Suppliers of frequency converter supporting equipment remind you that in traditional frequency control systems composed of general frequency converters, asynchronous motors, and mechanical loads, when the potential load transmitted by the motor is lowered, the motor may be in a regenerative braking state; Or when the motor decelerates from high speed to low speed (including parking), the frequency may suddenly decrease, but due to the mechanical inertia of the motor, it may be in a regenerative power generation state. The mechanical energy stored in the transmission system is converted into electrical energy by the motor and sent back to the DC circuit of the inverter through the six freewheeling diodes of the inverter. At this time, the inverter is in a rectified state. At this point, if no measures are taken to consume energy in the frequency converter, this energy will cause the voltage of the energy storage capacitor in the intermediate circuit to rise. If the braking is too fast or the mechanical load is a hoist, this part of energy may cause damage to the frequency converter, so we should consider this part of energy.
In general frequency converters, there are two most commonly used methods for processing regenerated energy:
(1) Dissipation into the "braking resistor" artificially set in parallel with the capacitor in the DC circuit is called the dynamic braking state;
(2) If it is fed back to the power grid, it is called feedback braking state (also known as regenerative braking state). There is another braking method, namely DC braking, which can be used in situations where accurate parking is required or when the brake motor rotates irregularly due to external factors before starting.
Many experts have discussed the design and application of variable frequency drive braking in books and publications, especially in recent times, there have been many articles on "energy feedback braking". Today, the author presents a new type of braking method, which has the advantages of four quadrant operation with "feedback braking" and high operating efficiency, as well as the benefits of "energy consumption braking" for pollution-free power grid and high reliability.
Energy consumption braking
The method of using the braking resistor set in the DC circuit to absorb the regenerative electrical energy of the motor is called energy consumption braking.
Its advantage is simple construction; No pollution to the power grid (compared to feedback control), low cost; The disadvantage is low operating efficiency, especially during frequent braking, which will consume a large amount of energy and increase the capacity of the braking resistor.
Generally, in general frequency converters, low-power frequency converters (below 22kW) are equipped with a built-in brake unit, which only requires an external brake resistor. High power frequency converters (above 22kW) require external brake units and brake resistors.
Feedback braking
To achieve energy feedback braking, conditions such as voltage control at the same frequency and phase, feedback current control, etc. are required. It adopts active inverter technology to invert regenerated electrical energy into AC power of the same frequency and phase as the power grid and return it to the grid, thereby achieving braking.
The advantage of feedback braking is that it can operate in four quadrants, and electric energy feedback improves the efficiency of the system. Its disadvantages are:
(1) This feedback braking method can only be used under stable grid voltage that is not prone to faults (with grid voltage fluctuations not exceeding 10%). Because during the operation of power generation braking, if the voltage fault time of the power grid is greater than 2ms, commutation failure may occur and the components may be damaged.
(2) During feedback, there is harmonic pollution to the power grid.
(3) The control is complex and the cost is high.
New braking method (capacitor feedback braking)
Main circuit principle
The rectification part uses a common uncontrollable rectifier bridge for rectification, the filtering circuit uses a universal electrolytic capacitor, and the delay circuit uses either a contactor or a thyristor. The charging and feedback circuit consists of a power module IGBT, a charging and feedback reactor L, and a large electrolytic capacitor C (with a capacity of about a few tenths of a meter, which can be determined according to the operating system of the frequency converter). The inverter part is composed of power module IGBT. The protection circuit is composed of IGBT and power resistor.
1) Electric motor power generation operation status
The CPU monitors the input AC voltage and DC circuit voltage (μ d) in real-time, and determines whether to send a charging signal to VT1. Once μ d is higher than the corresponding DC voltage value (such as 380VAC -530VDC) of the input AC voltage, the CPU turns off VT3 and charges the electrolytic capacitor C through pulse conduction of VT1. At this time, the reactor L and the electrolytic capacitor C are divided to ensure that the electrolytic capacitor C operates within a safe range. When the voltage on electrolytic capacitor C approaches a dangerous value (such as 370V) while the system is still in a power generation state, and the electrical energy is continuously sent back to the DC circuit through the inverter, the safety circuit plays a role in achieving energy consumption braking (resistance braking), controlling the turn off and turn on of VT3, and thus realizing the consumption of excess energy by resistor R. Generally, this situation does not occur.
(2) Electric motor operation status
When the CPU detects that the system is no longer charging, it pulse conducts VT3, creating an instantaneous left positive and right negative voltage on reactor L. Combined with the voltage on electrolytic capacitor C, the energy feedback process from the capacitor to the DC circuit can be achieved. The CPU controls the switching frequency and duty cycle of VT3 by detecting the voltage on electrolytic capacitor C and the voltage in the DC circuit, thereby controlling the feedback current and ensuring that the DC circuit voltage ν d does not become too high.
System difficulties
(1) Selection of reactors
(a)、 We consider the particularity of the operating conditions and assume that a certain fault occurs in the system, causing the potential energy load carried by the motor to accelerate freely and fall. At this time, the motor is in a power generation operation state,
The regenerated energy is sent back to the DC circuit through six freewheeling diodes, causing an increase in ∆ d and quickly putting the inverter in a charging state. At this time, the current will be very high. So the selected reactor wire diameter should be large enough to pass the current at this time.
(b)、 In the feedback loop, in order to release as much electrical energy as possible before the next charge of the electrolytic capacitor, selecting a regular iron core (silicon steel sheet) cannot achieve the goal. It is best to choose an iron core made of ferrite material. Looking at the current value considered above, it can be seen how large this iron core is. It is unknown whether there is such a large ferrite iron core on the market. Even if there is one, its price will definitely not be very low.
So the author suggests using one reactor for each charging and feedback circuit.
(2) Difficulties in control
(a)、 In the DC circuit of the frequency converter, the voltage ν d is generally higher than 500VDC, while the withstand voltage of electrolytic capacitor C is only 400VDC, indicating that the control of this charging process is not like the control method of energy braking (resistance braking). The instantaneous voltage drop generated on the reactor is, and the instantaneous charging voltage of electrolytic capacitor C is ν c=ν d - ν L. In order to ensure that the electrolytic capacitor operates within a safe range (≤ 400V), it is necessary to effectively control the voltage drop ν L on the reactor, which in turn depends on the instantaneous change rate of inductance and current.
(b)、 During the feedback process, it is also necessary to prevent the discharge of electrical energy from electrolytic capacitor C from causing excessive DC circuit voltage through the reactor, resulting in overvoltage protection in the system.
Main application scenarios
It is precisely because of the superiority of this new braking method (capacitor feedback braking) of frequency converters that many users have recently proposed to equip this system based on the characteristics of their equipment. With the expansion of the application field of frequency converters, this application technology will have great development prospects. Specifically, it is mainly used in industries such as mine hoists (for carrying people or loading materials), inclined mine cars (single or double tube), and lifting machinery. In any case, energy feedback devices can be used in situations that require them.