A soft starter is a novel motor control device that integrates soft starting, soft stopping, light-load energy saving, and multiple protection functions. It is known internationally as a SoftStarter. The soft starter uses three anti-parallel thyristors as voltage regulators, connected between the power supply and the motor stator. This circuit resembles a three-phase fully controlled bridge rectifier circuit. When starting the motor with a soft starter, the thyristor output voltage gradually increases, and the motor gradually accelerates until the thyristors are fully conducting. The motor then operates at its rated voltage mechanical characteristics, achieving smooth starting, reducing starting current, and preventing overcurrent tripping. Once the motor reaches its rated speed, the starting process ends, and the soft starter automatically replaces the thyristors with a bypass contactor, providing the rated voltage for normal motor operation. This reduces thyristor heat loss, extends the soft starter's lifespan, improves its efficiency, and prevents harmonic pollution of the power grid. The soft starter also provides a soft stopping function, which is the reverse of the soft starting process. The voltage gradually decreases, and the speed gradually drops to zero, avoiding torque surges caused by free stopping.
Motor soft starter installation process:
Soft starter installation -> Main circuit terminal wiring -> Control circuit wiring -> Startup parameter setting -> Simulation test -> Load test run
Choose a location with good dust protection and heat dissipation for the soft starter installation; during installation, the minimum distance between the installation space and the surrounding environment should be considered according to the instruction manual.
The main circuit terminal wiring can be divided into two types: internal connection and external connection (see figure). During construction, connect the motor and soft starter wiring according to the design drawings.
Normally, the control circuit of the distribution panel cabinet is already fully connected. Use a multimeter to check the control circuit against the drawings. Note that you should not use an insulation megohmmeter to test the insulation resistance of the control circuit, so as not to damage the electronic components inside the soft starter.
Soft starters cannot be used for no-load simulation tests because there is no current or voltage output under no-load conditions, which will cause the relay protection to malfunction. A light load test should be performed to check whether the soft starter parameter settings meet the design requirements. Here is a recommended economical and simple method for engineering sites: three 200W incandescent light bulbs can be used to simulate the three-phase windings of the motor as a light load for the test.
During the trial operation phase, pay attention to whether there is any deviation between the actual measured value and the set value, analyze the cause of the deviation and make continuous adjustments until the requirements are met; and make corresponding test records and operation reports.
Classification and Selection of Motor Soft Starters
Classification
1. Run the soft launcher online:
In the last century, soft starters were mainly sold in the Chinese market by foreign brands such as AB, ABB, Schneider Electric, and Siemens, but they all operated online. During application, the following disadvantages of online operation were discovered: 1. High power consumption due to long-term online operation of the thyristor leads to energy waste. 2. The large heat dissipation of the thyristor requires mechanical cooling, posing significant challenges to system integration. 3. Long-term online operation of the thyristor introduces high-order harmonic pollution to the power grid. 4. The reliability of the thyristor as a main switching element is far lower than that of mechanical switches over long periods. 5. The high cost is unacceptable to users. 6. The large size of the thyristor is due to its larger size and the need for heat dissipation. Its advantages are: 1. It integrates motor starting, protection, and control, with a powerful intelligent controller fully functioning. 2. The use of mechanical cooling makes it suitable for frequent starting applications. 3. The circuit is simple, facilitating maintenance and repair.
2. Bypass operation of the soft starter:
Towards the end of the last century and the beginning of this century, considering the drawbacks and technical difficulties of online operation, domestic manufacturers directly developed bypass-type soft starters, where the motor bypasses the contactor after starting. Its advantages include avoiding the disadvantages of online operation of thyristors, especially eliminating the need for mechanical cooling. However, it also brings disadvantages: 1. Increased circuit complexity and reduced system reliability; 2. Inability to fully utilize powerful intelligent controllers, some of which cannot protect the motor; 3. Increased size and cost of the complete unit; 4. Increased difficulty of maintenance and repair. After comprehensive comparison, most manufacturers in the market still adopt bypass operation. Even for soft starters using online operation, designers still add a bypass contactor to ensure bypass operation, thus avoiding the defects of online thyristor operation.
3. Built-in thyristor bypass type online soft starter:
At the 2003 Hansenberg International Electrical Technology Exhibition, German companies Möhler and ABB (limited to 200kW and above) launched in-line soft starters with built-in SCR bypass. In 2004, Tianjin Norhaden Electric Manufacturing Co., Ltd. (a Sino-US joint venture) developed in-line soft starters with built-in SCR bypass for 15-400kW. Currently, many domestic and international electrical companies are developing in-line soft starters with built-in SCR bypass. An in-line soft starter with built-in SCR bypass (referred to as a built-in bypass soft starter) has a set of mechanical contacts connected in parallel with the SCR inside. During the soft start and soft stop of the motor, the SCR operates, and the mechanical contacts open. When the motor is running normally, the SCR closes, and the mechanical contacts close. This entire process is automatically completed by an internal controller; from the perspective of external wiring, it is a single device, hence the term "in-line operation."
It can also be called a bypass-type soft starter, which integrates the external contactor into the soft starter while maintaining a single unit size. Its advantage is that it combines the advantages of both types mentioned above while avoiding their respective disadvantages.
1. The circuit is simple.
2. Natural air cooling.
3. The thyristor only handles startup and shutdown, avoiding the power consumption and heat dissipation caused by the thyristor running online.
4. Small size (comparable to the general size of bypass type).
5. The powerful intelligent controller can be fully utilized to start, stop, protect, and control the motor.
6. Saves space for complete sets.
7. Due to the integrated design of the thyristor and mechanical contact, the intelligent controller achieves arc-free mechanical contact, making the electrical life of the mechanical contact equal to its mechanical life. This solves a long-standing problem for contactors and greatly improves system reliability compared to bypass types.
8. Energy saving: This energy saving refers to the soft starter itself, which is negligible compared to the online thyristor type. It reduces energy consumption by 60% compared to the bypass type. This is because the mechanical contacts of the built-in bypass type use arc-free control, significantly reducing the hardness of the silver points and the contact resistance, thus greatly reducing the closing pressure of the mechanical contacts. The magnetic attraction mechanism of the mechanical contacts is reduced by half, halving energy consumption. The contact energy consumption of the mechanical contacts is also reduced by half. Overall, the energy consumption of the mechanical contacts and magnetic mechanism is halved compared to the bypass contact. Combined with the energy savings from the thermal relay, the overall energy saving is 60% compared to the bypass type.
Selection
Based on the above analysis of the principles, advantages, and disadvantages of soft starters, the built-in bypass type appears to have the strongest applicability, the most advantages, and the fewest disadvantages. In practical applications, the specific product brand's functions and technical parameters, as well as the product's verifiability, must be considered, and the selection should be based on the actual usage. It is necessary to distinguish between frequent and infrequent starts. For soft starters, generally, if the starting interval does not exceed 2 minutes and the number of starts per hour does not exceed 30, it can be considered an infrequent start. Less than this number should be considered frequent starts. Fans and pumps are generally considered infrequent starts. Mechanical transmissions can have both frequent and infrequent starts. For example, belt conveyors and ball mills can be considered infrequent starts, while starters or motors in large mechanical equipment requiring reversible functions are often considered frequent starts. In environments with frequent operation, the soft starter should be selected based on the motor's starting current, as soft starter manufacturers generally select a thyristor current that is 2.5 times the motor's rated current. The maximum starting current of the motor is limited to 4.5 times the rated current. To fully utilize the short-time overload capacity of the thyristor under infrequent operation, the capacity of the soft starter should be increased under frequent starting conditions, typically between 1.2 and 1.5 times the rated current, depending on the frequency. Furthermore, due to the frequent operation of the thyristor, the soft starter must be equipped with mechanical cooling to dissipate the large amount of heat generated. Ideally, one mechanically cooled soft starter should be installed per switchgear, and the switchgear should also be equipped with mechanical ventilation.
Types of main wiring and common faults of motor soft starters
Main wiring method for motor soft starter:
1. Online type:
All soft starter controllers have motor overload protection. When the soft starter is running online, the controller can protect the motor from overload; therefore, do not install a thermal overload relay. Because the harmonic current after the thyristor is very large, an electronic thermal overload relay cannot be installed; otherwise, a malfunction of the thermal relay will prevent the system from working properly. Since thyristors are relatively expensive and difficult to replace, fast-acting fuses should be used to protect them from short circuits at the lower end of the soft starter, which could burn out the thyristor. Figure 4A shows a frequently used location where no contactor is added to the upper end of the soft starter. Figure 4B shows a less frequently used location where the soft starter power is disconnected after stopping.
2. Bypass type:
The soft starter cannot operate without the bypass contactor, so it is present in both main wiring schemes. The function of the contactor at the upper terminal of the soft starter is the same as in online operation mode, and will not be repeated here. Special attention should be paid to the thermal relay; it is best to install it at the lower terminal of the bypass contactor without carrying the starting current, especially for electronic thermal relays, as the current harmonics after passing through the soft starter are significant and can interfere with the electronic thermal relay, causing it to malfunction and stop the motor. Furthermore, because the thyristor's short-time operation does not require a fast-acting fuse, it is not included in the main wiring scheme.
3. Built-in bypass type:
Its main wiring is roughly the same as the online type. The only advantage is that because the thyristor operates for short periods, there is no need to install a fast-acting fuse. Overload protection for the motor is achieved by a soft starter controller, which not only surpasses electronic thermal relays in function and performance, but also will not malfunction due to harmonic currents in the main circuit or external interference.
Common faults of motor soft starters
Soft starter malfunctions can be broadly categorized as follows:
1. The motor won't start:
There are generally two reasons why an electric motor might not start:
First, if one of the six thyristors fails to trigger reliably or does not conduct, then one phase of the circuit will be connected to half-wave DC. The DC current flowing through the two phase windings of the motor will act as a brake on the motor. Not only will the motor fail to start, but in severe cases, the motor and the thyristor may burn out.
Secondly, unsuitable starting parameters or starting curves can cause the motor to fail to start, which is a common fault. The former can occur during use, but its probability is lower than the failure rate of contactors. The latter mostly occurs during the initial commissioning and will not occur after proper commissioning. Most manufacturers do not have this problem; their starting programs have good performance and their factory settings are highly applicable. Only a few manufacturers' products require manufacturers to perform the commissioning themselves.
2. The silicon controlled rectifier (SCR) burns out:
Thyristor breakdown or explosion is a type of fault that can occur regardless of whether it is a domestic or foreign brand and varies from manufacturer to manufacturer, but the failure rate is lower than that of contactors. The main problem lies in the installation process of the disc-type thyristor.
3. Controller burnout:
Compared to soft starters, controller burnout is the most serious fault. For some manufacturers, this type of failure results in a return rate exceeding 30%. Imported or joint-venture manufacturers rarely encounter this problem. The main issues are the controller's power supply, trigger circuit, and input circuit, all of which are prone to burnout.
4. Soft starter malfunction:
It is not uncommon for motors to stop while running due to interference with the soft starter, and to start while stopped due to interference with the soft starter. The former is more common, while the latter has only occurred with two brands. The reasons are twofold: product quality issues and wiring layout. However, imported or joint-venture soft starters do not exhibit this phenomenon, while this problem is more prevalent among domestic brands.
5. Poor contact of internal connectors in the soft starter:
The selection of internal connectors for soft starters is not inherently an issue, but it's a problem that domestic manufacturers often overlook, frequently leading to malfunctions. Imported or joint-venture manufacturers do not make this mistake.
