All budget options for an angle grinder have several drawbacks. Firstly, there is no soft starter system. This is a very important option. Surely all of you included this powerful power tool in the network, and at startup, watched how the glow of the bulb, which is also connected to this network, falls.
This phenomenon occurs due to the fact that powerful electric motors at the time of startup consume huge currents, due to which the mains voltage sags. This can damage the tool itself, especially Chinese-made ones with unreliable windings that may one day burn out during start-up.
That is, a soft start system will protect both the network and the tool. In addition, a powerful kickback or push occurs at the moment the tool is launched, and in the case of the introduction of a soft start system this, of course, will not happen.
Secondly, there is no speed regulator that will allow you to work with the tool for a long time without loading it.
The diagram below is from an industrial design:
It is introduced by the manufacturer in expensive appliances.
Not only an “angle grinder” can be connected to the circuit, but also, in principle, any devices. drill, milling and turning machines. But taking into account that the collector motor should be in the tool.
With induction motors, this will not work. There you need a frequency converter.
So, you need to make a printed circuit board and proceed with assembly.
You can download the board at the following link at the bottom of the article.
A dual operational amplifier LM358 is used as a regulating element, which controls a power triac using a transistor VT1.
So, the power link in this circuit is a powerful triac type BTA20-600.
During operation, the triac will warm up, so it is necessary to install a heat sink on it.
Video: RPM Angle Grinder Without Power Loss
So that there are no questions about the fact that the motor at start-up can consume currents that significantly exceed the maximum current of the triac, and the latter can simply burn out, remember that the circuit has a soft start, and starting currents can be ignored.
Surely everyone knows the phenomenon of self-induction. This effect is observed when the circuit to which the inductive load is connected is opened.
The same thing in this scheme. When the power supply to the motor stops abruptly, the self-induction current from it can burn the triac. And the snubber chain extinguishes self-induction.
The resistor in this circuit has a resistance of 47 to 68 ohms, and a power of 1 to 2 watts. Film capacitor at 400 V. In this embodiment, self-induction as a side effect.
Resistor R2 provides current suppression for the low-voltage control circuit.
To some extent, the circuit itself is both a load and a stabilizing link. Due to this, after the resistor, you can not stabilize the power. Although the network has the same circuits with an additional zener diode, it makes no sense to use it, since the voltage at the power terminals of the operational amplifier is within normal limits.
Possible replacement options for low power transistors can be seen in the following picture:
The printed circuit board that was mentioned earlier is only a board for the soft starter, and it does not have components for adjusting the speed. This is done on purpose, because in any case, the regulator must be removed using wires.
The controller is tuned using a 100 kΩ multi-turn trimming resistor.
And the main adjustment is already using the resistor R5. It is worth saying that a circuit of this kind will not allow adjustment from zero, only from 30 to 100%.
If you need a more powerful controller, then it can be assembled according to the following scheme:
This scheme allows you to adjust the power from almost zero, but for the "angle grinder" this does not make sense.
At first, the circuit must be checked for operability by connecting a 40-60 W 220 V bulb as a load.
If everything is in order, then after disconnecting from the network immediately you need to check the triac to the touch. it should be cold.
Next, the board connects to the "angle grinder" and starts up.
If everything works fine. the "angle grinder" starts up smoothly and the speed is regulated. then it’s time to start the tests under load.