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Often when we are talking about robots, the word "servo" really means an remote control (RC) servo motor. This remote control (RC) servo motor is a small box designed for use in hobby airplanes and cars. 

There are several types of servo motors. But below just introduce a simple DC type. 

Because stepper motors can be controlled by turning coils on and off, it's easy to control with digital circuitry and microcontroller chips.  

First, on the servo motor, duty cycle (on-time vs. off-time) has no meaning whatsoever. All that matters is the absolute duration of the positive-going pulse, which corresponds to a commanded output position of the servo shaft.  

The function of the servo is to receive a control signal, which represents a desired output position of the servo shaft, and apply power to the DC motor until its shaft turns to the position. The servo uses the position-sensing device to determine the rotational position of the shaft. 

With the DC motor, its speed (power level) is controlled using a technique of pulse width modulation (PWM). This is idea of controlling the motor's power level by strobing the power on and off. The key concept is duty cycle - the percentage of "on time" versus "off time." If the power is on only 1/2 of the time, the motor runs with 1/2 the power of its full-on operation.

Where "h" is the harmonic order, "k" is any integer, and "q" is the pulse number of the device/circuit. 

This can also be achieved by using dual-wound transformers, where the primary is delta and the secondary is wound with two output windings: a wye and a delta. The result is on transformation being 30° apart, creating the equivalent of a 12-pulse unit. The same result is achieved by using two transformers, one wired delta-delta, and the other delta-wye. The figure above shows this application. 

When installed close to the line-side terminals of the equipment, filters provide the lowest impedance path for the harmonics currents to flow. Thus these currents will not be introduced into the rest of the distribution system, where they can cause wave distortion and damage or tripping of power factor correction capacitors. The figure above shows this concept.  

The 5th harmonic is a negative sequence harmonic, and the resulting magnetic field revolves in a direction opposite to that of the fundamental field at a speed five times that of the fundamental field. And the 7th harmonic is a positive sequence harmonic, with a resulting magnetic field revolving in the same direction as the fundamental field at a speed of seven times the fundamental. The net effect is a magnetic field revolving at a speed of six times the speed of the rotor.