Comparison of starting methods for very large motors

Medium and high voltage (3-10kV) motors typically have a large capacity, often exceeding 200kW. In recent years, with the rapid industrialization in China, many industries have expanded their production capabilities, leading to an increase in the power of drive motors used in their equipment. For instance, in the steel and chemical industries, the use of motors rated above 10,000 kW has become increasingly essential. As ultra-large motors (ranging from 10,000 kW to 50,000 kW) are being widely deployed, the challenges associated with starting these massive machines have become more pronounced. Pumped storage power stations, for example, often include several generating motors ranging from 40,000 kW to 300,000 kW. According to China's recent development plans, there are numerous steel companies with over ten thousand tons of production capacity and millions of tons of ethylene projects under construction, along with many new pumped storage power station projects. When combined with other industrial sectors, the number of ultra-large high-voltage motors is substantial, making the issue of starting such motors a critical concern. In the past, ultra-large motors were not commonly used, and fewer studies were conducted on their starting methods. For these large motors, the disadvantages of reduced-voltage starting became more apparent. Therefore, this method was mainly applied to motors between 10,000 kW and 20,000 kW. However, for motors larger than 20,000 kW, soft starting would require expensive high-voltage inverters, which many choose to avoid due to cost concerns, opting instead for direct transformer power supply. Full-voltage starting, while sometimes necessary, is not the preferred method because the risks associated with direct full-voltage starting are more significant for very large motors. ### The Risks of Direct Voltage Starting and the Benefits of Soft Starting 1. **Impact on the Grid** The large current during starting can be comparable to a three-phase short circuit, causing power oscillations and grid instability. Additionally, high-order harmonics in the starting current may lead to resonance with grid parameters, resulting in relay protection malfunctions or control system failures. 2. **Damage to Motor Insulation and Reduced Lifespan** Repeated Joule heating from large currents accelerates insulation aging, reducing motor life. Mechanical forces from the current can cause wire friction, further deteriorating insulation. 3. **Overvoltage from Switch Closure** The contact jitter when closing a high-voltage switch can generate operational overvoltage on the motor’s stator winding. 4. **Damage to Electrical Components** Large currents create strong mechanical forces on the stator coil and rotor cage, potentially causing structural issues like loose clamping, coil deformation, or squirrel cage breakage. 5. **Harm to Mechanical Equipment** A full-voltage start produces about twice the rated torque, which can cause excessive wear on gears, belts, blades, and other components, even leading to mechanical failure. When using reduced-voltage starting, the risks are somewhat mitigated, but soft starting eliminates most of these dangers. On the other hand, direct starting through an independent transformer only reduces grid voltage fluctuations, leaving other hazards unchanged. Given the high value and critical role of ultra-large motors in industrial operations, it is essential to implement robust protective measures. ### Comparison of Starting Methods #### 1. Autotransformer Reduced-Voltage Start This method involves connecting the motor to the low-voltage side of a transformer, reducing the primary current and minimizing line voltage drop. However, it has several drawbacks: - During the start-up process, the voltage is switched 2–3 times, causing torque fluctuations that can harm precision machinery. - The large current during switching may induce overvoltage on the autotransformer winding, damaging insulation and reducing its lifespan. This method is generally used for motors between 10,000 kW and 20,000 kW and rarely for larger ones. #### 2. Capacitor-Assisted Reduced-Voltage Start This traditional method uses a capacitor on the secondary side of a self-coupled transformer to reduce inductive current flow into the grid. While it helps in small-capacity grids, it does not significantly lower the motor’s starting current, so the impact on the motor and load remains high. Key concerns include: - High harmonics during start-up may shorten capacitor lifespan. - Large inrush currents when capacitors are closed make this method unsuitable for frequent starts. - If the motor trips during start-up, mechanical oscillation could occur, endangering equipment. - If the capacitor is not removed timely at the end of the start, over-compensation may occur, increasing voltage fluctuation. #### 3. Independent Transformer Power Supply (TD Group) This method essentially involves full-voltage starting, using a high-impedance transformer to isolate the motor from the grid. Although it reduces grid voltage fluctuations, the motor and mechanical equipment still face damage. Moreover, it leads to higher energy losses compared to shared grid systems. For example, a 20,000 kW/10 kV motor using an independent transformer results in significant power loss, costing around 580,000 yuan annually in electricity. Using a shared network can save capital investment and provide better economic benefits. #### 4. Static Inverter Mode Static frequency conversion devices offer the best performance among soft-start methods, providing smooth operation with no mechanical shock. However, they are still in early development stages, with high switching losses and reliability issues. Control complexity also makes them challenging for general technicians. Additionally, the long starting time (up to 3–5 minutes) is unfavorable for applications requiring quick response, such as pumped storage units. Currently, large-capacity static inverters are not produced domestically, relying heavily on imports. Key technologies are controlled by foreign firms, making them expensive. Maintenance costs after operation are also high. #### 5. Switching Transformer-Based Soft Starter This device uses thyristor switching characteristics to continuously adjust the transformer’s output voltage, enabling smooth motor starting. It improves upon traditional thyristor-based soft starters by replacing the thyristor string with the high-voltage winding of a switching transformer, placing the thyristors on the low-voltage side. This enhances reliability and reduces harmonic distortion. It offers full-range voltage and current adjustment, supports arbitrary waveforms, and allows for closed-loop control with fast response. It also has low startup losses, can handle continuous starting, and is cost-effective for large motors. The switching transformer-based soft starter is currently the most efficient solution for ultra-large high-voltage motors, offering excellent cost-performance. ### Application Example: Catalytic Cracking Unit in a Petrochemical Plant A 12,000 kW high-voltage motor was successfully started using a switching transformer-based soft starter. The motor had a rated power of 12,000 kW, a rated current of 1,322 A, and a rated voltage of 6 kV. The fan used was an AV-50 series axial compressor. The power supply came from a 20,000 kVA transformer with a voltage class of 35 kV/6 kV. During the start-up process, the maximum starting current reached 3,000 A (2.27 times the rated current), with a starting time of 10.4 seconds and a 15% voltage drop on the 6 kV busbar. Subsequent tests showed similar performance, confirming the effectiveness of the soft starter. This successful implementation marks the birth of a new generation of ultra-large soft starters with independent intellectual property rights, allowing China to develop its own solutions for large motors. ### Conclusion Ultra-large high-voltage motors are expensive and play a vital role in industrial operations. Despite their short working time, their achievements in reducing electrical and mechanical stress cannot be ignored. They should be given serious attention by engineers and managers. From an energy-saving perspective, soft starting eliminates the need for an independent power transformer, offering significant energy savings. This makes it highly relevant in today’s push for energy efficiency and emissions reduction. The switching transformer-based soft starter represents a major technical advancement, offering advanced performance, high reliability, and cost-effectiveness, thus providing a new option for ultra-large motor starting.

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