【Daily Question 20090217】What is the difference between an inverter and a soft starter?

A soft starter is a novel motor control device that integrates functions such as soft starting and stopping of the motor, energy saving at light loads, and various protection features; it is referred to as a Soft Starter abroad. Its main components are three-phase parallel thyristors connected in series between the power supply and the motor to be controlled, along with their electronic control circuit. By using different methods to control the conduction angle of the three-phase anti-parallel thyristors, the input voltage to the motor being controlled can be varied according to various requirements, thereby enabling the implementation of different functions. ??Soft starters and frequency converters are two products with completely different purposes. An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency ； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. Inverters possess all the functions of soft starters, but they are much more expensive and have a much more complex structure.

An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged.

When the motor is started directly at line frequency, it will generate 7 to 8 times the motor’s rated current. This current value will **increase the electrical stress on the motor windings and generate heat, thereby reducing the motor’s lifespan. Variable frequency speed control, on the other hand, allows for startup at zero speed and zero voltage (with torque enhancement can be applied as appropriate). Once the relationship between frequency and voltage is established, the inverter can drive the load to operate using V/F or vector control methods. The use of variable frequency speed control can significantly reduce the starting current and enhance the strength of the windings. The most direct benefit for users is that the maintenance costs of the motor will be further reduced, while its lifespan will increase accordingly. The soft starter uses three-phase parallel thyristors as voltage regulators, which are connected between the power supply and the motor stator. When starting a motor using a soft starter, the output voltage of the thyristors increases gradually, causing the motor to accelerate slowly until the thyristors are fully conductive. At that point, the motor operates according to its mechanical characteristics at the rated voltage, enabling a smooth start, reducing the starting current, and preventing tripping due to excessive starting current. Once the motor reaches its rated speed, the starting process is completed. The soft starter automatically replaces the thyristors that have completed their function with bypass contacts, thereby providing the motor with the rated voltage needed for normal operation. This reduces the thermal losses of the thyristors, extends the service life of the soft starter, improves its efficiency, and also helps to prevent harmonic pollution in the power grid. The soft starter also provides a soft shutdown function, which is the reverse of the soft start process: the voltage is gradually reduced and the speed drops gradually to zero, thereby avoiding torque shocks that can occur during sudden shutdowns.

Soft starters and frequency converters are two products with completely different purposes. An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency ； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. Inverters possess all the functions of soft starters, but they are much more expensive and have a much more complex structure.

The main difference lies in control theory; soft starting primarily involves changing voltage without changing frequency. So the starting torque is relatively low. The starting current is also relatively high at the same time. Its starting methods include ramp voltage starting, current-limited starting, step-and-ramp starting, and full-voltage starting. The frequency converter enables frequency and voltage conversion during starting, providing a high starting torque. At the same time, since the flux remains essentially constant, the current is generally low (except under some special heavy loads).

A soft starter is a novel motor control device that integrates functions such as soft starting and stopping of the motor, energy saving at light loads, and various protection features; it is referred to as a Soft Starter abroad. Its main components are three-phase parallel thyristors connected in series between the power supply and the motor to be controlled, along with their electronic control circuit. 　　By using different methods to control the conduction angle of the three-phase anti-parallel thyristors, the input voltage to the motor under control can be varied according to various requirements, thereby enabling the implementation of different functions. 　　Soft starters and frequency converters are two products with completely different purposes. An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency ； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. Inverters possess all the functions of soft starters, but they are much more expensive and have a much more complex structure. What are the additional functions of the soft starter MCC control cabinet? 　　By further combining the soft starter MCC control cabinets, various composite functions can be achieved. For example: by adding control logic to two control cabinets, a \"one in use and one as backup\" configuration can be created, which is suitable for building fire protection systems as well as systems such as sprinkler pumps and domestic water pumps. If equipped with a PC (programmable controller), it is possible to carry out automatic inspections of the fire pump at regular intervals (such as every half month) and to shut it down automatically on schedule ； With the appropriate control logic in place, it is possible to conduct regular checks on whether the fire pumps and various systems are operating properly by running them at a low speed and low water pressure on a scheduled basis (without water output) ； During fire fighting, it operates at full speed and with a full load. By combining several motors with control logic, a domestic pump system or other specialized systems can be created. The motors can be turned on one by one according to demand, or the number of motors can be reduced step by step to achieve optimal operational efficiency. Upon the customer’s request, it is also possible to enable automatic switching between multiple motors at each time, so that all motors have an equal service life. Why are some soft starters equipped with a bypass contactor? 　　Most soft starters have bypass contactor contacts on both sides of the thyristors, and the advantages are: (1) the control cabinet offers two starting methods (direct start, soft start). 　(2) Once the soft start is complete, the bypass contactor closes, taking the soft starter out of service; it is reactivated only when shutdown occurs. This not only extends the lifespan of the soft starter, but also prevents harmonic pollution in the power grid, as well as reducing the heating losses caused by the thyristors in the soft starter. What protection functions does a soft starter have? Overload protection function: The soft starter incorporates a current control loop, which enables it to continuously monitor changes in the motor current. The overload protection function is achieved by increasing the setting of the overload current and employing a reverse-time control mode, which allows the thyristor to be turned off and an alarm signal to be generated when the motor is overloaded. (2) Phase loss protection function: During operation, the soft starter continuously monitors the changes in the current of the three phase wires; should any phase lose its current supply, it can immediately initiate phase loss protection. (3) Overheat protection function: The temperature of the thyristor radiator is monitored by the thermal relay built into the soft starter; once the radiator temperature exceeds the allowable value, the thyristors are automatically turned off and an alarm signal is generated. (4) Other functions: Through the combination of electronic circuits, various other interlock protections can also be implemented in the system. What is a soft start MCC control cabinet? 　　The MCC (Motor Control Center) control cabinet is the motor control center. The soft starter MCC control cabinet consists of the following components: (1) a circuit breaker at the input side, (2) the soft starter (including the electronic control circuit and three-phase thyristors), (3) a bypass contactor for the soft starter, (4) a secondary-side control circuit that enables selection and operation of functions such as manual start, remote start, soft start, and direct start). It is equipped with displays for voltage and current, as well as indicator lights for showing fault, operation, and working status. How do soft starters achieve energy savings under light load conditions? The cage-type asynchronous motor is an inductive load; during operation, the current in the stator coil windings lags behind the voltage. If the operating voltage of the motor remains constant, the power factor is low under light load and high under heavy load. Soft starters enable energy savings under light load conditions by reducing the voltage at the motor terminal, improving the power factor, and thereby reducing the copper and iron losses of the motor ； When the load is heavy, the voltage at the motor side is increased to ensure normal operation of the motor. What is the soft stop of a motor? When the motor stops, traditional control methods rely on an instantaneous power outage to achieve this. However, there are many applications where instant shutdown of the motor is not allowed. For example, in the pump systems of high-rise buildings and large structures, an immediate shutdown can cause a severe \"water hammer\" effect, which can damage the pipes and even the pumps themselves. To reduce and prevent the \"water hammer\" effect, it is necessary for the motor to shut down gradually, that is, through a soft shutdown; using a soft starter can meet this requirement. In pump stations, the use of soft shutdown technology can prevent damage caused by slamming, thereby reducing maintenance costs and workload. The soft stop function in a soft starter involves the thyristors gradually reducing their conduction angle from a fully conductive state once a stop command is received, and over a certain period of time transitioning to a fully closed state. The parking time can be adjusted from 0 to 120 seconds according to actual needs. What is the difference between soft starting and traditional reduced-voltage starting? Traditional voltage-reduction starting methods for cage motors include Y-q starting, autotransformer voltage-reduction starting, and reactor starting. All these starting methods belong to stepped voltage reduction starting, and they have a significant drawback: a secondary inrush current occurs during the starting process. The difference between soft starting and traditional reduced-voltage starting is: (1) no inrush current. When starting a motor, a soft starter increases the conduction angle of the thyristors gradually, allowing the motor’s starting current to rise linearly from zero to a set value. (2) Constant current start. Soft starters can introduce current closed-loop control, allowing the motor to maintain a constant current during startup and ensuring a smooth start. (3) Based on the load conditions and the characteristics of the power grid’s relay protection, it can be freely adjusted in steps to achieve the optimal starting current. What is soft starting of a motor? What are the different startup methods? 　　By using a soft starter connected between the power supply and the motor to be controlled, the conduction angle of its internal thyristors is regulated, allowing the voltage applied to the motor to increase gradually from zero according to a predefined functional relationship, until startup is complete and the full voltage is applied to the motor. This is what is known as soft starting. During this process, the starting torque of the motor increases gradually, as does its speed. Soft starting generally includes the following starting methods. 　　(1) Ramp-up soft start. This starting method is the simplest; it lacks current closed-loop control, and only adjusts the conduction angle of the thyristor to increase it in a certain functional relationship with time. Its drawback is that, due to the lack of current limitation, a large inrush current can occur during motor startup, which may damage the thyristors and have a significant impact on the power grid; as a result, it is rarely used in practice. 　　(2) Ramp constant-current soft start. In this starting method, the starting current gradually increases during the initial stage of motor startup; once it reaches a pre-set value, it remains constant (during the t1 to t2 phase) until the startup is complete. During startup, the rate of change of current rise can be adjusted according to the motor load. A higher current rise rate results in greater starting torque and a shorter starting time. This starting method is the most widely used, especially suitable for starting fan and pump loads. 　　(3) Step start. Starting means enabling the starting current to reach the set value as quickly as possible upon power-up, which is known as step starting. A fast start-up can be achieved by adjusting the starting current setting. 　　(4) Pulse impact start. At the beginning of startup, the thyristor is allowed to conduct at a high current for a short period of time before its current level drops, after which it rises linearly back to the preset value, thereby entering constant-current startup mode. This starting method is less commonly used with normal loads, and is suitable for starting under heavy loads where it is necessary to overcome significant static friction

A soft starter is a novel motor control device that integrates functions such as soft starting and stopping of the motor, energy savings at light loads, and various protection features; it is referred to as a Soft Starter abroad. Its main components are three-phase parallel thyristors connected in series between the power supply and the motor to be controlled, along with their electronic control circuit. By using different methods to control the conduction angle of the three-phase anti-parallel thyristors, the input voltage to the motor under control can be varied according to various requirements, thereby enabling the implementation of different functions. Soft starters and frequency converters are two products with completely different purposes. An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency ； A soft starter is essentially a voltage regulator; during motor startup, it only changes the voltage without altering the frequency. Inverters possess all the functions of soft starters, but they are much more expensive and have a much more complex structure. Soft starters are used in applications that require reduced voltage for starting and stopping. The rotation speed of the electrode remains unchanged. Inverters are used in applications where speed control or constant voltage is required; the frequency determines the speed. The biggest difference between a soft starter and an inverter is that an inverter allows the operating frequency to be set arbitrarily, while a starter only serves to enable a smooth start and stop of the motor. An inverter changes both the output frequency and voltage, which in turn alters the n0 value on the motor’s operation curve, causing the curve to shift downward parallelly. Therefore, an inverter allows the motor to start with a lower starting current, while simultaneously enabling the motor’s starting torque to reach its maximum value; in other words, an inverter can start motors under heavy load conditions. Soft starting only changes the output voltage, not the frequency; in other words, it does not alter the value of n0 on the motor’s operating curve. Instead, it increases the steepness of this curve, thereby making the motor’s characteristics more gentle. When n0 remains constant, the various torques of the motor (rated torque, maximum torque, stall torque) are all proportional to the square of its terminal voltage. Therefore, soft starting is used to reduce the motor’s starting torque; as a result, soft starting is not suitable for motors that need to start under heavy loads. A soft starter is a device that is not needed after the motor has started up on short-term use. The cost of low-power devices is relatively high. Unless the power exceeds 100 kW, frequency converters are irreplaceable by soft starters in terms of functionality; when it comes to starting up, the differences in working principles mean that their performances are not comparable at all. One can understand this by reviewing relevant technical documents. Here I will mention just one key point: frequency converters enable constant torque starting, which means that they can provide the same torque at low speeds as they do at high speeds, something that soft starters cannot achieve

A soft starter is a novel motor control device that integrates functions such as soft starting and stopping of the motor, energy saving under light load, and various protection features. Its main components are three-phase thyristors connected in parallel between the power supply and the motor to be controlled, along with its electronic control circuit. By using different methods to control the conduction angle of the three-phase anti-parallel thyristors, the input voltage to the motor under control can be varied according to various requirements, thereby enabling the implementation of different functions. Soft starters and frequency converters are two products with completely different purposes. An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency ； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. Inverters possess all the functions of soft starters, but they are much more expensive and have a much more complex structure.

　1. Inverter: The inverter enables precise control of all relevant variables such as speed, torque, and power throughout each operation cycle of the motor, from startup to normal operation to shutdown； Another important advantage is that its control devices are static, meaning there are no moving parts. Its reliability is thus improved as well, with very little maintenance required. 　　The disadvantage of frequency converters is their relatively high initial investment cost, which limits their use in many fields, especially in those devices where timed control is not actually required during normal operation. 　　2. Soft starters Soft starters were introduced to the market in the late 1970s and early 1980s. Similar to frequency converters, they are also based on electronics and thyristors. It can be said that it bridges the gap between the functional practicality of star-delta starters and frequency converters, on one hand, and price, on the other. By using a soft starter, it is possible to control the voltage of the motor, allowing it to increase gradually during startup, thereby naturally limiting the starting current. This means that the motor can start smoothly, with mechanical and electrical stresses also being reduced to a minimum ； This device also has an additional feature, which allows it to be used for a \"soft\" shutdown. 　　Since this starter uses an electronic circuit, its basic functions can be enhanced relatively easily through safety and fault indicator lights, improving motor protection and simplifying fault detection such as phase loss, overcurrent, and over-temperature protection, as well as providing indications for normal operation, full voltage on the motor, and certain faults. All setting values, such as ramp voltage and initial voltage, can be easily set on the starter panel. 　　In addition to fully meeting the basic requirement of smooth motor startup, soft starters also have many advantages, such as high reliability, low maintenance needs, good motor protection, and simple parameter setting. 　　However, soft starters still have one drawback: they cannot be used for long periods of time to drive electric motors that require high starting torque. This limitation is mainly due to the fact that soft starters actually function by ramping their own voltage up to its maximum value (and then gradually reducing it to the set shutdown level during shutdown). Since torque is proportional to the square of voltage, motors connected directly cannot achieve maximum torque from the start; therefore, soft starters are more suitable for lightweight devices that are easy to start, such as water pumps, fans, conveyor belts, and elevators. 　　In summary, an inverter is a power control device that converts alternating current from the mains supply into electricity at another frequency by using the on-off action of power semiconductor devices. A soft starter is essentially a thyristor-based AC voltage regulator; by changing the firing angle of the thyristors, it is possible to adjust the output voltage of this voltage regulation circuit. Throughout the starting process, the output from a soft starter provides a smooth increase in voltage (and it can also have a current-limiting function), until the thyristors are fully conductive and the motor operates at its rated voltage. “Soft starting” not only reduces significantly the starting shock experienced by the transmission system itself and extends the service life of key components, but it also **shortens the duration of the motor’s starting current surge, reduces the thermal stress on the motor as well as the impact on the power grid. This helps to save energy and extend the motor’s operational life.

A soft starter is a novel motor control device that integrates functions such as soft starting and stopping of the motor, energy saving at light loads, and various protection features; it is referred to as a Soft Starter abroad. Its main components are three-phase parallel thyristors connected in series between the power supply and the motor to be controlled, along with their electronic control circuit. By using different methods to control the conduction angle of the three-phase anti-parallel thyristors, the input voltage to the motor under control can be varied according to various requirements, thereby enabling the implementation of different functions. Soft starters and frequency converters are two products with completely different purposes. An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency ； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. Inverters possess all the functions of soft starters, but they are much more expensive and have a much more complex structure.

An inverter is used in applications where speed control is required; its output not only changes the voltage but also the frequency； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. Inverters possess all the functions of soft starters, but they are much more expensive and have a much more complex structure.

Soft starting mainly involves changing the voltage without changing the frequency. The frequency converter changes both voltage and frequency.

Soft start is similar to the previous step-down starting method; the difference is that the former involved step-by-step voltage reduction, while soft start features continuous voltage reduction. In both cases, however, the voltage is changed without altering the frequency. When a frequency converter starts a motor, it does so by changing the frequency in order to adjust the motor’s excitation current, thereby starting the motor based on the level of load on it. Of course, the functions of inverters go far beyond those of soft starters, but some soft starters (such as AB’s SMC DELT series) provide a starting process that is similar to that of inverter-driven starting, with smooth start-up and stop-up curves. Their differences mainly lie in the control theory. Soft starting involves changing the voltage without changing the frequency; as a result, the starting torque is relatively low, while the starting current is high. The various starting methods include ramp voltage starting, current-limiting starting, step-and-ramp starting, and full-voltage starting. The inverter enables variable-frequency and variable-voltage starting, providing high starting torque; meanwhile, since the flux remains essentially constant, the current is generally low (except in some special heavy-load situations).

Those in the process engineering field know that soft starters have the function of adjusting voltage to protect motors when starting or stopping equipment. Inverters also possess soft-start capabilities, but their main function is to adjust frequency for speed control. I’m not sure about the more detailed aspects; I’ll learn from the posts written by the experts upstairs. Some of the soft starters in our workshop often malfunction. It’s unclear whether the problem lies with the equipment itself (given that we’re likely a state-owned enterprise), or with the maintenance staff who aren’t competent enough. The electrical engineers replaced them with frequency converters. It’s probably related to the heavy-load starting issue mentioned by the poster *anpangpang*. This post was last edited by Pseudo Xiao Bao on 2009-2-17 at 13:11

Soft starters and frequency converters are two products with completely different purposes. Frequency converters are used in applications where speed control is required; their output allows not only the voltage to be changed but also the frequency； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. An inverter has all the functions of a soft starter, but it is much more expensive and has a much more complex structure.

Soft starters and frequency converters are two products with completely different purposes. Frequency converters are used in applications where speed control is required; their output allows not only the voltage to be changed but also the frequency； A soft starter is essentially a voltage regulator; during motor startup, it alters only the voltage while leaving the frequency unchanged. An inverter has all the functions of a soft starter, but it is much more expensive and has a much more complex structure.

Soft starting is used only for motor startup; by adjusting the voltage, the starting current is kept at 2-3 times the rated current, thereby reducing the impact on the power grid. It is employed in situations where the transformer capacity or line capacity is limited, and it offers some energy-saving benefits, though they are not significant. Inverters are used throughout the entire operation of motors; by changing the frequency, they enable control over the output power of the load. The need for valves in the load’s output pipeline is eliminated, which reduces unnecessary energy consumption and leads to energy savings. The starting impact of an inverter is even smaller than that of a soft starter. However, whether energy is saved mainly depends on the effectiveness of the process calculations.