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HIGH TORQUE INDUCTION MOTORSDeep Bar Cage Induction Motor Double Cage Induction Motor Drawbacks of Conventional Squirrel Cage Induction Motor i.e. with Low Rotor Resistance Low Starting Torque High Starting Current Low Operating p.f. Advantages of Slip Ring Induction Motor High Staring Torque Low Starting Current High Operating p.f. High resistance in the rotor is needed for good starting torque Low resistance in the rotor is needed for normal operation Hence High Torque Induction machines were developed. © DEE_JRN
DOUBLE CAGE INDUCTION MOTOR / BOUCHEROT MOTOR Constructional details: Stator – Exactly similar to Three phase Induction Motor Rotor – Squirrel Cage type, but with 2 cages (i) Outer Cage – High Tst & Low Ist Designed with High Resistance & Low Reactance Rotor bars made of Brass, Aluminium or Bronze (iI) Inner Cage – for obtaining good running condition Designed with Low Resistance & High Reactance Rotor bars made of Copper Cage windings are embedded in two rows of slots separated by narrow slit. Staggered slots are also available. © DEE_JRN
Leakage Reactance of INNER CAGE is HIGH. Impedance also HIGH. At Starting: Leakage Reactance of INNER CAGE is HIGH. Impedance also HIGH. Most of the Starting Current is confined to OUTER CAGE. High Resistance OUTER CAGE develops HIGH Starting Torque and LOW Starting Current When Motor Picks up speed: Impedance of INNER CAGE lowers. Most of the rotor current now flows through INNER CAGE INNER CAGE develops greater portion of the Running Torque © DEE_JRN
Torque Speed Characteristics – DOUBLE CAGE MOTORTwo Cages develop 2 separate torques Resultant torque is equal to the sum of the Two Cage Torques Resultant Torque Speed Characteristics can be changed by modifying the individual Cage Resistances and Leakage Reactances. Resistances can be changed by changing the c.s.a of bars. Leakage reactance – Changing the depth & width of the slot opening. © DEE_JRN
DEEP BAR CAGE INDUCTION MOTORConstructional details: Stator – Exactly similar to Three phase Induction Motor Rotor – Squirrel Cage Rotor, with deep and narrow bars. Bars of narrow width are laid down in deep semi-closed slots. Depth to Width Ratio ( to 1). The rotor bar can be imagined to be composed of elementary strips in parallel – topmost and bottom-most strips. Magnetic leakage flux pattern set up by the bar current is shown. Less on top part and more on bottom part. © DEE_JRN
Bottom Part offers more impedance to the current than Top Part. At Starting: Bottom Part offers more impedance to the current than Top Part. Maximum current flows through the top layers and minimum through bottom layers. Unequal distribution of current. Effective resistance of the bar increases. High Rotor Resistance develops HIGH Starting Torque and LOW Starting Current. When Motor Picks up speed: Reactances of all the layers of the bar are reduced. Impedances of all the layers of the bar are nearly equal. Almost equal distribution of current. Effective resistance of the bar decreases. Low Rotor Resistance develops Good Running Torque and provides good efficiency at low slips. © DEE_JRN
Torque Speed Characteristics – DEEP BAR CAGE MOTORDeep Bar Cage Motors meet the desired starting and running performances. A Deep Bar construction is employed for HIGH Starting Torque applications. Double Cage construction for still higher starting requirements. Double Cage Induction Motors are highly useful when frequent starting under heavy loads is required, such as Compressors. © DEE_JRN
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