1. HIGH TORQUE INDUCTION MOTORS
Deep 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

2. 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

3. 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

4. Torque Speed Characteristics – DOUBLE CAGE MOTOR
Two 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

5. DEEP BAR CAGE INDUCTION MOTOR
Constructional 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

6. 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

7. Torque Speed Characteristics – DEEP BAR CAGE MOTOR
Deep 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