why the torque of wiper motor increase for low speed?

Question

as i get the information from internet,an unscientific test (using one wiper motor and a torque wrench) found that at 12 volts, on high speed, the motor has 13.5 pound-feet and on low speed, has 17.5 pound-feet of stall torque. why this happen while the current drains to the motor during no load for high speed is higher than low speed? (high speed no load current is 1.95A; low speed no-load current is 0.9A) the higher the current mean higher power input, but y the torque is less?

Answer 1

because at high speed you still only have 12V

Answer 2

Wiper Motor Torque

Answer 3

Here is a paper from an MIT student, using a motor very similar to the kind used for wipers. He plots curves of torque vs. speed, and torque vs. current.
http://pergatory.mit.edu/2.007/kit/actuator/Tamiya/data/9_volts/tamiya_9V_AA.htm
Current is a proportional function of torque, but speed is inversely proportional to torque.

The basic DC motor equations are:
Torque = [motor constant] * [magnetic flux] * [armature current]
Speed = [motor constant] *[armature V - (armature I * motor R)] / [magnetic flux]

Notice the '-' sign in front of the armature current in the Speed equation.
These equations apply to all DC motors, series, shunt or compound.
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