There are several disadvantages to this: • The machines must be close together. • The ratio by which the slave follows the master requires physically replacing gears to change. • Shifting the "phase" between master and slave requires a complicated design. • Decoupling undesirable mechanical characteristics from one machine to the other is difficult. Let's see how master/slave overcomes these restrictions. Consider the basic closed motion control loop:

 

 

The magnetic field generated in the stator induces an EMF in the rotor bars. In turn, a current is produced in the rotor bars and shorting ring and another magnetic field is induced in the rotor with an opposite polarity of that in the stator. The magnetic field, revolving in the stator, will then produces the torque which will “pull” on the field in the rotor and establish rotor rotation.

 

 

 

In addition to being classified by their step angle stepper motors are also classified according to frame sizes
which correspond to the diameter of the body of the motor. For instance a size 11 stepper motor has a body diameter of approximately 1.1 inches. Likewise a size 23 stepper motor has a body diameter of 2.3 inches (58 mm), etc. The body length may however, vary from motor to motor within the same frame size classification. As a general rule the available torque output from a motor of a particular frame size will increase with increased body length.

 

Power levels for IC-driven stepper motors typically range from below a watt for very small motors up to 10 –
20 watts for larger motors. The maximum power dissipation level or thermal limits of the motor are seldom
clearly stated in the motor manufacturers data. To determine this we must apply the relationship P␣ =V ×␣ I.
For example, a size 23 step motor may be rated at 6V and 1A per phase. Therefore, with two phases energized the motor has a rated power dissipation of 12 watts. It is normal practice to rate a stepper motor at the power dissipation level where the motor case rises 65°C above the ambient in still air. Therefore, if the motor can be mounted to a heatsink it is often possible to increase the allowable power dissipation level. This is important as the motor is designed to be and should be used at its maximum power dissipation ,to be efficient from a size/output power/cost point of view.