"H-type bipolar mode PWM control plays a significant role in improving the low-speed characteristics of turntable servo system, and it is simple and easy. H-type bipolar mode PWM control can improve the low-speed characteristics of the servo system because an alternating current always flows through the armature circuit of the motor controlled by H-type bipolar mode PWM. This current can make the motor vibrate at high frequency, which is conducive to reducing static friction, so as to improve the low-speed characteristics of the servo system. However, due to its large power loss, H-type bipolar mode PWM control is only suitable for medium and small power servo systems. Therefore, it is necessary to design a power conversion circuit of H-type bipolar mode PWM control which can reduce power loss, so that H-type bipolar mode PWM control can be applied to high-power servo system.
Power loss of H-type bipolar mode PWM control
As shown in Figure 1, H-type bipolar mode PWM control is generally composed of four high-power controllable switches (v1-4) and four freewheeling diodes (VD 1-4). The four high-power controllable switches are divided into two groups, V1 and V4 are one group, and V2 and V3 are one group. The two high-power controllable switches of the same group are turned on and off at the same time. The two groups are turned on and off alternately, that is, the driving signal U1 = U4, U2 = U3 = - U1. The direction of armature current changes in turn according to directions 1, 2, 3 and 4 in Figure 1 in a width modulation wave cycle. Since the current is allowed to reverse, the armature current is always continuous when the H-type bipolar mode PWM control works. The armature current always continuously produces the additional power consumption of the motor, the conduction power consumption and switching power consumption caused by the high-frequency opening and closing of high-power controllable switches, which are the main sources of power loss in H-type bipolar mode PWM control. The main factor determining the additional power consumption of the motor is the switching frequency of PWM. The larger the switching frequency is, the smaller the additional power consumption is. The main factors that determine the dynamic power consumption of high-power controllable switch are the on-off time of high-power controllable switch and the switching frequency of PWM. The longer the on-off time, the greater the dynamic power consumption, and the greater the PWM switching frequency, the greater the dynamic power consumption.
Fig. 1H bipolar mode PWM control schematic diagram
The additional power consumption of armature circuit and the dynamic loss of high-power controllable switch make the power loss of H-type bipolar mode PWM control very large and not suitable for application in high-power servo system. In order to solve this problem, this paper will design the power conversion circuit of H-type bipolar mode PWM control based on the principle of reducing the additional power consumption of motor armature circuit and the dynamic power consumption of high-power switch, so as to apply H-type bipolar mode PWM control to high-power servo system.
Design of power conversion circuit for H-type bipolar mode PWM control
The core of designing the power conversion circuit of H-type bipolar mode PWM control is the selection of power conversion devices, the design of driving circuit and protection circuit.
Power converter
The commonly used high-power controllable switches mainly include high-power bipolar transistor (GTR), high-power power MOSFET and IGBT. The main disadvantages of GTR are: long opening and closing time, high switching power consumption, low working frequency, poor thermal stability and easy damage. The main disadvantages of MOSFET are: large on state voltage drop and large power loss when the tube is turned on. IGBT (insulated gate bipolar transistor) integrates the advantages of GTR and MOSFET. It not only has the characteristics of low on state voltage, high voltage resistance, large current bearing and low power loss, but also has the characteristics of high output impedance, fast speed and good thermal stability. Therefore, IGBT has broad engineering application prospects.
The power conversion circuit in this paper adopts 2mb1300d-140 IGBT as the power converter. Its schematic diagram is shown on the right side of Figure 2. G is the gate (gate) electrode, C is the collector and E is the emitter. The relationship between IGBT driving conditions and IGBT characteristics is shown in Table 1. Vces, ton, toff, VCE and R are Collector Emitter Saturation Voltage drop, on time, off time, Collector Emitter Voltage and grid resistance respectively, ↑, -and↓ represent increase, unchanged and decrease respectively. As can be seen from table 1:
① With the increase of forward gate voltage + VGE, Vces and ton decrease, and the dynamic power consumption of IGBT decreases;
② Increasing the reverse gate voltage - VGE, toff decreases and the dynamic power consumption of IGBT decreases;
③ With the increase of R, the ton and toff of IGBT increase, and the dynamic power consumption of IGBT increases.
Table 1 Relationship between IGBT driving conditions and IGBT characteristics
Therefore, to reduce the dynamic power consumption of IGBT, it is necessary to increase the forward gate voltage + VGE, increase the reverse gate voltage - VGE, and reduce ton and toff. However, the higher the VGE is, the better. The reason is that the current increases when the VGE is too high, which is easy to damage the IGBT. Generally, + VGE does not exceed + 20V. During IGBT shutdown, due to the interference of other parts of the circuit, some high-frequency oscillation signals will be generated on grid g. these signals will either make the IGBT that should be closed in the micro on state and increase the power consumption of IGBT, or make the inverter circuit in the short-circuit through state. In order to prevent these phenomena, the greater the reverse grid voltage - VGE, the better. According to the above relationship, the requirements of IGBT for driving circuit mainly include: strong dynamic driving ability, appropriate forward and reverse grid voltage, strong input and output electrical isolation ability, no delay in input and output signal transmission, and certain protection function.
In order to reduce the dynamic power consumption of IGBT, ensure the circuit safety and meet the driving requirements of IGBT, it is necessary to reasonably determine the values of + VGE, - VGE and r. These need to be realized by designing the driving circuit.
Driving circuit design
Designing a driving circuit with good performance can make IGBT work in an ideal switching state, shorten switching time, reduce switching power consumption and improve the operation efficiency of power conversion circuit. IGBT grid driving methods mainly include transformer driving method, direct driving method and optocoupler isolation driving method. Transformer driving method is conducive to the isolation of driving signal and small driving power loss, but it limits the use frequency and is not conducive to the transmission of PWM signal. Direct drive method is suitable for small capacity unprotected IGBT. The optocoupler isolation driving method has high requirements for the optocoupler, which requires fast optocoupler speed, insulation withstand voltage higher than the power supply voltage and large common mode rejection ratio.
The skhi22ah4 module of Semikron company is a driver applying the transformer driving principle. When skhi22ah4 module drives IGBT, its maximum working frequency can reach 100kHz, which completely solves the problem of limiting the use frequency. The circuit schematic diagram of skhi22ah4 module driving IGBT is shown in Figure 2. The dotted box in Figure 2 is the structural diagram of skhi22ah4 module. The module is divided into primary and secondary parts, which are insulated, so that the driving circuit has good input and output electrical isolation ability; The module has two inputs and two outputs. One input corresponds to one output. Input is the primary and output is the secondary of the transformer; Skhi22ah4 module also has measurement device and error information storage device for errors such as short circuit, overcurrent and voltage instability, which are used to realize a variety of circuit protection functions. The working principle of skhi22ah4 module is: PWM control signal is added to the primary of transformer, and the secondary of transformer outputs amplified drive signal to drive IGBT. The power supply voltage of skhi22ah4 module is + 15V. When it drives 2mb1300d-140 IGBT, the on voltage and off voltage of its drive output can reach + 14.2v and - 2V, which fully meets the requirements of + VGE and - VGE for reducing IGBT dynamic power consumption. In order to reduce ton and toff, Ron = 3.38 and Roff = 3.38 are taken within the allowable range. Under the principle of reducing power loss, the peripheral components are selected in the process of designing circuit protection function.
Fig. 2skhi22ah4 schematic diagram of IGBT driven by module
Main circuit protection function design of skhi22ah4:
1) Short circuit protection function
Short circuit is easy to occur between C pole and e pole. In case of short circuit, the current increases, and the power loss of IGBT increases rapidly (with the increase of the square of the current), which will cause damage to IGBT in serious cases. Therefore, IGBT needs short-circuit protection. As shown in Fig. 2, the short circuit protection between C pole and e pole is realized by comparing the voltages of C pole and e pole. To realize short-circuit protection, it is necessary to reasonably determine the values of rce and CCE. The specific steps are as follows:
① Determine the value of Vces. Vces can neither be too large nor too small. Too large will increase the dynamic power loss of IGBT. Too small will weaken the short-circuit protection ability, generally 5.6v. In order to reduce the dynamic power loss of IGBT, it can be appropriately reduced, but not less than 3.5V. Here, Vces = 4V.
② Determine rce. Rce = 13 Ω is obtained from formula (1).
③ Determine Tmin. According to the characteristics of skhi22ah4 module, tmince = 470pf.
2) Interlock protection function
Skhi22ah4 module has interlocking function to prevent two IGBTs on the same side arm of H bridge from conducting at the same time. The interlock function is: among the two IGBTs on the same side arm of H bridge, there must be a delay after one IGBT is closed, and the other IGBT can be opened. Locking time of interlock TTD = 2.7 + 0.13rtd (RTD is interlock resistance), 2.7 μ S is generated because an interlock resistor has been integrated in the skhi22ah4 module. Take RTD = 08, then TTD = 2.7 μ s。
3) Error monitoring
Skhi22ah4 module has error monitoring function, which can monitor errors such as short circuit, overcurrent and voltage instability. When an error occurs, skhi22ah4 module stops running and stores the error signal in errormemory. It can not run again until the error is eliminated.
According to the above driving circuit design, the driving waveform of skhi22ah4 module can be obtained, as shown in Figure 3.
Fig. 3 waveform diagram of input and output of skhi22ah4 module
H-type bipolar mode PWM controlled power conversion circuit
After the above design, the power conversion circuit schematic diagram of H-type bipolar mode PWM control is obtained, as shown in Figure 4. According to the experimental test, in the power conversion circuit corresponding to Fig. 4, if the ton of IGBT is 1.8ls and toff is 1.4ls, the switching time of IGBT is 3.2ls.
Fig. 4H schematic diagram of power conversion circuit of bipolar mode PWM control
experiment
After designing the power conversion circuit of H-type bipolar mode PWM control, it is necessary to determine the reasonable PWM switching frequency in order to further reduce the power loss and realize the application of H-type bipolar mode PWM control in high-power servo system.
Calculation of PWM switching frequency
Reasonable switching frequency can not only further reduce power loss and improve efficiency, but also make the system performance almost the same as that of continuous system. On the whole, the determination of switching frequency is determined by many contradictory factors:
① In order to improve the influence of static friction on the low-speed performance of the servo system and make the motor in the dynamic lubrication state at zero position, the switching frequency considering micro vibration characteristics during bipolar mode PWM control shall meet formula (4);
② In order to prevent the switching frequency from adversely affecting the dynamic performance of the system, the frequency should be much greater than the passband FC of the servo system itself, and generally meet the empirical formula (5);
③ In order to avoid resonance, the switching frequency should be higher than the resonant frequency of all circuits in the system;
④ In order to improve the utilization rate of the motor, the current pulsation $ia must be limited, which should meet equation (6);
⑤ The upper limit of switching frequency shall be limited by the switching loss and switching time of IGBT, and shall meet empirical formula (7).
Taking the azimuth axis servo system of a three-axis flight simulation turntable as an example, the turntable is one of the most powerful turntables in China, with a power of 11000w, of which the power of azimuth axis servo system is 7200W. The azimuth axis motor parameters of the three-axis flight simulation turntable are as follows: torque coefficient KT = 82.3n? M / A, power supply voltage us = + 120V, armature resistance RA = 2.48 Ω, armature inductance La = 0.019h, static friction torque TF = 21010n? M on the motor axis, system passband frequency fc = 34hz, rated current in = 60A, starting current is ≈ in, α s=Is/IN≈1，Te=La/Ra=0.0079。
The switching frequency range is 340hz determined by equations (4) to (7)
Figure 5 power loss curve
test result
When the reversible unipolar PWM control is adopted in the azimuth axis servo system of a three-axis flight simulation turntable, the minimum stable speed of the azimuth axis servo system is 0.05 ° / S; When using the H-type bipolar mode PWM control of the power conversion circuit designed in this paper, the minimum stable speed that can be started is 0.01 ° / s, as shown in Fig. 6 (abscissa axis is the sampling point and the sampling frequency is 400Hz), the low-speed characteristics of azimuth axis servo system have been significantly improved. Fig. 6 starting speed curve of azimuth axis servo system of a three-axis turntable. Due to the pulse momentum of armature current, the motor will vibrate at high frequency, and the minimum stable speed of the system will also fluctuate; However, the pulsation is very small, less than 0.00025 ° / s, which is only 2.5% of the velocity value.
conclusion
The power conversion circuit of H-type bipolar mode PWM control designed in this paper reduces the power loss of bipolar mode PWM control; By calculating the reasonable switching frequency, the power consumption is further reduced. H-type bipolar mode PWM control is applied to high-power servo system. The practical engineering application shows that its application in the azimuth axis high-power servo system of a three-axis flight simulation turntable significantly improves the low-speed characteristics of the servo system; This method to improve the low-speed characteristics of the system has the advantages of simplicity and feasibility in engineering practice. This power conversion circuit design has good practical value in improving the low-speed characteristics of high-power servo system., Technology Zone
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