Angle grinder speed controller

[mike_mike]

Hello, I have a Makita GA5030R (125 mm, 720 W) angle grinder and I wanted to built a speed controller. I built the attached schematic, but as I am lowering the speed, appears a buzzing sound from the controller.
I have the following questions:
1. It is safe to use a dimmer to control the rpm of an angle grinder ?
2. Can this buzzing sound be removed ?
3. Can you please recommend a better schematic ?

The angle grinder will be used for grinding wood, using an Velcro adapter and 125 mm diameter sand paper.

Thank you in advance.

[Zero999]

• This circuit is phase controller, which is used in lamp dimmers. Assuming it's a universal motor, then yes, it can be used to vary the speed.
• No, the phase controller produces a spiky wavefrom which will cause the motor to vibrate.
• If it's a universal motor, the best way to control the speed is to rectify the AC to DC and use PWM.

[mike_mike]

I found that the motor it's an universal AC motor and it has a soft start circuit.
Is there any way to control the RPM of this type of motor, without eliminating the soft start circuit ?

[Zero999]

I found that the motor it's an universal AC motor and it has a soft start circuit.
Is there any way to control the RPM of this type of motor, without eliminating the soft start circuit ?

We don't know what sort of soft start circuit it uses, which could have been contributing to, or causing the buzzing noise.

The speed controller should be, after the soft start circuit.

[Doctorandus_P]

The schematics you posted are for a standard phase control light dimmer.
"Improved" versions have some extra filtering to compensate for the inductive part of the motor, and excessive noise generated by the brushes. These also used to be used for for example halogen lights with old style heavy iron transformers.

The soft start circuit will also be a very similar and simple circuit.
Circuits like that are on the border between working just good enough and not working reliably.

How two of those simple and cheaped out circuits influence each other is beyond simple prediction.

Universal motors can indeed also be run from a DC voltage.
Beware though that those motors have an inductive part in their coils, and a DC voltage / current mostly ignores that, and therefore the maximum DC voltage is less then the AC voltage.
Also, Fully rectified mains voltage is about 40% higher than the AC RMS voltage.

Maybe you can modify your "soft starter" to also do the speed regulation.

[themadhippy]

aside from the electronics, running the motor slower will also reduce motor cooling ,often these tools are already running at the minimum cooling required, slowing things down will only make things worse.

[Jwillis]

Theirs really no advantage to control the speed on an angle grinder since it requires the torque of the spinning wheel to do the work. And since the wheels are impregnated with diamond or ceramic heat is not an issue . Unlike a drill where the various drill sizes require a different speed to be the most effective. High speed for small sizes to low speed for large sizes. Same thing applies to machining and saws because your don't want to produce heat which can temper the metal causing the machine bit ,drill or saw to dull and stop cutting. With the addition of liquid cooling cutting becomes more effective .
There are different wheels designed for different application for angle grinders ranging from cutoff , polishing , general grinding , brushing and sanding . Why reinvent the wheel.

[shved]

I have done a similar circuit, works fine.