How to make a soft starter for power tools with your own hands

Soft starter is widely used in the safe starting of electric motors. During the start of the motor there is an excess of the rated current (In) by a factor of 7. As a result of this process is a reduction in the operating period of the motor, namely, the stator windings and a significant load on the bearings. It is because of this reason and it is recommended to make a soft starter for power tools with their own hands, where it is not provided.

The stator of an electric motor is an inductor coil, therefore, there are resistances with an active and reactive component.

When electric current flows through the radio elements, which have a resistance with an active component, there are losses associated with the conversion of some power into heat type of energy. For example, the resistor and the stator windings of an electric motor have a resistance with an active component. It is easy to calculate the active resistance, because the current (I) and voltage (U) phases coincide. Using Ohm’s law for a circuit section, we can calculate the active resistance: R = U/I. It depends on the material, the cross-sectional area, the length, and its temperature.

If current flows through a reactive element type (with capacitive and inductive characteristics), then reactive R. An inductor coil, which has practically no active resistance (the calculation does not take into account the R of its windings). This type of R is created by the Electromotive Force (EMF) of self-induction, which is directly proportional to the inductance and frequency of I flowing through its turns: Xl = wL, where w is the angular frequency of alternating current (w = 2Pif, where f is the frequency of mains current) and L is the inductance (L = n n / Rm, n is the number of turns and Rm is the magnetic resistance).

When you turn on the electric motor, the starting current is 7 times the rated current (the current consumed during tool operation) and the stator windings are heated. If the stator coil is old, then an inter-turn fault can occur which will result in the failure of the power tool. To do this you need to use a soft starter for the power tool.

One method of reducing inrush current (In) is to switch the windings. Its implementation requires 2 types of relays (time and load) and the presence of three contactors.

A motor with star winding can only be started if two contactors are not closed at the same time. After a certain time interval, which is set by the time relay, one of the contactors drops out and another one which has not been activated before is energized. Thanks to this alternation of winding closing the inrush current is reduced. This method has a significant disadvantage, since when two contactors are closed simultaneously, short-circuit current occurs. However, with this method, the windings continue to heat up.

Another way to reduce inrush current is frequency control of motor starting. The principle of this approach is the frequency variation of the supply U. The main element of this type of soft starters is the frequency converter, which consists of the following elements:

Rectifier is made of powerful diodes or thyristors, which performs the role of network power supply U conversion into a direct pulsating current. Intermediate circuit smooths the pulsating direct current at the rectifier output, which is collected in the capacitors of high capacity. The inverter is required for direct conversion of the signal at the output of the intermediate circuit into a signal of the amplitude and frequency of the alternating component. Electronic control circuitry is needed to generate the signals needed to control the rectifier, inverter.

Soft starter

The standard angle grinder soft-start circuit consists of a triac, rectifier unit and a set of capacitors. In order to increase the operating frequency, resistors are used which carry current in one direction. The starter is protected by a compact filter. The nominal voltage of the models is kept low. However, in our version a lot depends on the power limit of the motor, which is installed in the angle grinder.

Making a soft starter

The circuit is assembled on a circuit board with dimensions of 45 x 35 mm and the board is arranged as compactly as possible so that it can be built into the housing of a tool that requires a soft starter. It is better to solder power wires directly to the board, but if the load is not much power, you can install terminal blocks, as I did. The board is made by LUT method, photos of the process are presented below.Download the circuit board:

connect, soft, starter, angle, grinder

It is desirable to tin the tracks before soldering the parts, this will improve their conductivity. The chip can be installed in a panel and then it can be easily removed from the board. First the resistors, diodes and small capacitors are soldered, and afterwards the largest components. After you finish assembling the board, you should always check it for proper installation, test the tracks, wash off any leftover flux.

Operating principle

Soft starter in the angle grinder factory design is located inside the body of the angle grinder and is connected by contacts with the on button and the stator windings of the drive. It takes a certain time for the angle grinder to reach nominal mode and the electromagnetic field created by the evenly increasing current and voltage through the stator windings forces the angle grinder drive armature to smoothly gain speed.

connect, soft, starter, angle, grinder

For angle grinders, where the manufacturer does not provide such a device, usually in very rare cases it is possible to hide it under the body of the angle grinder. The most common is in the form of a separate unit installed in a breakout circuit of a power cable. But the principle of operation does not change.

Model for lathes with a 15A triac

Soft start for angle grinder with 15A triacs is universal and is often found in models of low power. The difference between the devices is the low conductivity. The scheme (device) of soft start of angle grinder assumes application of transceivers of contact type, which work at frequency of 40 Hz. Many models use comparators. These elements are installed with filters. The nominal voltage of starters starts at 200 V.

Why do we need a soft starter?

It’s a very important thing to have a soft starter. When starting a powerful electric tool connected to the network, there is a surge of starting current, which is many times higher than the rated motor current, the voltage in the network drops. Although this throw is short-lived, it causes increased wear on the brushes, motor manifold, and all parts of the tool where it is flowing. It can be the reason of failure of the tool itself, especially Chinese, with unreliable windings, which can burn out at the most inopportune moment during the power on. There is also a big mechanical jump at startup, which leads to rapid wear of the reducer. This start-up extends the life of the power tool and increases the level of comfort at work.

Electronic unit allows to combine speed controller and soft starter into one whole. The electronic circuit is based on the principle of pulse-phase control with a triac opening phase that increases gradually. Angle grinders of various capacities and price categories can be equipped with such a unit.

Types of machines with electronic unit: see table for examples

Name Power, W Maximum speed of the disc, rpm Weight, kg Price,
Felisatti AG125/1000S 1000 11000 2,5 2649
Bosch GWS 850 CE 850 11000 1,9 5190
Makita SA5040C 1400 7800 2,4 9229
Makita PC5001C 1400 10000 5,1 43560
Flex LST 803 VR 1800 2400 6,5 91058

Angle grinders with electronic unit: popular in the photo

Felisatti AG125/1000S Bosch GWS 850 CE Makita SA5040C Makita PC5001C Flex LST 803 VR

What applications require a variable speed angle grinder

AEG 451410 WS13-125XE angle grinder in operation. 220Volt photo

The advantages of an angle grinder with speed control are evident in the following applications:

  • processing of plastic or thin sheets of steel and other metals, without allowing the material to melt or warp;
  • When machining stone or ceramic tiles it helps avoid chipping on the surfaces of these materials;
  • diamond machining tool at lower speeds will not overheat, and will last longer;
  • Grinding and polishing of some materials at optimal speed for quality results.

Checking Operation

Checking the device in operation. Watch out! All elements of the circuit are directly connected to 220 volts! Life Threatening! Finishing assembling the device.

In order not to confuse this device with a simple extension cord, it should be marked in any way. I did this with a self-adhesive price tag and duct tape.

Please watch the test video of this device. A visual representation of the changing behavior of the device on startup. Good luck with your business and worries.

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