Do-it-yourself soft start for power tools: circuit, device, electric motor, on a triac

How to make a starter using a triac TS-122-25?

Making a soft start for an angle grinder with a triac TS-122-25 with your own hands is quite simple.
First of all, it is recommended to prepare a contactor resistor. Capacitors will be required of the single-pole type. A total of three elements are installed in the starter. The capacitance of one capacitor should not exceed 5 pF. To increase the operating frequency, a contactor is soldered on the plate. Some experts say that filters can increase conductivity. The rectifier block is used with a conductivity of 50 microns. It is capable of withstanding large overloads and will be able to provide high speeds. Next, to assemble a soft start on an angle grinder with your own hands, install a thyristor. At the end of the work, the model is connected via an adapter.

Models for angle grinder 600 W

For 600 W angle grinders, starters with contact triacs are used, in which the overload does not exceed 10 A. It is also worth noting that there are many devices with plates. They are distinguished by their security and are not afraid of elevated temperatures. The minimum frequency for 600 W angle grinders is 30 Hz. In this case, the resistance depends on the installed triode. If it is used of the linear type, then the above parameter does not exceed 50 Ohms.

If we talk about duplex triodes, then the resistance at high speeds can reach up to 80 Ohms. Very rarely, models have stabilizers that operate from comparators. Most often they are attached directly to the modules. Some modifications are made with wired transistors. Their minimum frequency starts from 5 Hz. They are afraid of overloads, but are able to maintain high speeds at a voltage of 220 V.

Read also: Silicone insulated wire marking

Making a soft start

The circuit is assembled on a printed circuit board measuring 45 x 35 mm, the board is laid out as compactly as possible so that it can be built inside the body of a tool that requires a soft start. It is better to solder the power wires directly into the board, but if the load power is small, then you can install terminal blocks, as I did. The board is made using the LUT method, photographs of the process are presented below.

It is advisable to tin the tracks before soldering the parts, this will improve their conductivity. The microcircuit can be installed in the socket, then it can be easily removed from the board. First, resistors, diodes, small capacitors are sealed, and only then the largest components. After completing the assembly of the board, it is necessary to check it for correct installation, ring the tracks, and wash off the remaining flux.

The simplest scheme

A soft starter with speed control on a thyristor KU 202 is widely used due to its very simple design (diagram 1). Connecting it does not require any special skills. Radio elements for it are very easy to obtain. This regulator model consists of a diode bridge, a variable resistor (acts as a U regulator) and a thyristor tuning circuit (supplying U to the control output with a nominal value of 6.3 volts) from a domestic manufacturer.

Due to the size and number of parts, this type of regulator can be built into the body of a power tool. In addition, the variable resistor knob should be removed and the speed controller itself can be modified by integrating a button in front of the diode bridge. The basic principle of operation is to regulate the speed of the electric motor of the tool by limiting the power in manual mode. This circuit allows you to use power tools with a power of up to 1.5 kW. To increase this indicator, it is necessary to replace the thyristor with a more powerful one (information about this can be found on the Internet or in a reference book). In addition, you need to take into account the fact that the thyristor control circuit will differ from the original one. KU 202 is an excellent thyristor, but its significant drawback is its configuration (selection of parts for the control circuit). To implement a soft start in automatic mode, scheme 2 is used (soft starter on a microcircuit).

Why was a soft engine start needed?

So, the problem is that the boiler room has pumps for feeding the boiler with water. There are only two pumps, and they are turned on by command from the system for monitoring the water level in the boiler. Only one pump can operate at a time; the pump is selected by the boiler room operator by switching the water taps and electrical switches.

The pumps are driven by conventional asynchronous motors. 7.5 kW asynchronous motors are switched on through conventional contactors (magnetic starters). And since the power is high, the start-up is very hard. Every time you start, there is a noticeable water hammer. The engines themselves, the pumps, and the hydraulic system deteriorate. Sometimes it feels like the pipes and taps are about to shatter into pieces.

In addition, when the boiler has cooled down and hot water is suddenly supplied to it (more than 95 ° C), then unpleasant phenomena occur, reminiscent of explosive seething. It happens the other way around, water with a temperature of 100 °C can be cold - when there is dry steam in the boiler with a temperature of almost 200 °C. In this case, harmful water hammer also occurs.

There are two identical boilers in the boiler room, but the second one has frequency converters for pumps. Boilers (more precisely, steam generators) produce steam with a temperature of more than 115 ° C and a pressure of up to 14 kgf/cm2.

It is a pity that the design of the boiler in the electrical circuit did not provide for smooth activation of the pump motors. Although the boilers are Italian, it was decided to save money on this...

I repeat that to smoothly switch on asynchronous motors we have the following options to choose from:

star-delta circuit
smooth start system (soft start)
frequency converter (inverter)

In this case, it was necessary to choose the option that would require minimal intervention in the operating boiler control circuit.

The fact is that any changes in the operation of the boiler must be agreed upon with the boiler manufacturer (or a certified organization) and with the supervisory organization. Therefore, changes must be made quietly and without unnecessary noise. Although, I don’t interfere with the security system, so it’s not so strict here.

My regular readers know that now, after passing the exams at Rostekhnadzor, I have every right to perform instrumentation and automation work in the boiler room.

Star connection and its advantages

Reversible motor circuit 380 to 220 Volts

Each of the three working windings of the electric motor has two terminals - the beginning and the end, respectively. The ends of all three windings are connected to one common point, the so-called neutral.

If there is a neutral wire in the circuit, the circuit is called 4-wire, otherwise, it will be considered 3-wire.

The beginning of the terminals is connected to the corresponding phases of the supply network. The applied voltage on such phases is 380 V, less often 660 V.

The main advantages of using the star scheme:

Stable and long-term non-stop operation of the engine;
Increased reliability and durability by reducing equipment power;
Maximum smooth start-up of the electric drive;
Possibility of exposure to short-term overload;
During operation, the equipment housing does not overheat.

There is equipment with internal connection of the ends of the windings. Only three pins will be output to the block of such equipment, which does not allow the use of other connection methods. Electrical equipment made in this form does not require competent specialists for its connection.

Connecting a three-phase motor to a single-phase network using a star circuit

Types of soft start devices

They can be divided into four categories.

Regulating starting torque. Their operating principle is such that they control one phase. But when controlling a soft start, the starting currents are not reduced. Therefore, their range of applications is limited.
Regulating voltage with no feedback signal. They work according to a given program and are among the most common in use.
Regulating voltage with feedback signal. Their operating principle is the ability to change the voltage and regulate the current within a given range.
Regulating current with the presence of a feedback signal. They are the most modern of all devices of this type. Provide the greatest control accuracy.

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 starting option allows you to achieve a current that is almost a third lower than when starting the 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.

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.

Using such circuits, it is usually possible to reduce the starting current to two to three rated ones. 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 are 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. Its versatility allows you to implement a lot of functions, such as: dynamic brake, reverse pickup or balancing of magnetic field and current limitation.

An important element of the last of the mentioned circuits is the bypass contactor, which was mentioned earlier. It allows you to ensure the correct thermal conditions of the soft start system of the electric motor, 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.

Operating principle

During the start-up of a commutator-type electric motor, a significant short-term increase in current consumption occurs, which causes premature failure of the power tool and requires it to be repaired. Electrical parts wear out (current exceeds 7 times) and mechanical parts (sharp start). To organize a “soft” start, soft start devices (hereinafter referred to as soft starters) should be used. These devices must meet the basic requirements:

Smooth increase in load.
Possibility of starting the engine at certain time intervals.
Providing protection against linear surges of U, phase loss (for a 3-phase electric motor) and various interference of the electrical component.
Significantly increased service life.

The most widely used are triac soft starters, the operating principle of which is smooth regulation of U by adjusting the opening angle of the triac junction. The triac must be connected directly to the motor windings and this allows you to reduce the starting current from 2 to 5 times (depending on the triac and control circuit). The main disadvantages of triac soft starters are the following:

Complex schemes.
Overheating of windings during prolonged startup.
Problems with starting the engine (leads to significant heating of the stator windings).

The circuits become more complicated when using powerful engines, however, with light loads and idle speed, simple circuits can be used.

The simplest single-phase control circuits are executed on a single triac and are used for instruments with a power of up to 12 kW. There are more complex circuits that allow you to adjust the power parameters of an engine with a power of up to 260 kW. When choosing a factory-made soft starter, it is necessary to take into account the following parameters: power, possible operating modes, equality of permissible currents and the number of starts in a certain period of time.

Choosing a scheme

There are many soft start schemes, we will try to choose something suitable and most affordable for us.

On discrete elements

The regulator, the diagram of which is presented below, is assembled on a symmetrical thyristor (triac) KU208G and allows for smooth start-up of power tools with a power of up to 2 kW.

Triac soft start circuit

Immediately after applying voltage to the circuit (toggle switch SA1), capacitor C1 is discharged, triac VS1 is closed and motor M does not rotate. Next, the capacitor is gradually charged through the diode VD1 and resistor R2, the triac begins to open, but with a long delay from the beginning of the half-wave of the mains voltage. A small initial voltage is applied to the motor and it starts at minimum speed.

As the capacitor charges, the opening delay of the triac decreases, the voltage on the motor increases, and therefore the speed increases. As soon as the capacitor is fully charged, the symmetrical thyristor will open at the beginning of each half-wave, supplying full mains voltage to the motor, and the latter will reach full speed.

The soft start time can be adjusted by selecting the capacitance of capacitor C1. At the indicated ratings (500 µF), the instrument will return to operating mode approximately 2-3 seconds after switching on.

On a microcircuit and triac

This circuit is assembled on the domestic universal microcircuit KR1182PM1. With its help, you can build both a soft starter and a voltage regulator. In the diagram below, the microcircuit is turned on in soft start mode.

Soft start circuit on IC KR1182PM1

Since the microcircuit has a relatively low output power - up to 150 W - it is equipped with a powerful output switch, which is a symmetrical thyristor TS122-20-10, which can withstand current up to 20 A. The time it takes for the engine to reach operating mode depends on the capacitance of capacitor C1. Such a circuit can operate without a radiator at a load power of up to 1 kW.

Integral regulator

The circuit based on discrete elements is quite simple and does not contain scarce elements, but it is too bulky and will have to be placed in a separate case, especially if the power tool is powerful and a radiator is required. In this regard, it is much more convenient to use ready-made integrated soft start units. The most convenient option for us is KRRQD20A.

Soft starter KRRQD20A

The compact integrated soft start unit (SPU) is designed for current up to 20 A and is capable of switching power up to 4 kW. The module has 2 outputs and is connected to a break in one of the power wires of the tool motor. If you equip an extension cord with it (for some reason many people call it a carrier), then the power tool connected through it will start smoothly when you press the power button.

Connection diagram of the KRRQD20A module to the extension cord

The photo clearly shows that the module is designed for installation on a radiator, but if the power of the power tool does not exceed 1 kW, then a radiator is not required.

Soft starter XS-12/D3

The connection diagram is printed directly on the body of the device and it is obvious that it can only be used by installing it after the switch in the power tool itself. This is also a good option, but, firstly, the extension cord is a more universal solution (you can connect any tool or even a lamp), and, secondly, by disassembling the tool, we lose warranty service.

How to connect, installation

For users of grinders who do not have electrical installation skills, you can purchase a separately sold soft start unit. You just need to install it correctly. There are two options for placing the starting device - inside the body of the angle grinder and, if not possible, outside.

In the next video, the author placed one of the purchased blocks inside it with the help of a slight modification of the angle grinder body. The two wires of the starting unit are connected according to the following diagram: one wire to the switch contact, the other to the stator winding of the electric drive.

In another video, the author also managed to place the purchased block inside the grinder. However, he chose a different connection scheme - to break the network. In this case, it is not important to consider where to connect the “zero” and where the “phase”.

How to make a soft start without carrying

If you don’t want to assemble a bulky structure from a separate carrying case + an additional socket for it, this soft-start device can be assembled in one rubber socket of this shape.

It is called a portable socket with a protective cover - RBp13-1-0m.

The KRRQD12A block fits perfectly inside and does not interfere with anything.

By connecting a long KG cable with a plug to it, you will get a ready-made socket with a power supply unit, which additionally plays the role of an extension cord.

With such a dust-proof, waterproof socket, you can safely work outdoors without fear of rain, snow and extraneous splashes of water.

In both cases, you will get a compact mobile soft starter through which you can connect any tool with a motor up to 2.5 kW.

Of course, a tool without such a PP can and will survive for many years if the manufacturer initially included an increased safety margin in it. An example is Soviet drills.

Many of them were inherited in working condition. But today not every manufacturer has such an approach to tools. Quite the contrary, they save on all components, reducing the cost of production.

And the logic here is simple - the sooner it burns, the sooner they will come for a new one. A resource of 5-10 years is considered a good period for some. For others, this is not acceptable.

The master must work with the tool in such a way that he only has to periodically change the brushes and lubricant and do not forget to clean it. With the above socket this could very well become a reality for you.

How to check

At home, before assembling an angle grinder with a soft starter, it’s a good idea to check it for an open circuit. The following video tests a three-pin device. Usually there is a connection diagram . There are two network wires here, one goes to the electric drive. If you assemble a circuit with an indicator light and include a starter in it, then you can determine the break in it by the indicator light coming on/not coming on .

How to make a soft start circuit for an angle grinder with your own hands

The popular circuit is implemented on the basis of the KR118PM1 phase control control microcircuit, and the power part is made of triacs. Such a device is quite easy to install, does not require additional settings after assembly, and therefore, it can be made by a master without specialized education, just be able to hold a soldering iron in his hands.

Electrical circuit for adjusting soft start for an angle grinder

The proposed unit can be connected to any power tool designed for an alternating voltage of 220 volts. A separate remote power button is not required; the modified power tool is turned on with a standard key. The circuit can be installed either inside the body of the angle grinder or in the break of the power cable in a separate case.

The most practical thing is to connect the soft starter to the socket from which the power tool is powered. The input (XP1 connector) is supplied with power from a 220 volt network. A consumable socket is connected to the output (connector XS1), into which the angle grinder plug is plugged.

When the start button of the angle grinder is closed, voltage is supplied to the DA1 chip via the common power circuit. There is a smooth increase in voltage across the control capacitor. As it charges, it reaches a working value. Due to this, the thyristors in the microcircuit do not open immediately, but with a delay, the time of which is determined by the charge of the capacitor. Triac VS1, controlled by thyristors, opens with the same pause.

Watch the video with a detailed explanation of how to do it and what scheme to use

In each half-cycle of alternating voltage, the delay decreases in an arithmetic progression, as a result of which the voltage at the input to the power tool gradually increases. This effect determines the smooth start of the angle grinder engine. Consequently, the disk speed increases gradually, and the gearbox shaft does not experience inertial shock.

The time it takes for the speed to reach the operating value is determined by the capacitance of capacitor C2. The value of 47 uF ensures a smooth start in 2 seconds. With such a delay, there is no particular discomfort when starting to work with the tool, and at the same time, the power tool itself is not subject to excessive loads from a sudden start.

After turning off the angle grinder, capacitor C2 is discharged by resistor R1. At a nominal 68 kOhm, the discharge time is 3 seconds. After which the soft starter is ready for a new start cycle of the angle grinder. With a little modification, the circuit can be upgraded to an engine speed controller. To do this, resistor R1 is replaced with a variable one. By adjusting the resistance, we control the engine power by changing its speed.

Thus, in one housing it is possible to make an engine speed controller and a soft start device for a power tool.

The remaining details of the circuit work as follows:

Resistor R2 controls the amount of current flowing through the control input of triac VS1;
Capacitors C1 and C2 are control components of the KR118PM1 microcircuit, used in a typical switching circuit.

For simplicity and compactness of installation, resistors and capacitors are soldered directly to the legs of the microcircuit.

The VS1 triac can be anything with the following characteristics: maximum voltage up to 400 volts, minimum throughput current 25 amperes. The amount of current depends on the power of the angle grinder.

Due to the smooth start of the angle grinder, the current will not exceed the rated operating value for the selected power tool. For emergency cases, for example, a jammed angle grinder disk, a current reserve is required. Therefore, the nominal value in amperes should be doubled.

The ratings of the radio components used in the proposed electrical circuit were tested on an angle grinder with a power of 2 kW. There is a power reserve of up to 5 kW, this is due to the peculiarity of the operation of the KR118PM1 microcircuit. The scheme is working, executed many times by home craftsmen.

If you have an old angle grinder in your arsenal, don’t rush to write it off. Using a simple electrical circuit, the device can be easily upgraded by adding the function of changing the speed. Thanks to a simple regulator that you can actually assemble with your own hands in a few hours, the functionality of the device will increase significantly. By reducing the rotation speed, the grinder can be used as a grinding and sharpening machine for various types of materials. New opportunities are emerging for the use of additional attachments and accessories.

How to choose a miter saw

According to the scope of application, miter saws are divided into:

household ones - have low strength, cutting accuracy, and are not able to cope with high loads. They are often light in weight and have modest technical characteristics.
Professional - designed for long-term continuous operation, have increased strength, and all components are tightly fitted to perform high-precision cuts

Characteristics

Power – 1.1 kW.
Rotation speed – 4800 rpm.
Cut parameters: height – 50 mm, width – 110 mm.
Disc parameters: diameter – 190 mm, mounting hole – 30 mm.
Operating angles: tilt – up to 45°, rotation – up to 46°.
Weight – 8 kg.

ELITECH PT 1825K

According to the main characteristics, this model differs from the previous miter saw in the speed of rotation of the circle, but the difference is not so significant, so it is capable of processing the same materials - thin aluminum, wood, plastic. It can be used to process workpieces up to 305 mm wide. The use of a laser pointer provides increased accuracy.

Long workpieces are fixed using extension brackets. For processing various materials, it is possible to use discs with a thickness of up to 2.8 mm, while their diameter should not exceed 255 mm. Protection is provided by a casing that prevents mechanical stress and dust from reaching the disk.

The tilt and rotation angle is 45 degrees.
Long power cable for connecting to the mains.
Includes 60 tooth disc.
Soft start/braking function.
Adjustable cutting depth.

The foot slides on the glossy surface, so it is difficult to firmly fix the angle of rotation.

BOSCH PCM 8S

Great miter saw

with a broach and a durable aluminum body that is resistant to deformation. The lightweight and compact device ensures precise cutting and is used even for large-sized products. The immobility is due to the use of a special clamp. Table extenders, lasers, and a depth limiter are also used in the cutting process.

For working with wide workpieces there is a broaching function. The laser pointer ensures maximum accuracy when sawing, it is not clogged with sawdust, so the line and direction remain visible under any conditions.

The grooves are sawed using a depth stop.
Dust bag included.
Compact size, light weight and a handle for easy portability.
Spindle lock for disc replacement.
Carbide saw blade included.

BISON ZPT-255-1800 PL

A good miter saw with a broach, designed for precise cutting of workpieces. It was made by a Russian company, so finding parts for it is very easy. The trouble-free service life is at least 5 years, and with proper care it will be several decades. This is a reliable, durable unit that can be used to work with metal, wood, and plastic. The multi-tooth saw blade is thicker, allowing for a wider range of applications.

The laser pointer ensures cutting accuracy and allows you to draw an even, clear line. The cut is made at any angle; for this purpose, the table can be rotated and can be fixed in any position. The workpiece is secured with a clamp.

Automatic protective cover.
Keeping your work area clean by connecting a vacuum cleaner.
5 year warranty.
Good build quality and materials.

There is no soft start.

Soft start - what is it for?

To reduce the excessive load on the mechanics of a power tool during startup, measures can be taken on the power supply side. Instead of supplying the electric motor with full voltage from the source (mains), a reduced voltage can be supplied using a soft start. But where can I get it? We are talking about mass application. In some cases, specialists and craftsmen could solve this problem, but this was not available to most ordinary consumers.

There are three ways to limit the starting torque of a power tool and achieve a smooth start:

Application of rheostats;
Application of transformers;
Application of semiconductor switches.

It can be used on both direct and alternating current.

A significant part of the power is lost to heating the rheostat resistance. If the task is limited only to a soft start, then this is quite tolerable. If you regulate the operating speed of the electric motor in this way, then this will result in unnecessary heating of the environment and consumption of electricity. In any case, the device turns out to be bulky.

The second method is much better and more economical. Suitable for AC power only. It can also improve electrical safety when using power tools. The disadvantage is that classic transformers are now very expensive. Even if you make it yourself, since they use a lot of expensive copper. The device is also quite large and heavy.

Transformer

The third soft start method is the most modern and cheapest. It relies on the massive use of semiconductors. At one time, huge amounts of money were invested in research and development of industrial production of semiconductor devices. But the cheapness of the materials from which they are produced and the mass production have already managed to pay for everything. Due to their low cost, such devices are available to everyone.

The main feature of semiconductor switches is that there are no mechanical contacts and they operate at enormous speed (switching frequency). The currents they switch can reach large values at high voltages in the off state. At the same time, such devices practically do not heat up and do not consume excess energy, like rheostats, and are excellent for modern power tools.

Types of semiconductor switches

Thyristors and triacs

The resistance of an open switch reaches millions of ohms, and practically no current flows through it.

The resistance of the closed switch is within units and tenths of an ohm.

Although a significant current can flow, the voltage drop across the switch is too small for it to generate much heat, according to the Joule-Lenz law. In both cases it remains practically cold.

This applies to any of the types of power switches, of which there are three:

Thyristors and triacs;
MOSFET field effect transistors;
IGBT transistors.

Historically, thyristors were the first to appear. With their help, they regulated the power in alternating current circuits, controlling the unlocking phase of the device.

By adjusting the phase of the control voltage (duration t1), it is possible to influence the moment the triac is unlocked in each half-cycle (t3) and thus the share of energy entering the load and, accordingly, the electric motor.

With the advent of powerful field effect transistors with an insulated MOS gate (metal-oxide-semiconductor, or in English Metal-Oxide-Semiconductor Field Effect Transistor), the current in the circuit began to be controlled by changing the width of the opening pulses. This method is very effective in direct current circuits, for which it is first rectified, and is used in welding inverters, frequency converters, etc.

For the most powerful power tools, IGBTs are used - insulated gate bipolar transistors. This is a combination of a field-effect transistor with a bipolar one.

To regulate the electric motor, an already established, long-used solution using triacs is currently used. More advanced solutions are not yet very common.

Homemade options

There are many schemes for modernizing power tools using soft starters. Among all varieties, devices based on triacs are widely used. A triac is a semiconductor element that allows you to smoothly regulate power parameters. There are simple and complex circuits that differ in design options, as well as in the supported power of the connected power tool. The design includes internal ones, which allow them to be built inside the case, and external ones, manufactured in the form of a separate module, which acts as a speed limiter and starting current when starting the angle grinder directly.

The simplest scheme

The basic principle of operation is to regulate the speed of the electric motor of the tool by limiting the power in manual mode. This circuit allows you to use power tools with a power of up to 1.5 kW. To increase this indicator, it is necessary to replace the thyristor with a more powerful one (information about this can be found on the Internet or in a reference book). In addition, you need to take into account the fact that the thyristor control circuit will differ from the original one. KU 202 is an excellent thyristor, but its significant drawback is its configuration (selection of parts for the control circuit). To implement a soft start in automatic mode, scheme 2 is used (soft starter on a microcircuit).

Soft start on a chip

The best option for manufacturing a soft starter is a soft starter circuit with one triac and a microcircuit that controls the smooth opening of a pn type transition. The device is powered from a 220 V network and is easy to assemble yourself. A very simple and universal soft start circuit for an electric motor also allows you to regulate the speed (diagram 2). The triac can be replaced with the same one or with characteristics exceeding the original ones, according to the reference book of semiconductor-type radioelements.

Scheme 2. Scheme for soft start of a power tool

The device is implemented on the basis of the KR118PM1 microcircuit and a triac. Due to the versatility of the device, it can be used for any tool. It does not require configuration and is installed into the power cable.

When the electric motor starts, U is supplied to KR118PM1 and the charge of capacitor C2 gradually increases. The thyristor opens gradually with a delay depending on the capacitance of the control capacitor C2. With a capacitance of C2 = 47 μF, there is a delay at startup of about 2 seconds. It depends directly proportionally to the capacitance of the capacitor (with a larger capacitance, the startup time increases). When the angle grinder is turned off, capacitor C2 is discharged using resistor R2, the resistance of which is 68 k, and the discharge time is about 4 seconds.

To regulate the speed, you need to replace R1 with a variable resistor. When changing the parameter of the variable resistor, the power of the electric motor changes. R2 changes the amount of current flowing through the triac input. The triac needs cooling and, therefore, a fan can be built into the module housing.

Thus, to start electric motors of various tools, it is necessary to use factory-made or home-made soft starters. Soft starters are used to increase the service life of the tool. When starting the engine, there is a sharp increase in current consumption by 7 times. Because of this, the stator windings may burn out and the mechanical part may wear out. Soft starters can significantly reduce the starting current. When making a soft starter yourself, you need to follow safety rules when working with electricity.

A soft starter is 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.

Disadvantages of power tools and service life

It is well known that not every tool is equipped with such soft start circuits. They mainly come in expensive models from well-known brands Bosch, Hilti, and DeWalt. Moreover, both in the network line and in the battery one.

A power tool without such a device has a bunch of disadvantages:

armature sparking on the commutator with armature lamellas burning out

brushes burn out and wear out faster

rotor and stator windings fail more often

current surge into the general electrical network

gears hitting each other and operating faster

dangerous jerk when starting, tearing the tool out of your hands and increasing the risk of injury

When working with a miter saw with a PP, the blade will not stray from the prepared cutting point. Which is important for non-professional carpenters.

If at your dacha or in your house at the initial stage of construction there is no electricity yet and you use a generator, then sooner or later you will understand that without a soft start unit (soft start unit) with sharp initial currents, the generator will not last long. Therefore, such a thing can save not only the tool, but also emergency power supplies.

You can, of course, independently build a BPP into the inside of the same grinder or trimmer, but not everyone wants to disassemble the equipment and tinker with the insides.

Plus, opening a new case will void the warranty. Therefore, the best use for the KRRQD12A unit is external connection.

Just keep in mind that it is suitable for commutator motors. For asynchronous ones, a frequency generator with different regulation principles is needed.

This box is designed for a current of 12 Amps.

There is also a more powerful 20A model.

What is typical is that they have the same dimensions, but the difference in price is a couple of tens of rubles.

It would seem better to take it, but for a standard 16A socket the first option is more profitable. There will be no desire to connect a more powerful load and thereby burn all the contacts.

DIY craftsmen, of course, assemble similar circuits with their own hands, based on VTA 12-600 or other thyristors, capacitors, a dinistor and a couple of small resistors. You can find many examples of schemes on the Internet.

But for the average user of the instrument, it is much easier to buy all this in a ready-made compact case. You can order a similar block using the link here.

Characteristics of the soft starter

The main criteria for choosing a soft starter are the current limit range, the degree of protection of the housing, the permissible number of starts per unit of time, rated current and voltage, permissible motor power, and the possibility of parallel connection of a shunt electrical device. The choice of device is carried out according to standard methods.

When choosing a soft starter, it is also necessary to take into account the presence of the following functions:

Trigger as a function of current or voltage. Soft starters with this function are used when the power of the supply network is limited. Such soft starters allow you to regulate the current and avoid overheating of cables, tripping of protection, and stopping of generators that are sensitive to sudden fluctuations in the current consumed by the load. For process equipment where fast starting with increased torque is unacceptable, a soft starter with starting as a function of voltage is used. Such devices smoothly increase the voltage in the windings of electrical machines. For more precise adjustment, a soft starter with current and voltage feedback is used.
Number of phases. To start electric motors, soft starters are used with adjustment of electrical parameters in one, two and three phases. Devices of the first two types are used to drive equipment with infrequent start-up, since an asymmetrical load at the time of start-up negatively affects the operation of the electric machine.
Availability of a bypass contactor. At the end of the transition process, it is advisable to turn off the current supply through the soft starter to prevent overheating of the triacs. This is achieved by connecting a contactor in parallel to the circuit, which closes the power contacts after the electric motor accelerates. There are soft starter models that do not provide for parallel connection of contactors, however, for a powerful motor it is better to choose a device with a shunt switching device.
Protection functions. Many soft starters have built-in protection against overheating of the device itself, changes in the frequency of the supply voltage, a decrease in the output current, as well as load shutdown functions when the acceleration time is exceeded, phase failure, or uneven load. In some models it is also possible to connect a heating sensor for the electric motor windings. To protect a drive with a soft starter from short circuits, fuses or circuit breakers are required.
Speed control functions. There are soft starters where the possibility of reducing the rotation speed of the electric motor is implemented. However, soft starters do not replace a frequency converter. Speed adjustment is carried out in steps. When operating for a long time at a reduced speed, the soft starter overheats greatly. The soft starter does not ensure long-term operation of the engine at reduced speed. Such modes are used when adjusting and setting up production equipment.
Braking mode. For drives of inertial equipment, you should select a soft starter with a braking function. In this mode, voltage is applied to the electric motor windings, causing the electric machine to brake. Such devices are used for lifts, conveyors, traction fans.
Monitoring the status of the bypass contactor. When the power contacts of the bypass contactor are not closed, upon reaching the rated speed of the electric motor rotor, the soft starter switches off the drive.
Start with maximum torque. Soft starters with this function supply the rated mains voltage to the windings. After a sharp starting push, the voltage is limited. Further acceleration of the electric machine is carried out in a smooth mode. A soft starter with this function is used for equipment drives that are switched on under significant load.

Frequent malfunctions, causes

The grinder may not gain the speed required by the regulatory regime, work with spontaneous changes, with jerks, and in extreme cases, stop rotating altogether. If there is a speed control option, it may not respond to changes in the position of the operating mode indicator.

Some types of malfunctions and methods for eliminating them are discussed in the presented videos.

Not gaining momentum

The Makita model grinder, powered by batteries, did not reach operating speed in the next video. The first reason that comes to mind is that the batteries are discharged, but this has not been confirmed. When disassembling the tool, faulty brush holders were discovered. The ingress of foreign oily liquid into the brush-collector assembly caused destruction of the brushes and melting of the plastic structure of the brush holders.

The author replaced the defective one with a new brush mounting unit.

Important: graphite dust from damaged brushes should be carefully removed from the stator and rotor, the surface of the commutator should be cleaned with emery cloth and preferably polished

Does not develop speed and smokes

The stability of the rotation speed of the spindle working shaft largely depends on the presence of malfunctions in the components of the electrical part of the angle grinder. In the next video, the stator winding burned out, resulting in smoke and the grinder stopped rotating. Stator failure can be easily determined visually: broken wires, burnt insulation.

The author had a used, but fully functional stator in stock, so he eliminated the defect by replacing the failed one.

It's getting warm

Poor contact in the connections of the electrical part of the angle grinders leads to strong heating of the body, the formation of increased spark strength due to deviations from the nominal values of the electrical parameters. In the next video, the contacts on the brush holders burned out. Perhaps this happened due to a violation of the location of the brushes, which led to a large formation of graphite dust and the creation of additional resistance on the commutator unit. The author suggested a more reliable connection of contacts and corrected the location of the guides for the brushes.

Loses speed under load

In the next video, the grinder spontaneously changed speed during operation, and from time to time there were jerks. At the same time, there was no option for adjusting the speed on the angle grinder. One such jerk caused the cutting disc to break. After which it was decided to find and fix the problem. Diagnostics of the electrical part when testing with a multimeter revealed that the power (start) button was faulty.

Since such buttons are usually not suitable for repair (when disassembling it, a completely defective contact was discovered), the author found a replacement. Functionally, the new button duplicated the old one, but was slightly different in size. The author successfully chose a new location for it and the grinder restored its functionality.