Sulphur HexaFluoride gas

Sulphur Hexafluoride is a gas that finds widespread use in Electric Switchgear.  

Its introduction over forty years ago led to a revolution in the design of electric switchgear.  Its excellent dielectric and arc-quenching properties led to drastic reduction in the size of electric switchgear. 

It is non-toxic, chemically inert and high thermal stability.  These characteristics have made it an ideal gas for use in switchgear to quench arcs which may for when circuit breakers operate.  Sulphur hexafluoride is also used in Gas insulated switchgear (Switchgear which are constructed inside a cylinder containing SF6 gas).  These switchgear are compact and occupy very less space.

Sulphur Hexafluoride is not a naturally occurring gas.  It is synthesized by the exposure of Sulphur and Fluorine.  Unfortunately, the SF6 gas is a greenhouse gas an thus should never be directly released into the atmosphere.  Hence, alternatives to SF6 gas are being explored.

When circuit breakers operate and current is interrupted, arcing occurs.  This arcing can cause the sulphur hexafluoride to decompose.  The products thus formed can be corrosive. Most of the products of decomposition recombine to form sulphur hexafluoride.  However, one of the products, Disulphur decafluoride is highly toxic.  Protective equipment for breathing and eye protection should be used when working in proximity to sulphur hexa fluoride gas. 

Plugging in Motors

Plugging is a method of braking used in induction motors.  Plugging involves interchanging the supply to two of the stator phase windings.  This method is used in applications which require immediate stop applications.  When the phase supply is reversed, torque is produced in the opposite direction.  This leads to braking of the electric motor.

Motors which are operated this way have a plugging switch.  This switch operates when the stop command is given to the motor circuit.  The operation of this switch applies reverses the supply to two of the windings.  When the motor comes to a complete halt, this reversed supply is disconnected. 

However, this method of braking generates a large amount of heat in the rotor, even greater than that produced by a locked rotor.  This is due to the kinetic energy of the rotor and the coupled load.

Plugging should not be used frequently as the heat produced in this method of braking can cause the rotor bars to overheat and, sometimes, melt.

Split Phase Motors

A single phase induction motor is not self starting.  This is because the ac supply creates a pulsating magnetic field in the stator core and not a rotating magnetic field which is required for an induction motor to be self starting.

Many methods are used to start a single phase motor.  One such method is the split phase method.  The split phase motor has a main winding in the stator.  In addition to this, it has a starting winding wound in the stator.  The starting winding is connected parallel to the main winding.  When the supply to the windings.  The starting winding is displaced 90 degrees from the main winding.

This is because the starting winding has a higher resistance and occupies less space in the stator.  It is wound over a few slots in the stator and is usually placed above the main winding in the slots. The main winding has many turns and thus has a higher reactance.  Thus it lags behind the starting winding. 

Split phase motors are used where low to moderate starting torque is sufficient.  They are designed up to 1/3 hp. 

The starting winding is only used when the motor is starting.  The winding is connected through a centrifugal switch.  When sufficient rpm is reached, the centrifugal switch operates and isolates the winding. 

Coreless DC motors

Coreless dc motors are small motors which do not have a core.  The armature winding is self-supporting wound in the form of a basket. 

These motors can have extremely high acceleration due to reduced weight of the rotor.  The rotor armatures also have low inductance.  This helps in extending the life of the brushes and the commutator.

These rotors have low inertia and low noise and vibration and thus can be easily controlled.

The downside to these motors is that the rotor gets heated quickly as it does not have the core which can serve as a heat sink.  These motors can be used only for low power application such as the motor in magnetic tape devices, optical rotary encoders, etc..

image courtesy: www.micromo.com