I am trying to modify a Harbor Freight 20V cordless string trimmer ($55) for a special use. My only problem is that the 8000 rpm motor is way too fast and I need to slow it down to maybe 1700 rpm. I got a 10-50V 40A DC Motor Speed Control PWM on Ebay and connected it to 1 and 4. It barely turned the motor and the lead wires got hot. I tried 8 alkaline D cell in series to 1 & 4 pins but It turned very slowly as well. I'm not sure what pins 2 and 3 do, battery protection perhaps. Any ideas?
BLDC? It's the no load speed spinning a string.
Martaine2005
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BrushLess Direct Current motor.
Brushless DC electric motor - Wikipedia
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Assuming it is this Bauer tool: the website nowhere explicitly states "brushless". Since it does state "brushless" for other tools and also elaborates (on another page) on the advantages of brushless motors, there is an indication that the motor is not brushless. Which does not mean that the motor isn't brushless anyway.
Anyway:
The four pins on the battery seem to be for power ("+" and "-") and additional sensing and/or charge balancing. Which leads to the conclusion that the output is pure DC and the motor and its associated control circuitry expect a pure DC power supply. The circuitry will malfunction with a pwm signal as supplied from the speed controller you used. You will also not be able to connect any other type of speed controller to these power lines.
To control the motor rpm electronically, you will have to open the case of the trimmer and connect directly to the motor. Either with pwm controller or with a BLDC controller, depending on the type of motor.
Another option: keep the trimmer as it is and add a mechanical gear to reduce rpm. This will, incidentally, increase torque. I assume the original motor directly drives the wire spool and does not have much punch. Some additional torque may come in handy.
Anyway:
The four pins on the battery seem to be for power ("+" and "-") and additional sensing and/or charge balancing. Which leads to the conclusion that the output is pure DC and the motor and its associated control circuitry expect a pure DC power supply. The circuitry will malfunction with a pwm signal as supplied from the speed controller you used. You will also not be able to connect any other type of speed controller to these power lines.
To control the motor rpm electronically, you will have to open the case of the trimmer and connect directly to the motor. Either with pwm controller or with a BLDC controller, depending on the type of motor.
Another option: keep the trimmer as it is and add a mechanical gear to reduce rpm. This will, incidentally, increase torque. I assume the original motor directly drives the wire spool and does not have much punch. Some additional torque may come in handy.
73's de Edd
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You say . . . .I am trying to modify a Harbor Freight 20V cordless string trimmer for a special use.
Specifically, what is that NEW use ? . . . . . .since we have no idea whether its powering a mini go kart or an airborne drone ?
Or is that " string ", being coiled onto a drum as it is reeling in a kite ?
Specifically, what is that NEW use ? . . . . . .since we have no idea whether its powering a mini go kart or an airborne drone ?
Or is that " string ", being coiled onto a drum as it is reeling in a kite ?
I am needing it to selectively remove weeds from my lawn based on my published patent. I built a machine to do the yard but it leaves a border area and I'd like to modify this trimmer to do the same thing for this border area.
Harold's post is informative.
GrandpaEd
Harold's post is informative.
GrandpaEd
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I love it when a plan comes together... especially when PATENTS are involved. We hire a lawn boy to mow our yard every week for about fifty bux a pop. We have a nice electric mower, but he brings his gasoline-powered internal-combustion-engine (ICE) mower and gets the job done much faster while only slightly warming the planet. I would mow the yard myself, but I am officially retired. As for modifying Harbor Freight tools... buyer beware! A lot of HF power tools are made in China, so good luck trying to maintain a reliable supply chain if you get your "border area" problem solved. I think the suggestion to open the weed whacker up and drive the motor directly has promise, but not if it depends on electronics to operate. In that case, if all you need is the string-driven whacker part, just replace the existing motor with another one that YOU control. I am guessing you want a Roomba-style weed whacker that is "smart" enough to "find" your border area. Best wishes and a lot of luck trying to penetrate that market!
I hadn't thought of making a product to sell yet, I was hoping for this to be an add on feature for myself and still keep it's original features. I'll take the string spool off and add my attachment. However at 8000 rpm it removes eveything down to dirt.
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Maybe replace the existing motor, too? Can you upload a picture of what you are trying to accomplish or have already accomplished? I can never get a string-mower to work for more than a few minutes before the string jams... yet another reason to hire someone else to take care of the lawn. <sigh>
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Sometimes we just have to accept reality and bite the bullet.
As @Minder stated, it is LIKELY that your string-mower has a BLDC (BrushLess Direct Current) motor because the 8000 rpm speed of the motor you have (measured?) cannot be accomplished with a brushed DC motor with a copper commutator and carbon-based brushes. Or, if it could, it would not last very long at that speed without quickly wearing out the carbon brushes or maybe even throwing commutator segments off the armature because of excessive centrifugal forces.
The problem with speed control of a BLDC motor is electronic. The switching necessary to make the motor run depends on a feedback mechanism to determine rotor position which in turn affects the timing. Unless you are prepared to design your own BLDC controller, and have knowledge of how the existing BLDC controller works, it appears (at least to me) that you will have to replace the existing motor with a motor that operates at the speed you desire.
I doubt that "gearing down" from 8000 rpm is practical, even with multi-stage planetary gear reduction. I have had brushed DC gear-motors that used several planetary stages to achieve very low speed, but this it probably not something the average weekend garage mechanic could construct as a DIY project. My motors were also no where nearly as powerful as the motor required for a string-mower. Still, if you can, a gear reducer for your existing motor is probably the simplest solution. Good luck with that.
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Wow, @Minder! You really are old if you remember universal-wound AC/DC motors. And, yeah, I remember the "runaway" problem with series field motors. Are these even manufactured anymore? The OP needs to either replace his motor or gear it down. Do you see any other alternatives?
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Thanks for providing a list of manufacturers for Universal motors. But do you have a solution to the original problem of the existing string-mower's motor running too fast?
I would tend to look to a 5:1 planetary gear box, with it also comes an increae in torque!
There are several reasonably priced out there. !
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