A frequency converter simultaneously changes both the output frequency and voltage, which alters the n0 on the motor's operating curve, causing the curve to shift downwards. Therefore, a frequency converter can enable a motor to start with a smaller starting current while simultaneously achieving its maximum starting torque, meaning it can start heavy loads.
Soft starters only change the output voltage, not the frequency, meaning they don't change n0 on the motor's operating curve. Instead, they increase the steepness of the curve, making the motor characteristics softer. When n0 remains constant, all the motor's torques (rated torque, maximum torque, stall torque) are proportional to the square of its terminal voltage. Therefore, soft starters significantly reduce the motor's starting torque, making them unsuitable for motors starting under heavy loads.
In terms of functionality, frequency converters cannot be replaced by soft starters. Frequency converters can achieve constant torque start, meaning that they can have the same torque at low speeds as at high speeds, which is something that soft starters cannot achieve.
I. Working principle of soft starter
Three anti-parallel thyristors (silicon controlled rectifiers) are connected in series between the three-phase power supply and the motor. Utilizing the thyristor phase-shifting control principle, during startup, the motor terminal voltage gradually increases from zero as the thyristor conduction angle increases, and the motor speed gradually increases until the starting torque requirement is met, thus ending the startup process. At this time, the bypass contactor is activated (to prevent harmonic pollution to the power grid during motor operation and extend the thyristor lifespan), and the motor enters a steady-state operating state. When stopping, the bypass contactor is first disconnected, and then the conduction angle of the thyristor in the soft starter gradually decreases, causing the three-phase power supply voltage to gradually decrease, and the motor speed to gradually decrease from high to zero, completing the stopping process.
During startup, the thyristor's conduction angle starts at 0 and gradually moves forward. The motor's terminal voltage, starting from zero, gradually increases according to a preset function until it reaches the required starting torque, allowing the motor to start smoothly. Then, the motor operates at full voltage. From a working principle perspective, the soft driver is essentially a voltage regulator; it only changes the voltage output and does not change the frequency. This differs from a frequency converter.
II. Advantages of Soft Starters
The advantages of soft start are:
1) There is no inrush current during startup. By gradually increasing the thyristor conduction angle, the startup current rises linearly from zero to the set value.
2) It is a constant current starter. The soft starter can introduce current closed-loop control to keep the motor at a constant current during the starting process, ensuring that the motor starts smoothly.
3) The starting current can be selected according to the load conditions and the characteristics of the power grid relay protection, and can be freely and steplessly adjusted to the optimal starting current.
4) The motor can be started frequently, with soft starts allowed up to 10 times per hour, without causing the motor to overheat.
(Traditional reduced-voltage starting methods for squirrel-cage motors include: Y-Δ starting, autotransformer reduced-voltage starting, reactor starting, etc., all of which belong to stepped reduced-voltage starting. Their disadvantage is that a secondary inrush current will occur during the starting process.)
Once the motor is running at full speed, a bypass contactor replaces the soft starter that has completed its task, reducing thyristor heat dissipation and improving system efficiency. In this mode, one soft starter can start multiple motors.
I. Inverter Concept
From the formula for the speed of an asynchronous motor, n=60f(1-s)/p (where: n-motor speed, f-power supply frequency, p-number of pole pairs of the motor, s-slip rate), it can be seen that when p remains constant, uniformly changing the frequency of the power supply can continuously change the synchronous speed of the motor. This speed regulation is called variable frequency speed regulation.
The torque formula for a three-phase asynchronous motor is:
SR2
Formula M=CU12[2]
R22+（SX20）2
C: a constant related to the characteristics of the motor itself; U1: input voltage;
R2: Rotor resistance; X20: Rotor leakage reactance; S: Slip
We know that M∝U12, and the torque is proportional to the square of the power supply voltage. Let the load torque at normal input voltage be M2. A voltage drop causes a significant decrease in the electromagnetic torque M. Since M2 remains constant, the balance is disrupted because M is less than M2, leading to a decrease in motor speed and an increase in slip S. This, in turn, causes a change in the rotor voltage balance equation, resulting in an increase in rotor current I2. This means the stator current I1 increases accordingly (as can be seen from transformer relationships). Simultaneously, the increase in I2 also causes the torque M delivered to the motor shaft to rise again until it equals M2. At this point, the motor speed tends to a new stable value.
The torque formula for a three-phase asynchronous motor is:
SR2
Formula M=CU12[2]
R22+（SX20）2
C: a constant related to the characteristics of the motor itself; U1: input voltage;
R2: Rotor resistance; X20: Rotor leakage reactance; S: Slip
T=9550*P/n
P(KW)n(r/min)T(N*m)
II. Classification and Principles of Frequency Converters
The key component of a variable frequency speed control device is the motor variable frequency speed controller, also known as a frequency converter.
Frequency converters include AC-AC frequency converters, which are composed of a reversible bridge of three anti-parallel thyristors and adopt the principle of natural commutation of the power grid.
The concept of a soft starter and its difference from a frequency converter
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. Its main components are three anti-parallel thyristors connected in series between the power supply and the controlled motor, along with their electronic control circuitry. By controlling the conduction angle of the three anti-parallel thyristors using different methods, the input voltage of the controlled motor can be varied according to different requirements, thus achieving different functions. Soft starters and frequency converters are two completely different products. Frequency converters are used where speed regulation is required; their output changes both voltage and frequency. A soft starter is essentially a voltage regulator; when used for motor starting, its output only changes voltage, not frequency. Frequency converters possess all the functions of soft starters and have rich protection features, but they are much more expensive and have a much more complex structure. The performance and applications of soft starters...
At the moment a motor starts, the current is 5-7 times its rated current. When a 30kW motor starts, it becomes 150kW-210kW, which has a significant impact on the overall electrical load and the load-bearing capacity of components. How can we reduce the starting current of a motor? Before soft starters, star-delta step-down starters and autotransformer step-down starters were used to achieve this. They can reduce the starting current by 3-5 times, but these devices use more components, have a more complex structure, and occupy a lot of space. A step-down starter cabinet for a motor of 150kW or more would probably require a whole cabinet, and in environments with a large number of motors, installing the cabinet would require a fairly large workshop.
Both star-delta and autotransformer step-down starters switch between star and delta configurations to reduce voltage and current. Each start-stop cycle requires a switch, resulting in noticeable noise. In applications with frequent start-stop cycles, components are prone to damage, increasing maintenance workload. Motor soft starters simplify the process of starting motors with reduced voltage. Also known as "solid-state soft starters," they utilize thyristors and integrated components. They are lightweight, compact, easy to install, and reliably provide smooth motor start-stop. Soft starters are available in domestic and imported versions. Imported soft starters include ABB, AB, and Schneider Electric; domestic soft starters include Heping, Renault, Aoto, Xinuoke, and Xichi. The price of domestic soft starters is comparable to that of autotransformer step-down switchgear. Compared to motor frequency converters, they share some similarities: smooth start and stop. The difference lies in the speed control: frequency converters allow for motor speed adjustment, while soft starters do not. Soft starters are primarily used for high-power motors that do not require speed control. Its main purpose is to reduce the starting current. While a frequency converter can achieve the same goal, it significantly increases the cost. Star-delta and autotransformers also have many shortcomings. Soft starting is the most economical and simplest method. Large motors in various operating conditions can use soft starting.
Several ways to start a soft boot
The soft start of a motor mainly adopts the following methods:
1. Start by reducing the power supply voltage.
2. Reduce power supply frequency during startup.
3. Reduce excitation current during startup.
Soft starters mainly use thyristor phase shifting to reduce motor voltage and achieve soft starting.
Soft starting of a motor essentially means starting the motor slowly with a lower current. This reduces the impact on the power grid, lowers the load margin of the transformer and control circuit, and extends the service life of the equipment. When an AC motor is started directly, the starting current is 6 to 10 times the trial run current, while with soft starting technology, the starting current is reduced to 1 to 3 times.
Frequency converter:
Definition: A frequency converter is a device that transforms alternating current with fixed voltage and frequency into alternating current with variable voltage and frequency.
Function: To reduce the impact load during motor startup, control motor speed, lengthen startup time, and smooth out the current flow, achieving soft starting. It also improves the efficiency of the power grid and the motor. In practice, frequency converters are mainly used for energy saving by adjusting and changing the output voltage, current, and frequency. Generally, speed-regulating motors use frequency converters.
shortcoming:
1. It is expensive to manufacture and costs much more than microcomputer protection.
2. Since most current frequency converters use PWM control, this pulse modulation method causes high-order harmonic currents to be generated on the power supply side during operation, resulting in voltage waveform distortion. When the power system is polluted by harmonics, it will affect the operating efficiency of the system at best, and damage the equipment or even endanger the safe operation of the power system at worst.
3. Overload causes the frequency converter to trip more frequently. When an overload occurs, the motor generally has a strong overload capacity, and as long as the motor parameters in the frequency converter's parameter table are set properly, the motor is unlikely to overload. However, the frequency converter itself has a poor overload capacity and is prone to overload alarms.
Soft starter:
Definition and Function: Connected in series between the power supply and the controlled motor, this system uses a microcomputer to control the conduction angle of its internal thyristors to achieve AC voltage regulation. This allows the motor input voltage to gradually increase from zero according to a preset function until the start-up is complete, providing the motor with full voltage – this is called soft start. During soft start, the motor's starting torque and speed gradually increase until the thyristors are fully conducting, and the motor operates at its rated voltage mechanical characteristics, achieving smooth starting and reducing starting current. This avoids overcurrent tripping during startup. The starting process ends when the motor reaches its rated speed, providing the rated voltage for normal motor operation.
shortcoming:
1. The power supply frequency cannot be adjusted, so the motor cannot be started from zero voltage and zero frequency, and zero-impact start cannot be achieved.
2. Speed cannot be adjusted.
3. The soft starter exits the system after starting the motor and loses its protection function.
1. A frequency converter is used in applications where speed regulation is required. The output of a frequency converter can change not only the voltage but also the frequency.
2. A soft starter is actually a voltage regulator that protects the motor. When used for reduced-voltage starting of the motor, the output of the soft starter only changes the voltage and does not change the frequency.
3. Variable frequency drives have all the functions of soft starters, but their structure is complex and their price is much higher than that of soft starters.
4. A frequency converter controls electrical equipment by adjusting the frequency, allowing for speed regulation and starting, such as variable frequency pumps used in industry. Soft starting essentially involves changing the resistance connected in series in the circuit, gradually increasing the current. This method is used to start high-power motors, reducing the fluctuations in the power grid caused by directly starting a high-power motor, or causing problems with unloading some unimportant loads.
Disclaimer: This article is a reprint. If there are any copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
