I understand now, the regulator just absorbs the extra voltage when it exceeds the maximum allowed battery voltage.
A three phase rectifier with a 15V power zener connected across the output should do the job, a power transistor and lower power transistor might be more cost effective than a power zener.
Gregory,
Did you try to power the rectifier/regulator with the battery disconnected? That would certainly destroy the regulator because it isn't designed to absorb all the power generated by the motor.