Brushed DC motor won't run at it's normal polarity. - Page 1
Forum section: electronics / repair

author: jdraughn
author_rank: Regular Contributor
timestamp: June 07, 2017, 10:38:02 pm
content: I have a 36v weed wacker that I have been trying to get going off/on over the last 4 yearsI bought it from the thrift store, it had no batteries, figuring I would just use a couple of laptop adapters in series and an extension cord until I hacked in some kind of battery pack. But the thing didn't work. The motor would barely move a tiny bit, then nothing.Fast forward to now. I figured the brushes were worn out so I removed the motor, but the brushes were fine. The armature was dark, so I cleaned it, and the brushes surface with an eraser knowing that finally, I fixed the problem, but nope, made no difference. When I swapped the on/off switch for the electric assembly out of a 36v drill, it's reverse direction works great and powerfully, but it's standard rotation still won't turn.The motor and brushes appeared fine, but still no go. What would cause this? Is it because one of the brushes have moved a tiny bit, throwing off the timing? Maybe I need to somehow bend the assembly that holds the brushes back a tiny bit? Any other ideas?I am so tired of my mixed-gas weed eaters constantly breaking down and requiring gas, I just want this to work so I can cut down the weeds in my yard.

author: retrolefty
author_rank: Super Contributor
timestamp: June 08, 2017, 12:54:56 am
content: Well consider me rude if you find this question questionable, but how did you determine what is 'normal polarity'?Possibly the direction of polarity were it runs is the 'normal polarity?

author: james_s
author_rank: Super Contributor
timestamp: June 08, 2017, 12:56:35 am
content: If it runs at all, I really doubt there is anything wrong with the motor itself.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 08, 2017, 01:56:44 am
content: I'm also tired of gas=powered weed eaters. They break down constantly, and mixing the oil and gas is a mess.To your problem. If the brushes angle is off a little, the brushes would spark more but the motor would still run.Most likely there is an open circuit somewhere. With your multimeter measure continuity from the batery wires to the motor itself. Then measure the motor's resistance.If the rotor shows infinite resistance? What most likely may have happened is that a winding buried deep in the rotor must have opened up.

author: jdraughn
author_rank: Regular Contributor
timestamp: June 09, 2017, 12:40:18 pm
content: Hi, sorry for the late reply.I know it's normal polarity because it's obvious which way the spool is supposed to spin.After I put it all back together I measured the ohms between the terminals of the motor and very slowly turned it to see if there were any dead spots or shorts, and it appeared fine.I am guessing that it's a mechanical issue, where if it turns one direction wear causes it to bind up. The motor is bolted to a metal plate which also has a large gear and the weed eater spool hub. The assembly feels like there is very little play however, it looks nice and tight. I am going to take a closer look at the mechanical aspect since no one can hazard a guess of what could electrically cause a brushed motor to work one direction, but not the other.

author: Brumby
author_rank: Unknown
timestamp: June 09, 2017, 02:54:44 pm
content: One suggestion I would like to offer depends on how free the brushes are to be manipulated.The suggestion is simple. Since most brushes are rectangular, slip each one out of its slot, rotate by 180º and slip back into place.Since the unit does run well in reverse, it would seem the commutator and windings are OK, so the theory is that the brushes may have been worn and polished to the point where they aren't making positive contact. Reversing the direction of the wear profile will (I believe) be the same as the unit running in reverse.I might also suggest checking the spring pressure on the brushes as another measure - but if it runs strongly in reverse, it would seem that might be OK.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 10, 2017, 03:36:34 am
content: I'm not aware of anything "electrical" that could cause this to happen. Even poor brush contact shouldn't cause the motor to not run at all in one direction. There could be all sorts of possible mechanical reasons though.Have you checked current draw when the motor is trying to run in forward and not going anywhere? Have you tried turning the motor in the forward direction by hand while it's powered? The brushes could be hanging up in comm slots; the armature could be pushed out by emf when running forward and snagging on something - when run in reverse it would be pulled in and free. All kinds of possibilities.

author: P90
author_rank: Frequent Contributor
timestamp: June 10, 2017, 05:14:30 am
content: Cordless tool motors draw many amps, more than your SMPS and its circuit is going into self protection.

author: oldway
author_rank: Super Contributor
timestamp: June 10, 2017, 07:33:08 am
content: What kind of brushed motor is this ?Series field motor or universal motor does only run in one direction even if you reverse the polarities.What you say is impossible: a series field motor does not run in opposite direction without inverting or the armature, or the field (not both !)You are making something horribly wrong with the connections of the motor.There are two field coils and you are connecting these coils in a wrong way so you have two poles with the same polarity instead of two poles with opposite polarities.The motor does not rotate for this reason.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 10, 2017, 08:00:43 am
content: I'm running under the assumption that it's a PMDC motor since it's in a battery operated weedwacker.

author: oldway
author_rank: Super Contributor
timestamp: June 10, 2017, 09:41:44 am
content: Serie field motor may also run with battery....The starter of your car is an example.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 10, 2017, 10:05:34 am
content: Yup, but not likely in a cordless (battery operated) hand tool - PMDC is way more efficient in that application. Guess we need to wait to hear back from the OP.

author: oldway
author_rank: Super Contributor
timestamp: June 10, 2017, 10:24:38 am
content: Quote from: eKretz on June 10, 2017, 10:05:34 amBut a field serie motor is far more cheaper than a PMDC motor....and has a better curve speed/torque for this application.

author: jdraughn
author_rank: Regular Contributor
timestamp: June 10, 2017, 11:54:24 am
content: Hi guys, thanks for the replies. I plan on removing the motor assembly today and doing some more testing and experimenting. In the meantime, I found this parts schematic online that shows the entire assembly. I will post actual pictures later. I don't think it's the power supplies going into protection mode, because it does run fine in reverse.

author: P90
author_rank: Frequent Contributor
timestamp: June 10, 2017, 12:27:28 pm
content: sounds like the motor is biased for forward torque. I remember some brushed motor drills had more torque in forward direction than reverse, it's a function of positioning of brushes in relation to the commutator. I have a Bosch drill that actually rotates the brushes to change direction rather than reversing a switch, it has equal power in forward and reverse, and same amount of sparking. I suspect this has something to do with it.

author: mikerj
author_rank: Super Contributor
timestamp: June 10, 2017, 11:52:12 pm
content: Quote from: P90 on June 10, 2017, 12:27:28 pmIt's actually the position of the brushes in relation to the field magnets rather than the commutator (the commutator turns so it can't have a fixed position relative to the brushes!). It's a bit like setting the ignition timing in an engine, there's an optimal position for a commutator winding to be energised depending on direction. The expensive upgrade motors for older radio controlled car models had an end plate holding the brushes that could be adjusted to 'tune' the motor.Sometimes the magnets are bonded into position with epoxy, and occasionally they have nothing but a spring clip holding them in place. Either way it's often possible for them to move if the motor has suffered mechanical shock.

author: eKretz
author_rank: Frequent Contributor
timestamp: June 11, 2017, 03:35:22 am
content: It would have had to have taken a hell of a hit to move the magnets or brushes that far out of whack though. On these cheap dime-a-dozen motors the brush housing is usually keyed into place for quick and easy assembly.

author: vltr
author_rank: Contributor
timestamp: June 11, 2017, 05:12:11 am
content: That motor looks like either a RS550 or 775 which are pretty cheap, usually about 20 dollars. With some google fu you might be able to turn up one with the right pinion already installed. These motors are not generally serviceable, but that doesn't stop some peopleSecondly did you ensure that your drill trigger actually works in both directions? Would hate to find you have a problem at the wrong end

author: tablatronix
author_rank: Regular Contributor
timestamp: June 11, 2017, 03:22:47 pm
content: I am back on gas after my $$$ ryobi motor burnt up, after opening it up i found out why. No protection on the motor whatsoever.Shit engineering.

author: P90
author_rank: Frequent Contributor
timestamp: June 11, 2017, 03:56:48 pm
content: Quote from: tablatronix on June 11, 2017, 03:22:47 pmIt's a Ryobi, what were you expecting?