【Daily Question 20090312】Parameter Setting of Inverters (VI)?

Braking principle of the inverter: If the output frequency of the inverter decreases, the motor speed will also decrease accordingly, resulting in a braking process. The power generated by this braking is sent back to the inverter side, and this power can be dissipated as heat through resistors. When used to increase the load, during the descent phase, the energy (potential energy) also needs to return to the inverter (or power supply) side for braking. This operation method is called \"regenerative braking\", and it can be applied to inverter braking. During deceleration, the method in which the generated power is not dissipated through heat loss, but instead returned to the power supply side of the inverter is called the \"power return regeneration method\". In practice, this application requires the \"energy feedback unit\" option. Therefore, to improve braking performance, one cannot rely on increasing the capacity of the inverter to solve the problem. Please select options such as \"braking resistor\", \"braking unit\", or \"power regeneration converter\" to improve the braking capacity of the inverter

Braking a motor using an inverter involves switching the motor from an electric mode to a power-generation mode in order to bring it to a stop within a specified time. During this process, the DC voltage of the inverter increases; therefore, it is common practice to connect a discharge resistor on the DC side, thereby converting the motor’s kinetic energy into heat energy that is dissipated in the discharge resistor. This state differs from free parking, as free parking involves cutting off the inverter’s control over the motor.

Principle: After the electric motor is disconnected from the AC power supply, the rotor continues to rotate due to inertia. Immediately, direct current is applied to the two-phase stator windings, thereby creating a stationary magnetic field in the stator. The conductive bars in the rotor cut through this stationary magnetic field, thereby generating an induced current, which is subject to electromagnetic forces within the stationary magnetic field. The torque generated by this force acts in the opposite direction to the rotor’s inertial rotation, and is known as braking torque; it forces the rotor’s speed to decrease. When the rotor speed drops to 0, the rotor no longer cuts through the magnetic field, the motor stops rotating, and braking comes to an end. This method utilizes the energy from the rotor’s rotation to cut through the magnetic flux, thereby generating a braking torque; essentially, the kinetic energy of the rotor is dissipated in the resistance of the rotor circuit, which is why it is called energy-consuming braking. When a motor driven by an inverter or other inductive loads comes to a stop, energy-consuming braking is generally used for this purpose; it involves dissipating the kinetic energy of the motor as well as the magnetic energy stored in its coils through another energy-consuming element, thereby enabling rapid stopping. When power supply is interrupted, the inverter circuit of the frequency converter conducts in reverse, feeding this remaining electrical energy back into the DC bus of the frequency converter. As a result, the voltage on the DC bus rises. When it reaches a certain level, the braking resistor of the frequency converter comes online, dissipating this electrical energy through heat generation, thereby maintaining the voltage on the DC bus at a normal level. This post was last edited by lihy on 2009-3-12 12:40]

Braking a motor using an inverter involves switching the motor from an electric mode to a power-generation mode in order to bring it to a stop within a specified time. During this process, the DC voltage of the inverter increases; therefore, it is common practice to connect a discharge resistor on the DC side, thereby converting the motor’s kinetic energy into heat energy that is dissipated in the discharge resistor. This state differs from free parking, as free parking involves cutting off the inverter’s control over the motor. To improve braking performance, one cannot rely on increasing the capacity of the inverter to solve the problem. Options such as \"braking resistor\", \"braking unit\", or \"power regeneration converter\" can be used to improve the braking capacity of the inverter

Braking a motor using an inverter involves switching the motor from an electric mode to a power-generation mode in order to bring it to a stop within a specified time. During this process, the DC voltage of the inverter increases; therefore, it is common practice to connect a discharge resistor on the DC side, thereby converting the motor’s kinetic energy into heat energy that is dissipated in the discharge resistor. This state differs from free parking, as free parking involves cutting off the inverter’s control over the motor.

Law of conservation of energy! When kinetic energy is exhausted, motion stops. Working process: During deceleration, the synchronous speed is lower than the actual speed of the motor; the motor is in power generation mode, with mechanical energy being converted into electrical energy. This electrical energy is fed back to the inverter’s DC bus through the freewheeling diodes built into the IGBTs. When the voltage on the DC bus exceeds a certain value, the braking unit activates, converting the electrical energy into heat energy through braking resistors. In reality, it is the magnetic force that causes the load to stop moving; this is similar to the motor startup process, except that during startup the speed of the magnetic field is higher than the actual rotational speed, while during stopping the speed of the magnetic field is lower than the actual rotational speed. It is used for motor braking; when the motor slows down, it enters a power-generation mode, and the brake resistor is responsible for consuming the excess electrical energy generated by the motor and sent back to the inverter, thereby preventing the bus voltage inside the inverter from rising too high and triggering protection mechanisms.

Working principle of inverters: An inverter is mainly composed of components such as rectification (converting AC to DC), filtering, re-rectification (converting DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit. 1. Why can the rotation speed of a motor be changed freely? 　 1: r/min – The unit for motor rotation speed; it represents the number of rotations per minute, and can also be expressed as rpm. For example: a 2-pole motor at 50Hz has a rotation speed of 3000; a 4-pole motor at 50Hz has a rotation speed of 1500. In conclusion, the rotation speed of a motor is proportional to the frequency. The motors referred to in this text are induction AC motors, which are the type of motors most commonly used in industry. The rotational speed of an induction AC motor (hereinafter referred to as the motor) is approximately determined by the number of poles and the frequency of the motor. The number of poles of a motor is fixed, as determined by its working principle. Since this pole value is not a continuous number (it is a multiple of 2, such as 2, 4, 6), it is generally not suitable to adjust the motor’s speed by changing this value. 　　Additionally, the frequency can be adjusted outside the motor before being supplied to it, allowing the rotation speed of the motor to be controlled freely. 　　Therefore, inverters designed for frequency control are the preferred devices as motor speed control equipment. 　　n = 60f/p 　　n: Synchronization speed 　　f: Power supply frequency 　　p: Number of pole pairs in the motor 　　Conclusion: Changing the frequency and voltage is the optimal method for motor control. 　　If only the frequency is changed while the voltage remains unchanged, a decrease in frequency can cause the motor to experience overvoltage (over-excitation), which may lead to damage to the motor. Therefore, the inverter must change the voltage simultaneously while changing the frequency. When the output frequency is above the rated frequency, the voltage cannot increase any further; it can at most reach the motor’s rated voltage. 　　For example: to reduce the rotational speed of the motor by half, the output frequency of the inverter needs to be changed from 50Hz to 25Hz; in this case, the output voltage of the inverter must also be changed from 400V to around 200V. 2. What happens to the output torque of the motor when its rotational speed (frequency) changes? 　　　　1: Power supply at line frequency 　　　　The power source provided by the electrical grid (commercial power supply) 　　　　2: Starting current 　　　　The output current of the inverter when the motor starts to operate 　　　　The starting torque and maximum torque when driven by an inverter are lower than those when the motor is driven directly by a power supply at line frequency. When powered by a line-frequency power supply, the starting and acceleration shocks on the motor are quite large; however, these shocks are reduced when an inverter is used for power supply. Direct starting at line frequency generates a large starting current. When a frequency converter is used, the output voltage and frequency of the converter are applied to the motor gradually, so the starting current and shock on the motor are reduced. 　　Generally, the torque generated by a motor decreases as the frequency decreases (i.e., as the speed drops). The reduced actual values are specified in some frequency converter manuals. 　　By using an inverter with flux vector control, the torque deficiency of the motor at low speeds will be improved, allowing the motor to generate sufficient torque even in low-speed ranges. 3. When the frequency converter adjusts the speed to a value higher than 50Hz, the output torque of the motor will decrease. Ordinary motors are designed and manufactured for a voltage of 50Hz, and their rated torque is also specified within this voltage range. Therefore, speed control at the rated frequency is referred to as constant-torque speed control. (When T=Te and at 60 Hz, X decreases accordingly.) For a motor, T=K*I*X, where K is a constant, I is the current, and X is the flux; thus, the torque T decreases as the flux X decreases. Meanwhile, when the frequency is below 50 Hz, since I*R is very small, the flux (X) remains constant when U/f = E/f. Torque T is proportional to the current. This is why the overcurrent capacity of an inverter is often used to indicate its overload (torque) capacity. This type of control is also known as constant-torque speed control (since the rated current remains unchanged, the maximum torque also remains unchanged). Conclusion: As the output frequency of the inverter increases above 50 Hz, the output torque of the motor decreases. 5. Other factors related to output torque Heat generation and heat dissipation capabilities determine the inverter’s ability to handle current, which in turn affects its output torque capacity. 　　Carrier frequency: The rated current specified for typical inverters is the value that can be maintained under the highest carrier frequency and highest ambient temperature. Reducing the carrier frequency does not affect the current in the motor. But the heat generation of the components will decrease. 　　Ambient temperature: Just as the protection current value of the inverter is not increased simply because a low ambient temperature is detected. Altitude: As altitude increases, it has an impact on both heat dissipation and insulation performance. Generally, altitudes below 1000 meters can be ignored; for higher altitudes, the capacity should be reduced by 5% for every 1000 meters. 6. How does vector control improve the motor’s output torque capability? 　　*1: Torque enhancement This feature increases the output voltage of the inverter (especially at low frequencies), in order to compensate for the loss in output torque caused by voltage drops across the stator resistances, thereby improving the motor’s output torque. 　　 　 Technologies to improve insufficient low-speed output torque of motors: By using \"vector control\", it is possible for the motor to achieve a output torque at low speeds – such as 1 Hz (for a 4-pole motor, this corresponds to a speed of around 30 rpm) – that is equivalent to the torque it can produce when operated at 50 Hz (approximately 150% of its rated torque). 　　In conventional V/F control, the voltage drop of the motor increases relatively as the motor speed decreases, which results in insufficient excitation and prevents the motor from obtaining adequate rotational force. To compensate for this deficiency, the inverter needs to increase the voltage in order to make up for the voltage drop caused by the decrease in motor speed. This function of the inverter is called “torque boost” (*1). 　　The torque boost function is to increase the output voltage of the inverter. However, even when the output voltage is increased significantly, the motor torque does not increase proportionally to its current. Because the motor current includes the torque component generated by the motor as well as other components (such as the excitation component). 　　"Vector control allocates the current values of the motor, thereby determining the values of the motor current components that generate torque and other current components such as the excitation component. 　　"Vector control can provide optimized compensation by responding to the voltage drop at the motor terminal, allowing the motor to generate high torque without an increase in current. This feature is also effective in reducing the temperature rise of the motor at low speeds. Inverter structure: An inverter is a device that converts mains power (50Hz or 60Hz) into AC power of various frequencies, thereby enabling variable-speed operation of motors. Among them, the control circuit is responsible for controlling the main circuit; the rectifier circuit converts alternating current into direct current, the DC intermediate circuit smooths and filters the output of the rectifier circuit, and the inverter circuit converts the direct current back into alternating current. For inverters that require extensive calculations, such as vector-controlled inverters, a CPU for torque calculation and some corresponding circuits are sometimes also needed. 1. Rectifier, which is connected to a single-phase or three-phase AC power supply to generate a pulsating DC voltage. 2. The intermediate circuit serves three functions: a. It stabilizes or smooths out the pulsating direct current voltage to supply it to the inverter. b. Power each control circuit through a switching power supply. c. Filtering or braking devices can be configured to improve the performance of the inverter. 3. Inverter, which converts a fixed DC voltage into an AC voltage with variable voltage and frequency. 4. Control circuit, which sends signals to the rectifier, intermediate circuit, and inverter, while also receiving signals from these components. Its main components are: the output drive circuit and the operation control circuit. The main functions are: a. Using signals to switch the semiconductor devices of the inverter. b. Provide various control signals for operating the frequency converter. c. Monitor the operating status of the inverter and provide protection functions.

In a variable-frequency speed control system, the speed reduction process corresponds to a decrease in frequency. From the perspective of energy balance, speed reduction involves converting kinetic energy into regenerative electrical energy. The electrical energy generated by the motor during regenerative braking is rectified into direct current through full-wave rectification using diodes adjacent to the inverters in the inverter circuit, and this direct current is fed back into the DC circuit, thereby creating a voltage rise (this voltage rise is referred to as a pump-up voltage since it is pulsating in nature), which increases the DC voltage. When this voltage reaches a certain level, the electrical energy is dissipated through braking resistors or braking units. Since kinetic energy is consumed by the DC resistance during the deceleration process, the operating mode of the entire variable-frequency speed control system is one of energy-consuming braking. When selecting brake resistors, calculating their resistance values can be quite complicated. Generally, the manufacturer’s manual for the frequency converter provides reference values. In order to reduce the number of different resistance values required, motors of different capacities can use the same resistance value. The selection of resistance and capacitance takes into account the temperature rise curve of the resistor and the average power during the deceleration process.

There are two modes for braking and stopping a frequency converter: inertial stopping and ramp stopping. In inertial mode, upon receiving a stop command, the motor slows down inertially until it comes to a stop. In slope mode, upon a stop command being given, the motor slows down according to the set deceleration parameters; if the amount of regenerative energy is high, an external braking resistor must be used to accelerate the stopping process