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I have disassembled an old tape deck to use its motor for a home project. Turns out that depending which wire I connect to + or - on my power source the motor goes a lot stronger/faster. So far I have only encountered motors that change direction if I switch + and - around. Why is this one different?

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closed as primarily opinion-based by Andy aka, DoxyLover, Dmitry Grigoryev, laptop2d, Daniel Grillo Jan 19 '17 at 10:30

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Is it just the supply and motor? Are there any other circuitry attached? \$\endgroup\$ – Umar Jan 15 '17 at 11:37
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    \$\begingroup\$ If several people offer plausible but different answers, how will you decide which answer to formally accept? Do you see the problem here? Your question doesn't provide sufficient information to do anything but solicit opinions. \$\endgroup\$ – Andy aka Jan 15 '17 at 11:58
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There are a couple of reasons why this might happen.

1) Many DC motors have the brushes arranged for maximum torque and/or speed when the motor is spinning in one particular direction. Something like ignition timing "Advance" in a gasoline internal combustion engine.

This is done to compensate for the time that it takes the magnetic field to decay as the brushes / commutator breaks the current to one armature winding and moves to the next.

I used to play with this when I was very young - it was easy to change the brush phase angle in some older DC permanent-magnet motors intended for toys. You would simply adjust the phase angle by rotating the rear plate holding the rear bushing and brush assembly until the motor was spinning at the fastest possible speed. Then re-crimp the tabs that hold the plate in place.

The motor would always spin faster in one direction compared to the other.

2) (more likely) The motor that drives the capstan in many older tape decks has an internal centrifugal speed governor. This is a weight attached to a set of contacts that is part of the armature. When the motor reaches the correct speed, the contacts open and either reduce or remove power from one or more windings on the armature.

This very simple speed regulator is surprisingly effective.

But: it only regulates speed when the motor is spinning in one particular direction.

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