Small DC motors have best speed control by armature voltage control.

Armature Voltage Control:
Armature voltage control is a method of controlling the speed of a DC motor by varying the voltage applied to the motor's armature. This method provides the best speed control for small DC motors. Here's why:

1. Speed-Torque Characteristics:
The speed-torque characteristics of a DC motor are influenced by the armature voltage. By varying the armature voltage, the speed-torque curve of the motor can be adjusted to achieve the desired speed control. Lowering the armature voltage reduces the torque produced by the motor, resulting in a decrease in speed. Conversely, increasing the armature voltage increases the torque, leading to an increase in speed.

2. Proportional Control:
Armature voltage control provides proportional control over the speed of the motor. The speed is directly proportional to the armature voltage. By adjusting the armature voltage, the speed of the motor can be precisely controlled. This makes armature voltage control ideal for applications that require accurate speed control, such as in robotics, conveyor systems, or precision machinery.

3. Efficiency:
Armature voltage control is an efficient method of speed control for small DC motors. The power loss in the motor is primarily due to the armature resistance and the voltage drop across it. By reducing the armature voltage, the power loss is minimized, resulting in improved efficiency. This is particularly important for small DC motors where energy efficiency is crucial.

4. Simplicity:
Armature voltage control is a relatively simple method of speed control. It involves varying the voltage applied to the motor's armature, which can be easily achieved using a variable resistor or a pulse width modulation (PWM) controller. The simplicity of this control method makes it suitable for small DC motors, where complex control systems may not be necessary.

In conclusion, armature voltage control provides the best speed control for small DC motors due to its ability to adjust the speed-torque characteristics, provide proportional control, improve efficiency, and its simplicity of implementation.

In DC motor field winding use as pole.So armature winding/rotor winding is used to speed control.
If it's small DC motor so it has only armature winding