Unit4 Three Phase Induction Motor Part -2 Air Gap, Rotor Design N.Pandiarajan Associate Professor/EEE SSNCE

Unit4 Three Phase Induction Motor

Part -2 Air Gap, Rotor Design

N.PandiarajanAssociate Professor/EEE

SSNCE

Prepared By N.Pandiarajan Associate Professor/EEE SSNCE

Length of the Air GapLength of the Air Gap is decided by the following factors

Power factor Over-load capacity Pulsation loss Unbalanced magnetic pull Cooling Noise


Power factor Length of air gap that primarily determines

the magnetizing current drawn by the machine.

Air gap is long – High value of no load current – Results in poor power factor


Over-load capacity Greater is the length of air gap, greater is

the overload capacity

Pulsation loss Pulsation loss is less with larger air gap

Unbalanced magnetic pull Unbalanced magnetic pull is less with

larger air gap


Cooling Cooling is better with larger air gap

Noise Noise is increased with larger air gap


Length of the Air Gap So When we need good power factor, then

the length of the Air Gap is as small as mechanically possible in order to keep down the magnetising current.

But if we need higher over load capacity, better cooling, reduction in noise or reduction in unbalanced magnetic pull length of the Air Gap should be large.


DESIGN OF ROTOR DESIGN OF SQUIRREL CAGE ROTOR

DESIGN OF SLIP RING ROTOR


DESIGN OF SQUIRREL CAGE ROTOR

Rotor bar current Area of Rotor bar Rotor end ring Rotor Slots Rules for selecting rotor slots Area of rotor end ring


ROTOR BAR CURRENT


AREA OF ROTOR BAR


Shape of rotor slots In case of squirrel cage motor the cross-section of

bars will take the shape of the slot Insulation is not used between bars and rotor

core. The rotor slots for squirrel cage rotor may be

either closed or semi – enclosed types. The semi enclosed type provides better over load

capacity.


Advantages of closed slots Low reluctance Less magnetizing current Quieter operation Large leakage reactance and so starting

current is limited


Disadvantage of closed slots Reduced over load capacity


Rectangular Bars & Slots Generally the rotor slots and slot bars are

rectangular in shape. In rectangular bars, during starting most of

current flows through top portion of the bar and so the effective rotor resistance is increased. This improves the starting torque.





ROTOR END RING CURRENT






















DESIGN OF SLIP RING No of rotor slots No of rotor conductors Rotor current C.S of rotor conductor Depth of rotor core


Parts of SLIP RING ROTOR

Laminated Core Semi-enclosed Slots Three Phase Winding Slip rings and brushes


Number of Rotor Slots Same Procedure as done in choice of

squirrel cage rotor slots


Rotor windings Small Motors - Mush Windings – i.e several

wires in parallel per turn Large Motors - Double Layer Bar Type

Wave Winding


Number of Rotor Turns



Slip rings and brushes The wound rotor consists of three slip rings

mounted on the shaft. The rings are made of either brass or

phosphor bronze. The cross-section of the slip-ring will be

rectangle.


Slip rings and brushes The brushes used are made of metal

graphite. The metal graphite is an alloy of copper

and carbon with very low resistance and high mechanical strength .

The dimensions of the brushes are decided by assuming a current density of 0.1 to 0.2 A/mm2.


DISPERSION COEFFICIENT



REDUCTION OF HARMONIC TOROUES

Chording Integral slot winding Skewing Increasing the length of air gap.


Documents

Unit4 Three Phase Induction Motor Part -2 Air Gap, Rotor Design N.Pandiarajan Associate Professor/EEE SSNCE