Special Electric Machines

Special Electric MachinesGeneral Information

Motor BasicsWhat is a motorConverts electrical energy into kinetic energy

Where did it come fromIdentification of rotating magnetic field principle by Nicola Tesla in 1882Introduction of Electric Motor by Nicola Tesla in December 1889 (U.S. Patent 0416194)

How It WorksWhen electric current passes through a coil in a magnetic field, the magnetic force produces a torque which turns the motor.

Force in Motor:F=ILBF = Force B = Magnetic FieldL = Length of ConductorI = Current in Conductor

Torque in Motor:T = IBA sin A = LWL = Length of WindingW = Width of Winding

Images courtesy of Wikipedia(http://en.wikipedia.org/wiki/Electric_motor)3A simple DC electric motor. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation. The armature continues to rotate. When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.

Brushless DC MotorNo Commutators

Position of Coils with respect to the magnetic field is sensed electronically.

Current is commutated through electronic switches to appropriate phases. How it WorksHalls Sensors sense the position of the coils

The Decoder Circuit turns appropriate switches on and off

The voltage through the specific coils turns the motor

Images courtesy of Servo Magnetics(http://www.servomag.com/flash/2-pole/2pole-bldc-motor.html)

AdvantagesIncreased Reliablilty & Efficiency

Longer Life

Elimination of Sparks from Commutator

Reduced Friction

Faster Rate of Voltage & Current

Precision Voltage & Current Applied to Field Coils

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9Switched Reluctance Motors

IntroductionThe switched reluctance motor (SRM) is an electric motor in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized.SRM is a type of synchronous machine. It has wound field coils of a DC motor for its stator windings and has no coils or magnets on its rotor.

It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient, singly excited machine.