Nov 10, 2020
The basic characteristics of a high-quality inverter and the composition of a voltage inverter
Part 1: When choosing a good inverter, you should pay attention to the following issues:
1. Whether it is produced by a regular manufacturer: A regular product must have a Chinese trademark and manufacturer, and the manufacturer’s address and phone number are printed on the package. There are a lot of products in China, but they don’t even have a trademark or manufacturer. They are called imported products. It’s best not to buy such products. If there is a quality problem, you don’t even know who you are looking for. Count on the seller to guarantee you after-sales service.
2. Be sure to use metal shell products: The car inverter has a large power and generates heat. If the internal heat cannot be dissipated in time, it will affect the life of the components in the light of it, and it may cause fire. The metal shell has good heat dissipation characteristics on the one hand, and it will not burn on the other hand. It is best not to use plastic case products. Even if a fan is added to help dissipate heat, it will increase the noise during use, and the service life of the fan will generally be relatively short, which will reduce the reliability of the whole machine. If it stops running someday, the consequences can be serious.
3. You must choose a split product. You cannot choose an integrated product: the integrated product has four disadvantages. First, because the car cigarette lighter socket is not very deep, the rear part of the integrated product is heavier, and the car is Bumps can easily shake off the inverter or cause poor contact. Second, the integrated inverter is plugged into the cigarette lighter socket, and the rear is exposed longer, which will affect the shifting operation of the right hand. This is closely related to driving safety, so be careful. Third, because it is directly plugged into the cigarette lighter socket, the rear row of the integrated inverter cannot be used. Fourth, the integrated housing is made of plastic. Everyone knows that plastic is not resistant to high temperatures. Therefore, the contact part of the integrated inverter with the cigarette lighter socket will generate high temperature through high current, which will cause the head to deform or melt, and it will cause a fire. . Therefore, no matter from the perspective of safety or convenience, an integrated inverter cannot be used.
4. Whether the various protection functions are perfect: A good car inverter should have perfect protection functions, which can protect your car and power supply under various conditions. It should also have a sound and light alarm function to remind you to deal with various situations in time. Generally, the protection functions that should be possessed are input under-voltage, over-voltage protection, over-temperature protection, output overload, and output short-circuit protection.
5. Conversion efficiency: This is a very important indicator of the inverter. The higher the inverter efficiency, the longer the battery is used, and the less the inverter will generate heat. Generally, it should reach more than 90%. But users have no way to test this indicator, they can only rely on the conscience of the manufacturer to mark. The user can only make a simple judgment from the heat generation of the inverter through the load with high power. A good inverter should be able to work for a long time without being hot when the load is 80% of the nominal power.
6. The cigarette lighter plug should be made of bakelite, the plug wire should be thick, and the length should be convenient to use: the cigarette lighter plug made of bakelite can work under high temperature conditions without deformation or burning. The products on the market now do not There are very few products that use bakelite cigarette lighter plugs. Because of the safety in use, you should still pay attention. Bakelite cigarette lighter plugs look brighter, while plastic cigarette lighter plugs look duller and lack luster.
7. The electrical power is greater than 150W, and the battery clip must be used to directly take electricity from the battery: the car cigarette lighter socket can only pass about 10A current, so high-power inverters must be equipped with a battery clip line to directly take electricity from the battery . Some manufacturers nowadays, in order to save costs, 300W products do not use the bottle clamp cable, and they claim that they have passed the CE certification. The use of this product can burn out the wiring of the car or cause a fire. Please pay attention to the majority of car owners.
8. Pay attention to the form of the output socket: It is necessary to be able to use Chinese standard two-phase and three-phase plugs. 90% of the products on the market have only American standard three-phase sockets, which is very inconvenient to use.
Part 2: The composition of the voltage inverter
We call inverters with adjustable frequency and adjustable voltage.
1. Main circuit
The power conversion part that provides voltage and frequency power for asynchronous motors is called the main circuit. The main circuit is composed of three parts, the "rectifier" that converts the industrial frequency power into DC power, the "flat wave loop" that absorbs the voltage ripples generated in the converter and inverter, and the conversion of DC power into AC power. "Inverter". In addition, when the asynchronous motor needs to be braked, a "brake circuit" is sometimes added.
1. Rectifier Recently, a large number of diode converters have been used, which convert industrial frequency power to DC power. Two groups of transistor converters can also be used to form a reversible converter, which can be regenerated because of its reversible power direction.
2. Smoothing circuit In the DC voltage rectified by the rectifier, the pulsating voltage of 6 times the frequency of the power supply is included, and the pulsating current generated by the inverter also changes the DC voltage. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). If the device capacity is small, if the power supply and the main circuit components have a margin, the inductor can be omitted and a simple smoothing loop can be used.
3. Inverter In contrast to the rectifier, the inverter converts DC power into AC power at the required frequency, and turns on and off the 6 switching devices within a determined time to obtain a 3-phase AC output.
Voltage type inverter output voltage
4. Braking circuit When the asynchronous motor is used in the regenerative braking area (the slip rate is negative), the regenerative energy is stored in the smoothing circuit capacitor to increase the DC voltage. Generally speaking, the energy stored by the inertia of the mechanical system (including the motor) is greater than the energy stored by the capacitor. When fast braking is required, the inverter can be used to feed back to the power supply or set a braking circuit (switch and resistor) to regenerate The power is consumed to prevent the DC circuit voltage from rising.
5. Four-quadrant operation of asynchronous motor According to the type of load, the rotation direction and torque direction of the asynchronous motor required are different, and an appropriate main circuit must be constructed according to the load.
In quadrants I and III, the torque direction of the asynchronous motor is the same as the rotation direction, and it is electric. Quadrant I is electric operation in forward rotation, and quadrant III is electric operation in reverse rotation. In quadrants II and IV, the direction of torque is opposite to the direction of rotation, which is a regeneration state. Quadrant II is the regenerative operation of forward rotation, and quadrant IV is the regenerative operation of reverse rotation. When only electric operation is required, only power is supplied from the power supply to the asynchronous motor, and a non-inverter can be used. For loads that require braking force during deceleration, power must flow from the asynchronous motor to the inverter, and a braking circuit can be added to enable use in quadrants II and IV. In addition, for occasions that require rapid acceleration and deceleration and frequent acceleration and deceleration (such as elevators), or for applications where braking is the main purpose, an inverter can be used to achieve four-quadrant operation from I to IV. At this time, energy is fed back to the power source to save energy.
Four-quadrant operation with voltage-type inverter
a) When it is a non-inverter b) When it has a brake circuit c) When it is an inverter
Above, the structure of the main circuit has been explained using a voltage-type inverter as an example. For current-type inverters, as described in the previous chapter, four-quadrant operation can also be achieved with an irreversible rectifier.
2. Control circuit
The circuit that provides the control signal to the main circuit of the asynchronous motor (voltage and frequency adjustable) is called the control circuit. The control circuit consists of the following circuits: frequency and voltage "calculation circuit", main circuit "voltage and current detection circuit", motor "speed detection circuit", "drive circuit" that amplifies the control signal of the calculation circuit, and "Protection circuit" for inverters and motors.
The composition of the inverter
When only the control circuit A is used to form the control circuit, there is no speed detection circuit and it is open loop control. The speed detection circuit is added to the part B of the control circuit, that is, the speed command is added, which can carry out more accurate closed-loop control of the speed of the asynchronous motor.
(1) Calculation circuit Compares the external speed, torque and other commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter.
(2) Voltage and current detection circuit Potentially isolated from the main circuit to detect voltage and current.
(3) The drive circuit is a circuit that drives the main circuit device. It is isolated from the control circuit so that the main circuit device is turned on and off.
(4) Speed detection circuit Take the signal of the speed detector (TG, PLG, etc.) installed on the asynchronous motor shaft machine as the speed signal, and send it to the calculation circuit. According to the command and calculation, the motor can run at the command speed.
(5) Protection circuit Detects the voltage and current of the main circuit. When an abnormality such as overload or overvoltage occurs, in order to prevent damage to the inverter and asynchronous motor, stop the inverter or suppress the voltage and current value.
The protection circuit in the inverter control circuit can be divided into inverter protection and asynchronous motor protection.
1. Inverter protection
(1) Instantaneous overcurrent protection When the current flowing through the inverter device reaches an abnormal value (exceeds the allowable value) due to a short circuit on the load side of the inverter, etc., the inverter is stopped instantaneously and the current is cut off. When the output current of the converter reaches an abnormal value, the inverter operation is also stopped.
(2) Overload protection The output current of the inverter exceeds the rated value and continues to circulate for more than a specified time. In order to prevent damage to the inverter components, wires, etc., the operation must be stopped. Proper protection requires inverse time characteristics, using special relays or electronic thermal protection (using electronic circuits). Overload is caused by too large GD2 (inertia) of the load or blocking of the motor due to excessive load.
(3) Regenerative overvoltage protection When the inverter is used to quickly decelerate the motor, the DC circuit voltage of the regenerative power will increase and sometimes exceed the allowable value. You can stop the inverter operation or stop the rapid deceleration method to prevent overvoltage.
(4) Instantaneous power failure protection For instantaneous power failures within a few milliseconds, the control circuit works normally. However, if the instantaneous power failure is more than 10ms, usually not only the control circuit malfunctions, but the main circuit cannot supply power, so the inverter will stop running after detection.
(5) Grounding overcurrent protection When the load side of the inverter is grounded, in order to protect the inverter, a grounding overcurrent protection function is sometimes required. But in order to ensure personal safety, a leakage circuit breaker needs to be installed.
(6) Abnormal cooling fan A device with a cooling fan, when the fan is abnormal, the temperature in the device will rise, so the fan thermal relay or device heat sink temperature sensor is used to stop the inverter after the abnormality is detected. It can be omitted when the temperature rise is small and does not hinder the operation.