Electrical Machine Design -II — Summer 2019

Define dispersion co-efficient and discuss its effect on maximum power factor and overload capacity for 3-ph induction motor.

Define Short Circuit Ratio (SCR) and discuss its effect on the performance of synchronous machine.

Derive the equation for relationship between rating and size of the machine for 3-ph induction motor.

State and explain the steps for an auxiliary winding design of a resistance split 1-ph induction motor.

What is the role of damper winding synchronous machine? Derive the equation of MMF of damper winding.

Explain how selection of L\τ ratio, peripheral speed and ventilating duct affects the main dimension of 3-ph induction motor design.

Calculate the values of (i)) Current in rotor bar and end ring (ii) Area of rotor bar and end ring for 11 kW, 3-ph, 6 pole 50 Hz, 415 V, star connected induction motor having an efficiency of 0.86 and full load power factor of 0.85(lag). Assume number of stator slots = 54, conductor per stator slots = 9, stator slots = 57, rotor mmf = 85 percent of stator mmf, current density = 5.5 A/mm2.

Explain the factors that affects the selection of number of stator slots for 3- ph induction motor design.

Calculate the main dimensions of 15 kW, 3-ph, 415 V, 50 Hz, 2850 RPM, squirrel cage induction motor having an efficiency of 0.85 and full load power factor of 0.89(lag). Assume winding factor = 0.955, specific magnetic loading = 0.55 Wb/m2, specific electric loading = 25,000 A/m and peripheral speed = 25 m/s.

Explain the factors that effects the choice of specific magnetic loading for synchronous machine design.

A 1200 kVA, 3-phase, 50 Hz, 3.3 kV, star connected, 300 RPM synchronous generator with a concentric winding has B = 0.60 Wb/m2, ac av = 35,000 A/m, gap length = 5 mm, field turns per pole = 60, short circuit ratio = 1.2, effective gap area = 0.6 times the actual area, peripheral speed = 30 m/s. Find the stator core length, stator bore diameter, turns per phase1 and armature mmf per pole.

Explain the factors that effects the choice of air-gap length for synchronous machine design.

Estimate the diameter, core length, size and number of conductors of a MVA, 11 kV, 3 phase, 50 Hz, 3000 RPM, star connected cylindrical rotor alternator with the armature winding having a 60° phase spread. Assume B = 0.55 Wb/m2, ac = 36,000 A/m, δ = 5 A/mm2, peripheral speed = 160 av m/s.

Prove that the output of 1-ph induction motor is two third of that of 3-ph induction motor for same loadings, number of poles, power factor and efficiency.

State and explain the steps for rotor design of a 1-ph induction motor.

Derive the equation of capacitance to get maximum starting torque for capacitor start 1-ph induction motor.

Derive the equation of leakage reactance of 1-ph induction motor.