23-05-2012, 05:07 PM
Three Phase AC Motor Theory
Types of Induction motor
. Squirrel cage induction motor : Low starting torque, Less maintenance due to robust construction of rotor, Skewed bars for reducing magnetic locking and noise (magnetic humming). (skewing has dis advantage of increased rotor resistance, causing more cupper losses, reducing output torque etc)
Slip ring induction motor : The rotor of a slip-ring motor is wound with wire coils. The ends of the windings are connected to slip rings so that resistors or other circuitry can be inserted in series with the rotor coils to increase the starting torque. More friction losses at brush, higher maintenance etc.
Some facts about induction motor
Production of a resultant flux of constant magnitude 3/2 Øm due to interaction of three fluxes 120° apart from each other and rotating with synchronous speed
θ = π/2-ωt. Which lags from main flux by 90°. Due to various laws as indicated above, torque is developed.
% slip (s) = (Ns-Nr)*100/Ns
Frequency of Rotor current fr= s*f (Supply frequency)
Air Gap Power Pg=2πNs*Td (where Td is developed torque)
(this can be referred as total mechanical power transferred from stator to rotor)
Total Mechanical Power
developed by the Rotor Pmd = 2πNr*Td