Question 1:
A series DC motor is driven from a 750 V, 60 kW supply. It can be assumed that the field coil and armature resistances are so small that theu can be considered zero and the magnetic circuit is unsaturated in the analysis. The motor runs at 1100 rpm on full-load.
(a) Calculate the full-load torque and the armature back emf.
(b) Determine the full-load current drawn from the source.
(c) Calculate the DC series motor constant 'kc'
An 8 i2 resistor is placed in series with the field coil in the series DC motor to reduce the torque to 275 Nm.
(d) Determine the current flowing in the new motor configuration.
(e) Calculate the armature back emf.
(0 Find the motor's new speed (in rpm and rad/s).
Question 2: A variable voltage, 3-phase, 8-pole, 50 Hz supply induction motor has the following per phase parameters. A stator resistance of 0.2 0, a stator inductance of reactance 1.5 0, a rotor resistance of 0.1 0, a rotor inductance of reactance 0.2 0, a core resistance of 600 0 and a magnetizing inductance of reactance 200 0. It is a wound rotor motor with a stator to rotor turns ratio of 2. Determine the following per phase parameters:
(a) Zrs,
(b) ZTH, RTH, XTH
(C) VTH when the magnitude of the mechanical torque developed is 500 Nm with a motor speed of 7200 rpm.
(d) Find the mechanical torque developed under the following motor performance values:
i. VTH = 400 V, motor speed 6800 rpm,
ii. VTH = 350 V, motor speed 6500 rpm
iii. VTH = 300 V, motor speed 6000 rpm.