DC Motor driving and PWM

Hello Members,

I have a Pololu Dual Motor Shield Driver - VNH5019.
I have three motors,

  1. a Pololu 37D with a 131:1 gear ratio, and the encoder (64CPR)
  2. a mil spec motor rated at 12 V, shaft dia is just under .5 inch, with an encoder (32 PPR)
  3. another mil spec motor, more in line size-wise with the Pololu encoder: (100 PPR)

I am using the PID_v1 library for one drive program, and some code from the Arduino forum,
where a member used the Pololu motor and his own PID code (with just Kp and Kd!)

I have tested the Pololu and motor number 2 with the VNH5019 driver, and both work fine, from a fast
rotation (i.e. higher PWM value) and a very slow rotation (PWM value around 20, and a "speed" of about 3 to 4)

BUT - motor number 3 - will not run at lower PWM values (equates to slower rotation speed) using either the PID library or the non-library code from the Arduino post. It will run fine (more or less) at a higher to max PWM rate - from about 50 to 255.

I know the inductance of motor number 2 is about half the inductance of motor number 3 (1.3 and 2.3) - could this be the issue here? The resistance for motor number 3 is higher as well (3.2 vs. 1.2)

I do not know the similar values for the Pololu motor. Any ideas on how to get motor 3 to run as very
slow rotation? OR am I stuck with a min usable speed? I have not tried it on a variable DC power supply at yet.