent, and some frequency converters can be viewed or downloaded from any networked computer. They can act as remote I/O and provide a percentage load and Hz and can be started and stopped only through the communication cable from the PLC, thereby reducing installation costs. They have a built-in stall and overload protection, overcurrent protection, and are particularly useful when controlling high inertia motors.
The frequency converter acts as a barrier between all input voltage disturbances (such as harmonics, ripple, sags, surges, etc.) and prevents them from entering the motor. The resistive load can be easily controlled by changing the applied voltage. Controlling an inductive load like an AC motor is definitely not that simple, it can only be done with a frequency converter.
With the help of an LED indicator, control panel, and programmable inverter, you can effectively control many parameters and functions of users. By standardizing individual modules, the frequency converter can be integrated as a modular component in an existing SPS system, and can also be accessed through a serial interface or other analog output. Thanks to this modular approach and therefore improved design, installation and wiring can be completed faster.
The frequency converter will increase the current induced inside the rotor due to the magnetic field. For small motors, this does not cause important problems, but for large motors, this strength on the bearings can cause corrosion and wear on the ball bearings. To solve this problem, we must cut off these currents.
Motors that do not require modifications to the IEC 250 frame size (including). Due to the IEC 280 frame size, motors with isolated shafts or bearings must be ordered (please consult COSGRA)
The motor power must be too large in the minimum 10% range because the frequency converter will reduce efficiency.
A heat detector must be installed in the winding of the motor.
The motor efficiency (IE3 / IE2 / IE1) can be one level lower than the corresponding efficiency level.
The process of inverter capacity selection is actually the best matching process between the inverter and the motor. The most common and safer is to make the capacity of the inverter greater than or equal to the rated power of the motor, but the actual matching should consider the motor's What is the difference between the actual power and the rated power? Usually, the selected capacity of the equipment is too large, but the actual required capacity is small. Therefore, it is reasonable to select the inverter according to the actual power of the motor. Avoid choosing the inverter too large, which will increase the investment. For light loads, the inverter current should generally be selected according to 1.1 N (N is the rated current of the motor), or the maximum motor power matching the output power rating of the inverter indicated in the product by the manufacturer.