Marry

How to make a soft start on the angle grinder. Soft start and speed control of the angle grinder. Connecting the soft start function

Soft start for any power tool is very important for the following reasons. Firstly, it helps to protect the electrical device from breakdowns, which contributes to fewer trips to the repairmen, which means almost no downtime and increased productivity. Secondly, having a soft start for an electric motor saves you money that could be spent on paying repairmen or buying a new tool.

This article will consider the manufacture of a soft start of an electric motor with your own hands using the example of a grinder or, in other words, an angle grinder.

Why you need a soft starter

Due to some design features, starting the grinder leads to the appearance of dynamic loads to the device. Since the mass of the disk, with which useful work is carried out, is quite high, powerful inertial forces act on the collector electric motor and gearbox of the apparatus, which leads to the following negative factors:

During the start, which is especially abrupt, the inertia forces have a very strong effect on the body of the device, which can lead to injury: you just don't hold the tool and release it. Therefore, when starting the electric motor of the angle grinder, always hold it with both hands.
During the start, the motor is overloaded due to the high voltage applied. What does this lead to? First of all, the motor winding suffers and accelerated brush wear occurs, which will not happen if you make a soft start unit. Otherwise, be prepared for the fact that one day, not very fine, something will happen in the motor. short circuit caused by complete wear of the brushes. This, in turn, will force you to shell out for repairs or buy a new grinder.
The rapidly applied torque to the gearbox during start-up causes accelerated wear on the gears in the gearbox of your grinder.
Also keep in mind that a sharp start of the grinder can destroy the disc, the fragments of which can cause you serious harm, so never work without a casing for protection.

In order to make it more clear to you which elements of the grinder are most affected by a sharp start, look at the diagram below.

Of course, some companies that produce grinders, even at the factory, complete their devices with a soft start unit. However, soft start equipment is an unaffordable luxury for grinders included in the budget price segment, so if you do not want to buy an expensive power tool, then you are in danger of running into the problems that were described above.

Nevertheless, there is a way out and it is quite simple: make a device for a soft start using one of the possible schemes with your own hands. If there is free space in the case of your device, then you can use ready-made soft starter and put it in the grinder.

We make a soft start for the grinder with our own hands

One of the most commonly used schemes for the manufacture of a starting device is based on chip KR118PM1 and triacs that make up the power part. According to this scheme, you can make a soft start unit without having specialized skills and without deep knowledge in electrical engineering. The only important thing is that you know how to solder.

Graphically, this diagram looks like this:

Self made device you can connect to absolutely any power tool, designed for a voltage of two hundred and twenty volts. Soft starter, created on the basis of this scheme, does not have to be turned on with a separate button, but can be connected to the standard key of the grinder. If your grinder has free space inside the case, then you can install the unit in it or make a separate case for it and connect it to the power tool through a break in the power cable.

The best option for connecting a soft starter and your grinder will be as follows: at the input of the block (connector XS1), you supply voltage from a power supply with a voltage of two hundred and twenty volts. A plug from a grinder is connected to the block output (XP1 connector).

The principle of operation of the soft starter

After you press the power button on the grinder, voltage appears in the circuit, which is initially directed to the microcircuit, which is indicated as DA1 in the diagram above. The capacitor, which regulates the voltage value, gradually increases it until it reaches the operating value. Due to the operation of the capacitor, the thyristors in the microcircuit open with some delay and slowly transfer voltage to the power part in the triacs VS1.
The process described above occurs in periods that become shorter and shorter if you count them from the moment of launch. As a result, the voltage supplied to the grinder increases slowly, and not abruptly, which causes the smooth start of the electric motor.
The time during which the engine is gaining operating speed, depends on the capacitance of the capacitor used C2. As a rule, a capacity equal to forty-seven microfarads is enough for the grinder to start smoothly in two seconds. Usually this period of time is enough to remove the overload from the motor and gearbox.
After you finish your work and turn off your device, resistor R1 discharges capacitor C1 with its resistance. If the value of the resistor R1 is sixty-eight kilo-ohms, then the discharge takes only three seconds. You can then use the soft starter again, as it will be ready to start the grinder again.

If you want to upgrade the unit to a device, regulating the speed of the electric motor, then instead of the constant resistor R1, put a variable. In this case, you can adjust its resistance, and therefore influence the engine speed.

Triac VS1 in your block must meet the following specifications:

The current strength, the minimum passed by him, is twenty-five amperes.
The maximum voltage for which it is designed is four hundred volts.

This circuit, tested by many masters, was tested on a grinder with a power equal to two kilowatts, and has a power margin of up to five kilowatts, which is made possible thanks to the KR118PM1 microcircuit.

Soft starting an induction motor is always a difficult task, because starting an induction motor requires a lot of current and torque, which can burn the motor winding. Engineers are constantly suggesting and implementing interesting technical solutions to overcome this problem, such as the use of a switching circuit, an autotransformer, etc.

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

From physics, the principle of operation of an induction electric motor is known, the whole essence of which is to use the difference between the frequencies of rotation 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, while the torque value also increases following 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 help to protect the units from the initial high current and torque resulting from the slip effect of the induction motor.

Advantageous features of using a circuit with a soft starter (SCD):

reduction of starting current;
reduction of energy costs;
improving efficiency;
relatively low cost;
achieving maximum speed without damage to the unit.

How to smoothly start the engine?

There are five basic soft start methods.

High torque can be created by adding 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 easiest and cheapest way because the induction motor is connected directly to the power supply.
Connections on a special winding configuration - the method is applicable to motors intended for operation under normal conditions.

The use of SCP is the most advanced of all the methods listed. Here, semiconductor devices such as thyristors or SCRs that regulate the speed of an induction motor successfully replace mechanical components.

Collector motor speed controller

Most of the circuits of household appliances and electric tools were created on the basis of a 220 V collector electric motor. This demand is due to its versatility. The units can be powered by direct or alternating voltage. The advantage of the circuit is due to the provision of an effective starting torque.

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

Starting an electric motor with your own hands can be done, for example, in this way.

Who wants to strain, spend their money and time on the re-equipment of devices and mechanisms that already work perfectly? As practice shows - many. Although not everyone in life encounters industrial equipment equipped with powerful electric motors, they constantly meet, albeit not so voracious and powerful, electric motors in everyday life. Well, everyone used the elevator, for sure.

Are motors and loads a problem?

The fact is that virtually any electric motors, at the moment of starting or stopping the rotor, experience huge loads. The more powerful the engine and the equipment it drives, the greater the cost of running it.

Probably, the most significant load that falls 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 a few seconds of operation, when the electric motor reaches its nominal speed, the current consumed by it will also return to normal levels. To ensure the necessary power supply have to increase the capacity of electrical equipment and conductive lines which drives up their prices.

When a powerful electric motor is started, due to its high consumption, a “drawdown” of the supply voltage occurs, which can lead to malfunctions or failure of equipment powered with it from the same line. In addition, the service life of power supply equipment is reduced.

In the event of emergency situations that caused the engine to burn out or its severe overheating, properties of transformer steel may change so much so that after repair the engine will lose up to thirty percent of power. Under such circumstances, it is no longer suitable for further operation and requires replacement, which is also not cheap.

What is a soft start for?

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:

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 connection 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. It's not healthy at all, because may cause damage to its moving parts: gear systems, drive belts, conveyor belts, or just imagine yourself riding in a twitching elevator. In the case of pumps and fans, this is the risk of deformation and destruction of turbines and blades;
do not forget about products that may be on the production line. They may fall, crumble or break due to such a jerk;
Well, and probably the last of the points that deserve 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 tangible 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 layman. First of all, this applies to power tools.

The specifics of the use of such units as electric jigsaws, drills, grinders and the like involve multiple start and stop cycles within a relatively short period of time. This mode of operation, to the same extent, affects their durability and energy consumption, as well as their industrial counterparts. With all this, one should not forget that soft start systems unable to control engine speed or reverse their direction. It is also impossible to increase the starting torque or reduce the current below what is required to start the rotation of the motor rotor.

Video: Soft start, adjustment and protection of the collector. 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 nominal speed is set, it automatically switches to delta switching. This kind of start allows you to achieve a current almost a third lower than with direct start of the electric motor.

However, this method is not suitable for mechanisms with a small rotational inertia. These include, for example, fans and small pumps due to the small size and weight of their turbines. At the time of the transition from the "star" to the "delta" configuration, they will sharply reduce speed or stop altogether. As a result, after switching, the electric motor essentially restarts. That is, in the end, you will not only achieve savings on the engine resource, but, most likely, you will get an overrun of electricity.

Video: Connecting a three-phase asynchronous motor with a star or delta

Electronic motor soft starter

Soft start of the engine can be performed using triacs included in the control circuit. There are three schemes for such inclusion: single-phase, two-phase and three-phase. Each of them differs in its functionality and final cost, respectively.

These 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 contributes to an increase in the service life of electric motors. Due to the fact that the engine start is controlled by reducing the voltage, the acceleration of the rotor is carried out smoothly, and not abruptly, like in other schemes.

In general, several key tasks are assigned to engine soft start systems:

the main one - lowering the starting current to three or four nominal;
reduction of the motor supply voltage, in the presence of appropriate capacities and wiring;
improvement of starting and braking parameters;
emergency protection of the network against current overloads.

Single-phase starting circuit

This scheme is designed to start electric motors with a power of not more than eleven kilowatts. This option is used if it is necessary to soften the impact at start-up, and braking, soft start and lowering the starting current do not matter. First of all, because of the impossibility of organizing the latter in such a scheme. But due to the cheaper production of semiconductors, including triacs, they are discontinued and rarely found;

Two-phase starting circuit

Such a scheme is designed to regulate and start engines 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 asymmetric power supply of the phases, which can lead to overheating;

Three-phase starting circuit

This circuit is the most reliable and versatile soft start system for electric motors. The maximum power of the motors controlled by such a device is limited exclusively 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, flyback or magnetic field and current limiting balancing.

An important element of the last of the mentioned circuits is the bypass contactor, which was mentioned earlier. He allows to ensure the correct thermal regime of the soft start system of the electric motor, after the engine has reached its normal operating speed, preventing it from overheating.

The soft starters of electric motors that exist today, in addition to the above properties, are designed for their joint operation with various controllers and automation systems. They have the ability to turn on at the command of the operator or the global control system. Under such circumstances, at the time of switching on the loads, interference may occur that can lead to malfunctions in the automation, and therefore, it is worth taking care of protection systems. The use of soft start circuits can significantly reduce their impact.

DIY soft start

Most of the systems listed above are actually inapplicable in domestic conditions. First of all, for the reason that at home we rarely use three-phase asynchronous motors. But collector single-phase motors - more than enough.

There are many schemes for the smooth start of engines. The choice of a specific one depends solely on you, but in principle, having certain knowledge of radio engineering, skillful hands and desire, it is quite you can assemble a decent homemade starter which will extend the life of your power tools and home appliances for years to come.

The design features of some tools, such as angle grinders, lead to a high impact on the motor of the dynamic load device. To eliminate uneven loads on the electrical appliance and its components, it is recommended to purchase or make a soft starter (SCD) with your own hands.

general information

In power tools, in which the working part is represented by a disk that rotates at high speed, inertia forces act on the gearbox axis at the beginning of their work. This impact entails the following negative aspects:

The inertial jerk created as a result of the load on the axle during a sharp start can snatch the unit out of hand, especially if disks that are large in diameter and mass are used;

Important! Due to such inertial jerks, when working with steel and diamond discs, it is necessary to hold the tool with both hands and be prepared to hold it, otherwise you can be injured when the unit breaks down.

The sudden supply of operating voltage to the engine creates a large current overload, which occurs after the unit has reached the minimum speed. This leads to overheating of the motor windings and rapid wear of the brushes. Frequent switching on and off of the tool can lead to a short circuit, since there is a high probability of melting the insulating layer of the windings;
A sharp set of revolutions of an angle grinder or a circular saw due to a large torque leads to rapid wear of the gearbox gear. Sometimes it is possible to jam the gearbox or even break off its teeth;
The overload that the working disk perceives during a sharp start can lead to its destruction. The presence of a protective casing on such power tools is mandatory.

Important! When starting the grinder, the open section of the casing should be on the opposite side from the person in order to protect it from flying fragments in case of possible destruction of the working disk.

To reduce the detrimental effects of a sharp and dynamic start on a power tool, manufacturers produce models with a built-in soft start and speed control.

For information. Such devices are built into units from the middle and high price categories.

A soft starter and speed control are missing from many of the power tools that most households have. If you purchase powerful equipment (the diameter of the working disk is more than 20 cm) without a soft starter, a sharp start of the engine will lead to rapid wear of the mechanics and electrical parts, and it is also difficult to hold such a unit in your hands when turned on. Installing a soft starter is the only way out.

There are many models of ready-made soft starters and reverse regulators on the market for accessories for power tools.

A ready-made soft starter for power tools can be mounted both inside the case if there is free space, or connected to a break in the power cable. However, you can not purchase a finished product, but make it yourself, since the scheme of this device is quite simple.

Self-manufacturing SCP

To manufacture the most popular soft starter for power tools based on the KR1182PM1R board, you will need the following tools and materials:

soldering iron with solder;
phase adjustment chip KR1182PM1R;
resistors;
capacitors;
triacs;
other auxiliary elements.

In the device, which is obtained according to the scheme above, the control takes place through the KR1182PM1R board, and the triacs act as a power unit.

The advantages of this assembly of soft starters are the following features:

ease of manufacture;
no need for additional settings after the assembly of the soft starter;
the soft starter can be mounted in any type and model of a power tool, which is designed for an alternating voltage of 220 V;
no requirements for the removal of a separate power button - the modified unit is activated by a standard key;
the possibility of installing such a unit inside the equipment or in the break of the power cable with its own housing;
any home craftsman who has the basics of soldering and reading microcircuits can make such a device.

Recommendation. The most practical option for connecting the soft starter is to connect it to an outlet that serves as a power source for a power tool. To do this, you need to connect a power outlet to the device output (XS1 socket on the diagram), and supply 220V power to the input (XP1 socket on the diagram).

The principle of operation of the SCP

The principle of operation of such a soft starter installed in a grinder consists of the following processes:

After pressing the start key on the grinder, voltage is applied to the microcircuit;
On the control capacitor (C2), the process of a smooth increase in electrical voltage occurs: as this element is charged, it reaches operating indicators;
The thyristors that are part of the control board open with a delay, which depends on the time the capacitor is fully charged;
The triac (VS1) is controlled by thyristors and opens with the same delay;
In each half of the period of alternating voltage, such a pause decreases, which leads to its smooth supply to the input of the working unit;
After turning off the grinder, the capacitor element is discharged by the resistance of the resistor.

It is the processes described above that determine the smooth start of the grinder, which makes it possible to eliminate the inertial shock for the gearbox due to a gradual increase in the speed of the disk.

The time for which the power tool will pick up the working number of revolutions is determined only by the capacitance of the control capacitor. If, for example, the capacitor element has a capacitance of 47 microfarads, then a soft start will be provided in 2-3 seconds. This time is enough to start using the tool comfortably, and he himself was not subjected to shock loads.

If the resistor has a resistance equal to 68 kOhm, then the capacitor discharge time will be approximately 3 seconds. After this time period, the soft starter is completely ready for the next cycle of starting the power tool.

On a note. This circuit can be subjected to a slight refinement, which will add to the soft starter the function of a speed controller. To do this, you need to change the usual resistor (R1) to a variable version. By controlling the resistance, you can adjust the power of the electric motor by changing the number of revolutions.

Other elements of the scheme are intended for the following:

resistor (R2) is responsible for controlling the amount of electric current that flows through the input of the triac;
capacitor (C1) is one of the additional components of the control system of the KR1182PM1R board, used in a typical version of the switching circuit.

Tips for assembling the structure and choosing materials:

Ease of installation and compactness of the future product can be ensured by soldering condensing elements and resistors directly to the legs of the control board;
The triac must be selected with a minimum throughput electric current of 25 A and an electric voltage of not more than 400 V. The magnitude of the electric current will completely depend on the power indicator of the engine of the power tool;
Due to the soft start of the unit, the current will not exceed the nominal values that are set by the manufacturer. In some cases, for example, jamming of the working disk of the grinder, an additional supply of electric current may be required, respectively, it is better to choose a triac with a working current that is equal to twice the nominal value of the tool;
The power of an angle grinder or other type of tool when working with a soft starter according to the KR1182PM1R scheme should not exceed 5,000 W. This condition is due to the peculiarities of the board.

There are also other soft start schemes for power tools and a variety of motors, which are strikingly different from each other in all respects: from the installation method and appearance to the connection method and composite components.

Note. The above scheme is the simplest and is used everywhere, as it has proven its performance and reliability.

Soft starter for power tools - save money on repairs and fully protect the main components of the tool. Everyone has a choice: buy SCP or do it yourself. If you have some knowledge in electrical engineering and soldering of radio components, then it is recommended to perform self-assembly, as it is reliable and simple. Otherwise, you should purchase a ready-made soft starter for power tools at any specialized store or on the radio market.

Video

The disadvantage of small cheap angle grinders is the lack of a soft start and speed control. Everyone who turned on a powerful electrical appliance in the network noticed how the brightness of the network lighting drops at that moment. This is due to the fact that powerful electrical appliances at the time of startup consume a huge current, respectively, the voltage in the network sags. The instrument itself can fail, especially Chinese ones with unreliable windings.

The soft start system will protect both the network and the tool. Also, there will be no strong kickback (shock) at the moment of switching on. And the speed controller will allow you to work for a long time without overloading the tool.

The presented scheme is copied from an industrial design, installed on expensive devices. It can be used not only for a grinder, but also for a drill, milling machine, etc., where there is a collector motor. For asynchronous motors, the circuit is not suitable; a frequency converter is required there.

First, I drew a printed circuit board for a soft starter system, without components for speed control. This is done on purpose, because. In any case, the regulator must be wired. Having a diagram, everyone will figure out what to connect where.

In the circuit, the regulating element is the dual operational amplifier LM358, which controls the power triac BTA20-600 through the transistor VD1. I did not get it in the store and put BTA28 (more powerful). For a tool up to 1kW, any triac with a voltage of more than 600V and a current of 10-12A is suitable. Because the circuit has a soft start, then the starting currents will not burn such a triac. During operation, the triac heats up and should be installed on a radiator.

The phenomenon of self-induction is known, which is observed when the circuit is opened with an inductive load. In our circuit, the R1-C1 circuit extinguishes self-induction when the grinder is turned off and protects the triac from breakdown. R1 from 47 to 68 ohms, power 1-2W. Film Capacitor 400V.

Resistor R2 provides current limiting for the low voltage part of the control circuit. This part itself is both a load and, to some extent, a stabilizing link. Due to this, after the resistor, you can not stabilize the power. Although there is a variant of the same circuit with an additional zener diode. I did not install it, because. the supply voltage of the microcircuit, so, is within the normal range.

Possible replacements for low-power transistors are indicated below the diagram.

The adjustment of the regulator is done using a multi-turn resistor R14, and the main adjustment is done with a resistor R5. The circuit does not give power adjustment from 0, but only from 30 to 100%. If you need a simpler powerful regulator from 0, then you can assemble a version that has been proven over the years. True, for an angle grinder, getting the minimum power is pointless.