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'[EE]: 115 VAC fan? Now: Synchronous Motor Verifica'
2002\03\12@235923 by Jim

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  "I believe that your fan has a synchronous motor. "

Check this by verifying that the "rotor" is
magnetized.

My old solenoid-operated, tri-motor (2 take-up reel
motors and single "synchronous" capstan drive motor),
Dokorder model 7100 Reel-Reel tape recorder (circa the
late 70's) has a bona-fide "synchronous" motor which
drives the capstan and, indeed, the rotor *is*
magnetized (verified one day when I had it apart
changing belts (or was that the time when the
erase-bias oscillator failed?).

This is the means by which a motor becomes "synchronous"
in rotation with the AC mains.

Jim


{Original Message removed}

2002\03\13@010031 by Sean H. Breheny

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This is changing the topic slightly, but I have always wondered: how do DC
powered tape players/recorders maintain the proper tape feed speed? Do they
have some means of measuring the speed and perform feedback or do they
simply rely on the load and motor specs being relatively constant? Since
even a few percent change is quite noticeable, this would seem to be
inadequate for cheap motors.

Thanks,

Sean

At 10:58 PM 3/12/02 -0600, you wrote:
>My old solenoid-operated, tri-motor (2 take-up reel
>motors and single "synchronous" capstan drive motor),
>Dokorder model 7100 Reel-Reel tape recorder (circa the
>late 70's) has a bona-fide "synchronous" motor which
>drives the capstan and, indeed, the rotor *is*
>magnetized (verified one day when I had it apart
>changing belts (or was that the time when the
>erase-bias oscillator failed?).

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2002\03\13@011208 by Jim

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 "how do DC powered tape players/recorders maintain
  the proper tape feed speed"

I recall, as a kid (I used to take everything apart),
an 'arm' that was thrown further out on some cheaper
units as the speed increased - I think this was in
series (it's been years now!) with a winding on the
armature in some fashion ...

Jim


{Original Message removed}

2002\03\13@011709 by David Duffy

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Sean wrote:
>This is changing the topic slightly, but I have always wondered: how do DC
>powered tape players/recorders maintain the proper tape feed speed? Do they
>have some means of measuring the speed and perform feedback or do they
>simply rely on the load and motor specs being relatively constant? Since
>even a few percent change is quite noticeable, this would seem to be
>inadequate for cheap motors.

The really cheap ones had mechanical governors. Sort of like a centrifugal
switch that opens when the motor spins too fast. (on/off regulation)
New ones have the drive circuit built in to detect the spikes in motor current
as the brushes contact different portions of the armature. Higher quality
ones are direct drive (induction) with a hall effect or similar feedback.
Regards...

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2002\03\13@020542 by Peter L. Peres

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>   "I believe that your fan has a synchronous motor. "
>
>Check this by verifying that the "rotor" is
>magnetized.

Actually 80% of the consumer fans have shaded pole synchronous motors
(squirrel cage + shaded pole or capacitor start). The rotor is not
magnetized at all. Maybe you have a different experience.

Peter

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2002\03\13@023241 by Jim

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   "shaded pole 'synchronous' motors"

I believe we are not being correct here ...

Do you believe that all AC motors are 'synchonous'
motors?

It is *only* with a magnetized rotor that a motor
becomes a synchronized (synchonous) motor - otherwise
they all 'slip' (the rotor lags and *loses* entire
cycles in relation to the exciting AC) some ...

Jim


{Original Message removed}

2002\03\13@023513 by Jim

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PERHAPS I am not being clear -

- I am referring to a rotor which is PERMANENTLY
magnetized (without an applied AC field of any
kind) via a 'magnet' or magnetization during
manufacture. I have not disasembled my Dokorder's
motor BUT I can assure you that there is definite
magentic field present in and about the rotor.

Jim

{Original Message removed}

2002\03\14@151053 by Peter L. Peres

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>    "shaded pole 'synchronous' motors"
>
>I believe we are not being correct here ...
>
>Do you believe that all AC motors are 'synchonous'
>motors?

No, only those that do not employ a commutator of any kind and have a
defined number of poles in the rotor and stator ;-). F.ex. the motor in
the electricity wattmeter is not synchronous because the rotor does not
have a defined number of poles (it's a solid disc or cylinder).

>It is *only* with a magnetized rotor that a motor
>becomes a synchronized (synchonous) motor - otherwise
>they all 'slip' (the rotor lags and *loses* entire
>cycles in relation to the exciting AC) some ...

The squirrel cage is a shorted transformer secondary winding and is
'magnetized' by the induced primary current from the stator. The principle
is the same as in the physics experiment where a metal ring jumps off an
electromagnet when it is energized. The ring is not magnetized at all (it
can be made of copper) and it will jump quite far. Some people hope to
make weapons like this (see 'rail guns' etc). Squirrel cage shaded pole
motors are as synchronous as their 2 and 3 phase cousins. Except they are
usually designed to operate with considerable slip (and this is why they
usually run hot and have low efficiency).

Peter

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2002\03\14@152332 by Jim

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  "and is 'magnetized' by the induced primary
   current from the stator."

This is understood (IOW - Duh!).

I still think there is a misunderstnading on your
part on just *what* exactly constitutes a "synchronous"
motor, per se. A motor whose rotor is *in lock* with
the phase of the incoming AC mains.

Do a web search then report back. You will find mentioned
some means in establishing a *fixed*, non-fluctuating
(with the applied AC field) steady (DC?) magnetic field
in the *rotor* of said 'synchronous' motor.

Jim


{Original Message removed}

2002\03\14@171120 by Dave Dilatush

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Peter wrote...

>Squirrel cage shaded pole
>motors are as synchronous as their 2 and 3 phase cousins. Except they are
>usually designed to operate with considerable slip (and this is why they
>usually run hot and have low efficiency).

Synchronous motors have zero slip.  That's why they're called
"synchronous"-- they rotate synchronously with the applied AC voltage.

Common induction motors (squirrel-cage, etc.) have slip, and it's from
this slip that they get their torque; synchronous motors don't have any
slip, and get their torque from the angle by which the rotor's static
magnetic field lags behind the applied AC field.

I think.

Dave

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2002\03\14@191333 by hard Prosser

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Er,
I seem to remember that squirrel cage motors are induction motors - of
which the shaded pole design is a subset used for single phase operation.
The operating torque is roughly proportional to the slip which is the
difference between the synchronous speed (where there is zero torque) and
the actual running speed.

True synchronous motors, as used in clocks and timers, have a magnetised
(permanent magnet) rotor and the torque is proportional to the phase
difference between the rotor and the rotating field. This does mean they
can run in either direction and have low starting torque so a mechanical
arrangement is often used to ensure they can only run one way.

Larger syncronous motors may be different, and quite possibly have wound
rotors and comutators or slip rings.

Richard P










>    "shaded pole 'synchronous' motors"
>
>I believe we are not being correct here ...
>
>Do you believe that all AC motors are 'synchonous'
>motors?

No, only those that do not employ a commutator of any kind and have a
defined number of poles in the rotor and stator ;-). F.ex. the motor in
the electricity wattmeter is not synchronous because the rotor does not
have a defined number of poles (it's a solid disc or cylinder).

>It is *only* with a magnetized rotor that a motor
>becomes a synchronized (synchonous) motor - otherwise
>they all 'slip' (the rotor lags and *loses* entire
>cycles in relation to the exciting AC) some ...

The squirrel cage is a shorted transformer secondary winding and is
'magnetized' by the induced primary current from the stator. The principle
is the same as in the physics experiment where a metal ring jumps off an
electromagnet when it is energized. The ring is not magnetized at all (it
can be made of copper) and it will jump quite far. Some people hope to
make weapons like this (see 'rail guns' etc). Squirrel cage shaded pole
motors are as synchronous as their 2 and 3 phase cousins. Except they are
usually designed to operate with considerable slip (and this is why they
usually run hot and have low efficiency).

Peter

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2002\03\15@033438 by Vasile Surducan

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On Thu, 14 Mar 2002, Dave Dilatush wrote:

>
> Synchronous motors have zero slip.  That's why they're called
> "synchronous"-- they rotate synchronously with the applied AC voltage.
>
 Theoretically, yes. I was working about 5 years into a plant where the
compresed air was giving by some pumps driven by synchronously 6KV/300KW
motors.
They all have a small slip. It was a real problem to obtain the
syncronously revolution and keep it so.

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2002\03\16@035706 by John

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Hello Peter & PIC.ers,

I beg to differ with some of the bandiment around AC motors..

>
>>    "shaded pole 'synchronous' motors"

..
A shaded-pole motor has a rotating field on the (generally..) stator
which is synchronous to the incoming mains frequency.
No torque exists until the rotor slips somewhat.
The rotor turns due to the torque, but *not* at the synchronous speed.
It is called an `asynchronous motor', because at rated mechanical load the
rotor runs at a speed below synchronous.

This also applies to sq.cage motors.
..

>>
>>I believe we are not being correct here ...
>>
>>Do you believe that all AC motors are 'synchonous'
>>motors?

..
There *are* synchronous motors, and generators.  Power station generators
are all synchronous.., but it is possible to generate using an asynchronous
machine provided you already possess a solid-frequency grid to feed into.
Y'just overhaul the `motor' & suddenly it's a genny.

Synchronous motors aren't common, but were popular in spinning mills to keep
bobbins winding in sync., and did a real good job in mains-powered
mantelpiece clocks about a generation ago. Plug-in timers still use them
& this sort have permanently magnetised rotors, this lets them be
synchronous.

The biggies have the property that, by over-exciting the wound rotor (field)
you can create leading power-factor conditions, i.e. the current leads the
line voltage in phase.  This can be used to compensate for a customer's
other lagging p.f. loads & save him a pile of max. demand tariff costs.
..mmm.. I don't think anyone uses clock motors for this.. it's not viable
till you're in the megawatt range.
( now, a megawatt clock would really be something :)  wonder if Bob Blick's
listening )
..

{Quote hidden}

..
...but t t  ... the instant you have slip, you are no longer `synchronous'
...


>
>Peter
>

..
:)
Did I miss anything?


       best regards,   John


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