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  Coil Pitch

Introduction to Coil Span or coil pitch:

It is the distance among the two active sides of similar coil under adjacent opposite poles. It is described in terms of number of slots each pole or electrical degrees.

Full pitch coil:

A coil comprising a coil span equal to 1800ed is termed as a full pitch coil.

Short pitch coil:

A coil comprising a coil span less than 1800ed through an angle â, is termed as a short pitch coil, or fractional pitch coil. It is also termed as a chorded coil.

á= 0 for full pitch winding

á = xâ for short pitch winding

also á = 180

               S/p

Where á stands for = short pitch angle or an angle less than 1800 ε

            â stands for  = angle between adjacent slots

            X stands for = 1, 2, 3,...  an integer

Illustration 3: Find the angle between adjacent slots of a 3 phase, 6 pole motor having 36 slots.

Solution: slots per pole = 36/6= 6

                According to the equation 8 â = 180/(s/p)= 300 ε

Pitch Factor / coil span factor / chording factor, KP: while the two sides of similar coil are short pitched through an angle á, as displayed in the below diagram (b), the emf induced in the two coil sides comprise a phase angle difference of á0.  Because of phase angle difference, the actual emf is reduced through a factor cos α/2 and is termed as a pitch factor / coil span factor / chording factor.

KP = cos α/2

Illustration 4: Find out the pitch factor for a three phase 4 pole ac machine wound in 36 slots with a coil span of 1400ε.

Solution: by the diagram (b) below, short pitch angle, á = 1800 - 1400 = 400ε. From above equation:

Kp = cos (α/2) = cos (40/2) = 0.94

278_Coil Pitch Homework Help 2.jpg

Diagram - Full Pitch and Short Pitch coils

Distribution Factor, Kd: It is described like the ratio of phasor addition of emfs induced in every the coils distributed in m slots within 1 pole region to their arithmetic addition of emfs induced in all the coils distributed in m slots within 1 pole region.

Kd = [Sin(mβ/2)]/[m Sin(β/2)]

Where m = s/3p   = number of slots per pole per phase         

Illustrations 5: calculate the distribution factor for a 3 phase 4 pole ac machine wound in 36 slots with a coil span of 1400ε.

Solution: From equation, m = 36/(3x4) = 3

The angle between adjacent slots: â= 180/(s/p) = 180/(36/4) = 20o and Kd = 0.96

Winding Factor, KW : It is described like the product of pitch factor and distribution factor.

Equation: KW = KP KD                                            

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