PWM control for B&D Grass Trimmer

[Moses_C]

Hi everyone,

I new here, and i realized there are lot of people here with very good knowledge of electronics and can help solve many difficult problems. Hope mine can be solved as well

You see, I have a Black and Decker Grass Trimmer GL300. It is operating at 10000 rpm...rated at 300W...takes 240V. Because of its extremely high revolution, it produces far too much vibration for me to hand hold it and is causing my hand to go numb and painful. Furthermore, my small patch of lawn has a very soft and thin kind of grass. My plan is to use PWM to cut down its speed to maybe 3000rpm, or better still have a variable control to adjust the speed of my trimmer according to my need.

I am a beginner and have a bit of electronics knowledge but certainly not enough to build my own PWM for grass trimmer. So, i hope some of you can point me to the right site for circuit or explain whether its possible or not. I tried searching but cant find any useful related topic...Is there a way to build a PWM circuit for this kind of equipment, or at least is there a product i can buy online to solve this problem?

Thank you ahead for any help, positive or negative suggestions, etc. All are welcomed!

[RomanLini]

At 300W 240V there are a few cheap commercial "universal motor speed controllers" you can buy. If I remember right Harbour Freight had a motor speed controller for about $39 (although that might be a 120v USA version).

As your motor is fairly small and speed control is not critical (and you don't need big torque for a whippersnipper) these cheap speed controllers should provide a ready made plug-in solution for you.

[keebler303]

Easiest fix: Light dimmer switch.

Matt

[Al_The_Man]

Recent posting.
AC brushed motor, speed controller?
See last few posts for circuit.
Al.

[Moses_C]

Hi all,

Appreciate all your help! :-)

The Harbour Freight product looks great and is cheap too...however, it is at 120V...so not compatible...also, shipping it to Malaysia frm the USA would create custom and taxes problem..so, i think had to drop this option :-(

On another note, soldering and constructing my own circuitry would be my last resort method...many thanks for the suggested link and all! Learn quite a lot from there...thanks!

So, the simplest method, if i'm not wrong, is to use a light dimmer switch? Is it really going to work? If it is going to work well with the 300 Watts Trimmer, then what kind of dimmer should i start looking for...and how to assemble and integrate it together with the trimmer in the most convenient and effective way?

I really hope to find the simplest, cheapest, safest and most effective way to control its speed...and that's just about it...easy to use and simply effective.

So thanks again for all the recommendations guys! ;-D

[keebler303]

I'd get a rotary light dimmer, put it in an outlet box with a male cord end and a female cord end sticking out. Basically so it is like an extension cord with the dimmer switch in the middle. It should be wired just like it would be for a light in a house. If you don't know what you are doing a local electrician could surely help. If you have or could find a variac, or variable transformer, you could use that too.

Matt

[underthetire]

Got a old variable speed drill around?

[vger]

Moses,

Are you using 2 strings coming out of the trimmer head? If not it will be out of balance and will vibrate at about 166 Hz at 10,000 rpm.

Either way, the source of the vibration is probably because of out of balance issue either in the motor, trimmer head, or strings. Does it vibrate badly with no string in the head? If so, I could tell you how to balance it.

[gateman]

Cheaper to just buy a better one

[Moses_C]

I'd get a rotary light dimmer, put it in an outlet box with a male cord end and a female cord end sticking out. Basically so it is like an extension cord with the dimmer switch in the middle. It should be wired just like it would be for a light in a house. If you don't know what you are doing a local electrician could surely help. If you have or could find a variac, or variable transformer, you could use that too.

Matt

Thanks Matt...i just bought a dimmer with an on/off switch rated at 500W. Now, i just have to rig it up correctly..i think i'm able to do it fine.

Just only one concern, will it cause any damage to my trimmer motor, like when i play around with the rotary knob? Is it possible to burn the motor or sth like that with that dimmer...wanna know just to avoid further complication. Tq.

[Moses_C]

Got a old variable speed drill around?

Thanks for reply...

I do have one but its still in use. :-P I planned to go for the dimmer option. If got any positive or negative suggestion or comment on this, pls dont hesitate to tell me. Tq.

[Moses_C]

Moses,

Are you using 2 strings coming out of the trimmer head? If not it will be out of balance and will vibrate at about 166 Hz at 10,000 rpm.

Either way, the source of the vibration is probably because of out of balance issue either in the motor, trimmer head, or strings. Does it vibrate badly with no string in the head? If so, I could tell you how to balance it.

Ya, i'm using two strings..but have not tried removing the spool and run it...i do regularly lengthen it when it starts to shorten.

The vibration is rather bad i think...unlike any power tools i've used so far. :-P
I do notice the vibration reduced drastically when the trimmer hits a patch of grass in a rough way that the rpm drops to a much slower rate. So, that's when i thought of cutting down its rpm thru PWM or sth similar.

Is there a way to balance it as you say? Hope you wont mind sharing the various methods of doing it, as i've never thought of that. Tq.