from Hosfelt.com
5 VDC @ 1.6 AMP Adapter $US 4.75
Mfg. # AFK-520C1
2-7/8"x 2-1/4" x 1-7/8"
Input: 120 VAC, 60 Hz, 20 watt
Output: 5 VDC @ 1.6 amp. The enclosure and the
6 ft. cord are black. Center is negative for polarity.
Coaxial jack: OD - .215", ID - .082".
PN 56-602
When you put the 5 volts on to the motor, it will have inductance and resistance, the motor will not be at its full efficiency until it is drawing its maximum 1.6 amps.
Having a higher voltage allows the current in the motor to ramp up much more quickly to reach its maximum current in a faster time. When the motor is at its rated current, the limiter kicks in to stop the coil burning out.
If you google something like "chopper circuit" you'll get loads of info