These N20 12V 200RPM Micro Metal Gear Motor With Encoder is intended for use at 12 V. Though in general, these kinds of motors can run at voltages above and below this nominal voltage; so they should comfortably operate in the 3 – 12 V range (rotation can start at voltages as low as 0.5 V).

Lower voltages might not be practical, and higher voltages could start negatively affecting the life of the motor. The micro metal gear motors are available in a wide range of gear ratios—from 5:1 up to 1000:1. With the exception of 1000:1 gear ratio versions, all of the micro metal gear motors have the same physical dimensions; so one version can be easily swapped for another if your design requirements change.

Robomart.com have different varieties of motors with various RPM and operating voltages; 

Features :

• Model: GA12 N20.
• This is a DC Mini Metal Gear Motor, ideal for making robots
• Lightweight, high torque and low RPM.
• Fine craftsmanship, durable, not easy to wear.
• With excellent stall characteristics, can climb hills easily.
• You can also easily mount a wheel on the motor’s output shaft.
• Motor Size: 15 x 12 x 10 mm.
• Gearbox Length: 9 mm.
• Count: 3PPR(Pulse per Revolutions).