Direction of an AC motor revisited

Take a DC Motor. The brushes are directly connected to the + - connector. The permanent field is fixed. If you inverse the Batterie, the rotor will run in the other direction.

So do the AC Motor. However the brushes and the field is connected to the mains connector, so changing the mains connector will also change the field, therefor no change between field and rotor.

But you managed reversing by changing the internal brush connection. So the rotor is now to the opposite of the field direction.

Hope my Ginglisch is understandable :)

Wolfgang

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Wolfgang Allinger, anerkannter Trollallergiker :) reply Adresse gesetzt! 

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So these natives should try harder if even a bloody Kraut can outperform them :)

Wolfgang

--
Wolfgang Allinger, anerkannter Trollallergiker :) reply Adresse gesetzt! 

ihr Niveau herunter und schlagen mich dort mit ihrer Erfahrung! :p 
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The sad thing Wolfgang is that your post was far better than some from people whose native language is *supposed* to be English...

** 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

snipped-for-privacy@googlegroups.com

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 goe s on!

:)

On Friday, June 19, 2015 at 12:19:59 AM UTC-4, snipped-for-privacy@gmail.com wrote :

in reverse after being disassembled and then put back together. It turns o ut 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 or iginal 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 too l. It looks like he tried to pry the holders out without releasing the scre ws 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 t he 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 bee n working on it since. So in dis assembly the correct orientation of the fi eld then become unknown, and after replacing the brushes and holders we evi dently 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 consul ted 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 wo uld a 1.5 thousandth's out of round condition be enough to "bounce" the bru shes and make them arc? Brushes and holders are new and each brush is mount ed stationary, and it's relationship to the position on the commutator cann ot be altered. I can't figure out what else could be causing this? Lenny

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.

• Single phase AC induction motor: Direction determine by relative phase of run and start windings during starting. Start winding only used during starting, cut off by centrifugal switch, starting relay, or other starting device. Main winding conencted directly to mains; start winding through capacitor or has different inductance/resistance to provide phase shift.

• Split phase AC induction motor: Direction determined by relative phase of two windings. Main winding conencted directly to mains; phase winding through capacitor or has different inductance/resistance to provide phase shift. Both windings powered when running.

There are many variations. :)

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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.

On Friday, June 19, 2015 at 12:19:59 AM UTC-4, snipped-for-privacy@gmail.com wrote :

in reverse after being disassembled and then put back together. It turns o ut 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 a ll moved the same as a group it doesn't make any sense. 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.

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.

On Friday, June 19, 2015 at 12:19:59 AM UTC-4, snipped-for-privacy@gmail.com wrote :

in reverse after being disassembled and then put back together. It turns o ut 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 de grees apart. Is that correct? 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.

On Friday, June 19, 2015 at 12:19:59 AM UTC-4, snipped-for-privacy@gmail.com wrote :

in reverse after being disassembled and then put back together. It turns o ut 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 t ime. 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 win ding either open or intermittent at best. The connections on this commutato r, (and in fact others I've worked on over the past few years) are (and hav e been) clean and look like they were crimped. Is soldering to commutator s egments no longer done? 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.

By changing the connections, you change the magnetic poles of the armature. Just like changing the polarity on a permanent magnet DC motor, changes it direction.

It also test for shorts between coils, usually caused by a short between segments of the commutator.

If anyone could please comment on my "bounce" question below from an earlie r 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 consul ted 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 wo uld a 1.5 thousandth's out of round condition be enough to "bounce" the bru shes and make them arc? Brushes are now trued to the commutator, holders ar e new, and each brush is mounted stationary, and it's relationship to the p osition on the commutator cannot be altered. I can't figure out what else c ould be causing this? Lenny

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.