High inrush current is a problem for systems with maximum power limitations. The machine may trip and the uninterruptible power supply system may go into overload mode. What should I do?
A good solution would be to use a soft starter (soft starter). For example, we have a single-phase submersible pump with a power of 1 kW, located in a well at a depth of 50 meters. To start its engine, 4-6 times the starting current will be required, i.e. The system must withstand a short-term power of about 5 kW. Let's say, an inverter rated at 3 kW simply will not be able to start. The moment of start will also be accompanied by a sharp increase in pressure, which actually means a water hammer on the water supply system.
We will insert a soft starter into the line supplying the pump. The device will gradually increase the voltage within a specified time (usually up to 20 seconds), which will allow the pump to spin the impeller with acceleration, without jerking. As a result, we equated the starting current to the nominal value, i.e. it amounted to 1 kW and significantly extended the life of the submersible pump (the service life increases by about 2 times, taking into account the cost of the pump, the decision to use a soft starter, even in the absence of an energy backup system, becomes obvious):
Let's imagine a connection diagram that can be used with both single-phase and three-phase equipment:
Are there any restrictions for using a soft starter? Yes, there are some and you should know about them:
1) The soft starter cannot be used with refrigerators. High starting current is required to stall the compressor valves.
2) Likewise for air conditioners and other equipment
If you have any questions, I’ll be happy to answer in the comments!
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There are many reasons for turning on household pumps through a soft starter.
Typically, a submersible or surface pump is connected via an electromechanical or electronic relay, an automation unit or a magnetic starter. In all of the above cases, mains voltage is supplied to the pump by closing the contacts, that is, through a direct connection. This means that we supply full mains voltage to the stator windings of the electric motor, and the rotor is not yet rotating at this time. This leads to the appearance of an instantaneous powerful torque on the pump motor rotor.
This connection diagram is characterized by the following phenomena when starting the pump:
Current surges through the stator (and, accordingly, through the supply wires), since the rotor is short-circuited.
In a simplified understanding, we have a short circuit on the secondary winding of the transformer. In our experience, depending on the pump, manufacturer and shaft load, the pulse starting current can exceed the operating current from 4 to 8 times, and in some instances up to 12 times.
A sudden appearance of torque on the shaft.
This has a negative impact on the starting and operating stator windings, bearings, ceramic and rubber seals, significantly increasing their wear and reducing their service life.
The appearance of a sharp torque on the shaft leads to a sharp rotation of the well pump housing relative to the pipeline system.
We have repeatedly witnessed how, because of this, a well pump was disconnected from the pipelines and fell into the well. In the case of a pumping station based on a surface pump installed on a hydraulic accumulator platform, this leads to loosening of the fastening nuts and destruction of the weld points and seams of the hydraulic accumulator. Also, when the pump is turned on directly, the service life of the water supply and shut-off valves is reduced, especially at the points of their connection.
It is generally accepted that a hydraulic accumulator eliminates water hammer in the water supply system.
This is true, but water hammer disappears in pipelines only starting from the point where the hydraulic accumulator is connected. In the gap between the pump and the hydraulic accumulator, when the pump is directly connected, the hydraulic shock remains. As a result, in the interval from the pump to the accumulator we have all the consequences of water hammer on all parts of the pump and on the pipeline system.
In water filtration systems, water hammer that occurs when the pump is directly connected significantly reduces the service life of the filter elements.
If the local power grid weak, then your neighbors will also know that a pump with a power of more than 1 kW is running when directly connected by a sharp drop in voltage in the network at the moment the pump is turned on.
If local network EXTREMELY WEAK, and your neighbor also enjoys life by connecting all available electrical appliances to the network, then a well pump submerged to great depths may not start. Such a voltage surge can damage electronic devices connected to the network. There are known cases when, when the pump was started, an expensive refrigerator stuffed with electronics failed.
The more often the pump is turned on, the shorter its service life.
Frequent starts through direct connection lead to failure of the plastic couplings of well pumps connecting the electric motor to the pumping part.
We went over the problems that arise when starting a pump without soft start devices (SPD) .
It should be noted that even when turning off the pump without SCP There are some negative aspects with a direct connection diagram:
When the pump is turned off, a water hammer also occurs in the system, but now due to a sharp decrease in torque on the pump shaft, which is tantamount to the creation of an instant vacuum.
A sharp decrease in torque on the pump shaft also leads to rotation of the pump housing, but in the opposite direction.
Let's think about the pipelines and threaded connections of the pump.
In conventional household pumps, electric motors are asynchronous and have a pronounced inductive nature.
If we abruptly interrupt the flow of current through an inductive load, then there is a sharp jump in voltage across that load due to the continuity of the current. Yes, we open the contact and all the high voltage should remain on the pump side. But with any mechanical opening of the contact, the so-called “contact bounce” is present, and high voltage pulses enter the network, and therefore also enter the devices connected to the network at that time.
Thus, when the pump is directly connected, there is increased wear on the mechanical and electrical parts of the pump (both during startup and shutdown). Devices included in the same network also suffer, and the service life of filtration systems and plumbing fittings is reduced.
Usage soft start devices (“Aquacontrol UPP-2.2S”) allows you to smooth out most of the shortcomings described above. In device UPP-2.2S a specially calculated voltage rise curve on the pump has been implemented, which allows, on the one hand, to reliably start the pump in the most unfavorable operating conditions, and on the other hand, to smoothly increase the shaft rotation speed. This device also has built-in protection against low and high voltage mains to protect the pump from extreme operating conditions and switching on.
IN UPP-2.2S phase triac control is used. At the moment of starting, a part of the mains voltage is supplied to the pump, which creates a torque sufficient to ensure that the pump starts. As the rotor spins, the voltage on the pump gradually increases until the voltage is fully applied. After this, the relay turns on and the triac turns off. As a result, when using UPP-2.2S the pump is connected to the network through relay contacts, that is, the same as with a direct connection. But for 3.2 seconds (this is the soft start time), voltage is supplied to the pump through a triac, which ensures a “soft start”, without sparks at the relay contacts.
With such a start, the maximum starting current exceeds the operating current by no more than 2.0-2.5 times instead of 5-8 times. Using UPP-2.2S, we reduce the starting load on the pump by 2.5-3 times and extend the life of the pump by the same amount, ensuring more comfortable operation of devices connected to the electrical network. UPP-2.2S can be called a device with resource-saving technology.
A well pump, due to the need to provide high performance with fairly small transverse dimensions, is a complex device that operates under rather harsh conditions. And if we take into account that its installation (as well as dismantling) is quite labor-intensive work, then the reliability of the well pump becomes of paramount importance. One of the factors that has a decisive influence on the operating time of this unit is inrush currents. Due to the fact that the rotating parts of the electric motor and the pump itself have a certain inertia, in contrast to the current (that is, the current value can almost instantly reach very high values), when turned on, starting currents arise that are 4-10 times higher than the rated ones! What if the well pump also turns on frequently? For example, due to a small volume of a diaphragm accumulator or incorrect setting of the pressure switch? It is clear that, in the end, the insulation of the electric motor winding will not withstand such high thermal loads and a short circuit will occur, which will result in failure of the pump. To reduce starting currents, various soft starting systems are used.
Types of soft start
Currently, two soft start systems are mainly used for well pumps:
- 1.Smooth startSS. With this method, using electronics, a smoothly increasing voltage (and therefore a smoothly increasing current) is supplied to the electric motor. Voltage regulation is carried out by phase control. Many control stations (panels) for well pumps, both domestic and foreign brands, operate on this principle: Cascade, Vysota, Grundfos, Pedrollo, etc.
- 2. Soft start using frequency conversion. This method is the most advanced in terms of reducing inrush currents. Frequency conversion allows you to keep the starting current at the rated level. The main disadvantage of control stations (panels) with variable frequency drives is their high cost, comparable to the cost of the pump itself. Among domestic models, it is worth highlighting STEP, SU-CHE, SUN. ASUN. The most popular foreign models are SIRIO and SIRIO-ENTRY 230 of the Italian brand ITALTECNICA. It should be said that the SQ/SQE series well pumps have a built-in soft start system based on frequency conversion.
Benefits of soft starting
- Reduced starting currents (in the case of a variable frequency drive, starting currents are reduced to rated).
- Reduced mechanical loads on the impeller and bearings of the well pump.
- Reducing or completely preventing water hammer that occurs when the pump is turned on. Water hammer negatively affects not only the pump itself, but also the well, causing additional loads on the joints of the casing pipes and causing rapid wear of the filters. As a result, the well begins to sand.
Based on a frequency-controlled soft start system, it is possible to control the power of the pump by changing the rotation speed of its engine. That is, the control system precisely selects the rotational speed of the electric motor, and therefore its power, in accordance with the currently required performance, maintaining constant pressure in the network. In other words, the electric motor uses exactly the amount of electricity needed to provide the required performance and not a joule more. Such a system is implemented in Grundfos SQE series well pumps.
Who wants to strain, spend their money and time on re-equipment of devices and mechanisms that already work perfectly? As practice shows, many do. Although not everyone in life encounters industrial equipment equipped with powerful electric motors, they constantly encounter, albeit not so voracious and powerful, electric motors in everyday life. Well, everyone probably used the elevator.
Electric motors and loads - a problem?
Probably the most significant load placed on the engine at the time of start-up is a multiple, albeit short-term, excess of the rated operating current of the unit. After just a few seconds of operation, when the electric motor reaches its normal speed, the current consumed by it will also return to normal levels. To ensure the necessary power supply have to increase the power of electrical equipment and conductive lines, which leads to their rise in price.
When starting a powerful electric motor, due to its high consumption, the supply voltage “drops”, which can lead to failures or failure of equipment powered from the same line. In addition, the service life of power supply equipment is reduced.
If emergency situations occur that result in engine burnout or severe overheating, properties of transformer steel may change so much so that after repair the engine will lose up to thirty percent of its power. Under such circumstances, it is no longer suitable for further use and requires replacement, which is also not cheap.
Why do you need a soft start?
It would seem that everything is correct, and the equipment is designed for this. But there is always a “but”. In our case there are several of them:
- at the moment of starting the electric motor, the supply current can exceed the rated one by four and a half to five times, which leads to significant heating of the windings, and this is not very good;
- starting the engine by direct switching leads to jerks, which primarily affect the density of the same windings, increasing the friction of the conductors during operation, accelerates the destruction of their insulation and, over time, can lead to an interturn short circuit;
- the aforementioned jerks and vibrations are transmitted to the entire driven unit. This is already completely unhealthy, because may cause damage to its moving parts: gear systems, drive belts, conveyor belts, or just imagine yourself riding in a jerking elevator. In the case of pumps and fans, this is the risk of deformation and destruction of turbines and blades;
- We should also not forget about the products that may be on the production line. They may fall, crumble or break due to such a jerk;
- Well, and probably the last point that deserves attention is the cost of operating such equipment. We are talking not only about expensive repairs associated with frequent critical loads, but also about a significant amount of inefficiently spent electricity.
It would seem that all of the above operating difficulties are inherent only in powerful and bulky industrial equipment, however, this is not so. All this can become a headache for any average person. This primarily applies to power tools.
The specific use of such units as jigsaws, drills, grinders and the like require multiple start and stop cycles over a relatively short period of time. This operating mode affects their durability and energy consumption to the same extent as their industrial counterparts. With all this, do not forget that soft start systems cannot regulate engine speed or reverse their direction. It is also impossible to increase the starting torque or reduce the current below that required to start rotating the motor rotor.
Video: Soft start, adjustment and protection of the commutator. engine
Options for soft start systems for electric motors
Star-delta system
One of the most widely used starting systems for industrial asynchronous motors. Its main advantage is simplicity. The engine starts when the windings of the star system are switched, after which, when the normal speed is reached, it automatically switches to delta switching. This is the starting option allows you to achieve a current almost a third lower than when starting an electric motor directly.
However, this method is not suitable for mechanisms with low rotational inertia. These, for example, include fans and small pumps, due to the small size and weight of their turbines. At the moment of transition from the “star” to the “triangle” configuration, they will sharply reduce the speed or stop altogether. As a result, after switching, the electric motor essentially starts again. That is, in the end, you will not only not achieve savings in engine life, but also, most likely, you will end up with excessive energy consumption.
Video: Connecting a three-phase asynchronous electric motor with a star or triangle
Electronic motor soft start system
A smooth start of the engine can be done using triacs connected to the control circuit. There are three schemes for such connection: single-phase, two-phase and three-phase. Each of them differs in its functionality and final cost, respectively.
With such schemes, usually it is possible to reduce the starting current up to two or three nominal. In addition, it is possible to reduce the significant heating inherent in the aforementioned star-delta system, which helps to increase the service life of electric motors. Due to the fact that the engine starting is controlled by reducing the voltage, the rotor accelerates smoothly and not abruptly, as with other circuits.
In general, engine soft start systems are assigned several key tasks:
- the main one is to reduce the starting current to three to four rated ones;
- reducing the motor supply voltage, if appropriate power and wiring are available;
- improvement of starting and braking parameters;
- emergency network protection against current overloads.
Single-phase starting circuit
This circuit is designed to start electric motors with a power of no more than eleven kilowatts. This option is used if it is necessary to soften the shock at start-up, but braking, soft starting and reducing the starting current do not matter. Primarily due to the impossibility of organizing the latter in such a scheme. But due to the cheaper production of semiconductors, including triacs, they have been discontinued and are rarely seen;
Two-phase starting circuit
This circuit is designed to regulate and start motors with a power of up to two hundred and fifty watts. Such soft start systems sometimes equipped with a bypass contactor to reduce the cost of the device, however, this does not solve the problem of phase supply asymmetry, which can lead to overheating;
Three-phase starting circuit
This circuit is the most reliable and universal soft start system for electric motors. The maximum power of motors controlled by such a device is limited solely by the maximum temperature and electrical endurance of the triacs used. His versatility allows you to implement a lot of functions, such as: dynamic brake, flyback pickup or balancing of magnetic field and current limiting.
An important element of the last of the mentioned circuits is the bypass contactor, which was mentioned earlier. He allows you to ensure the correct thermal conditions of the electric motor soft start system, after the engine reaches normal operating speed, preventing it from overheating.
The soft start devices for electric motors that exist today, in addition to the above properties, are designed to work together with various controllers and automation systems. They have the ability to be activated by command from the operator or the global control system. Under such circumstances, when the loads are turned on, interference may appear that can lead to malfunctions in the automation, and therefore it is worth paying attention to protection systems. The use of soft start circuits can significantly reduce their influence.
Do-it-yourself soft start
Most of the systems listed above are actually not applicable in domestic conditions. Primarily for the reason that at home we extremely rarely use three-phase asynchronous motors. But there are more than enough commutator single-phase motors.
There are many schemes for smooth starting of engines. The choice of a specific one depends entirely on you, but in principle, having a certain knowledge of radio engineering, skillful hands and desire, it is quite you can assemble a decent homemade starter, which will extend the life of your power tools and household appliances for many years.
Soft starter- an electrical device used in asynchronous electric motors, which allows the motor parameters (current, voltage, etc.) to be kept within safe limits during startup. Its use reduces starting currents, reduces the likelihood of motor overheating, eliminates jerks in mechanical drives, which ultimately increases the service life of the electric motor.
Purpose
Control of the process of starting, operating and stopping electric motors. The main problems of asynchronous electric motors are:
- impossibility of matching engine torque with load torque,
- high starting current.
During start-up, the torque often reaches 150-200% in a split second, which can lead to failure of the drive kinematic chain. In this case, the starting current can be 6-8 times higher than the rated current, causing problems with power stability. Soft starters avoid these problems by making the engine accelerate and decelerate more slowly. This allows you to reduce starting currents and avoid jerks in the mechanical part of the drive or hydraulic shocks in pipes and valves when starting and stopping engines.
Operating principle of soft starter
The main problem with asynchronous electric motors is that the torque developed by the electric motor is proportional to the square of the voltage applied to it, which creates sharp jerking of the rotor when starting and stopping the motor, which, in turn, causes a large induced current.
Soft starters can be either mechanical or electrical, or a combination of both.
Mechanical devices directly counteract a sharp increase in engine speed, limiting torque. They can be brake pads, fluid couplings, magnetic locks, shot counterweights, etc.
These electrical devices allow the current or voltage to be gradually increased from an initial low level (reference voltage) to a maximum level in order to smoothly start and accelerate the electric motor to its rated speed. Such soft starters usually use amplitude control methods and therefore cope with starting equipment in idle or lightly loaded modes. More modern generation of soft starters (for example, EnergySaver devices) use phase control methods and are therefore capable of starting electric drives characterized by severe starting modes “rated to rated”. Such soft starters allow starting more often and have a built-in energy saving and power factor correction mode.
Selecting a soft starter
When an asynchronous motor is turned on, a short circuit current appears in its rotor for a short time, the strength of which, after gaining speed, decreases to the nominal value corresponding to the power consumed by the electric machine. This phenomenon is aggravated by the fact that at the moment of acceleration the torque on the shaft increases abruptly. As a result, protective circuit breakers may trip, and if they are not installed, then other electrical devices connected to the same line may fail. And in any case, even if an accident does not occur, when starting electric motors, increased energy consumption is noted. To compensate or completely eliminate this phenomenon, soft starters (SFDs) are used.
How is a soft start implemented?
To smoothly start the electric motor and prevent an inrush current, two methods are used:
- Limit the current in the rotor winding. To do this, it is made consisting of three coils connected in a star configuration. Their free ends lead to slip rings (collectors) mounted on the shaft shank. A rheostat is connected to the collector, the resistance of which is maximum at the time of start-up. As it decreases, the rotor current increases and the motor spins up. Such machines are called wound-rotor motors. They are used in crane equipment and as traction electric motors for trolleybuses and trams.
- Reduce voltage and current supplied to the stator. In turn, this is implemented using:
a) autotransformer or rheostat;
b) key circuits based on thyristors or triacs.
It is the key circuits that are the basis for the construction of electrical devices, which are usually called soft starters or soft starters. Please note that frequency converters also allow you to start an electric motor smoothly, but they only compensate for a sharp increase in torque without limiting the starting current.
The operating principle of the key circuit is based on the fact that the thyristors are unlocked for a certain time at the moment the sinusoid passes zero. Usually in that part of the phase when the voltage rises. Less often - when it falls. As a result, a pulsating voltage is recorded at the output of the soft starter, the shape of which is only approximately similar to a sinusoid. The amplitude of this curve increases as the time interval during which the thyristor is unlocked increases.
Softstarter selection criteria
In order of decreasing degree of importance, the device selection criteria are arranged in the following sequence:
- Power.
- Number of controlled phases.
- Feedback.
- Functionality.
- Control method.
- Additional features.
Power
The main parameter of the soft starter is the value I nom - the current strength for which the thyristors are designed. It should be several times greater than the current passing through the motor winding when it reaches rated speed. The frequency depends on the severity of the launch. If it is light - metal-cutting machines, fans, pumps, then the starting current is three times higher than the rated current. Hard starting is typical for drives with a significant moment of inertia. These are, for example, vertical conveyors, sawmills, and presses. The current is five times higher than the rated current. There is also a particularly difficult start-up, which accompanies the operation of piston pumps, centrifuges, band saws... Then the I rating of the softstarter should be 8-10 times greater.
The severity of the launch also affects the time it takes to complete. It can last from ten to forty seconds. During this time, the thyristors become very hot as they dissipate some of the electrical power. To repeat, they need to cool down, and this takes the same amount of time as the working cycle. Therefore, if the technological process requires frequent switching on and off, then choose a soft starter for heavy starting. Even if your device is not loaded and picks up speed easily.
Number of phases
One, two or three phases can be controlled. In the first case, the device mitigates the increase in starting torque to a greater extent than in current. The most commonly used are two-phase starters. And for cases of heavy and particularly difficult starting - three-phase.
Feedback
SCP can work according to a given program - increase the voltage to the nominal value within a specified time. This is the simplest and most common solution. The presence of feedback makes the management process more flexible. The parameters for it are the comparison of voltage and torque or the phase shift between the rotor and stator currents.
Functionality
Ability to work on acceleration or braking. The presence of an additional contactor, which bypasses the key circuit and allows it to cool, and also eliminates phase asymmetry due to a violation of the sinusoid shape, which leads to overheating of the windings.
Control method
It can be analog, by rotating potentiometers on the panel, and digital, using a digital microcontroller.
Additional functions
All types of protection, energy saving mode, the ability to start with a jerk, work at a reduced speed (pseudo-frequency regulation).
A correctly selected soft starter doubles the service life of electric motors, savesup to 30 percent electricity.
Why do you need a soft starter?
Increasingly, when starting electric drives of pumps and fans, a soft start device (soft starter) is used. What is this connected with? In our article we will try to highlight this issue.
Induction motors have been in use for over a hundred years, and during that time relatively little has changed in their functioning. The startup of these devices and the problems associated with it are well known to their owners. Inrush currents lead to voltage sags and wiring overloads, resulting in:
some electrical equipment may turn off spontaneously;
possible equipment failure, etc.
A timely installed, purchased and connected softstarter allows you to avoid unnecessary waste of money and headaches.
What is starting current
The principle of operation of asynchronous motors is based on the phenomenon of electromagnetic induction. The build-up of reverse electromotive force (emf), which is created by applying a changing magnetic field during engine starting, leads to transients in the electrical system. This transient may affect the power system and other equipment connected to it.
During startup, the electric motor accelerates to full speed. The duration of the initial transients depends on the design of the unit and the characteristics of the load. The starting torque should be the greatest, and the starting currents should be the smallest. The latter entail detrimental consequences for the unit itself, the power supply system and the equipment connected to it.
During the initial period, the inrush current can reach five to eight times the full load current. During motor starting, the cables are forced to carry more current than during the steady state period. The voltage drop in the system will also be much greater at start-up than during stable operation - this becomes especially obvious when starting a powerful unit or a large number of electric motors at the same time.
Motor protection methods
As the use of electric motors has become widespread, overcoming problems starting them has become a challenge. Over the years, several methods have been developed to solve these problems, each with its own advantages and limitations.
Recently, significant advances have been made in the use of electronics in power control for motors. Increasingly, soft starters are used when starting electric drives of pumps and fans. The thing is that the device has a number of features.
A special feature of the starter is that it smoothly supplies voltage to the motor windings from zero to the rated value, allowing the motor to smoothly accelerate to maximum speed. The mechanical torque developed by an electric motor is proportional to the square of the voltage applied to it.
During the startup process, the soft starter gradually increases the supplied voltage, and the electric motor accelerates to the rated rotation speed without large torque and peak current surges.
Types of soft starters
Today, for the smooth start-up of equipment, three types of soft starters are used: with one, two and all controlled phases.
The first type is used for a single-phase motor to provide reliable protection against overload, overheating and reduce the influence of electromagnetic interference.
As a rule, the second type of circuit includes a bypass contactor in addition to the semiconductor control board. Once the motor has reached rated speed, the bypass contactor is activated and provides direct voltage to the motor.
The three-phase type is the most optimal and technically advanced solution. It provides limitation of current and magnetic field strength without phase imbalances.
Why do you need a soft starter?
Due to their relatively low price, the popularity of soft starters is gaining momentum in the modern market of industrial and household appliances. A soft starter for an asynchronous electric motor is necessary to extend its service life. The big advantage of a softstarter is that the start is carried out with smooth acceleration, without jerking.
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