[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.
http://www.cnczone.com/forums/genera...ontroller.html
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.

[Moses_C]

Cheaper to just buy a better one

Yeah, i've considered that already long ago, to be honest...i was thinking of switching over to a small lawnmower instead of a handheld trimmer, as it aches my arms after each usage apart frm the numbing vibration..the only thing is, mowers are not cheap in my country and its actually sth like a specialty product...most people here like to do away with their lawn and tile it, to avoid the hassle of up keeping it. But love my little lawn... :-D

[underthetire]

Got a smart phone? There are some handy free apps that help balancing quite well. I have one I use to help balance my model jets.

[Moses_C]

Got a smart phone? There are some handy free apps that help balancing quite well. I have one I use to help balance my model jets.

Oh i see..however i dont have one. Thanks.

[Moses_C]

Ok guys, this it is the light dimmer i've bought and put togerther. I've attached some pictures of it...it is working well with a light bulb.

Now, i havent tested it with my trimmer yet...so, would like to know b4 hand, will it burn the trimmer or cause some serious problem with this type of dimmer? From what i do know is, if an electric motor runs too slow, it may cause the thing to burn..or sth bad may happen.

The dimmer is rated to handle 500W...while my trimmer is just 300W, running at 10000rpm at normal usage. As mentioned at the start of thread, i plan to reduce the speed to at least half, or to a level it wont generate numbing vibration.

Any suggestion or comment on this would be great...simply bcoz i dont wanna fry it so soon! (chair) ...that's all.

Thanks a lot again!

[Al_The_Man]

Probably the worst case scenario is the Triac in the dimmer would short or open, if so, either way it would not harm the trimmer?
Al.

[Moses_C]

Probably the worst case scenario is the Triac in the dimmer would short or open, if so, either way it would not harm the trimmer?
Al.

Hi Al! Good to hear from you again!

Thanks for telling...ok, i'll try later then.. ^__^

I just managed to resize and upload the photos...sorry bout that. There are some of the internal pictures of the dimmer assembly..hope it is somewhat useful to you and others. :-P

So, correct me if i'm wrong...when i use it, i should not turn it all the way down to zero...that is almost off, right? Also, how slow actually SHOULD i go? Is there a line i shouldnt cross??

Tq Al.

[keebler303]

If the motor has a fan on it, that is what is relied on to cool the motor. If you go too slow, the motor may still be drawing a large current, but the speed will be lower so the cooling will be less efficient and the motor will get hotter. I would say just monitor the motor and as long as it's not getting too hot you should be fine.

Matt

[Moses_C]

If the motor has a fan on it, that is what is relied on to cool the motor. If you go too slow, the motor may still be drawing a large current, but the speed will be lower so the cooling will be less efficient and the motor will get hotter. I would say just monitor the motor and as long as it's not getting too hot you should be fine.

Matt

Yep, thanks Matt...i did just that, and managed to reduce the speed to such a comfortable level. I'm so happy everything work out so perfectly...with no additional heat nor heated up odour coming from the motor! Yes, even after extended usage time...just so glad all the effort invested was not wasted.

Thanks to you Matt...and everyone who contributed to resolving my problem.

THANK YOU! :cheers: