Feb 20, 2021
Composition of 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 of a typical voltage inverter consists of three parts, the "rectifier" that converts the commercial frequency power into DC power, the "flat wave loop" that absorbs the voltage ripples generated in the converter and inverter, and the DC An "inverter" that converts power into AC power. In addition, when an asynchronous motor needs to be braked, a "brake circuit" is sometimes added.
An example of a typical voltage inverter
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 The DC voltage rectified by the rectifier contains a pulsating voltage with a frequency of 6 times the frequency of the power supply, 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 Contrary to the rectifier, the inverter converts DC power into AC power of the required frequency, and the 6 switching devices are turned on and off at a certain time to obtain a 3-phase AC output. Take voltage type PWM inverter as an example to show the switching time and voltage waveform.
Voltage type inverter output voltage
4. Braking circuit When the asynchronous motor is used in the regenerative braking area (the slip 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 required rotation direction and torque direction of the asynchronous motor are different, and the appropriate main circuit must be constructed according to the load.
is the relationship between the four-quadrant operation of the asynchronous motor driven by the voltage-type inverter and the main circuit configuration. 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 B part 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, send it to the calculation circuit, and make the motor run at the command speed according to the command and calculation.
(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. Table 1 is a list of protection functions.
List of protection functions
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., stop operation. 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 over-voltage protection When the inverter is used to decelerate the motor quickly, the regenerative power DC circuit voltage 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 also 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.