Electrical Drives and Controls (EDC) MCQs | Page - 1

 the armature is disconnected from the supply and then a braking resistor rb is immediately connected across it. the motor acts as a generator, driven by the inertia and stored kinetic energy dissipating power in rb. this is a simple method of bringing a motor nearly to a standstill.

 in plugging energy is wasted in braking resistance which is equal to starting resistance while running as a motor. in dynamic braking energy is generated but it is not fed back to supply. in regenerative method energy is sent back for reuse.

 in case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other. dc motor will be heated up and it may burn.

 if back emf vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply. if supplying unit didn’t trip down by this time, excess current in armature may heat up the armature.

 all other parts except brushes and commutator are same in ac machine when outer looks are only taken in consideration. commutator is used only in dc machine for providing mechanical rectification and not in ac machine.

 maximum temperature rise can be sustained by field winding, as it is not involved in rotary parts. field winding is present away from rotary parts of the machine, so temperature rise in the machine will not produce any effect in machine rotations.

 flemings laws can be summarized as whenever, a current carrying conductor comes under a magnetic field, there will be a force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor.

 name plate of the motor shows rated values i.e. rated speed, rated current, rated voltage. it also shows output power available at shaft when all other quantities are set to rated values.

 in case of dc motor for the constant output power, ea×ia = t×ω. as t×ω

 for a motor, from power equation it is known that,