1. It is quite necessary to develop ac transmission engineering locomotive for the need of railway speed raising. The design requirements of the car should have two kinds of operation: high-speed, long-distance traction operation state and ultra-low stable operation state. At present, the domestic use of engineering locomotives, on the one hand did not use ac transmission, on the other hand do not have this kind of performance requirements.

2. Characteristics of traction motor

The typical torque-speed characteristics of asynchronous motors are known from the motor principle. When the motor rotor is at synchronous speed, the torque is 0. When slip is very small, the torque changes almost in a straight line as the speed decreases, that is, as the slip increases. When slip rate S is positive, is the electric torque; When slip is negative, is the braking (power generation) torque. Slip is the ratio of slip frequency (frequency of rotor current) to stator current frequency 

The rotation of the rotor asynchronous motor frequency F2, if can measure calculated, according to the need of the counter rotating torque load, by the motor control characteristic, can find its corresponding slip frequency Δ F, the inverter output frequency of stator current F1 as:

Where, () of (2) corresponds to the state of electric traction, that is, the frequency F1 of stator current is greater than the frequency F2 of rotor rotation; (-) corresponds to the power generation braking state. At this time, the rotation frequency F2 of the rotor is greater than the frequency F1 of the stator current.

The graph is variable frequency traction asynchronous motor rated slip frequency characteristic curve of Δ F. The curve can be calculated according to the design parameters of traction asynchronous motor. The characteristic curve as torque setting (slip frequency Δ F) of the original basis. In the inverter - traction motor matching experiment to correct, in the engineering work locomotive field debugging, according to the needs of the appropriate speed adjustment.

3. Characteristics of traction frequency conversion speed 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 towed, it is generally a large inertial load, and its start/stop time is relatively long, and the response time of its torque has no requirement for rapidity. Therefore, variable frequency traction speed control adopts slip frequency control to realize the given control of torque and closed-loop control of speed difference, which can completely meet the requirements of traction control.

3.2 working mode of traction frequency conversion speed regulation system

The engineering operation vehicle runs on the track, usually is "double vehicle reconnection work". This configuration, on the one hand to increase the reliability of equipment; On the other hand, it can meet the requirements of different trailer loads, long ramps, high speed and long distance.

In view of the design and use requirements of ac drive internal-combustion locomotive, the traction frequency conversion speed control system should be designed in the following way:

• dual-vehicle parallel operation mode:

According to the given torque control (Δ F slip frequency control) way to work;

High - speed long - distance (long - haul ramp, etc.) traction operation.

The cycle works independently:

Operate in closed-loop or V/F open-loop frequency control mode according to rotation speed difference;

Stable operation at low speed.

3.2. 1 dual-vehicle parallel operation mode

The control system calculates the actual operating frequency F2 in the rotation of the traction motor rotor. If the rotor of the motor only needs to run with the locomotive at this time, only the rotor running frequency F2 is used as the given frequency F1 of the traction frequency converter (the frequency applied on the stator winding), that is, F1 = F2. When the need to impose a certain traction, electric torque, motor rotor control system will run at this time of a certain frequency F2 of slip frequency Δ F '(Δ F'/Δ F = actual torque/rated torque) combined with F2, namely the F1 = F2 Δ F ', the motor output torque. Through the mechanical transmission mechanism, the locomotive will get the corresponding traction. In order to give the diesel generator set a certain amount of regulation time, the traction/braking force is applied through a given slope time to cushion it. In this way, the traction frequency control system will be very stable.

In combination with the above figure, if the torque is controlled by (1 ~ 15) range (if the range is divided to scale), then 15 slip frequency characteristic curves are available for users to choose from.

3.2.2 independent working mode of bicycles

After understanding how torque works in a given mode, we will discuss the principle of closed-loop rotation difference. The input signal F2 in the PWM calculation box is the same as F2 in the torque given control mode, which is not described here. Speed regulator PI to VG, difference and PI arithmetic, VF output by Δ Δ F value F data limit, namely when the Δ F value within the rating (Δ F), output its actual value, more than the rated Δ F, limited to F2 frequency corresponding Δ F value, namely Δ F don't like a given torque control mode only 15 curve, but many more in the traction/braking workspace any curve.

V/F open-loop frequency (speed) control, that is, the speed signal directly as the traction frequency converter output frequency signal F1. Of course, V/F open-loop frequency control should take into account the characteristics required for traction control, far more than the general purpose inverter can be competent.

4. Main circuit design of traction frequency conversion speed regulation system

4.1 special requirements for traction frequency conversion speed regulation systems

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

(1) outstanding low disturbance rejection of diesel generator power supply system;

The particularity of traction motor power supply requirements;

The necessity of the traction converter maintenance free requirement.

4.2 main circuit analysis of traction frequency conversion speed regulation system

Traction frequency conversion speed regulation system main circuit circuit principle and operation control of the main circuit consists of: incoming circuit, rectifier, precharge circuit, filter, kinetic energy brake and inverter. The functions of each component are described as follows:

(1) my in-line loop

The input circuit is composed of knife switch K, input reactor LP and fuse RD. The functions of each electrical element are:

Knife switch K: the traction frequency conversion speed regulation system (device) is isolated from the power supply system of locomotive diesel-generator set.

Incoming reactor LP: make the rectifier ac current continuous and smooth, reduce the rectifier circuit to the diesel generator network interference.

Fuse RD: provides protection for diesel generator set.

2 rectifier

It consists of three thyristors and three rectifier diodes. The thyristor is triggered by level instead of phase control, that is, the thyristor is equivalent to a rectifier diode when it works.

We pre - charge circuit

Traction frequency control system, "ac - dc - ac voltage converter", the middle branch of the voltage link is composed of a number of large capacity electrolytic capacitor series/and, to protect the rectifier and capacitor from damage due to large charging current.

Electrical current damage.

The precharging circuit is composed of three-phase bridge rectifier ZL and charging current limiting resistor 2R1 and 2.

(4) filter

The filter is mainly composed of a number of large-capacity electrolytic capacitors connected in series/parallel. Voltage equalizing resistance 1R1 and 2 is used to carry out forced voltage equalizing, so that the voltage on the series capacitor is nearly consistent.

There are three functions of filter capacitor: 1. Filter the branch voltage ripple; 2. Provide reactive current for asynchronous motor; 3. It provides low resistance path for kinetic energy braking and full control power electronic switching elements in inverter.

5] kinetic energy braking circuit

When the traction motor is under electric braking, its output voltage and current reverse. In an inverter, the time of the IGBT transistor passing through the current in one output frequency cycle is less than the time of its passing through the relay diode current. With the help of the diode, the inverter transfers the electric energy from the load kinetic energy to the filter capacitor. This energy and cannot pass back to the rectifier ac power grid, the results on the capacitor energy, increasing voltage increasing, when the voltage reaches a certain value (such as 700 v), launched by the control system of IGBT transistor below, so that the dc power supply () by discharge brake resistance RB and the opening of the transistor, and power of (-) meet with, through the current IB = VDC/RB. Release the electrical energy stored in the capacitor. The inverter continuously sends back the electric energy from the motor, which makes the locomotive produce certain braking force.

[6] inverter

The inverter is composed of 6 IGBT transistor switches and 6 fast recovery reflow diodes in inverse parallel with 1GBT to realize two-way current flow. This circuit structure is a two-level control mode, with the help of PWM control technology, realize voltage/frequency (V/F) coordinated control (VVVF control).

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 (there is a phase Angle difference), and the inverter inverts the electric energy of the dc power supply to the ac electric energy, providing power to the motor. When the traction asynchronous motor makes the braking operation, the voltage/current at the stator winding end of the ac motor is reversed (there is a phase difference), and the inverter converts the ac electric energy generated by the motor into dc and sends it back to the filter capacitor. The alternating to dc rectifier transformation, its principle is described in the kinetic energy braking.

In addition to the main circuit components, there are auxiliary power control circuit, signal detection elements and control system weak current control unit.

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 all-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:

(1) low frequency (starting) large torque;

(2) 200% overload capacity and software inverse time limit characteristics;

In order to prevent dynamic overvoltage, overcurrent set up automatic stall control;

In addition, software can choose flexible PWM energy consumption braking;

5. Automatic voltage control (AVC) that maintains output voltage by overcoming grid fluctuation; Internal combustion engine, motor, frequency converter