Synchronous induction motor

Introduction :

In the applications where high starting torque and constant speed are desired then synchronous induction motors can be used. It has the advantages of  both synchronous and induction motors. The synchronous motor gives constant speed whereas induction motors can be started against full load torque.

connections and working principles :

Consider a normal slip ring induction motor having three phase winding on the rotor as shown in the Fig.

The motor is connected to the exciter which gives d.c. supply to the motor through slip rings. One phase carries full d.c. current while the other two carries half of the full d.c. current as they are in

parallel. Due to this d.c. excitation, permanent poles (N and S) are formed on the rotor.

Initially it is run as an slip ring induction motor with the help of starting resistances. When the resistance is cut out the motor runs with a slip. Now the connections are changed and the exciter is

connected in series with the rotor winding's which will remain in the circuit permanently.

 As the motor is running as induction motor initially high starting

torque (upto twice full load value) can be developed. When d.c.

excitation is provided it is pulled into synchronism and starts running at constant speed. The synchronous induction motor provides constant speed, large starting torque, low starting current and power factor correction.

Semiconductors and its types

A Semiconductor is a substance which has resistivity in between conductor and insulator .Copper is a good conductor which is resistivity is 1.7*10^-8.Germanium is a semiconductor which is reistivity is 0.6 ohm meter.A Glass is a good insulator which is resistivity is 9*10^ ohm meter.A Nichrome is a resistance material which is resistivity is 10^-4 ohm meter.

Comparing the resistivity of above materials it is apparent that resistivity of germanium is quite high as compared to copper but is quite low when compared with glass.This is shows that resistivity of a semiconductor lies in between conductors and insulators.

Types of semiconductors :

The semiconductors are classifieds as followed

(i) intrinsic semiconductors

(ii)Extrinsic semiconductors

In every elements,the atoms are tied together by the bonding action of valence electrons.Si and Ge atoms contains only 4 valence electrons.These electrons have tendency to fill the outer most orbit.In this way the electrons placed to in the last orbit of an atom share the electrons with their neighbor atoms. Similarly all the electrons are tied together with their neighboring atoms.

(i)Intrinsic Semiconductors :

A pure semiconductor is called intrinsic semiconductor.The silicon and Ge atoms contain only four electrons in the outermost orbit.So they are called tetravalent bonds.In an intrinsic semiconductors even at room temperature hole pair are created.

When electric field is applied across an intrinsic semiconductor ,the current conduction takes same place by two process which is free electrons and holes.Now no electrons get away from the co-valent bonds.So current flows is zero.

Metal oxide Semiconductor FET

Introduction :

A field effect transistor that can be operated in the enhancement -mode is called a MOSFET. A MOSFET is a important semiconductor device and can be used in any of the circuits covered for JFET.

The main draw back of a JFET is that gate must be reverse biased for proper operation of the device which means that negative gate operation for n-channel and positive gate operation for p-channel.This means that we can only decrease width of the channel.This type of operation is called depletion mode operation.There fore JFET can only operate in the depletion mode.

Symbols for MOSFET :

Types of MOSFET :

there are two basic types available in MOSFET 

1.D-MOSFET or Depletion type MOSFET .This type of MOSFET can be operated in both depletion mode and enhancement mode.

2.E-MOSFET or Enhancement type MOSFET.This type of MOSFET can be operated only in enhancement -mode.

1.D-MOSFET : 

The above figure shows that n-channel MOSFET is a piece of n-type material with p-type region on the right and insulated gate on the left side then free electrons are moving from source to drain must passes through the narrow channel between the gate and the n-type region.

It is usual practice to connect the substrate to the source internally so that a MOSFET has three terminals which are source,drain,gate.

gate insulated from the channel we can apply either positive or negative voltage to the gate.therefore D-MOSFET can be operated in both modes then JFET can operated only one mode which means that depletion mode.

The figure given in below the n-channel D-MOSFET 

2.E-MOSFET :

The E-MOSFET has no channel between source and drain unlike the D-MOSFET. The E-MOSFET requires a proper gate voltage to form a channel .It is reminded that E-MOSFET can operate only enhancement mode only.The constructional details of a n-channel E-MOSFET figure given in below.