Govt. Exams
Entrance Exams
Delta-Star (Dy) connection introduces a 30° phase shift. Secondary voltage leads primary by 30°, which is standard for Dy11 configuration.
Compound DC motors combine advantages of series (high starting torque) and shunt (stable speed) motors, making them ideal for cranes requiring both high starting torque and controlled speed.
Star-delta connection reduces neutral point problems on primary (generator side) and utilizes delta on secondary for better voltage utilization.
At starting, back EMF is zero. Starting resistance limits initial armature current to safe values: Ia = V/(Ra + Rstart)
Both star-delta and autotransformer starting reduce starting current. Star-delta reduces current by 1/3 of DOL current.
Ns = 120×50/4 = 1500 rpm; Nr = Ns(1-s) = 1500×0.95 = 1425 rpm
Back EMF in DC motor Eb = PΦZn/(60A), where P = poles, Φ = flux, Z = conductors, n = rpm, A = parallel paths
P = √3 × V × I × cosφ = 1.732 × 400 × 50 × 0.8 = 27,712 W ≈ 27.71 kW
R'2 = R1 × (N2/N1)² = 2 × (230/2300)² = 2 × 0.01 = 0.02 Ω
The damper winding (squirrel cage) in a synchronous motor provides the initial asynchronous torque for starting until synchronism is achieved.
