People who often use power tools sometimes encounter the following problem: the engine of a grinder, circular saw, planer or other equipment starts very abruptly. Such a sharp start is fraught with many troubles: firstly, there is a high starting current, which does not have the best effect on the wiring, secondly, a sharp start of the engine quickly wears out the mechanical parts of the tool, thirdly, ease of use decreases when starting the grinder you have to hold it tightly, it tries to break free from your hands. Expensive models already have a built-in soft start system that easily copes with all these troubles. But what to do if this system does not exist? There is a way out - to assemble a soft start circuit yourself. In addition, it can be used with incandescent light bulbs, because most often they burn out at the moment they are turned on. A soft start will significantly reduce the chances of a light bulb burning out quickly.
Scheme
On the Internet you can often find a soft start circuit built on a rather rare domestic microcircuit K1182PM1R, which is not always easy to get now. That is why I propose an equally effective circuit for assembly, the key link of which is the available TL072 microcircuit; you can also use LM358 instead. The time it takes for the engine to reach full speed is set by capacitor C1. The larger its capacity, the more time it will take to overclock; the best option is 2.2 µF. Capacitors C1 and C2 must be designed for a voltage of at least 50 volts. Capacitor C5 - at least 400 volts. Resistor R11 will dissipate a decent amount of heat, so its power should be at least 1 Watt. Any low-power transistors can be used in the circuit, T1, T2, T4 have an n-p-n structure, you can use BC457 or domestic KT3102, T4 has a p-n-p structure, BC557 or KT3107 are suitable in its place. T5 – any semistor suitable for power and voltage, for example, BTA12 or TS-122.
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.Download the board:
(downloads: 1089)
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.
First launch and tests
After the board is completely ready, you can check its functionality. First of all, you need to find a low-power light bulb of 5-10 watts and connect it to the board through a 220-volt network. Those. the board and the light bulb are connected to the network in series, and the OUT output remains unconnected. If nothing is burned on the board and the light does not light up, you can connect the circuit directly to the network. The same low-power bulb can be connected to the OUT output for testing. When connected, it should smoothly gain brightness to maximum. If the circuit works properly, you can connect more powerful electrical appliances. During prolonged operation, the semistor may become slightly warm - there is nothing to worry about. If there is free space, it would not hurt to install it on the radiator.Dangerous mains voltage is present on the board during operation, so precautions must be taken. Under no circumstances should you touch the board parts while it is connected to the network. Before turning on, make sure that the board is securely fastened and that no metal objects that can cause a short circuit will fall on it. For reliability, it is recommended to fill the board with varnish or epoxy resin, then even moisture will not harm it. Happy build!
Many power tools fail due to wear and tear on the motor. Modern models of angle grinders have a soft start device. Thanks to it, they are able to work for a long time. The operating principle of the element is based on changing the operating frequency. In order to learn more about the starter device, it is worth considering the diagram of the standard model.
Soft starter
The standard soft start circuit for an angle grinder consists of a triac, a rectifier unit and a set of capacitors. To increase the operating frequency, resistors are used that allow current to pass in one direction. The starter is protected by a compact filter. models maintain low. However, in this case, much depends on the maximum power of the motor installed in the angle grinder.
How to connect the model?
The soft start of the angle grinder is connected via an adapter. Its input contacts are connected to the rectifier block. In this case, it is important to determine the zero phase in the device. To secure the contacts you will need to check the functionality of the starter using a tester. First of all, negative resistance is determined. When installing a starter, it is important to remember the threshold voltage that the device can withstand.
Device diagram for an angle grinder with a 10 A triac
The do-it-yourself soft start circuit for an angle grinder involves the use of contact resistors. The polarity coefficient of modifications, as a rule, does not exceed 55%. Many models are produced with interlocks. A wired filter is responsible for protecting the device. Low frequency transceivers are used to pass current. The process of lowering the threshold voltage is carried out on a transistor. The triac in this case acts as a stabilizer. When connecting the model, the output resistance at an overload of 10 A should be about 55 Ohms. Covers for starters are suitable on a semiconductor basis. In some cases, magnetic transceivers are installed. They handle low speeds well and can maintain the rated frequency.
Model for grinders with a 15 A triac
Soft start for an angle grinder with 15 A triacs is universal and is often found in low-power models. The difference between the devices is their low conductivity. The grinder's soft start circuit (device) involves the use of contact-type transceivers that operate at a frequency of 40 Hz. Many models use comparators. These elements are installed with filters. The rated voltage of starters starts from 200 V.
Starters for angle grinders with a 20 A triac
Devices with 20 A triacs are suitable for professional grinders. Many models use contactor resistors. First of all, they are able to operate at high frequencies. The maximum temperature of the starters is 55 degrees. Most models have a well-protected case. The standard device circuit involves the use of three contactors with a capacity of 30 pF or more. Experts say that the devices stand out for their conductivity.
The minimum frequency for starters is 35 Hz. They are capable of operating in a DC network. Modifications are connected via adapters. Such devices are well suited for 200 W motors. Filters are quite often installed with triodes. Their sensitivity indicator is no more than 300 mV. Quite often there are wired comparators with a protection system. If we consider imported models, they have an integrated converter that is installed with insulators. Current conductivity is ensured at around 5 microns. With a resistance of 40 Ohms, the model is capable of stably maintaining high speeds.
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
Devices for angle grinders 800 W
800 W grinders work with low frequency starters. Triacs are quite often used at 15 A. If we talk about the circuit of the models, it is worth noting that they use expansion transistors, whose current carrying capacity starts from 45 microns. Capacitors are used with and without filters, and the capacitance of the elements is no more than 3 pF. It is also worth noting that starters differ in sensitivity.
If we consider professional grinders, then modifications of 400 mV are suitable for them. In this case, the current conductivity may be low. There are also devices with variable transistors. They warm up quickly, but are not able to maintain high speeds of the grinder, and their current conductivity is about 4 microns. If we talk about other parameters, the rated voltage starts from 230 V. The minimum frequency for models with wideband triacs is 55 Hz.
Starters for angle grinders 1000 W
Starters for these angle grinders are made using triacs with an overload of 20 A. The standard device circuit includes a triode, a stabilizer plate and three transistors. The rectifier unit is most often installed on a wire base. Capacitors can be used with or without a filter. The minimum frequency of the conventional model is 30 Hz. With a resistance of 40 ohms, starters are capable of supporting large overloads. However, problems may arise at low speeds of the angle grinder.
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.
Assembling a model with VS1 series triacs
You can assemble a soft start for an angle grinder using a VS1 triac with your own hands using several rectifier blocks. The capacitors for the device are suitable for the linear type with a capacity of 40 pF or more. You should start assembling the modification by soldering the resistors. Capacitors are installed in series order between insulators. The rated voltage of a high-quality starter is 200 V.
Next, to make a smooth start for the angle grinder with your own hands, take a prepared triac and solder it at the beginning of the circuit. Its minimum operating frequency should be 30 Hz. In this case, the tester must display a value of 50 ohms. If problems arise with overheating of the capacitors, then you need to use dipole filters.
Model for angle grinders with regulator KR1182PM1
To assemble a soft start for an angle grinder with the KR1182PM1 regulator with your own hands, you need a contact thyristor and a rectifier unit. It is more expedient to use a triode for two filters. It is also worth noting that to assemble the starter you will need three capacitors with a capacity of at least 40 pF.
The sensitivity indicator of the elements must be 300 mV. Experts say that the triac can be installed behind the lining. You also need to remember that the threshold voltage should not fall below 200 V. Otherwise, the model will not be able to operate at lower speeds of the angle grinder.
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.
Why does the grinder need low speeds?
The built-in disc speed control function allows you to delicately process materials such as plastic or wood. At low speeds, operating comfort and safety increases. This function is especially useful in electrical and radio installation practice, in car services and restoration workshops.
In addition, among professional users of power tools there is a strong opinion that the simpler the device is, the more reliable it is. And it is better to move the additional service stuffing outside the power unit. In this situation, equipment repair is greatly simplified. Therefore, some companies specially produce remote, separate electronic regulators that connect to the machine’s power cord.
Speed controller and soft start - what are they for?
Modern grinders use two important functions that increase the reliability and safety of the tool:
- speed controller - a device designed to change the number of engine revolutions in various operating modes;
- soft start - a circuit that ensures a slow increase in engine speed from zero to maximum when the device is turned on.
They are used in electromechanical tools that use a commutator motor in their design. Helps reduce wear on the mechanical part of the unit during switching on. They reduce the load on the electrical elements of the mechanism by putting them into operation gradually.
As studies of the properties of materials have shown, the most intense production of friction units occurs during a sharp transition from a state of rest to a mode of rapid movement. For example, one start of an internal combustion engine in a car is equivalent to 700 km of mileage in terms of wear on the piston group.
When the power is turned on, an abrupt transition occurs from a state of rest to rotation of the disk at a speed of 2.5–10 thousand revolutions per minute. Those who have worked with an angle grinder are well aware of the feeling that the machine simply “rips out of your hands.” It is at this moment that the overwhelming number of breakdowns associated with the mechanical part of the unit occur.
The stator and rotor windings experience no less load. The commutator motor starts in short circuit mode, the electromotive force is already pushing the shaft forward, but inertia does not yet allow it to rotate. A jump in starting current occurs in the coils of the electric motor. And although they are structurally designed for such work, sooner or later a moment comes (for example, during a power surge in the network) when the winding insulation cannot withstand and an interturn short circuit occurs.
When soft start circuits and changes in engine speed are included in the electrical circuit of the tool, all of the above problems automatically disappear. Among other things, the problem of voltage “dip” in the general network at the moment of starting a hand tool is solved. This means that the refrigerator, TV or computer will not be at risk of “burning out”. And the safety circuit breakers on the meter will not operate and cut off the current in the house or apartment.
The soft start circuit is used in angle grinders of medium and high price categories, the speed control unit is used mainly in professional models of angle grinders.
Adjusting the speed allows you to process soft materials with a grinder, perform fine grinding and polishing - at high speed, wood or paint will simply burn.
Additional electrical circuits increase the cost of the tool, but increase the service life and level of safety during operation.
How to assemble a regulator circuit with your own hands
The simplest power regulator, suitable for an angle grinder, soldering iron or light bulb, is easy to assemble with your own hands.
Electrical circuit diagram
In order to assemble a simple speed controller for an angle grinder, you need to purchase the parts shown in this diagram.
- R1 - resistor, resistance 4.7 kOhm;
- VR1 - trimming resistor, 500 kOhm;
- C1 - capacitor 0.1 µF x 400 V;
- DIAC - triac (symmetrical thyristor) DB3;
- TRIAC - triac BT-136/138.
Circuit operation
Trimmer resistor VR1 changes the charging time of capacitor C1. When voltage is applied to the circuit, at the first moment of time (the first half-cycle of the input sinusoid), triacs DB3 and TRIAC are closed. The output voltage is zero. Capacitor C1 charges and the voltage across it increases. At a certain point in time, specified by the R1-VR1 chain, the voltage on the capacitor exceeds the opening threshold of the triac DB3, the triac opens. The voltage from the capacitor is transmitted to the control electrode of the TRIAC triac, which also opens. Current begins to flow through the open triac. At the beginning of the second half-cycle of the sinusoid, the triacs are closed until capacitor C1 is recharged in the opposite direction. Thus, the output produces a pulse signal of complex shape, the amplitude of which depends on the operating time of the C1-VR1-R1 circuit.
Assembly order
Assembling this circuit will not be difficult even for a novice radio amateur. Spare parts are available and can be purchased at any store. Including desoldering from old boards. The procedure for assembling the regulator using thyristors is as follows:
How to connect the device to an angle grinder, options
The connection of the regulator depends on what type of device is selected. If a simple circuit is used, it is enough to mount it into the power supply channel of the power tool.
Installing a homemade board
There are no ready-made installation recipes. Anyone who decides to equip an angle grinder with a regulator places it in accordance with their goals and model of the tool. Some people insert the device into the handle of the holder, others into a special additional box on the body.
In different models, the space inside the angle grinder body may be different. Some have enough free space to install a control unit. In others, you have to take it to the surface and attach it in a different way. But the trick is that, as a rule, there is always a certain cavity in the back of the instrument. It is designed for air circulation and cooling.
This is usually where the factory speed controller is located. A DIY diagram can be placed in this space. To prevent the regulator from burning out, the thyristors should be installed on the radiator.
Video: soft start plus and engine speed adjustment
Features of installation of the finished block
When purchasing and installing a factory regulator inside an angle grinder, most often you have to modify the body - cut a hole in it to allow the adjustment wheel to come out. But this may adversely affect the rigidity of the casing. Therefore, it is preferable to install the device outside.
The numbers on the adjustment wheel indicate the number of spindle revolutions. This value is not absolute, but conditional. “1” - minimum speed, “9” - maximum. The remaining numbers serve as a guide for regulation. The location of the wheel on the body varies. For example, on the Bosch PWS 1300–125 CE, Wortex AG 1213–1 E or Watt WWS-900 angle grinders, it is located at the base of the handle. On other models, such as the Makita 9565 CVL, the adjustment wheel is located at the end of the housing.
The connection diagram of the regulator to the angle grinder is not complicated, but sometimes it is not so easy to stretch the cables to the button, which is located at the other end of the device body. The problem can be solved by selecting the optimal wire cross-section or by placing it on the surface of the casing.
A good option is to install the regulator on the surface of the device or attach it to a network cable. Not everything always works out on the first try; sometimes the device has to be tested and then some adjustments made. And this is easier to do when access to its elements is open.
Important! If you have no experience working with electrical circuits, it is more advisable to purchase a ready-made factory regulator or angle grinder equipped with this function.
Device instruction manual
The basic rule when operating an angle grinder with a homemade speed controller is to adhere to the work and rest schedule. The fact is that an engine operating at a “regulated” voltage gets especially hot. When grinding at low speeds, it is important to take frequent breaks so that the commutator windings do not burn out.
It is also highly recommended not to turn on the tool if the speed regulator is set to minimum - the reduced voltage will not be enough to rotate the rotor, the collector lamellas will remain in short circuit mode, and the windings will begin to overheat. Unscrew the variable resistor to the maximum, then, turning on the angle grinder, reduce the speed to the desired value.
Following the correct order of activation and adjustment will allow you to operate the angle grinder for an unlimited period of time.
In addition, you should understand that adjusting the speed of rotation on an angle grinder occurs on the principle of a water tap. The device does not increase the number of revolutions, it can only decrease them. It follows from this that if the maximum nameplate speed is 3000 rpm, then when a speed controller is connected, the angle grinder will operate in a range lower than the maximum speed.
Attention! If the angle grinder already contains electronic circuits, for example, it is already equipped with a speed controller, then the thyristor controller will not work. The internal circuits of the device simply will not turn on.
Video: homemade angle grinder speed controller
Equipping the angle grinder with a circuit for adjusting engine speed will increase the efficiency of using the device. and expand its functional range. This will also save the technological resource of the grinding machine and increase its service life.
Asynchronous electric motors, in addition to the obvious advantages, have two significant disadvantages - a large starting current (up to seven times more than the rated current) and a jerk at the start. These shortcomings negatively affect the condition of electrical networks, require the use of circuit breakers with an appropriate time-current characteristic, and create critical dynamic loads on equipment.
Everyone is familiar with the effect of starting a powerful asynchronous motor: “the voltage sags and everything around the electric motor shakes. Therefore, to reduce negative impacts, methods and schemes have been developed to soften the jerk and make the start of an asynchronous motor with a squirrel-cage rotor smoother.
Methods for smooth starting of asynchronous motors
In addition to the negative impact on the power circuit and the environment, the starting impulse of an electric motor is also harmful to its stator windings, because the moment of increased force during startup is applied to the windings. That is, the jerking force of the rotor puts intense pressure on the winding wires, thereby accelerating the wear of their insulation, the breakdown of which is called an interturn short circuit.
Illustration of the principle of operation of an asynchronous electric motor Since it is structurally impossible to reduce the starting current, methods, circuits and devices have been invented that provide soft start asynchronous motor. In most cases, in industries with powerful power lines and in everyday life, this option is not mandatory - since voltage fluctuations and starting vibrations do not have a significant impact on the production process.
Graphs of current changes during direct starting and using soft starters But there are technologies that require stable, not exceeding standard parameters, both power supply and dynamic loads. For example, this could be precision equipment operating on the same network with voltage-sensitive electricity consumers. In this case, to comply with technological standards for soft starting of the electric motor, various methods are used:
- Star-delta switching;
- Starting using an autotransformer;
- asynchronous motor soft start devices (USM).
The video below lists the main problems that arise when starting an electric motor, and also describes the advantages and disadvantages of various soft starters for squirrel-cage asynchronous electric motors.
In another way, UPPs are also called soft starters, from the English “soft” - soft. Below we will briefly describe the types and options offered in widely used soft starters. You can also familiarize yourself with additional materials on soft starters
Industrial soft starters for electric motors of various powers Introduction to the soft start principle
In order to smoothly start an asynchronous electric motor as efficiently as possible and at minimal cost by purchasing ready-made soft starters, you must first become familiar with the operating principle of such devices and circuits. Understanding the interaction of physical parameters will allow you to make the optimal choice of soft starter.
Using soft starters, it is possible to reduce the starting current to a value of three times the rated value (instead of a seven-fold overload)
For a smooth start of an asynchronous electric motor, it is necessary reduce starting current, which will have a positive effect on both the load on the electrical network and the dynamic overloads of the motor windings and drive mechanisms. They achieve a reduction in starting current by reducing the supply voltage of the electric motor. A reduced starting voltage is used in all three methods proposed above. For example, using an autotransformer, the user independently lowers the voltage at startup by turning the slider.
By lowering the voltage at the start, you can achieve a smooth start of the electric motor When using star-delta switching, the line voltage on the motor windings changes. Switching is carried out using contactors and a time relay designed for the time the electric motor starts. A detailed description of the soft start of an asynchronous electric motor with help is available on this resource at the specified link.
Star-delta switching circuit using contactors and time relays Theory of soft start
To understand the principle of a smooth start, it is necessary to understand the law of conservation of energy required to spin the rotor shaft of an electric motor. In a simplified way, we can consider the acceleration energy to be proportional to power and time, E = P*t, where P is power equal to current multiplied by voltage (P = U*I). Accordingly, E = U*I *t. Since in order to reduce the starting torque and reduce the load on the network it is necessary to reduce the starting current I, then while maintaining the level of energy spent, it is necessary to increase the acceleration time.
Increasing the acceleration time by reducing the starting current is only possible with a small load on the shaft. This is the main disadvantage of all UPPs
Therefore, for equipment with difficult starting conditions (large load on the shaft during startup), special electric motors with a wound rotor are used. You can learn about the properties of these engines from the corresponding section on this resource by following the link.
Motor with phasic rotor, required for heavy-duty equipment It is also necessary to take into account that during a soft start, increased heating of the windings and electronic power switches of the starting device occurs. To cool semiconductor switches, it is necessary to use massive radiators, which increase the cost of the device. Therefore, it is appropriate to use a soft starter for short-term acceleration of the engine with further bypassing of the switches with direct mains voltage. Similar mode ( bypass switching) makes the electronic soft start device for asynchronous motors more compact and cheaper, but limits the number of starts in a certain interval due to the required time for cooling the keys.
Block diagram of shunting power semiconductor switches (bypass) Main parameters and characteristics of the soft starter
Below in the text there will be diagrams of soft-start devices for study and self-manufacturing. For those who are not ready to soft-start an asynchronous electric motor with their own hands, relying on a finished product, information about the existing types of soft starters will be useful.
Example of an analog and digital soft starter, in modular design (mounted on a DIN rail) One of the main parameters when choosing a soft starter is the power of the electric motor being serviced, expressed in kilowatts. Equally important is the acceleration time and the ability to adjust the start interval. All existing soft starters have these characteristics. More advanced soft starters are universal and allow you to configure soft start parameters in a wide range of values relative to engine characteristics and process requirements.
An example of a universal softstarter Depending on the type of soft starter, they may contain various options that increase the functionality of the device and allow you to control the operation of the electric motor. For example, with the help of some soft starters it is possible not only to smoothly start an electric motor, but also to brake it. More advanced soft starters carry out engine protection from overloads and also allow you to adjust the rotor torque during start-up, stop and operation.
An example of differences in the technical characteristics of different soft starters from the same manufacturer Types of soft starters
According to the connection method, soft starters are divided into three types:
Do-it-yourself SCP
For self-production of a soft starter, the do-it-yourself soft start circuit for an asynchronous motor will depend on the capabilities and skills of the craftsman. Independent mitigation of starting overloads using an autotransformer is available to almost any user without special knowledge, but this method is inconvenient due to the need to manually adjust the start of the electric motor. On sale you can find inexpensive soft start devices that you will have to connect to the power tool yourself, without having in-depth knowledge of radio engineering. An example of work before and after the soft starter, as well as its connection, is shown in the video below:
For craftsmen with general knowledge of electrical engineering and practical electrical installation skills, a star-delta switching circuit is suitable for performing a smooth start with their own hands. These schemes, despite their advanced age, are widespread and are successfully used to this day due to their simplicity and reliability. Depending on the qualifications of the master, you can find SCP diagrams on the Internet for repeating with your own hands.
An example circuit of a relatively simple two-phase soft starter Modern soft starters have inside a complex electronic filling consisting of many electronic parts operating under the control of a microprocessor. Therefore, to manufacture a similar soft starter with your own hands According to the schemes available on the Internet, not only the skill of a radio amateur is required, but also the skills of programming microcontrollers.
A cheap angle grinder is quite easy to upgrade in order to significantly increase its service life so that its gearbox does not jam and the armature winding wires do not burn out. Typically, these problems are typical when starting an inexpensive grinder abruptly.
The entire modernization consists only of assembling a simple circuit in an external box.
The prototype design in the figure below was used to adjust the filament of lamps, that is, to operate a purely active load.
The basis of the design is the K1182PM1R microcircuit. It is highly specialized, and no matter how strange it sounds today, it is domestically produced. If necessary, the start time can be increased by installing a larger capacitance of capacitor C3. While this capacitor is charging, the electric motor smoothly increases speed to maximum. A 68 kOhm resistor is optimally chosen for our circuit. If you want to make a power regulator, then you need to replace resistance R1 with a variable one. Resistance is 100 kOhm or more.
A standard junction box is suitable as an excellent housing made of insulating material. A socket is screwed onto it and a cable with a plug is connected, which makes this design very similar to a DIY extension cord.
If you want, you can assemble a slightly more complex soft start circuit. It is typical for the XS–12 module. It is installed in power tools during factory production by many companies.
If you want to regulate the speed of the connected electric motor, then the design becomes a little more complicated: because a trimming resistor of 100 kOhm and an adjusting resistance of 50 kOhm are installed.
In order to save money, you can equip a standard angle grinder with a speed controller. Such a regulator for grinding housings of various electronic equipment is an indispensable tool in the arsenal of a radio amateur.