Generator rotation

[donQ]

I was checking a number of model A generators. First I checked to see if they would run as a motor. All but one ran clockwise when facing the pulley. This one ran CCW. I then used my motor fixture to see if they would generate. Most charged at 3-4 amps and the voltage at the battery increased. The one that motored CCW, showed - 9 amp on the ammeter and dropped the voltage at the battery.
Is this simply a case of the residual magnetism being of the wrong polarity, have the field winding connections been interchanged, or is there some other problem?
Thanks
DonQ

[MikeK]

They should rotate in the same direction.

Interestingly, even with a generator internally swapped so it runs backwards, if the field core has the correct 'residual' orientation it will still run as a generator with the same output polarity.

The problem then becomes the third brush, which acts 180 degrees out of phase, and the very real possibility that transients while disconnecting and stopping will reverse the desired polarization direction of the iron field core.

Many people successfully convert from +6V to -12V ground simply by repolarizing, but the above mentioned problems may be troublesome. Best to switch the field to armature connections to establish the correct run direction.

[Tom Wesenberg]

A guy in our club installed new field windings and had the two wires wrong for ground and third brush. It acted just as you described the one generator running backwards.

[donQ]

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Thanks to Mike K and Tom. I'll follow up on the field winding connections and go from there.
donQ