Design of variable frequency speed regulation cont

Design of diesel locomotive traction variable frequency speed regulation control system

Abstract: according to the use requirements of AC drive internal combustion locomotive, a set of traction variable frequency speed regulation control system with two working modes is designed, namely, double vehicle parallel working mode and single vehicle independent working mode. The main circuit and control system of the variable frequency speed regulation system are designed and analyzed. Finally, the system is tested by experiments, and the expected performance requirements are achieved

key words: variable frequency speed regulation system, torque, inverter, slip frequency

1 introduction

for the needs of railway speed increase, it is quite necessary to develop AC drive engineering locomotive. The design requirements of the vehicle should have two operation modes: high-speed, long-distance traction operation state and ultra-low speed stable operation operation state. At present, the engineering locomotives used in China, on the one hand, do not use AC drive, on the other hand, do not have this performance requirement

2 characteristics of traction motor

it is known from the motor principle that the typical torque speed characteristics of asynchronous motor. When the motor rotor is at synchronous speed, the torque is 0; When the slip rate is very small, the torque changes almost linearly with the decrease of speed, that is, the increase of slip rate. When slip ratio S is positive, it is electric torque; When the slip is negative, it is the braking (power generation) torque. Slip rate is the ratio of slip frequency (frequency of rotor current) to stator current frequency f1:

s= Δ F/F1 (1)

if the rotation frequency f2 of the asynchronous motor rotor can be measured and calculated, the corresponding slip frequency can be found according to the load's demand for torque and the control characteristics of the motor Δ f. Then the stator current frequency f1 output by the frequency converter is:

f1 = F2 ± Δ F (2)

in the formula (2) () corresponds to the electric traction state, that is, the frequency f1 of stator current is greater than the frequency f2 of rotor rotation; (-) corresponding to the power generation braking state, at this time, the frequency f2 of rotor rotation is greater than the frequency f1 of stator current. [4]

the figure shows the rated slip frequency of variable frequency traction asynchronous motor Δ Characteristic curve of F. The curve can be calculated according to the design parameters of traction asynchronous motor. This characteristic curve is used as the torque setting (slip frequency Δ f) The original basis of. It shall be corrected during the frequency converter traction motor matching test, and the appropriate speed regulation shall be carried out as required during the on-site commissioning of the engineering locomotive. [1]

3 characteristics of traction variable-frequency speed regulation control system

3.1 control mode of variable-frequency traction speed regulation system

due to the large weight of the locomotive itself and the trailer it is towing, it is generally a large inertia load, its start/stop time is long, and its torque response time has no rapidity requirements. Therefore, variable frequency traction speed regulation adopts slip frequency control to realize torque given control and speed slip closed-loop control, which can fully meet various requirements of traction control. [2]

3.2 working mode of traction variable frequency speed regulation system

engineering operation vehicle runs on the track, which is usually "double vehicle combined operation". This configuration, on the one hand, increases the reliability of the equipment; On the other hand, it can meet the requirements of different Trailer loads, long ramps, high-speed and long-distance operation

according to the requirements of the design and use of AC drive diesel locomotive, the traction variable-frequency speed control system should be designed according to the following working mode: [2]

⑴ double car parallel working mode:

control according to the given torque (slip frequency Δ F) control mode operation

high speed and long distance (heavy load and long ramp, etc.) traction operation

⑵ single vehicle independent working mode:

work according to the speed slip closed-loop or v/f open-loop frequency control mode

stable operation at low speed

3.2.1 dual vehicle parallel working mode

the control system calculates the actual operating frequency f2 in the rotation of the traction electric rotor. If the rotor of the motor only needs to run with the locomotive at this time, only take the rotor running frequency f2 as the given frequency f1 (the frequency applied on the stator winding) of Trump's "gun" buzzer of the traction transformer on March 23, that is, F1 = F2. When a certain traction force (electric torque) needs to be applied, the control system only needs to adjust the slip frequency corresponding to a certain frequency f2 of the motor rotor at this time Δ f′（ Δ f′/Δ F = actual torque/rated torque) plus F2, i.e. F1 = F2 Δ F ', so that the motor will output the corresponding torque. Through the mechanical transmission mechanism, the locomotive will get the corresponding traction. In order to give the diesel generator set a certain adjustment time, the application of traction/braking force should be buffered after a given ramp time. If the torque control is carried out in this way, the traction variable frequency speed regulation system will be very stable. [3]

in combination with the above figure, if the torque is controlled according to (1 ~ 15) gears (such as in proportion), that is, there are 15 slip frequency characteristic curves for users to choose from

3.2.2 single vehicle independent working mode

after understanding the working principle of torque setting mode, we will discuss the working principle of speed slip closed loop. The input signal F2 in the PWM calculation box is the same as F2 in the torque given control mode, which will not be repeated here. The speed regulator PI calculates the difference between VG and VF, performs PI operation, and outputs Δ F value is affected by Δ F data limiting, that is, when Δ F value at rated value（ Δ f) Within, output its actual value, exceeding the rated Δ F, limited to the corresponding frequency of F2 Δ F value, i.e Δ F unlike the torque given control mode, which has only 15 curves, there are countless curves in the traction/braking working area. [3]

v/f open-loop frequency (speed) control, that is, the speed signal is directly used as the output frequency signal F1 of the traction converter. Of course, the v/f open-loop frequency control should take into account the characteristics required for traction control, which is far beyond the competence of general-purpose frequency converters

4 main circuit design of traction variable frequency speed regulation system

4.1 special requirements of traction variable frequency speed regulation system

according to the particularity of the application of this variable frequency speed regulation system, the main circuit design needs to consider the following factors:

⑴ the prominence of low immunity of diesel generator power supply system

⑵ particularity of traction motor power supply requirements

⑶ necessity of maintenance free requirements for traction converter. [5]

4.2 analysis of main circuit of traction variable frequency speed regulation system

main circuit principle and operation control main electrical route of traction variable frequency speed regulation system: incoming circuit, rectifier, precharge circuit, filter, kinetic energy braking and inverter, etc. The functions of its components are briefly described as follows: [6]

⑴ incoming circuit

incoming circuit is composed of knife switch K, incoming reactor LP and fuse Rd. the functions of various electrical components are:

knife switch K: isolate the traction frequency conversion speed regulation system (device) from the power supply system of locomotive diesel generator set

incoming reactor LP: make the AC incoming and outgoing current of rectifier continuous and smooth, and reduce the interference of rectifier circuit to the power of diesel generator set

fuse Rd: provide protection for diesel generator set

⑵ rectifier

a three-phase bridge semi controlled rectifier circuit is composed of three thyristors and three rectifier diodes. Here, the thyristor is triggered by level instead of phase control, that is, when the thyristor works, it is equivalent to a rectifier diode

⑶ precharge circuit

the benefits of traction transformer raw material industry continue to rise frequency speed regulation system, which is "AC-DC-AC voltage type frequency converter", and the middle tributary voltage link is composed of multiple high-capacity electrolytic capacitors in series/parallel, so as to protect the rectifier and capacitor from damage due to large charging current

⑷ filter

the filter is mainly composed of multiple high-capacity electrolytic capacitors connected in series/parallel. The voltage sharing resistors 1r1 and 2 are used for forced voltage sharing to make the voltage on the series capacitors almost consistent

there are three functions of filter capacitor here: 1 Filter the voltage ripple of the tributary; 2. Provide reactive current for asynchronous motor; 3. Provide a low resistance path for the kinetic energy braking and the commutation of the fully controlled power electronic switching elements in the inverter. [3]

⑷ kinetic braking circuit

when the traction motor is in power generation braking, its output voltage and current are reverse. In the inverter, the time for IGBT transistors to pass the current is less than the time for them to pass the freewheeling diode current within an output frequency cycle time. With the help of diodes, the inverter sends the electric energy converted from the load kinetic energy to the filter capacitor. This energy can not be sent back to AC through rectifier. As a result, the energy on the capacitor continues to increase and the voltage continues to increase. When the voltage reaches a certain value (such as 700V), the control system turns on the IGBT transistor below. In this way, the () pole of the DC power supply is connected with the (-) pole of the power supply through the discharge braking resistor Rb and the turned on transistor, and the flow current IB = VDC/RB. Release the electric energy stored in the capacitor. The inverter constantly sends back the electric energy sent by the motor, so that the locomotive produces a certain braking force

⑥ inverter

the inverter is composed of six IGBT transistor switches and six fast recovery freewheeling diodes in reverse parallel with 1GBT to realize the bidirectional flow of current. The circuit structure is a two-level control mode. With the help of PWM control technology, voltage/frequency (v/f) coordinated control (VVVF control) is realized

when the traction asynchronous motor is operated by electric traction, the voltage/current at the stator winding end of the AC motor is in the same direction (with a phase angle difference), and the inverter inverts the electric energy of the DC power supply into AC energy to supply power to the motor. When the traction asynchronous motor is in braking operation, the voltage/current at the stator winding end of the AC motor is reversed (there is a phase difference), and the inverter rectifies the AC energy generated by the motor into DC and sends it back to the filter capacitor. The principle of AC to DC rectification transformation is described in (5) kinetic energy braking. [2]

in addition to the components of the main circuit, there are auxiliary power control circuits, signal detection elements and weak current control units of the control system

5 control system design

the control system adopts 16 bit single chip microcomputer of Intel company as the main control chip, and adopts space voltage vector wave control mode to realize the full digital slip frequency control mode suitable for single machine operation and multi machine linkage

the control system has the following characteristics through the combination of software and hardware:

⑴ low frequency (start) large torque

⑵ 200% overload capacity and software inverse time limit characteristics

⑶ automatic stall control is added to prevent dynamic overvoltage and overcurrent

⑷ software selectable flexible PWM energy consumption braking

⑷ automatic voltage control (AVC) to overcome the problem of maintaining the output voltage by radio waves

⑥ anti locked rotor limiting characteristics (excavator characteristics)

⑺ zero braking function suitable for natural conversion of electric parking on slopes or emergency braking

8 dynamic speed tracking can ensure the power input of the system during vehicle operation

⑼ overload protection, peak overcurrent protection, overheating protection, undervoltage protection, phase loss protection and fault memory, fault self recovery function

⑽ S-shaped acceleration curve suitable for heavy-duty traction of vehicles

6 conclusion analysis

this system belongs to the transmission control level. The main rectifier adopts three-phase bridge semi controlled rectifier circuit; The motor control strategy is vector control, which is a high-performance asynchronous motor control technology with fast dynamic response and excellent steady-state performance. Low harmonic content

as an independent whole, the AC drive control system of diesel locomotive can