Case Study on HIVERT MV VFD Master-Slave Application in Baoshan Mining

1.   Introduction

AC Drives have been widely used in various areas of Industries due to its stable performance, precise speed regulation, and energy-saving effect. Hiconics, after 10 years’ growth, now is able to manufacture the high performance vector control drives: one VFD can drive two or more motors, with master-slave control function and synchronous/asynchronous vector master-slave control. Hiconics has now become the leading player in the Chinese MV AC drive Market.

2.   AC drive speed regulating synchronous control solution

In the field application, there are many cases in which one load has to be driven by two or more motors. A perfect solution to the synchronous control is essential to the synchronous frequency regulating system. According to the difference of the field processes, different variable frequency control solutions are available. Motor synchronous operation means to create a certain link among motor speed regulating controls, to coordinate the relations of speed and torque of different motors.

1.1 One VFD drives multiple motors

This solution is to use one VFD to drive multiple motors, connecting the stator winding in parallel to the output side of one VFD.

In this solution, because the motor stators are connected directly to the output side of the same VFD, which results one driving speed, and the speed synchronous performance is the best. However, the torque synchronization is the worst, because VFD cannot conduct independent control on the motor torque, the output power of each motor is determined by the motor parameter, machine parameter and the variations of the loads.

There are three disadvantages in one VFD driving multiple solution:

1.    VFD cannot distribute the powers of the different motors efficiently.

2.    If the VFD has a fault, the whole system has to stop, the system has redundancy difference, difficult for maintenance and restore.

3.    VFD cannot protect every motor efficiently.

1.2         Master-slave control solution

This solution uses multiple VFDs to drive multiple motors, connecting the stator winding accordingly to the output sides of different motors.

Master-slave synchronous operation variable frequency system is to use several independent VFDs through master-slave synchronous communication mode to ensure the balance of the rotation speed and torque among the motors. Any motor can be a master motor, and the others are slave motors.

In the master-slave control solution, VFD can do independent control on the motor torque. Master-slave VFD communication uses optic fiber, which has the features of strong anti-interference ability, fast speed of communication, building appropriate load distribution relations in the synchronization running between motors, makes the best of the output torque control ability of each motor. And the master and slave VFDs can automatically regulate its own output rotation speed and torque.

3.   HIVERT MV VFD Master-slave control

When several motors drive one load at the same time, there will be problems of unsynchronized rotating speed and torque. The motors with fast speed and big torque will bear heavy load, therefore the synchronized running of motors is the key element in the control process. The master-slave control connection modes have two following types:

HIVERT Series MV VFD (for flexible connection load, shown as Figure 1), when the master drive receives the signal of normal starting, it will start the VFD, at the same time, sending the operation signal, speed and torque current to the slave drive. Slave drive conducts normal starting, operation and output torque control according to the signals received from the master drive. The slave drive itself is able to regulate the torque current, and at the same time compare its own torque current and the current sent from the master drive, and the slave drive will automatically regulate its own output rotation speed to reach the torque balance among the motors.

HIVERT Series MV VFD (for rigid connection load, as Figure 2 shows),

when the master drive receives the signal of normal starting, it will start the VFD, at the same time, sending the operation signal, speed and torque current to the slave drive. Because the master and slave drives are rigid connection, the running speeds of the two are almost the same. Slave drive conducts normal starting, operation and output torque control according to the signals received from the master drive. The slave drive itself is able to regulate the torque current, and at the same time compare its own torque current and the current sent from the master drive, and the slave drive will automatically regulate its own output rotation speed to reach the torque balance among the motors.

4.   HIVERT MV VFD master-slave application in Baoshan Mining in Kuancheng town, Chengde City

Baoshan Mining is a low grade vanadium-titanium magnetite ore mining, attached to Chengde Jingcheng Mining Group. Its motor driving structure is as following:

4.1 Baoshan Mining motor specification and conveyor specification

                                                                                         Table 1  Baoshan Mining motor specification and conveyor specification

4.2 Baoshan Mining HIVERT MV VFD master-slave control

Baoshan Mining 1th section of belt has four motors, among which motor 1 and motor 2 belong to the same group, motor 3 and motor 4 belong to another group, as the rigid connection. The connection between the two groups is flexible connection. Its control difficulty point is the three slave-motors control mode track.

Four HIVERT NV VFDs correspond with four motors. According to motors 1~4, we define the VFDs as HIVERT1～HIVERT4, HIVERT1 is master drive, HIVERT2～HIVERT4 are slave drives.

HIVERT2 is slave drive 1, using rotation speed and rotation and torque double closed loop regulating. After slave 1 receives the operation signal of the master drive, comparing its own torque current to the master drive torque current. In this process, the slave drive will automatically regulate its own output voltage, to reach the torque balance between the master drive and slave drive 1.

HIVERT3 is slave 2, using torque single closed loop regulation. After slave 2 receives the signal from the master drive, it runs, following the data sent from the master drive and output the torque control. Slave 2 compares its own torque current to the master torque current, and slave drive will automatically regulate its own output rotation speed to reach the torque balance between master drive and slave drive 2.

HIVERT3 is slave drive 3, using torque single closed loop for regulation, rotation speed is limiting; after the slave drive 3 receives the operation signal from the master drive, it runs following the data sent from the master drive and output torque control, slave drive 3 compares its own torque current with the master torque current, and the slave will automatically regulate its own output rotation speed to reach the torque balance between master drive and slave drive 3. At the same time, through rotation speed limiting, reach the speed balance with the slave drive 2. Onsite running is shown as figure 4, the current difference between master and slave drives is shown as figure 5:

4.3  Baoshan HIVERT MV Vfd redundancy design

The onsite conveyor belongs to the second and the third part transportation. If the VFD or motors has faults, it will cause the whole system stop running. Considering this phenomenon, VFD has added the redundancy design, that is when any VFD or motor has faults, the master-slave control structure will not be damaged. Permitted by the belt system, the rest of the VFDs and motors will continue to drive the conveyor continue to manufacture. After the VFD and motors being maintained and ready to start, the VFD will automatically restore the operation.

In Baoshan mining, the four VFDs are running, if any slave drive has serious faults, it will disjunct its own medium voltage breaker. In this way, it will not affect the normal operation of the other three master-slave drives. The client can immediately maintain the broken VFD, after the broken VFD restores to its own normal operation, the VFD will follow the current rotation speed of the motor and output the relevant rotation speed; light fault will not affect the master-slave operation, only the giving fault alarming.

When master drive has serious fault, it will disjunct its own medium voltage breaker and inform the slave drives free stops. The slave drives will be standby, and the clients can replace any VFD as new master drive, the system will immediately restores to normal operation.

5.   Conclusion

In Baoshan Mining project, HIVERT VFD runs smoothly since it’s been put into operation, the master and slave drives reached power balance, performing well; after the onsite application, Baoshan Mining Belt conveyor has realized smooth and stable start/stop, reduce the lash on the belt, and the wear and tear of the belt, thus improving the manufacturing efficiency and system’s reliability.

Hiconics always plays a leading role in the automation industry, offering solutions to different speed regulation operation condition, which improves the automation level of the system and also responds to the energy saving policy of the Chinese government.