How to make a smooth start of an angle grinder. DIY soft start for power tools

As a rule, budget angle grinders (angle grinders), popularly called grinders, do not have adjustable electronic modules in their design, which include an engine speed controller and a soft start. Over time, owners of such grinders begin to understand that their absence sharply reduces the functionality of the tool. In this case, you can modify the angle grinder by installing homemade devices on it.

When power is applied to the grinder motor, sudden increase in speed from zero to 10 thousand or more. Those who have worked with an angle grinder know well that it is sometimes difficult to hold it in your hands when starting, especially if a large diameter diamond disk is installed.

It is precisely because of such abrupt increases in engine speed that the mechanics of the device most often fail.

Also, during startup, a huge load is applied to the rotor and stator windings of the electric motor. Since the grinder has a commutator motor, it starts in mode short circuit: the electromagnetic field is already “trying” to turn the rotor, but it remains motionless for some time, since the force of inertia prevents this from happening. As a result, the engine coils sharply increase starting current. Despite the fact that the manufacturer has invested a certain safety margin for the coils, taking into account overloads at start, sooner or later the insulation cannot withstand it, which leads to an interturn short circuit.

In addition to starting problems, the lack of speed control causes some discomfort. For example, an angle grinder's speed regulator may come in handy for certain types of work:

• when grinding or polishing any surfaces;
• when installing large diameter tools;
• for cutting certain materials.

In addition, when stripping with wire brushes, there is a high probability of wire jamming in any gap. If the spindle speed was high, then the angle grinder may simply be torn out of your hands.

If you connect a power (speed) regulator with a module to the angle grinder soft start, then all the problems described above will disappear, the service life of the device will increase and the safety of its use will increase.

Homemade regulator circuit

One of the most popular schemes for smoothly starting an angle grinder engine with the ability to adjust the speed is presented below.

The basis of this regulator is the KR118PM1 microcircuit, as well as triacs, which are the power part of the device. Using this circuit, you can make a power regulator with your own hands, even without special knowledge in radio electronics. The main thing is that you know how to use a soldering iron.

This block works as follows.

1. After pressing the unit start button electric current begins to flow, first of all, to the chip (DA1).
2. The control capacitor begins to charge smoothly and after a while reaches the required voltage. Thanks to this, the opening of the thyristors in the microcircuit occurs with some delay. It depends on the time it takes to fully charge the capacitor.
3. Since the VS1 triac is controlled by the microcircuit’s theristors, it opens just as smoothly.

The processes described above occur in periods that become shorter each time. Therefore, the voltage supplied to the motor windings does not increase abruptly, but slowly, resulting in a smooth start of the angle grinder.

The time it takes for the electric motor to reach full speed depends on the capacitance of capacitor C2. The capacitor capacity of 47 uF allows you to start the engine in 2 seconds. When the angle grinder is turned off, capacitor C1 is discharged using 60 kOhm resistor R1 for 3 seconds, after which this electronic module is again ready to start.

If resistor R1 is replaced with a variable one, then you will get a speed controller that will allow you to reduce engine speed.

It is important that triac VS1 has the following characteristics:

• the minimum current for which it is designed must be 25 A;
• The triac must be designed for a maximum voltage of 400 V.

This circuit and the regulators made according to it have been repeatedly tested by many craftsmen on grinders with power up to 2000 Watt. It is worth noting that this device, thanks to the KR118PM1 microcircuit, it is designed for power up to 5000 W. So he has a considerable margin of safety.

Ideally, to solder the speed controller for an angle grinder, you will need to draw printed circuit board, etch the contacts with acid and then tin them, drill holes and solder the radio components. But everything can be made simpler:

• solder all the parts of the circuit by weight, that is, leg to leg;
• Attach a radiator to the triac (can be made from sheet aluminum).

A regulator soldered in this way will take up less space and can be easily placed in the body of the angle grinder.

How to connect a regulator to an angle grinder

To connect a homemade power regulator, no special knowledge is required, and any home handyman will cope with this task. The module is being installed into a break in one wire, through which power goes to the grinder. That is, one wire remains intact, and a regulator is soldered into the gap of the second.

In the same way, you can connect a factory power regulator that costs about 150 rubles, which is often purchased by craftsmen in China.

If there is very little space in the grinder, then the regulator can be placed outside the instrument, as shown in the following photo.

Also, the regulator can be placed in a socket and used to reduce the speed not only of an angle grinder, but also of other electrical appliances (drill, sharpener, milling or lathe on wood, etc.). This is done as follows.

The regulator is connected as described above - at a break in one of the wires of the power cable.

The following photos show what a finished outlet will look like, having a built-in speed controller for an angle grinder, which can also be used for other electrical appliances.

Instead of distribution box you can use any plastic case suitable size. You can also make the box yourself by gluing pieces of plastic together with a glue gun.

Soft start induction motor is always a difficult task because starting an induction motor requires a lot of current and torque, which can burn out the motor winding. Engineers are constantly proposing and implementing interesting technical solutions to overcome this problem, for example, using a switching circuit, autotransformer, etc.

Currently, similar methods are used in various industrial installations for the uninterrupted operation of electric motors.

The principle of operation of an induction electric motor is known from physics, the whole essence of which is to use the difference between the rotation frequencies of the magnetic fields of the stator and rotor. The magnetic field of the rotor, trying to catch up with the magnetic field of the stator, contributes to the excitation of a large starting current. The motor runs at full speed, and the torque value also increases along with the current. As a result, the winding of the unit may be damaged due to overheating.

Thus, it becomes necessary to install a soft starter. Soft starters for three-phase asynchronous motors allow you to protect units from the initial high current and torque that arise due to the sliding effect when operating an induction motor.

Advantages of using a circuit with a soft starter (SPD):

1. reduction of starting current;
2. reduction in energy costs;
3. increasing efficiency;
4. relatively low cost;
5. achieving maximum speed without damaging the unit.

How to start the engine smoothly?

There are five main soft starting methods.

• High torque can be created by adding an external resistance to the rotor circuit as shown in the figure.

• By including an automatic transformer in the circuit, the starting current and torque can be maintained by reducing the initial voltage. See the picture below.

• Direct starting is the simplest and cheapest method because the induction motor is connected directly to the power source.
• Connections using a special winding configuration - the method is applicable for motors intended for operation under normal conditions.

• Using SCP is the most advanced method of all the methods listed. Here, semiconductor devices such as thyristors or SCRs, which control the speed of an induction motor, successfully replace mechanical components.

Commutator motor speed controller

Most circuits for household appliances and electrical tools are based on a 220 V commutator motor. This demand is explained by its versatility. The units can be powered from DC or AC voltage. The advantage of the circuit is due to the provision of effective starting torque.

To achieve a smoother start and have the ability to adjust the rotation speed, speed controllers are used.

You can start an electric motor with your own hands, for example, in this way.

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?

The fact is that virtually any electric motor, at the moment of starting or stopping the rotor, experiences enormous loads. The more powerful the engine and the equipment it drives, the greater the costs of starting it.

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 level. 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. Maximum power, controlled by such a device of motors, is limited solely by the maximum thermal and electrical endurance of the triacs used. His versatility allows you to implement a lot of functions such as: dynamic brake, kickback or limit balancing magnetic field and current.

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 living 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 tool and household appliances for many years.

Everyone who has been using an angle grinder for more than one year has had it break. At first, each master tried to repair the sparkling grinder on his own, hoping that it would work after replacing the brushes. Usually, after such an attempt, the broken instrument remains lying on the shelf with burnt windings. And to replace it, a new grinder is bought.

Drills, screwdrivers, hammer drills, and milling cutters are necessarily equipped with a speed control. Some so-called calibration grinders are also equipped with a regulator, while ordinary grinders only have a power button.

Manufacturers do not deliberately complicate low-power grinders with additional circuits, because such a power tool should be cheap. It is clear, of course, that the service life of an inexpensive tool is always shorter than that of a more expensive professional one.

The simplest angle grinder can be upgraded so that its gearbox and armature winding wires will no longer be damaged. These troubles mainly occur during a sharp, in other words, shock start of the angle grinder.

All modernization is just a matter of assembly electronic circuit and securing it in the box. In a separate box, because there is very little space in the handle of the sander.

The tested, working diagram is posted below. It was originally intended to regulate the intensity of lamps, that is, to operate on an active load. Her main advantage? simplicity.

1. The highlight of the soft starter, the circuit diagram of which you see, is the K1182PM1R microcircuit. This microcircuit is highly specialized, domestically produced.
2. The acceleration time can be increased by choosing a larger capacitor C3. While this capacitor is charging, the electric motor accelerates to maximum speed.
3. There is no need to replace resistor R1 with a variable resistance. A 68 kOhm resistor is optimally selected for this circuit. With this setting, you can smoothly start the angle grinder with a power of 600 to 1500 W.
4. If you are going to assemble a power regulator, then you need to replace resistor R1 variable resistance. A resistance of 100 kOhm or more does not reduce the output voltage. By short-circuiting the pins of the microcircuit, you can completely turn off the connected grinder.
5. By inserting into power circuit semistor VS1 type TS-122-25, that is, at 25A, you can smoothly run almost any commercially available grinder with a power from 600 to 2700 W. And there remains a large reserve of power in case the grinder jams. To connect angle grinders with a power of up to 1500 W, imported semi-sectors BT139, BT140 are sufficient. These less powerful dongles are cheaper.

The semisistor in the above circuit does not open completely; it cuts off about 15V of the mains voltage. This voltage drop does not affect the operation of the grinder in any way. But when the semistor heats up, the speed of the connected instrument decreases greatly. This problem can be solved by installing a radiator.

This simple circuit has one more drawback - it is incompatible with the speed controller installed in the tool.

The assembled circuit must be hidden in a plastic box. A housing made of insulating material is important because you need to protect yourself from mains voltage. You can buy a junction box at an electrical supply store.

A socket is screwed to the box and a cable with a plug is connected, which makes this design look like an extension cord.

If experience allows and there is a desire, you can assemble a more complex soft start circuit. Below circuit diagram is standard for the XS–12 module. This module is installed in the power tool during factory production.

If you need to change the speed of the connected electric motor, then the circuit becomes more complicated: a 100 kOhm trimmer and a 50 kOhm adjustment resistor are installed. Or you can simply and roughly introduce a 470 kOhm variable between the 47 kOhm resistor and the diode.

It is advisable to connect a 1 MΩ resistor in parallel with capacitor C2 (it is not shown in the diagram below).

The supply voltage of the LM358 microcircuit ranges from 5 to 35V. The voltage in the power circuit does not exceed 25V. Therefore, you can do without an additional zener diode DZ.

Whatever soft start circuit you assemble, never turn on the tool connected to it under load. Any soft start can be burned if you rush. Wait until the grinder unwinds, and then work.

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Smooth start of the grinder the circuit, which is built on the KR1182PM1 microcircuit (phase control microcircuit), allows you to smoothly and safely start not only the angle grinder, but also any powerful power tool. The soft start circuit is quite simple and does not require any configuration.

It is possible to connect to the circuit without any changes any power tool that operates on a 220-volt power supply. Starting and turning off the electric motor of the angle grinder is carried out by an electric button on the power tool itself.

The soft start circuit for an angle grinder is shown in the figure below. Connector XP1 is connected to a 220 volt power outlet, and a grinder plug is plugged into XS1 (socket). It is possible to install and connect in parallel several sockets for power tools that operate alternately.

When you press the power tool button, the circuit closes and power supply is supplied to DA1 (phase regulator). In this case, capacitor C2 begins to charge, which leads to a smooth increase in voltage across it. The result of this is a delay in the opening of the thyristors (inside) the regulator, and with them the VSI triac. The delay decreases in each half-cycle of the mains voltage, as a result of which the voltage flowing through the electric motor of the angle grinder gradually increases and, consequently, its speed gradually increases.

With the capacitance value of capacitor C2 indicated in this diagram, a smooth increase in speed from the minimum amount to the nominal one takes about 2 seconds, which is quite enough to protect the power tool from dynamic and thermal shock, and at the same time ensure comfortable work with the angle grinder.

After turning off the electric motor of the angle grinder, capacitance C2 is discharged through resistance R1 and after 3 seconds the soft start circuit of the angle grinder is ready for a new start. By replacing the constant resistance R1 with a variable one, it is possible to smoothly change the power supplied to the electric motor. Resistance R2 reduces the current flowing through the control electrode of the triac, and capacitances C1 and SZ are radio components of a typical connection circuit for the KR1182PM1 microcircuit.
All resistances and capacitances are soldered directly to the terminals of the KR1182PM1 microcircuit.

It is possible to use any triac, with a maximum operating voltage of more than 400 V and with maximum current at least 25 amperes (depending on the power of the grinder). Due to the smooth start of the electric motor of the angle grinder, its starting current is not more than the rated current. The current reserve is needed only in case of jamming of the power tool.
The soft start circuit has been tested with tools with power up to 2.2 kW. Since the KR1182PM1 microcircuit guarantees the flow of current in the circuit of the electrode (control) triac VS1 during the entire active phase of the half-cycle, there are no restrictions on minimum power connected load.