Re: Sewing machine motor - reversing direction

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Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Hi:

Thanks for all the answers.
I tried to reverse the connections of the brushes, and the motor 
really reversed.
But it also started to get much hotter than the usual.
I see two reasons for this: 
The first is trivial: The motor has a fan, that normally blows
air from the front of the motor to the back of it, where the
brushes are. Reversed, it blows the hot air of the back area
across the motor, heating it.
The second is more interesting, and has some (remote) relation
with Tesla coils:
An universal motor is normally wired so the coils at the stator
and the rotor attract, rotating the motor in a sense that increases
the inductance of the series combination of the stator and the
rotor. This is obtained by increasing the coupling between the
stator and rotor coils (Ltotal=Lstator+Lrotor+2M).
With the connections of the rotor reversed, the force is now
of repulsion, and again the motor turns in the direction that
increases the inductance of the series combination. but now 
the mutual inductance is negative, and this is obtained by
-reducing- the coupling.
In the normal connection, the inductance of the motor is always
greater than the sum of the inductances of the stator and the
rotor. In the reversed connection, it is always smaller.
As the motor is powered by AC, smaller inductance causes greater
current, and greater losses in the resistance of the wiring and
at the brushes. The effect is more clearly seen when the motor
is running at low speed.

Conclusion: I left the motor as originally wired and changed
my plans.

Antonio Carlos M. de Queiroz