crazy idea: running ac line trimmer off dc

[kefa]

hi all,

i've got a ryobi line trimmer that runs off mains and its a real pain to have to lug the cord around everywhere. i had this crazy idea wouldn't it be great if i could run this thing off my ebike batteries!

i stumbled upon this guy on instructables who claims i can just plug the thing with batteries and it should work fine cause the motor is a universal motor.

http://www.instructables.com/id/Run-AC-Tools-on-Batteries-Directly,-without-an-Inv/

anybody on this forum ever tried this? EE's out there - is this guy just a nutter or would this actually work?

 

[liveforphysics]

Vaccuum cleaners, angle grinders, die grinders, leaf blowers, some hand drills, and a bunch of other tools that use universal motors all run equally well, if not better on DC than they do on AC. If your line trimmer has a uni-motor in it, it will be just as happy humming away on DC.

 

[kefa]

how do i know what voltage to run? i guess i can just try and see how much power it has.

 

[heathyoung]

I would say it would run off 240V DC ~ deadly...

 

[kefa]

hi heath,

i tried putting 12v through it yesterday and the motor spun. not very fast mind you! when i get some time i'll up the voltage a bit more and see how much power its got. i recently got a turnigy watt meter so i can wire it up to that and see how much watts it chews up. the line trimmer is rated 1100w!

i'll try a few voltages - i'll start low and go up and see how it goes. i'm hoping it'll run alright at maybe 60-70v.

 

[liveforphysics]

hi heath,

i tried putting 12v through it yesterday and the motor spun. not very fast mind you! when i get some time i'll up the voltage a bit more and see how much power its got. i recently got a turnigy watt meter so i can wire it up to that and see how much watts it chews up. the line trimmer is rated 1100w!

i'll try a few voltages - i'll start low and go up and see how it goes. i'm hoping it'll run alright at maybe 60-70v.

It will want roughly the RMS voltage of the AC power it was being fed to have equal performance.

 

[John in CR]

Can a brushed controller control the speed of such motors?

 

[amberwolf]

Yes. Any brushed motor that is either a permanent-magnet field or a series-wired wound-field will work with one. Parallel-wired wound-fields might work, too.

I have a Ryobi AC line trimmer that I did in fact run off of a string of car batteries as a test, but since it was intended for 110VAC and all I had was three car batteries at 12VDC each, it didn't run nearly well enough to actually cut thru anything but the thinnest grass.

If the motor hadn't died from overheating while in use as a regular AC-powered trimmer one day a few summers back, I would be able to run that from these LiPo cells I have.


There have been a couple of ebikes powered by 110VAC-designed circular saws, running off DC batteries instead. Don't know how well they worked or what voltages they were at, but if I had more than one circular saw I'd probably try it.


I have a 1/3 HP weedeater motor from some Craftsman (I think) trimmer, and it is actually a permanent-magnet type, which simply has a diode in series with the brush input to the armature winding, since it is also intended to run on 110VAC power. I've considered trying to use it on an ebike, but to get decent power out of it I'd have to have more than 100VDC on there, probably more like 200VDC, and then a reduction system to take the extremely high RPM it'd be at down to something usable to drive a wheel with torque. Problem is, I think the RPM it'd be at at usefully-high-voltages would end up destroying it.


I also have an old Black & Decker lawnmower that uses a wound-field motor, so if I were to power the field separately from the armature, I could control it to a useful speed without having to run it at over 100VDC. I don't recall it's power rating, though.


I looked into using some of the vacuum-cleaner motors I have, which are extremely high RPM, but at the time I did not know how I might create a speed-to-torque reduction system for them. I am curious if the Recumpence reduction would work for this, as well as it works for RC motors.

 

[liveforphysics]

Can a brushed controller control the speed of such motors?

Even better John, you can run them off a dimmer switch to control speeds . Dimmer switches are SCR based pwm choppers, so you gotta get the kind that un-latches on its own, not the kind that unlatches when the voltage crosses zero.

 

[heathyoung]

Never seen one with an SCR, they are pretty much always a TRIAC (basically a dual SCR) with a DIAC for firing. Unlatching is a problem for SCR and TRIACs, thats how they work - the only way to unlatch either is to disconnect the power or on zero crossing. You want one with something like a IGBT, not exactly common.

 

[Jay64]

Seems to me like it would be a lot more of a hassle to always be carrying around 120v worth of batteries then it would be to deal with a cord. If you really want to go cordless, you would probably be better off just getting a cordless one. Those motors are designed to run good at much lower voltages. Lowe's sells some that range from 18v up to 24v. Also, getting the 18v model for about $100 seems cheaper then getting 120v of super light batteries. I know this kinda goes against the spirit of this forum, but it seems like you're trying to force an issue that is less convienient.

Or if you really just want a project, maybe try to tinker with the shaft to see if you can change the "gearing" to make it spin better at a lower voltage.