i Overview:

 

There are 2 ways for mine lifting: perpendicular shaft and incline shaft, mine hoist is the key equipment in the mine production, is called the throat of mine. It is be responsible for lifting the mineral, waste rock, equipment, material and operators.

For example, some company’s inclined shaft hoist(model# 3-2), use the traditional speed control mode by rotor winding series resistors. This mode has the following problems:

1) It takes too long time to judge and remedy the faults due to too many electromagnetic relays. After a long time running, the malfunction rate is high, not only revise the repair costs, but also decrease the mine productivity.

2) Low degree of automation, big noise when running, improve the room temperature is high, hoist driver is easy to fatigue, great labor intensity.

3) When continuously lift heavy weights with low-speed, resisters heat seriously, often burned out resistor connection, large energy consumption.

4) Speed control is stepwise regulation, and thus the mechanical impact on the wire rope, the speed reducer is very large.

ii Transformation plan and system composition

2.1 Upgrading scheme

Use inverter, PLC, Energy Feedback Unit and HMI etc. to make up an AC speed adjustable system, replacing the old series-resistors speed regulation system.

 

2.2 System composition 

 

 

Frequency conversion circuits:

Knife switch, circuit breaker, frequency inverter, contactor and energy feedback unit. The energy feedback unit constitutes a cabinet.

Control circuit:

It is mainly composed of main PLC, auxiliary PLC, HMI, relay auxiliary control, and operation station. The main PLC control and the auxiliary PLC control are mutually active and standby, the relay auxiliary control, the main and auxiliary PLC control are installed in the same control cabinet, and the touch screen is installed as the upper computer on the operation station. The encoder acts as a position and speed feedback signal and is mounted on the motor and reel. The speed encoder signal installed on the motor is fed back to the inverter to form a PG vector control. Another encoder signal is fed back to the PLC for speed and position calculations. Speed motor signal, oil temperature, oil pressure signal and other switching signals enter PLC system.

 

iii Working principle:

3.1 Functions in four states: system stop, manual, automatic, and slide:

Stop: In this state, the system stops working, all operations cannot be performed, and the winch is in the brake shoe brake state;
 

Manual: In this state, when the system meets the requirements of safe operation, checking the parameters displayed on the meter and the touch screen, manually operate the main controller to command the winch up and down.

Auto: In this state, the up and down of the winch is controlled by the up and down buttons. During the travel, it can be manually interrupted by the stop button, and can be started again by the up and down buttons.

Sliding: When the system has serious faults and cannot perform normal up and down operations, the function switch should be switched to this position, and the winch will slide down to the determined position by the master controller.

Through the dual PLC digital control technology, the double-wire protection and control of the winch specified in the “Safety Regulations for Coal Mines” was realized. The double-wire protection and control of the winch has the following characteristics:

(1) High precision speed regulation, wide speed regulation range, and speed regulation accuracy higher than 0.05%.

(2) Two-wire protection: Use one hardware safety circuit and two software safety circuits to offer necessary safety margin, making the system safer and reliable.

(3) Two-wire control: In dual PLC systems, one PLC monitors the other one. When one of the PLC systems fails, another PLC system is put into use for emergent working, which improves the operational reliability of the winch system.

3.2 Protection and chain function:

1）Working together with the safety valve of the hydraulic station, safe braking system makes the hoist realize the first or second level braking, and the inverter enters the energy-feedback brake state.

2）Under any circumstance, the hoist can only be operated after the driver receives the driving signal.

3）When the lubricating oil pressure is too high or too low during the lifting process, or the lubricating filter and hydraulic station oil filter is blocked or the oil temperature is high, the corresponding alarm information is displayed on the touch screen, and the corresponding error indicators are lit on to inform that the driver can complete the current lifting travel but not could not operate the next lifting task until the alarm is removed.

4）When the hoist stops in the middle due to a fault, and if the lifting container is located in the the deceleration section, after the fault is removed, the driver is allowed to drive according to the direction of the last driving, and can only drive at a low speed: if the lifting container is not in the deceleration section, The wellhead sends a driving signal to allow the driver to drive at a high speed.

5） When the whole mine is out of power, the PLC system ensures that the lifting function can achieve the second-level braking, and the backup protection of the lifting machine is completed.

6）The working brake torque of the disc brake is adjustable. The emergency brake (safe brake) can generate secondary brake to avoid mechanical shock.

3.3 Travel sectional control function:

The travel sectional control is completed by the PLC system, which mainly divides the lifting process of the hoist into different speed travel sections. The frequency inverter speed reference value is changed according to different speed requirements in different travel sections, and the inverter uses the closed loop control to smoothly adjust the lifting speed. The travel sectional control not only controls the speed of the entire lifting process, but also controls the stop and braking of the lifter. The sectional control can prevent accidents such as over-winding, over-unwinding, derailment and rollover of the hoist, especially for special inclined wells with curves and branch roads.

3.4 brake control function:

The normal braking of the hoist has energy-feedback braking and clutching braking. The energy-feedback braking is realized by adding inverting into DC process of variable frequency drive. When the actual running speed of the hoist is higher than the given running speed, the motor works as a generator, the DC link voltage is increased, the inverter is supplied with energy and fed back to the grid, and the motor automatically runs in the braking state. This provides accurate parking of the frequency converter and prevent mechanical shock and rapid decline. Clutching braking is generally used to stop the lift. When the lift runs to the parking space, the travel controller sends a stop signal to the inverter and send a brake control signal to the clutch-brake to implement the braking. When an accident such as derailment occurs, the operation control performs emergency clutch-braking.

The energy-feedback braking mode is adopted by the system. During the braking process, the hoist runs smoothly, and the braking energy is fed back to the grid, which has certain energy-saving effects.

3.5 Main devices functions

3.5.1 PLC

All control except the emergency protection function of the system is controlled by PLC which can adapt to harsh industrial environment and strong anti-interference ability and has two communication ports that can be used simultaneously (programmer port and universal port). Through communication with the LCD touch screen, PLC can intuitively and conveniently display the system operating status, fault status, operating parameters, display all switching elements and various faults and alarms. PLC receives the signals transmitted from the voltage transmitter, current transmitter, encoder, etc. and do the calculation and processing to control, monitor and protect the winch. In emergency braking (safe braking), PLC controls disc brake to have secondary braking to avoid mechanical shock. Simplified PLC control diagram is as follows:

 

 

3.5.2 Frequency inverter (VFD) functions

The frequency converter receives the speed feedback signal from the rotary encoder to form a PG vector control to realize the step-less speed regulation of the motor. This system is equipped with AD300 series inverter and energy feedback unit:

1) integrated solution: the combination of motor drive and lifting logic control, eliminating the PLC in the original system, reducing potential fault points, simplifying system wiring, debugging and simple and flexible, representing the future development direction of crane control;

2) Wide voltage design can better meet the harsh power supply condition

3) Professional logic design of crane: With years of experience in hoist industry application, we design the control and safety logic of the expert-level, widely used lifting mechanism, including timing sequence coordination, anti-slip hook, etc. to provide perfect solution for customers.

4) In the mining, metallurgy, road construction industry, the feeding system has a special fixed length control to make the control safer and reliable. Precise torque control offers stall prevention and fault reset. Adding the rotary encoder can realize full frequency domain flux current vector control, keeping the motor 150% output torque at 0.5Hz.

Main technical parameters of frequency inverter:

(1) input power supply voltage AC304-456V, frequency 50HZ; voltage allowable fluctuation range / -20%; frequency allowable fluctuation range: 48-65HZ;

(2) the output frequency range is continuously adjustable from 0 to 50 Hz;

(3) rated heavy-duty output power: 160kW.

(4) overload capacity 150%, 1 minute; 180%, 20s

(5) Inverter has a high power factor cos φ> 0.98;

(6) When running at low frequency, there is automatic torque boosting function and slip compensation function to ensure 150% rated torque;

(7) the inverter is equipped with overvoltage, under-voltage, overcurrent, overload, power component over-heating and motor phase-loss protection. The fault memory function can retain the latest 3 faults error codes and the parameters of the last fault;

(8) Total harmonic content THD < 5%.

PLC control flow, as shown in Figure 2:

 

 

3.5.3 function of energy feedback unit

The main function is to improve the inverter’s braking ability, and feed back the braking energy of the weight of the declining winch to the power grid for energy saving purposes.

3.5.4 ​​the main function of the operator station

The operator station is equipped with a manually operated master controller. It can manually adjust the braking force and winch running speed. It is equipped with manual and pedal emergency brake switches for quick braking in case of emergency.

3.5.5 the function of the LCD touch screen

 
Through the communication with the PLC to visually display the system operating status, fault status, operating parameters(such as winch running speed, lifting container position, motor current and voltage, etc.) as well as the status of all switching elements and various faults and alarm information. It is also used to modify the PLC internal parameters setting.

iv System configuration:
 

4.1 the inverter selection

The motor driven by the inverter is: JR 127- 8 115k W

Rated voltage: 380V

Rated Current: 227A

Refer to principle that the rated output current of the inverter ≥ the motor rated current × 1.1

227×1.1=249.7A

Check the AD300 series inverter manual:

The rated output current of 132kW is: 250A

The rated output current of 160kW is: 300A

Considering the environment and transportation conditions under the pit, the inverter 160kW is finally selected.

 

4.2 PLC selection

Comprehensive consideration of over-rolling, over-unrolling, manual up, manual down, automatic up, automatic down, normal stop, emergency stop, safety circuit, hydraulic station control, disc brake control, work indicator, report indicator, etc. And the interlocking control between the various conditions, and consider certain redundancy.

4.3. Energy feedback unit selection

4.3.1 brake voltage selection

700V braking voltage is generally selected for 380V system.

 

4.3.2  braking current selection

Braking current refers to the DC current flowing through the braking resistor and the braking unit during braking.

Selection basis: Braking must fully absorb the regenerative energy of the motor

Brake absorption power (U × I) = motor regenerative energy (Watt) = 1000 × P × η

P: motor rated power (kW)

U: brake unit DC operating point, usually take 700V

I: Braking current (A)

η:  The conversion efficiency of mechanical energy. Usually take η=0.7

     Calculated: I=115

4.3.3, power selection

Motor regenerative energy must be absorbed and fed back to the grid

PR=P×Kf×η×ε

PR: energy feedback unit power (Kw)

η: The conversion efficiency of mechanical energy. Usually take η=0.7

ε: braking power consumption safety factor. Take ε = 1.4

Kf: braking frequency, which refers to the proportion of time of regeneration process accounts for the entire motor working time. Kf takes 60%

Calculated PR = 67.62 kW

Select feedback unit power 70 kW

The application of the variable frequency drive in well inclined shaft lifting system can solve the problems of speed regulation and starting, realizing the soft start and soft stop, reducing the mechanical impact and make the operation more stable and reliable. Current shock is very small in start and accelerate, which reduces the impact on the power grid, simplifies the operation and reduces the labor intensity. The S-curve running speed makes the acceleration and deceleration smooth and has no impact. The safety protection function is complete. Besides the general protections for over/under-voltage, over-load, SC, over-heat etc., it also provides interlock protection, automatic speed limit protection and so on.

Promote the variable frequency drives replacing old resistors-series speed regulation system is a new way to improve underground mining inclined shaft hoisting and energy saving.