Abstract: This paper describes the characteristics of multi-motor driven belt conveyor transmission and analyzes the scheme of using frequency converter control for synchronous transmission of multiple motors. It also analyzes the solution of BENSHAW using the MVRSM type high-voltage solid-state soft starter with speed feedback control, based on the soft-start control system of a 15,000t coal storage silo belt conveyor in the largest coal mine in North Antelope, USA. The paper points out that this solid-state soft starter solution can ensure coordinated operation of each motor without overload, enabling smooth starting of the belt conveyor and preventing excessive tension in the belt. Furthermore, it is far cheaper than the frequency converter solution, making it the most cost-effective solution for belt conveyor transmission. English abstract: The paper expatiate on the features of belt conveyer, analyze a multi-motors synchronously driven by inverters scheme; and combining with the application example of a belt conveyer of 15,000 Ton Storage Silos in North Antelope the American largest Coal Mine, the paper analyze Benshaw's solution method adopt MVRSM type solid state soft starters with speed feed back control. The paper point out that with this solid state soft starter solution we can not only start the belt conveyer smoothly but also make all motors work cooperatively without overload and there is no exorbitant tension in the belt. Its price is far lower than inverter's solution. Therefore the solid state soft starter solution is optimized in performance/price ratio. Keywords: solid state soft starter transmission control belt tension control frequency conversion speed regulation 1. Introduction In my previous article, "Transmission Control of Single-Motor Driven Belt Conveyors," I discussed the requirements of belt conveyors for their transmission systems. After analyzing various transmission control methods, I concluded that a solid-state soft starter with torque closed-loop control and speed feedback closed-loop control is the best choice. Benshaw, an American company, already possesses such a solid-state soft starter, which is widely used in practice. Multi-motor driven belt conveyors, in addition to considering the general requirements of single-motor driven belt conveyors (such as large load variations on the belt during startup and avoiding large tension surges), also need to address the synchronization and load balance of each motor during startup and operation. Many belt conveyors in China frequently experience overload tripping of one motor during startup, sometimes even causing damage to the motor or belt, due to extreme load imbalance among the motors. 2. Variable Frequency Drive (VFD) Control Scheme Figure 1 shows a VFD control scheme for a belt conveyor driven by two motors. If each AC motor of the belt conveyor is controlled by a vector control type VFD, the system shown in Figure 1 exhibits the best performance. Figure 1 shows a control system for a belt conveyor driven by two electric motors. Since the two motors are connected by the same belt, they have the same speed; if their speeds differ, belt slippage will inevitably occur. In this system, the speed feedback to the speed regulator of frequency converter 1 forms a closed-loop speed system. The output of its speed regulator not only provides a current setpoint to its own current regulator but also to the current regulator of frequency converter 2. This ensures that both frequency converters output the same current to their respective motors, thus guaranteeing load balance between the two motors. If the frequency converter has an energy-dissipating braking resistor, this system is also suitable for downward-sloping belt conveyors. When a downward-sloping belt conveyor stops, the motors need to return energy to the frequency converter system due to gravity; therefore, the frequency converter must be connected to a braking unit and braking resistor to operate. While frequency converter-driven belt conveyors are technologically advanced, they are too expensive. Is there a method that meets the technical requirements of belt conveyors while also being more cost-effective? Benshaw's RSM series (MVRSM series for medium voltage) can solve the control problems of multi-motor driven belt conveyors due to its speed feedback control and current closed-loop control functions. 3. Solid-state soft starter control scheme with speed feedback closed-loop control Taking Benshaw's solution using RSM and MVRSM series solid-state soft starters as an example. North Antelope Coal Mine is the largest coal mine in the United States. It has a 15,000-ton coal storage silo, which uses a 72-inch wide belt to transport coal to the silo (see Figure 2). This belt conveyor is driven by three 4160V 1250Hp motors, each driven by a Benshaw MVRSM-4160V-1250Hp medium-voltage solid-state soft starter. The system diagram is shown in Figure 3. Figure 2 shows a 72-inch wide belt conveyor transporting coal to the coal bunker. Figure 3 shows a control scheme for a belt conveyor driven by three medium-voltage motors using Benshaw's MVRSM solid-state soft starters. In the system shown in Figure 3, all motors are of the same model and have essentially the same mechanical and electrical performance. Each motor is controlled by an MVRSM solid-state soft starter with speed feedback closed-loop control. The system only has one tachometer (speed encoder) ST, which outputs pulses with a frequency proportional to the speed. Since all soft starters need to share the same speed feedback signal, the speed feedback signal needs to be sent to a frequency/voltage converter FVC, which converts the speed feedback pulses into a 0-5V or 0-20mA signal. Its output signal passes through three signal isolators SI1-SI3 and is sent to the speed feedback input terminals of the solid-state soft starters respectively. Of course, if three speed encoders are used in the system for speed feedback signals, then connecting a frequency/voltage converter to the output of each speed encoder is the same. However, it is crucial to ensure that the speed feedback signal of the MVRSM soft starter is within the range of 0-7V. If the signal exceeds 7V, it will cause permanent damage to the MVRSM speed control board. The speed closed-loop control block diagram of the MVRSM solid-state soft starter is shown in Figure 4. Due to the presence of the speed control closed loop, it ensures that the controlled motor speed follows the given speed. If the controlled motor speed is much lower than the given speed, the speed regulator will output the maximum current, allowing the motor to accelerate with the maximum current. If the controlled motor speed is equal to or higher than the given speed, the speed regulator will output the minimum current, allowing the motor to operate with the minimum current. Figure 4: Speed ​​feedback control principle block diagram of the MVRSM solid-state soft starter. In the case of multiple motors/multiple soft starters, during startup, one motor (and soft starter) will typically bear a greater load than the others. In severe cases, only some motors will rotate, while the rest will not, resulting in motor tripping or belt breakage. This is because even motors of the same model from the same manufacturer will not have completely identical mechanical and electrical performance. Furthermore, when the motor and soft starter are in different positions, the varying power supply voltage drop and impedance result in different voltages for each device. Using the aforementioned speed closed-loop control, the MVRSM solid-state soft starter prevents any motor's current from falling below 30% of its full-load current. This keeps the soft starter's thyristors conducting and ensures the motor is always ready to bear the load. In this system, all solid-state soft starters have identical settings, such as motor full-load current, overload protection level, and tachometer full-speed voltage, and are set to speed ramp operation mode with the same speed ramp and the same maximum and minimum currents. So how does the speed control closed loop function when load imbalance occurs? Suppose that at startup, one motor (soft starter) carries most of the load until the current reaches its maximum current setting. Because this motor can no longer increase torque, the other motors (soft starters) will share the necessary load, and their currents will gradually increase to increase torque and allow the speed to follow the given value. For most users, the current distribution process described above is acceptable because the speed is tightly controlled, and the current is within the tolerance range of each motor, allowing the load to be started appropriately. It is important to note that in this system, the load distribution needs to be relatively balanced after the motors reach full speed, as the starters have entered bypass operation and can no longer regulate load distribution. 4. Conclusion Compared to frequency converter control, the biggest advantage of the solid-state soft starter control scheme with speed feedback closed-loop control is its lower cost. For a belt conveyor driven by three 4160V 1250Hp motors, the price is only about 40% of that using a frequency converter, making it a truly cost-effective solution for belt conveyor drives.