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

Prepared By N.Pandiarajan Associate Professor/EEE SSNCE Length of the Air Gap Length 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

ROTOR END RING CURRENT

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

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

REDUCTION OF HARMONIC TOROUES Chording Integral slot winding Skewing Increasing the length of air gap.

Prepared By N.Pandiarajan Associate Professor/EEE SSNCE