VACUUM CIRCUIT BREAKERS

Circuit breakers are devices which can automatically break a circuit under fault conditions and it can make the circuit manually after the fault. Circuit breakers are classified into different types. One of the main circuit breaker among them is Vacuum circuit breaker. Here vacuum is used to extinct the arc. Arc extinction is the main process of a circuit breaker. Before that, it is needy to explain how an arc is produced. Circuit breaker has two contacts- Fixed and movable, otherwise called as electrodes. Normally the contacts remain closed but if any fault occurs these contacts open automatically and of course can be opened manually.
           We know that circuit breakers work under the instructions from relay. When the trip coils from relay energize, circuit breaker identifies fault occurred and hence the contacts are pulled apart. A high intensity current flows through the contacts of the circuit breaker before they are opened by the system. At the same instant contacts begin to separate and causes heavy heat and temperature in between the contacts. This is termed as an arc. This heat is highly dangerous and can cause heavy damage to the whole system. So it has to be extinguished in no time.
         Vacuum is having superior qualities over other arc quenching medium. It has high insulating strength. When the contacts are opened in vacuum, the arc is produced in between them. It got extinguishes quickly since the metallic vapours and ions produced during the arc rapidly condense on the surface of the circuit breaker contacts, resulting in quick recovery of dielectric strength.
The contacts of Vacuum circuit breaker and arc shied is mounted inside a vacuum chamber. The chamber is connected to the control mechanism by means of stainless steel bellows. A glass vessel is used as the outer insulating body. This is all about its construction.
      Vacuum circuit breaker is really the most reliable one with low cost, no fire hazards, no gas generation, low arc energy, little maintenance and low inertia.

DC MOTOR TYPES AND CONTROL





dc motor



DC motor principles

              DC motors consist of rotor-mounted windings (armature) and stationary windings (field poles). In all DC motors, except permanent magnet motors, current must be conducted to the armature windings by passing current through carbon brushes that slide over a set of copper surfaces called a commutator, which is mounted on the rotor.  

     The commutator bars are soldered to armature coils. The brush/commutator combination makes a sliding switch that energizes particular portions of the armature, based on the position of the rotor. This process creates north and south magnetic poles on the rotor that are attracted to or repelled by north and south poles on the stator, which are formed by passing direct current through the field windings. It's this magnetic attraction and repulsion that causes the rotor to rotate

The greatest advantage of DC motors may be speed control. Since speed is directly proportional to armature voltage and inversely proportional to the magnetic flux produced by the poles, adjusting the armature voltage and/or the field current will change the rotor speed.

            Today, adjustable frequency drives can provide precise speed control for AC motors, but they do so at the expense of power quality, as the solid-state switching devices in the drives produce a rich harmonic spectrum. The DC motor has no adverse effects on power quality.

Major types of dc motors

§        Self excited dc motor

§        Series dc motor

§        Shunt dc motor

§        Compound dc motor

§        Separately excited dc motor

§        Permanent magnet dc motor

Series motors

          Series motors connect the field windings in series with the armature.

Series motors lack good speed regulation, but are well-suited for high-torque loads like power tools and automobile starters because of their high torque production and compact size.

Shunt motors

Shunt motors use high-resistance field windings connected in parallel with the armature.

Varying the field resistance changes the motor speed.  Shunt motors are prone to armature reaction, a distortion and weakening of the flux generated by the poles that results in commutation problems evidenced by sparking at the brushes. Installing additional poles, called interpoles, on the stator between the main poles wired in series with the armature reduces armature reaction.

Compound motors

In this motor the concept of the series and shunt designs are combined.

Separately Excited Motor

There is no direct connection between the armature and field winding resistance. DC field current is supplied by an independent source (such as battery or another generator or prime mover called an exciter)

Permanent Magnet motors

PMDC is a dc motor whose poles are made of permanent magnets.

Do not require external field circuit, no copper losses

No field winding, size smaller than other types dc motors

Disadvantage: cannot produce high flux density, lower induce voltage