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Chapter: Electrical machines - Electromechanical Energy Conversion and Concepts in Rotating Machines

Mmf of Distributed Windings

1. Alternating Field Distribution 2. Rotating field 3. Three-phase winding 4. Determination of slot mmf for different moments (temporal)

Mmf Of Distributed Windings

 

1. Alternating Field Distribution

 

Spatial field distribution and zerocrossings remain the same, whereasthe field strength amount changes periodically with current frequency.This kind of field is called alternating field.

 




The fundamental wave of the square-wave function (Figure. 131 etc.) can be determined byFourier analysis. This results in an infinite count of single waves of odd ordinal numbers andanti-proportional decreasing amplitude with ordinary numbers. The amplitudes offundamental waves and harmonics show proportional dependency to the current, zerocrossings remain the same. These are called standing wave. The existence of harmonics isto be attributed to the spatialdistributions of the windings.The generating current is ofpure sinusoidal form, notcontaining harmonics. it necessarily needs to be distinguished between

 

wave: spatiotemporal behaviour,

oscillation: pure time dependent behavior

 



2. Rotating field

 

Rotating fields appear as spatial distributed fields of constant form and amount, revolving with angular speed w1:


A sinusoidal alternating field can be split up into two sinusoidal rotating fields. Their peak value is of half the value as of the according alternating field, their angular speeds are oppositely signed

 

3. Three-phase winding

Most simple arrangement of a three-phase stator consist of:

Score stack composed of laminations with approximately 0,5 mm thickness, mutual insulation for a reduction of eddy currents

2. The number of pole pairs is p=1 in Fig.138. In case of p>1, the configuration repeats p-times along the circumference.



 

4. Determination of slot mmf for different moments (temporal)

·                    quantity of slot mmf is applied over the circumference angle.

 

·                    line integrals provide enveloped mmf, dependent on the circumference angle.

 

·                    total mmf is shaped like a staircase step function, being constant between the slots. At slot edges, with slots assumed as being narrow, the total mmf changes about twice the amount of the slot mmf,the air gap field results from the total mmf


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