Machine Design

Question 1

Explain distributed windings?

Answer 1

Concentrated windings make poor use of available space and air gap

Number of stator slots depended on radius of machine

Each phase has multiple coils occupying several slots around air gap

Air gap flux density distribution for distributed windings will have a better shape (more sinsusoidal) reducing space harmonic content, increasing effciency.

Question 2

What is the winding distribution factor Kd?

Answer 2

The space shift of coils in distributed windings results in a time shift in each coil

Therefore total EMF induced in a phase is vector sum of Emf in each coil

This is accounted for by kd

M is number of slots each phase band occupies

ß is angle between slots of slot pitch

Question 3

What is short Pitching?

Answer 3

Improves on single distibuted windings

allows length of end connections of phase coils to be shortened

Reducing harmonic content of air gap further

Reduces flux cut by the coil

therfore phase voltage is reduced by a factor Kp “Pitch Factor”

alpha is angle by which coil is short pitched realtive to a full pole pitch

note: double layer winding necessary for short pitching

Question 4

What is Specific Magnetic Loading?

Answer 4

B is defined as average flux density (B) measured over one pole pitch

Magnitude of B must be limited otherwise magnetic saturation will cause excessively large magnetising currents and increased iron losses

Question 5

What is specific electrical Loading J?

Answer 5

Defined as total effective current per unit length averaged around the air gap.

Magnitude of J must be limited to keep conduction losses within specified bounds

Question 6

How do you calculate peak tooth flux density?

Answer 6

Realtionship can be found by assuming that the air-gap flux over one tooth plus slot pitch passes only through the tooth

Question 7

Explain Core flux?

Answer 7

FLux that passes radially through one magnetic pole splits into two equal components and travel ciurcumferentially around the stator or rotor iron cores to the next magnetic pole of oppsoite polarity.

Flux density in stator or rotor core can be related to B by conservation of flux

Question 8

To work out turns per coil

Answer 8

Nph/coils per phase

For short pitched coils = slots

For non- short pitched cols= slots/2

Question 9

Vph =

Answer 9

k*Kw*N*Brms

Question 10

K=

Answer 10

wld/p