Re: SIngle phase ac motor question


"HardySpicer" <gyansorova@xxxxxxxxx> schreef in bericht
news:b4c1043a-2a01-49ef-ba4a-4cf3c51d2bce@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Mar 13, 11:54 am, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Thu, 12 Mar 2009 15:42:16 -0700 (PDT), HardySpicer



<gyansor...@xxxxxxxxx> wrote:
On Mar 13, 11:14 am, "Phil Allison" <philalli...@xxxxxxxxxx> wrote:
"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

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

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 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=r3c27IaomA0C&pg=PA262&lpg=PA262&dq=single+phase+motor+principle&source=bl&ots=-2qc_wURGP&sig=qeM_KuNasnBVcngiKWh1OJ8FYDQ&hl=en&ei=hpa5Sf77BJqqtQPm74lK&sa=X&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.

--
Bill Sloman, Nijmegen


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