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SIngle phase ac motor question

H

HardySpicer

Jan 1, 1970
0
My understanding is that you must have a rotating magnet field to
drive an ac motor. Hence often a capacitor start system is used with a
second winding (for single phase) which is switched off once the motor
is started. Essentially this is a two-phase motor for starting.Am I
therefore wrong with the idea that a single phase on its own will not
rotate a motor since there is a single phase and one coil only after
the thing has started? How does a single phase give a rotating field?


Hardy
 
P

Phil Allison

Jan 1, 1970
0
"HardySpicer"
My understanding is that you must have a rotating magnet field to
drive an ac motor. Hence often a capacitor start system is used with a
second winding (for single phase) which is switched off once the motor
is started. Essentially this is a two-phase motor for starting.Am I
therefore wrong with the idea that a single phase on its own will not
rotate a motor since there is a single phase and one coil only after
the thing has started? How does a single phase give a rotating field?


** No Google where you live ????

Most commonly, single phase motors use a capacitor to shift phase and make
the field rotate.

Low powered ( sub 100 watt input ) examples often use a "shaded pole" to
achieve a similar result.

Plenty of good, basic AC motor stuff here:

http://www.allaboutcircuits.com/vol_2/chpt_13/10.html



...... Phil
 
H

HardySpicer

Jan 1, 1970
0
"HardySpicer"




** No Google where you live ????

Most commonly, single phase motors use a capacitor to shift phase and make
the field rotate.

Low powered ( sub 100 watt input ) examples often use a "shaded pole" to
achieve a similar result.

Plenty of good, basic AC motor stuff here:

http://www.allaboutcircuits.com/vol_2/chpt_13/10.html

..... Phil

I know that much. I was just asking as to whether without the
capacitor (we are back to single phase then and not simulated two-
phase) do we still have a rotating field.

Hardy
 
P

Phil Allison

Jan 1, 1970
0
"HardySpicer"...
"Phil Allison"
"HardySpicer"
I know that much. I was just asking as to whether without the
capacitor (we are back to single phase then and not simulated two-
phase) do we still have a rotating field.



** Did you even fucking bother to read the link I supplied ???

SHITHEAD !!!!




..... Phil
 
H

HardySpicer

Jan 1, 1970
0
A capacitor-start induction motor uses two coils and the phase-shift
cap to make a rotating field at startup. Once the rotor gets up to
near synchronous speed, the auxiliary winding and cap are usually cut
out by a centrifugal switch. The motor will continue to run in
single-phase induction mode. It's similar to a single-phase
synchronous (pm rotor) motor in that it runs on a single-phase (non
rotating) field as long as it's at speed.

Most of the caps and start windings would fry if you left them on for
long.

As Phil so delicately points out, details are on the web.

John

Obviously you have no idea either along with the otehr chappie who is
rude.

I found the answer here

http://books.google.co.nz/books?id=...&oi=book_result&resnum=10&ct=result#PPA264,M1

When the motor is at standstill, the induced voltages are equal and
opposite resulting in two equal and opposite torques which cancel each
other out. So the net torque will be zero. However, if the rotor is
given an initial rotation by auxilliary means in either direction the
torque due to the rotating field acting in the direction of initial
rotation will be more than the torque
due to the other rotating field. Therefore the motor will develop a
net positive torque in the same direction
as the original rotation. This follows from cross-field theory or
double revolving field theory for a pulsating magnetic field.
A pulsating magnetic field is the type you get with a single phase
coil.(and no starter coil)

I wasn't after a superficial understanding as was given in that link.
If you are not a professional engineer please don't bother answering.

Thanks
Hardy
 
P

Phil Allison

Jan 1, 1970
0
"HardySpicer"


** Tell this FUCKING MORON

to GO TO HELL !!




** This was your question:

" How does a single phase give a rotating field? "

Which I answered thoroughly.

But you did not read your own question to see what it said.


I wasn't after a superficial understanding as was given in that link.


** You never explained WHAT the HELL you were after here.

Cos you are a retarded autistic fuckhead with zero command of the English
language.

**** OFF

DAMN TROLL




...... Phil
 
B

Bill Sloman

Jan 1, 1970
0
HardySpicer said:
Obviously you have no idea either along with the other chappie who is
rude.

Phil is always rude, but he knows quite a lot about electronics.

John Larkin knows even more.

Both of them answered your question a level that was appropriate to
the quality of your question, which didn't suggest that you had a
particularly firm grasp of the subject.
I found the answer here

http://books.google.co.nz/books?id=...&oi=book_result&resnum=10&ct=result#PPA264,M1

When the motor is at standstill, the induced voltages are equal and
opposite resulting in two equal and opposite torques which cancel each
other out. So the net torque will be zero. However, if the rotor is
given an initial rotation by auxilliary means in either direction the
torque due to the rotating field acting in the direction of initial
rotation will be more than the torque
due to the other rotating field. Therefore the motor will develop a
net positive torque in the same direction
as the original rotation.

This is good - as far as it goes. It talks about voltages rather than
currents, which is a bit odd, since force and torque are generated
by currents flowing through conductors in a magnetic fields. Voltages
are only interesting as far as they drive currents.
This follows from cross-field theory or
double revolving field theory for a pulsating magnetic field.
A pulsating magnetic field is the type you get with a single phase
coil.(and no starter coil).

"Cross-field theory" and "double rotating field theory" don't ring
any bells with me. I imagine that some academic somewhere
swears by them, but practical enegineers tend to think more in
terms of magnetic fields and currents flowing through conductors
immersed in the relevant magnetic fields.
I wasn't after a superficial understanding as was given in that link.
If you are not a professional engineer please don't bother answering.

Professional engineers don't waste time getting a deep understanding
if a superficial understanding works. Studying takes time, and time is
money.
 
H

HardySpicer

Jan 1, 1970
0
"HardySpicer" <[email protected]> schreef in bericht




Phil is always rude, but he knows quite a lot about electronics.

John Larkin knows even more.

Both of them answered your question a level that was appropriate to
the quality of your question, which didn't suggest that you had a
particularly firm grasp of the subject.


This is good - as far as it goes. It talks about voltages rather than
currents, which is a bit odd, since force and torque are generated
by currents flowing through conductors in a magnetic fields. Voltages
are only interesting as far as they drive currents.


"Cross-field theory" and "double rotating field theory" don't ring
any bells with me. I imagine that some academic somewhere
swears by them, but practical enegineers tend to think more in
terms of magnetic fields and currents flowing through conductors
immersed in the relevant magnetic fields.


Professional engineers don't waste time getting a deep understanding
if a superficial understanding works. Studying takes time, and time is
money.

That's rubbish Bill abd you know it. Why the hell do you think
Wermher Von Braun managed to stabiise a rocket when others could
not?
He had a deep understanding.


Hardy
 
H

HardySpicer

Jan 1, 1970
0
"HardySpicer"

** Tell this FUCKING MORON

to GO TO HELL !!




** This was your question:

" How does a single phase give a rotating field? "

Which I answered thoroughly.

But you did not read your own question to see what it said.


** You never explained WHAT the HELL you were after here.

Cos you are a retarded autistic fuckhead with zero command of the English
language.

**** OFF

DAMN TROLL

..... Phil
You never answered this at all. You just showed the standard 2-phase
capacitor start info which I knew about.
The question was well stated - you miss-read and clearly are in need
of some tablets to calm you down.

Hardy
 
P

Phil Allison

Jan 1, 1970
0
"HardySpicer"


** Tell this FUCKING MORON

to GO STRAIGHT TO HELL !!



** This was your question:

" How does a single phase give a rotating field? "

Which I answered thoroughly.

But YOU did not read your own question to see what it said.

And YOU never explained WHAT the HELL you were after here.

Cos you are a retarded autistic fuckhead with zero command of the English
language.


So **** OFF

YOU ASD FUCKED TROLL !!!
 
P

Phil Allison

Jan 1, 1970
0
"HardySpicer"


** Tell this FUCKING MORON

to GO STRAIGHT TO HELL !!


** This was your question:

" How does a single phase give a rotating field? "

Which I answered thoroughly.

But YOU did not read your own question to see what it said.

And YOU never explained WHAT the HELL you were after here.

Cos you are a retarded autistic fuckhead with zero command of the English
language.


So **** OFF

YOU ASD FUCKED TROLL !!!
 
H

HardySpicer

Jan 1, 1970
0
gmail/googlegroups... what do you expect?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon athttp://www.analog-innovations.com| 1962 |

It's what you learn, after you know it all, that counts.

Don't give me that superiority bullshit - show me anywhere in the link
that was given by that other idiot

http://www.allaboutcircuits.com/vol_2/chpt_13/10.html

that answers the question? Nowhere. It gives the standard stuff about
how a capacitor start works with TWO phases!! I wanted info on a
single phase and not about a single phase that was split into two.
Surely he could understand that. I needed info on the pulsating
magnetic field essentially but I didn't know the terminology to use
until I read the book. So he didn't know he was just pretending to
know or miss-read my question.

Hardy
 
P

Phil Allison

Jan 1, 1970
0
"HardySpicer = Criminal **** HEAD "


** The above Kiwi TURD is a

FUCKING INSANE BLOODY LIAR !!!!!


Don't give me that superiority bullshit - show me anywhere in the link
that was given by that other idiot

http://www.allaboutcircuits.com/vol_2/chpt_13/10.html

that answers the question?


** You did NOT ever write your actual damn question down !!!!!!!!!!!

So it only existed in your ASD fucked pointy HEAD !!!

Now, you keep alluding to some fucking NON EXISTENT question.

FUCKHEAD !!



Nowhere. It gives the standard stuff about
how a capacitor start works with TWO phases!! I wanted info on a
single phase and not about a single phase that was split into two.


** What the **** do you think a " shaded pole " design does???

YOU FUCKING NUT CASE LIAR !!!




....... Phil
 
H

HardySpicer

Jan 1, 1970
0
"HardySpicer =  Criminal **** HEAD  "

**  The above  Kiwi  TURD  is a

 FUCKING   INSANE  BLOODY  LIAR  !!!!!


** You did NOT ever write your actual damn question down  !!!!!!!!!!!

 So it only existed in your ASD fucked pointy HEAD   !!!

 Now,  you keep alluding to some fucking  NON  EXISTENT  question.

 FUCKHEAD  !!


**  What the **** do you think a " shaded pole "  design does???

  YOU  FUCKING   NUT   CASE   LIAR   !!!

......  Phil

Bloody head - I can see your Aspergers is getting to you. What an
idiot.


Hardy
 
H

HardySpicer

Jan 1, 1970
0
OK, which direction would it rotate?

John

With the ac single field and no capacitor it does not rotate at all.
There is zero net torque.
However, if you spin it in one direction it stays there and starts
rotating and continues in that direction. My simple question was why?
With a capacitor you create two vectors of flux which add and give a
rotating magnetic field. That much is everywhere on the net and was
discovered well over 100 years ago. And that idiot also mentioned
shaded pole ac motors which also has a net rotating flux. It's my
first point that is not well explained. Actually even the book
explanation that I gave earlier isn't exactly compelling.

Hardy
 
P

Phil Allison

Jan 1, 1970
0
"HardySpicer = Criminal **** HEAD "

** The above Kiwi **** is a

INSANE FUCKING LIAR !!!!!



He did NOT EVER write his actual damn question down !!!!!!!!

So it only existed in his ASD fucked pointy HEAD !!!

Then the LYING ASS alludes over and over to a NON EXISTENT question !!!

What a COLOSSALLY STUPID, ASD FUCKED FUCKHEAD !!


So **** the HELL OFF

- you TROLLING

FUCKING NUT CASE

& DAMN LIAR !!!





...... Phil
 
H

HardySpicer

Jan 1, 1970
0
"HardySpicer =  Criminal **** HEAD  "

**  The above  Kiwi  TURD  is a

 FUCKING   INSANE  BLOODY  LIAR  !!!!!


** You did NOT ever write your actual damn question down  !!!!!!!!!!!

 So it only existed in your ASD fucked pointy HEAD   !!!

 Now,  you keep alluding to some fucking  NON  EXISTENT  question.

 FUCKHEAD  !!


**  What the **** do you think a " shaded pole "  design does???

  YOU  FUCKING   NUT   CASE   LIAR   !!!

......  Phil

"Am I
therefore wrong with the idea that a single phase on its own will not
rotate a motor since there is a single phase and one coil only after
the thing has started? How does a single phase give a rotating field?
"

That was my original question which the idiot Phil miss-understood. He
assumed that I meant how do I get a rotating field from single phase
to which the obvious answer is: use a capacitor and a second field!
Read the question carefully though...slowly. The reason I was confused
is that I knew that a single phase with no second quadrature phase
would give zero torque. How then would the machine keep rotating after
the capacitor field was removed and we go back to a single field?
Sp dont get your knickers in a twist - you are in the wrong.


Hardy
 
That's rubbish Bill abd you know it. Why the hell do you think
Werner  Von Braun managed to stabilise a rocket when others could
not?
He had a deep understanding.

Like I said "Professional engineers don't waste time getting a deep
understanding if a superficial understanding works". Superficial
understanding didn't work in von Braun's application, so he had to put
the time in to develop an understanding that was deep enough to work.
And he's a poor example of a professional engineer - his primary
interest was in space flight, rather than rockets as such, and he did
a Ph.D. on rocket design

http://en.wikipedia.org/wiki/Wernher_von_Braun

In many ways he's better seen as an obsessed hobbyist, who managed to
con the German and American governments into funding his hobby, by
selling them space rockets as ballistic missiles.

Some engineers don't always understand that their superficial over-
simplified models can break down if you try to use them in situations
where the simplifying assumptions are invalid, but that's another
story.
 
R

Robert

Jan 1, 1970
0
My understanding is that you must have a rotating magnet field to
drive an ac motor. Hence often a capacitor start system is used with a
second winding (for single phase) which is switched off once the motor
is started. Essentially this is a two-phase motor for starting.Am I
therefore wrong with the idea that a single phase on its own will not
rotate a motor since there is a single phase and one coil only after
the thing has started? How does a single phase give a rotating field?


Hardy

Another method is the Shaded-pole motor
http://en.wikipedia.org/wiki/Shaded-pole_motor

There is a phase difference between the fields from the shaded and unshaded
pole faces.

Robert H.
 
H

HardySpicer

Jan 1, 1970
0
With the ac single field and no capacitor it does not rotate at all.
There is zero net torque.
However, if you spin it in one direction it stays there and starts
rotating and continues in that direction. My simple question was why?
With a capacitor you create two vectors of flux which add and give a
rotating magnetic field. That much is everywhere on the net and was
discovered well over 100 years ago. And that idiot also mentioned
shaded pole ac motors which also has a net rotating flux. It's my
first point that is not well explained. Actually even the book
explanation that I gave earlier isn't exactly compelling.

Ah, finally you ask a coherent question.

Imagine that the rotor was a bar magnet, perpendictular to the shaft.
Apply the AC current to the single stator coil, making a single-phase
field. The magnet will just sort of buzz, with no usable torque
applied to the shaft, unless you want a very weak vibrator. [1]

But spin the shaft up to 3600 RPM. The magnet now aligns with the
field at both the positive and the negative peak of the 60 Hz AC
cycle. If it gets a little ahead of or behind the time peaks, a torque
develops that tries to pull it back into alignment. That's a
synchronous motor. It won't start by itself, but it does fine once
you're up to synchronous speed. Think of the field as a stroboscope
that's illuminating the rotor only at its current peaks; that image
looks like a DC field forcing the bar magnet into one position.

Now replace the magnet with a cylinder of moderately conductive stuff.
At 3600 RPM, the field essentially magnetizes the rotor every time it
peaks, and the magnetization makes it act like the magnet in the
synchronous motor. The "magnetization" is actually eddy currents
induced into the rotor, and the best rotor isn't solid, it's shorted
turns of copper over a laminated steel core. Since the "magnetization"
isn't fixed like the PM rotor, it sort of slides around, so the
induction motor doesn't, and doesn't have to, run at synchronous
speed.

More detail tends to be mathematical.

Oh, don't thank me, I'm going to write a book on electronics for
beginners, and I need to practice explaining simple stuff to idiots.

John

[1] One interesting sort-of-countercase being the single-pole
induction motors that spin microwave oven turntables. They self-start
in a random direction.

Thank you for your long reply. That's good. I am still confused by
this however because to me you need a field that rotates in order to
get a motor and the single phase version does not.
In fact it just pulsates and as you said gives zero torque. Now I was
aware of synchronous machines and induction motors (not the microwave
example which was interesting). They have fields that rotate however.
Now that book which I references earlier had another explanation - if
you look back in this thread. Neither explanation seems that good to
me and there are no diagrams anywhere on the net which describe this
effect (other than the book which splits the torque into two parts).
Of course there are many animations showing a rotating magnetic field
and the induction motor.


Hardy
 
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