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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
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Cooling Cooling is better with larger air gap
Noise Noise is increased with larger air gap
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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.
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
DESIGN OF ROTOR DESIGN OF SQUIRREL CAGE ROTOR
DESIGN OF SLIP RING ROTOR
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
ROTOR BAR CURRENT
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
AREA OF ROTOR BAR
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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.
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Advantages of closed slots Low reluctance Less magnetizing current Quieter operation Large leakage reactance and so starting
current is limited
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Disadvantage of closed slots Reduced over load capacity
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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.
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
ROTOR END RING CURRENT
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
DESIGN OF SLIP RING No of rotor slots No of rotor conductors Rotor current C.S of rotor conductor Depth of rotor core
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Parts of SLIP RING ROTOR
Laminated Core Semi-enclosed Slots Three Phase Winding Slip rings and brushes
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Number of Rotor Slots Same Procedure as done in choice of
squirrel cage rotor slots
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Rotor windings Small Motors - Mush Windings – i.e several
wires in parallel per turn Large Motors - Double Layer Bar Type
Wave Winding
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Number of Rotor Turns
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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.
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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.
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
DISPERSION COEFFICIENT
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
REDUCTION OF HARMONIC TOROUES
Chording Integral slot winding Skewing Increasing the length of air gap.
Prepared By N.Pandiarajan Associate Professor/EEE SSNCE
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