There are many ways to start an electric motor, including direct start, autotransfer, Y-Δ voltage reduction, soft starter, frequency converter and so on.
In the case that both the grid capacity and the load allow full-voltage direct starting, full-voltage direct starting can be considered.
The advantage is that it is easy to control, simple to maintain and more economical.
It is mainly used for starting small power electric motors.
From the perspective of saving electricity, electric motors larger than 11kW should not be used in this way.
2、Autotransformer reduced voltage starting
This method is often used to start larger capacity electric motors, as it can adapt to the needs of different loads and obtain a larger starting torque.
The main advantage is the high starting torque, which can reach 64% of the direct starting torque when the winding is tapped at 80%.
The starting torque can be adjusted by the tap.
It is still widely used today.
3. Y-Δ start
For a squirrel-cage asynchronous electric motor with a triangular stator winding in normal operation.
If the stator winding is connected to a star when starting and then to a triangle after starting, the starting current can be reduced and the impact on the grid reduced.
This is called star-delta reduced voltage starting, or simply star-delta starting (Y-Δ starting).
If the direct starting current is 6-7 Ie, the starting current is only 2-2.3 times higher when the star-delta starting is used.
This means that with star-delta starting, the starting torque is also reduced to 1/3 of that of a direct start with a triangular connection.
This means that the starting torque is reduced to 1/3 of that of a direct triangular connection, which is suitable for no-load or light-load applications.
The star-delta starter is the simplest and cheapest in comparison with any other reduced voltage starter.
In addition to this, the star-delta starting method has the advantage that the three phase induction motor can be operated with a star connection when the load is light.
In this case, the rated torque can be matched to the load, which improves the efficiency of the induction motor and saves power consumption.
4、Soft starter for servo motors
This uses the phase-shifting principle of the SCR to achieve voltage regulation and start the servo motor.
Due to the use of silicon controlled components, harmonic interference during the operation of the SCR is high and has a certain impact on the grid.
In addition, fluctuations in the power grid can also affect the conductivity of the thyristor element, especially when there are multiple thyristor devices in the same grid.
Therefore, the failure rate of silicon controlled components is higher, because it involves power electronics, so the requirements of maintenance technicians are also higher.
5、Inverter
The frequency converter is the most technically advanced, versatile and effective motor control device in the modern motor control field.
It regulates the speed and torque of the vibrating motors by changing the frequency of the grid.
Because they involve power electronics and microcomputer technology, they are costly and require a high level of maintenance technicians.
So they are mainly used in areas where speed regulation is required and where high speed control is required.
Comparison of the advantages and disadvantages of reduced voltage start, soft start and variable frequency start
The disadvantage is that the starting torque is small and it is only suitable for no-load or light-load starting.
The advantage is that it is cheap.
Soft start, where the start time and initial torque can be set to achieve a soft start and soft stop, and the start current can be limited.
Inverter start, which allows smooth starting at a set time and allows the machine to run at a set frequency, is more expensive for starter motor .
Comparison of the performance principles of pressure reducing start, soft start and frequency conversion start
1, soft starter is a combination of thyristor AC voltage regulation technology and power factor control technology.
It is through the thyristor voltage regulation to achieve soft start, soft stop motor, does not have speed control function.
2、Inverter is a motor control (speed control) device that uses the on-off action of power semiconductor devices to convert the frequency power supply to another frequency.
Through the frequency conversion control motor operation (voltage also changes with frequency, such as v/f constant), is a truly efficient way to regulate speed, high efficiency.
The frequency converter is able to achieve true soft start, soft stop and efficient speed regulation.
The biggest advantage of the autotransformer is that the starting torque is larger.
When the motor windings tap is at 80%, the starting torque can reach 64% of the direct starting torque.
The starting torque can be adjusted by the tap.
They are still widely used today.
The Y-Δ is suitable for unloaded or light load starting.
It is also the simplest and cheapest of all reduced voltage starters to be worked with other equipment.
In addition to this, the star-delta starter has the advantage that the motor can be operated in the star configuration when the load is light.
In this case, the rated torque can be matched to the load, which increases the efficiency of the motor and saves power consumption.
Comprehensive analysis of pressure reducing start, soft start and frequency conversion start
1、Price problem
Naturally, frequency converters are the most expensive, while Y-Δ and autotransfer reduced voltage starters are relatively cheap.
For projects with smaller inputs, economy will be the preferred choice.
2, controllable problem for auto transformer
Y-Δ, autotransfer reduced voltage start is simple, but only start.
However, in the case of high automation, it is expected to be used less often, even for soft start.
The control of the simple motor by means of a frequency converter, including speed and voltage, is far from being comparable to a reduced voltage start or soft start.
This is why frequency converters are preferred in large or highly automated production lines to do motor works.
3、Network communication
The inverter itself can be networked and monitored through its own integrated or extended communication port.
Soft start can also do some monitoring, but to achieve real-time monitoring of the stepper motors, but also the reduced voltage start, soft start can not be compared to.
4、Maintenance
As Y-Δ and autotransfer reduced voltage starters are relatively simple, they are naturally the easiest to maintain.
I am actually very opposed to the use of soft start, if not choose the inverter, will certainly choose directly Y-Δ or autotransfer reduced voltage start.
The inverter is capable of soft starting and soft stopping of the motor, so for relatively large loads, Y-Δ, autotransformer or soft start is no match for the inverter.
Additional knowledge comparison
1. Soft starters and frequency converters
Both frequency converters and soft start devices belong to the category of reduced voltage starting.
Although frequency converters will mostly reduce voltage after frequency reduction, the constant torque is full voltage.
Soft start is the process of starting from voltage 0 to full voltage by changing the conduction angle of the thyristor.
The frequency converter is controlled throughout and can be controlled by instrumentation signals to control the motor speed at any time.
The soft starter can only be used for the purpose of voltage reduction when the new motor is started and stopped.
2. Electric Motors starting methods in general
Common methods of new motor starting: full voltage direct start, autotransfer reduced voltage start, Y-Δ start, soft start, variable frequency start, etc.
Where both the grid and the load allow, direct starting is preferred as it is easy to manipulate and control and is more economical.
Although the autotransformer is an old-fashioned starting device, it can be adapted to the needs of a wide range of loads by using the multiple taps of the autotransformer to reduce the voltage and obtain a larger starting torque.
The star-delta starting method has good current characteristics and poor torque characteristics, so it is only suitable for no-load or light-load starting, but this method is the simplest and cheapest, and can save power consumption in light-load operation.
All of these starting methods are classified as graded reduced voltage starting, with the obvious disadvantage that a secondary inrush current occurs during the starting process.
3. Comparison between soft start and traditional reduced voltage starting methods
①No inrush current
When starting a dc motor, the soft starter gradually increases the thyristor conduction angle, causing the motor starting current to rise linearly from zero to the set value.
The soft starter has no impact on the dc motors, which improves the reliability of the power supply, smooths the start, reduces the impact torque on the load machinery and extends the service life of the machine.
2.Soft stop function
That is, smooth deceleration and gradual stopping, it can overcome the disadvantages of instantaneous power-off stopping, reduce the impact on heavy-load machinery, avoid the water hammer effect of high-range water supply systems and reduce equipment damage.
③Adjustable starting parameters
The starting current can be freely and steplessly adjusted to the optimum starting current, depending on the load and the relay protection characteristics of the grid.
Soft starters and frequency converters are two completely different products.
Frequency converters are used where speed regulation is required and their output not only changes the voltage but also the frequency.
An inverter has all the features of a soft starter, but it is much more expensive and complex than a soft starer.
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