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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Direction of an AC motor revisited
Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny
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#2
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Direction of an AC motor revisited
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#3
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Direction of an AC motor revisited
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#4
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Direction of an AC motor revisited
On Friday, June 19, 2015 at 5:14:57 AM UTC-4, Wolfgang Allinger wrote:
Hope my Ginglisch is understandable The sad thing Wolfgang is that your post was far better than some from people whose native language is *supposed* to be English... |
#5
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Direction of an AC motor revisited
wrote:
Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? ** A regular DC motor is surrounded by a *permanent magne*t - right? If you reverse the external connections to the rotor, it spins the other way cos the relationship between the fixed and moving magnetic fields is reversed. With your " AC/DC" motor, the fixed field is provided by the same current that drives the rotor. When fed by an AC or DC supply, the fixed and rotor fields always *change polarity at the same time* so the rotation direction remains the same. You have to go inside and reverse the connection to the field or rotor to effect a change. .... Phil |
#6
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Direction of an AC motor revisited
On Friday, June 19, 2015 at 6:12:11 AM UTC-4, Wolfgang Allinger wrote:
On 19 Jun 15 at group /sci/electronics/repair in article (John-Del) wrote: On Friday, June 19, 2015 at 5:14:57 AM UTC-4, Wolfgang Allinger wrote: Hope my Ginglisch is understandable The sad thing Wolfgang is that your post was far better than some from people whose native language is *supposed* to be English... So these natives should try harder if even a bloody Kraut can outperform them Saludos (an alle Vernünftigen, Rest sh. sig) Wolfgang If we can only convince them to try harder. They seem to have no will, nor is there any incentive for them to do so.. Seems to get worse as time goes on! |
#7
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Direction of an AC motor revisited
On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote:
Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor.. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny Thanks everyone for the great explanations. It makes more sense now. The original owner of this thing apparently tried to replace the brushes himself. I can only surmise that he didn't realize that the bakelite brush holders were held into the housing with small set screws. So he did what any idiot would do. He used a "bigger hammer". Some people should never pick up a tool. It looks like he tried to pry the holders out without releasing the screws and cracked them into many pieces. The field is connected to the brush holders with push on connectors. That's a good thing too because to get it out he must have ripped it loose from the remnants of the brush holders. Luckily the field wasn't damaged. At this point he apparently gave up and gave the tool to my son who has been working on it since. So in dis assembly the correct orientation of the field then become unknown, and after replacing the brushes and holders we evidently switched the brush positions. So he now has the direction problem resolved but the new brushes arc really bad. Perhaps the old ones did too and maybe that's why Mr Wizard tried to replace them in the first place but we don't know. The commutator does not appear to have worn down much during it's lifetime. A growler test shows no shorted windings to ground, The areas between some of the the segments however appear to be a little ragged and opened a bit from the arcing, no doubt. A dial indicator on the commutator shows an out of round condition totaling 1.5 thousandth's, on each side for a total of three thousandth's for the entire piece. According to a machinist we consulted this doesn't seem like enough to warrant turning the commutator, but I've been considering something else. With this motor spinning at 9000 RPM would a 1.5 thousandth's out of round condition be enough to "bounce" the brushes and make them arc? Brushes and holders are new and each brush is mounted stationary, and it's relationship to the position on the commutator cannot be altered. I can't figure out what else could be causing this? Lenny |
#9
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Direction of an AC motor revisited
"Samuel M. Goldwasser" wrote in message ... Common (non-solid state/brushless) motors: * Series DC motor, shunt DC motor, universal AC DC motor: Direction determined by relative wiring of stator and brushes. * Permanent magnet motor: Direction determined by polarity of DC (or PWM etc.) input. * 3 phase AC induction motor: Direction determined by phase relationship of 3 connections. Swap any two pairs to reverse motor. One more thing to add to this that hapened where I worked. I don't recall the type of motor, but it was around 100 HP and had 4 sets of brushes and was driven by a varitable speed drive. Two people changed the brushes and said they did not touch the wires. It ran backwards. Several others looked at it and could not determin why it was running backwards. I went up and asked a few questions. I solved the problem for them. They had loosened the plate that holds the brushes and rotated it some to make it easier to replace the brushes. When they rotated it back, they had put it in the wrong place. By rotating it back to the correct position it ran the correct way. |
#10
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Direction of an AC motor revisited
On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote:
Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor.. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny I'm just curious, when this plate was moved did all the brushes rotate the same amount, or perhaps just two? Because if you think about that if they all moved the same as a group it doesn't make any sense. Lenny |
#11
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Direction of an AC motor revisited
wrote in message
... On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote: Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny Thanks everyone for the great explanations. It makes more sense now. The original owner of this thing apparently tried to replace the brushes himself. I can only surmise that he didn't realize that the bakelite brush holders were held into the housing with small set screws. So he did what any idiot would do. He used a "bigger hammer". Some people should never pick up a tool. It looks like he tried to pry the holders out without releasing the screws and cracked them into many pieces. The field is connected to the brush holders with push on connectors. That's a good thing too because to get it out he must have ripped it loose from the remnants of the brush holders. Luckily the field wasn't damaged. At this point he apparently gave up and gave the tool to my son who has been working on it since. So in dis assembly the correct orientation of the field then become unknown, and after replacing the brushes and holders we evidently switched the brush positions. So he now has the direction problem resolved but the new brushes arc really bad. Perhaps the old ones did too and maybe that's why Mr Wizard tried to replace them in the first place but we don't know. The commutator does not appear to have worn down much during it's lifetime. A growler test shows no shorted windings to ground, The areas between some of the the segments however appear to be a little ragged and opened a bit from the arcing, no doubt. A dial indicator on the commutator shows an out of round condition totaling 1.5 thousandth's, on each side for a total of three thousandth's for the entire piece. According to a machinist we consulted this doesn't seem like enough to warrant turning the commutator, but I've been considering something else. With this motor spinning at 9000 RPM would a 1.5 thousandth's out of round condition be enough to "bounce" the brushes and make them arc? Brushes and holders are new and each brush is mounted stationary, and it's relationship to the position on the commutator cannot be altered. I can't figure out what else could be causing this? Lenny The "ragged" wear and arcing would indicate open circuits in the armature windings. Put it back on the growler and drag a hacksaw blade across the gap in each slot. There should be an arc en you do that, if not most likely that winding is open. |
#12
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Direction of an AC motor revisited
The "ragged" wear and arcing would indicate open circuits in the armature windings. Put it back on the growler and drag a hacksaw blade across the gap in each slot. There should be an arc en you do that, if not most likely that winding is open. Also I forgot to mention you have to rotate the armature after testing each slot. |
#13
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Direction of an AC motor revisited
On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote:
Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor.. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny I don't have access to the growler any more but can't I use my Simpson? I'm thinking that I should have continuity between each of two segments 180 degrees apart. Is that correct? Lenny |
#14
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Direction of an AC motor revisited
wrote in message
... On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote: Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny I don't have access to the growler any more but can't I use my Simpson? I'm thinking that I should have continuity between each of two segments 180 degrees apart. Is that correct? Lenny Sure, use the Simpson on low ohms, once you have located a winding's bars go around and check each winding. Start with a winding with clean bar edges and then move up to the burnt ones. If you find an open or hi resistance check the crimp or soldering at those bars. |
#15
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Direction of an AC motor revisited
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#16
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Direction of an AC motor revisited
On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote:
Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor.. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny There was a term commonly used years ago which I haven't heard for a long time. Many commutators would fail because they "threw solder". The point on the commutator that was soldered to the individual rotor winding had heated, melted the solder, and "threw" it out of the connection, leaving that winding either open or intermittent at best. The connections on this commutator, (and in fact others I've worked on over the past few years) are (and have been) clean and look like they were crimped. Is soldering to commutator segments no longer done? Lenny |
#17
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Direction of an AC motor revisited
wrote in message
... On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote: Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny There was a term commonly used years ago which I haven't heard for a long time. Many commutators would fail because they "threw solder". The point on the commutator that was soldered to the individual rotor winding had heated, melted the solder, and "threw" it out of the connection, leaving that winding either open or intermittent at best. The connections on this commutator, (and in fact others I've worked on over the past few years) are (and have been) clean and look like they were crimped. Is soldering to commutator segments no longer done? Lenny I think they are all crimped these days, I am no longer active in that field. |
#18
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Direction of an AC motor revisited
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#19
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Direction of an AC motor revisited
wrote: On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote: Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny I don't have access to the growler any more but can't I use my Simpson? I'm thinking that I should have continuity between each of two segments 180 degrees apart. Is that correct? Lenny It also test for shorts between coils, usually caused by a short between segments of the commutator. https://en.wikipedia.org/wiki/Growle...ical_device%29 |
#20
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Direction of an AC motor revisited
If anyone could please comment on my "bounce" question below from an earlier post in this thread I would be very grateful. Lenny
The commutator does not appear to have worn down much during it's lifetime. A growler test shows no shorted windings to ground, The areas between some of the the segments however appear to be a little ragged and opened a bit from the arcing, no doubt. A dial indicator on the commutator shows an out of round condition totaling 1.5 thousandth's, on each side for a total of three thousandth's for the entire piece. According to a machinist we consulted this doesn't seem like enough to warrant turning the commutator, but I've been considering something else. With this motor spinning at 9000 RPM would a 1.5 thousandth's out of round condition be enough to "bounce" the brushes and make them arc? Brushes are now trued to the commutator, holders are new, and each brush is mounted stationary, and it's relationship to the position on the commutator cannot be altered. I can't figure out what else could be causing this? Lenny |
#21
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Direction of an AC motor revisited
Sam ! haven't heard from you for a while. Nice to know you are still alive and kicking, provided you are kicking the right people.
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#22
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Direction of an AC motor revisited
On Friday, June 19, 2015 at 4:01:50 PM UTC-7, wrote:
On Friday, June 19, 2015 at 12:19:59 AM UTC-4, wrote: Just wanted to get back to the group about this AC motor that was running in reverse after being disassembled and then put back together. It turns out that reversing the brushes did indeed reverse the direction of the motor. We can't argue with success, but I still don't fully understand why this is so. Can anyone please explain this to me? Thanks, Lenny I'm just curious, when this plate was moved did all the brushes rotate the same amount, or perhaps just two? Because if you think about that if they all moved the same as a group it doesn't make any sense. Lenny I think it makes sense. The commutator selects a winding. The rotation of brushes determines whether winding #1 puts a new rotor N pole CW of the stator's S pole, or winding #2 puts a new rotor N pole CCW of the stator's S pole |
#23
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Direction of an AC motor revisited
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#24
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Direction of an AC motor revisited
In article -berlin, all2001
@spambog.com says... On 21 Jun 15 at group /sci/electronics/repair in article (whit3rd) wrote: I think it makes sense. The commutator selects a winding. The rotation of brushes determines whether winding #1 puts a new rotor N pole CW of the stator's S pole, or winding #2 puts a new rotor N pole CCW of the stator's S pole on an AC Motor are no permanent S and N poles ;( Its because Alternating Current = AC! Saludos (an alle Vernünftigen, Rest sh. sig) Wolfgang Yeah but, it's a series motor so that means it can operate on AC or DC.. As for the communtator being out by 1.5" or so, that isn't anything to worry about because in the first short running time that should smooth out, unless there is some bearing play? You really don't want that. Jamie |
#25
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Direction of an AC motor revisited
It's good to know Sam Goldwasser is still around, his posts are almost always 100 correct!!!
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#26
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Direction of an AC motor revisited
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#27
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Direction of an AC motor revisited
There was a term commonly used years ago which I haven't heard for a long time. Many commutators would fail because they "threw solder". The point on the commutator that was soldered to the individual rotor winding had heated, melted the solder, and "threw" it out of the connection, leaving that winding either open or intermittent at best. A friend from my youth was an auto-electrician, and apparently, back then, this was a common occurrence on starter motors and the like. The term that he used for the condition was "flung" as in 'it's got a "flung" armature' I knew what that one meant, but the one that I never got in terms of where the phrase came from was "drop testing" which I believe referred to the resistance checking of the armature windings across opposite brass commutator segments, as described here by someone else. Arfa |
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