Type of construction of CT's:

Type of construction of CT's: 

Ring type CT or Window type CT:

This is the simplest type of CT. The core has three types of popular shapes (1) Rectangular shape (2) Oval shape (3) Ring shape.

Ring Type CT

Bar primary current transformers:

✥The core is of a Nickel-Iron Alloy, or grain oriented sheet steel. 
✥The core is continuously wound type. Before applying secondary winding,
the core is insulated by means of end collars and circumferential wraps.

Bar Primary Current Transformer

✥Recently the continuously wound cores are available in encapsulated form. Synthetic resins are used as encapsulating material.
✥The material is applied by fluidized beds or electrostatic spraying.

(A)Core of A Ring Type CT

(B) Core is taped with insulting tape

(C)Secondary winding wound on core by toroidal winding machine

(D)Exterior taping is completed with circumferential insulating wraps

✥The secondary winding conductor is then wound as the insulated core in the form of toroidal winding by hand winding or toroidal winding machine. 
✥The secondary winding is then completely wrapped by external tape with or without exterior ring ends and circumferential insulating wraps.

Current transformers for high voltage installation : 

✥Separately mounted post type CT's They are suitable for out door service.
✥They are usually installed in the outdoor switchyard.
✥The primary conductor is at voltage with respect to earth. Hence it insulated by means of an insulator column filled with dielectric oil. Alternatively SF6 gas at a pressure of 2 or 3 atmospheres is now being used.

CT for high Voltage

Types of Resistance Welding

Types of Resistance Welding:(A)Butt welding:

❉ Rods, pipes and wires are welded by this method. As shown in Fig. (A) there is a transformer, across secondary of the transformer two pieces of metal to be welded are held. The two pieces are held in clamps. One clamp is fixed and other is movable

          (A) Butt welding

❉ The pieces of pipe are made to touch each other and force/pressure is applied through springs. When supply is given to transformer, current passes through the pieces of pipes and sufficient heat is developed at the joint causing welding at the joint.

(B)Flash butt welding:

❉ This is modification of butt welding. The ends of the pipe to be welded are held together with a little force/pressure applied to them. Arcing at the joint is allowed to take place. This removes any unevenness at the joint. A weld having a fin around the joint is obtained Fig.(B)

       (B) Flash butt welding

❉ The fin can be easily removed. Two dissimilar metals can be effectively welded with this method. This welding method is faster than other methods.

(C)Spot welding:

❉ In spot welding, the welding is done at certain points on metallic sheets. The pieces to be welded are held between the two electrodes as shown in Fig.(C)

            (C) Spot welding

❉ When current passes through the electrodes a spot weld is produced between the sheets.

(D)Projection welding :

❉ It is a modified form of spot welding. The electrodes used are flat and not pointed as in case of spot welding.

         (D) Projection welding :

❉ Projections are made on one sheet so that heat is concentrated in that region. When current is passed through the electrodes. The current density and pressure required is less hence electrode life is more. The welding automatically takes place only at the projections. 

(E) Seam welding:

❉ It is similar to sport welding. In seam welding, a series of spots is produced by roller electrodes. As shown in Fig. (E)

          (E) Seam welding

❉ Two sheets are passed under the two rolling electrodes. Depending upon the number of welding current pulses are second and speed of electrodes, a series of spot welds is obtained.

(F)Energy storage welding/percussion welding:

❉ Welding is obtained by discharging the stored energy in capacitor. As shown in Fig. (F) it consists of a bridge rectifier, capacitor, switching arrangement.

    (F) Energy storage welding

❉ The pieces to be welded one kept at a certain distance apart (1.5 mm). Switch 'S' is put on position 1. So capacitor gets charged through the bridge rectifier.

Type of Transmission for Drive

Type of Transmission for Drive:

☞The motor power output available at the shaft is the transmitted to respective load through a transmission, media like chain, belt, gear, rope etc.

☞The choice is governed by most economical speed of motor. The dimensions and hence the cost of motor of given output is inversely proportional to the speed.

1. Belt drive:

This is a simple and cheap drive. Following types of belts are used: Leather belt, nylon

belt, terylene belt, rubberized cotton ply belt, hair belt.

  The belt drives are of two types:

   (1) Falt belt drive

   (ii) V-belt drive

(i)Flat belt drive :

It is a long distance drive. The belt is generally made up of leather. Other material listed in above can also be used for belt. It is preferred in horizontal position. The slack side of the belt is kept at top so that natural sag increases the area of contact of the two pulleys.

Advantages:

1. Simple.

2. Greater flexibility.

3. Any desired speed ratio can be obtained.

4. When sudden load change occurs there is a tendency of belt to slip on pulley. This is better for both driving and driven member.

Disadvantages:

1. More space is required.

2. Maximum power it can transmit is 150 to 200 kW. So transmission capacity is limited.

3. It exerts side pull on bearing causing wear and tear of bearings. Following are the methods of transmitting mechanical power of motor to the load:

(a) Direct drive: The motor shaft (driving member) is directly connected to the driven member. E.g. motor-generator set in electrical lab.

☞A flexible coupling is used for such purpose. This can be used only where speed requirement of driven member matches with speed of driving member.

(ii) V-belt drive: It is suitable for motors with rating upto 450 kW. Because of wedge effect, there is increased friction between the belt and groove. This gives a better grip of about 3 times as that of flat belt.

Advantages:

 1. Less wear and tear of bearings.

 2.Short distances can be maintained.

 3. Less noise.

2. Rope drive :

It is used for power ranges beyond the limit of V belt drive. It is a long centre drive. Main advantages of rope drive are negligible slip and ability to take sudden loads.

3. Chain drive:

It has no slip and no initial tension is present hence bending stress are eliminated. It can be used for high speed ratio (7:1). It is particularly suitable for damp and dirty conditions. The chain must run concentrically on the sprocket and at right angles to the shaft.

Chain Drive

4. Gear drive:

☞It is a short centre positive rive. Proper alignment is very essential otherwise motor shaft may bend.

Gear Drive

☞Modern straight cut gears and particular variation found in worm reduction drive are common features in many machines such as cloth calendar.

A.C. Welding Sets and Methods of AC Welding

A.C. Welding Sets and Methods of AC Welding

Welding transformer:

Single phase or three phase step down transformers are used. The transformer is designed for high leakage reactance. The secondary voltage on open circuit is about 80-100 volt. Some control device is also used along with this transformer.

The secondary winding is air cooled and made up of copper strip. Oil cooled transformers are also used.

The air cooled sets are cheaper in cost and lighter in weight and easier to maintain. It can be easily shifted from one location to other.

Oil cooled sets are more compact but these are heavy so suitable for stationary application, e.g. in workshops.

The current control in welding transformer can be obtained by following ways:

(i) Tapped reactor method : 

 A series reactor is used with tapping on it fig.(a)

(a) Tapped Reactor Method

The output current can be varied by selecting appropriate tap. Different values of currents can be obtained by this method.

(ii) Moving Coil Method:

The transformer used in shell type with a movable primary winding as show in Fig.(b)

(b) Moving Coil Method

The movement of primary is possible by rotating the handle. If primary is moved near to secondary the current increases and if it is moved away current decreases.

(iii) Magnetic Shunt Method:

A magnetic shunt is used to divert the flux. The position of magnetic shunt can adjusted. Hence, value of flux in transformer core changes and thus current can be changed. fig.(c)

(c) Magnetic Shunt Method

Core Induction Furnace: Construction, Operation & Disadvantages

Core Induction Furnace: Construction, Operation & Disadvantages

Construction Core Induction Furnace

It consists of magnetic core having thin laminations of silicon steel. The primary is wound on one limb and a crucible type non-conducting ring is provided on the secondary. The crucible provides housing for the metal to be heated. 

The crucible is made up of ceramic material so that I can withstand high temperatures. The core of the transformer is designed such that it does not saturate even if high current is passed

 (a) Core Type Induction

Operation  Core Induction Furnace

First of all the charge in the form of different sizes is poured into the crucible. Due to these different sizes of the material there is a possibility that the secondary circuit may not complete, to avoid it, this more conductive material of powdered form is put on the pieces of material.

When A.C. supply is given to the primary, alternating flux is produced which links with secondary. So c.m.f. is induced in the charge itself. Heavy current is circulated through the top

powdered layer of conductive material and heat is produced in the top layer of charge which is transferred to bottom layer of charge.

Disadvantages Core Induction Furnace

(i)Due to irregular shape of the charge, magnetic coupling between primary and secondary is poor.

(ii) High leakage reactance due to poor magnetic coupling causes poor power factor.

(iii) To avoid low power factor low frequency supply is necessary. So low frequency generator is required to be installed. The overall cost of the furnace is increased. 

(iv) Pinch effect may occur which causes open circuit to secondary side.

(v) Crucible is of odd shape and it is difficult to manufacture. 

Pinch-off effect in core type induction furnace :

The magnetic coupling between primary and secondary is very poor, resulting in high leakage reactance and low power factor to overcome this difficulty the frequency is kept small (as low as 10 Hz).

(a) Pinch-off Effect

The electromagnetic forces produce turbulence of the molten metal. This effect is dependent on frequency and current density. The turbulence effect is severe at high values of frequency and current density.

(b) Pinch-off Effect

Hence, frequency is kept upto 10 Hz and current density is 5 amp/mm². The current flowing around the melt interacts with the alternating flux and produces constrictive forces on a section of the metal. This squeezes the melt and results in complete interruption of the section circuit. This effect is known as Pinch off effect.

Buchholz Relay: Operation, Advantages & Disadvantages

Buchholz Relay: Operation, Advantages & Disadvantages

1. When the inci pant faults are occured in the transformer, no relay will detect such

faults due to very small magnitude of fault current.

2. To detect such slow developing faults, Buchholz relay is used.

3. When the transformer is totally oil immersed and having kVA rating above 750 kVA; Buchholz relay will provide protection against such faults

4. Basically Buchholz relay is a gas-actuated relay installed in oil-immersed transformers for protection against all kind of fault.

5. When there is inci pant fault in  transformer Buchholz relay give an alarm. When there is severe fault in transformer, it disconnects the transformer from supply.

6. It is usually installed in the pipe  connecting the conservator to the main tank.

Location of Buchholz  Relay Construction

 Fig (b) shows the constructional details of Buchholz relay. It consists of a domed vessel and located between the main tank and the conservator.

Buchholz relay has two elements. The upper element consists of a mercury type switch attached to a float and used to close an alarm circuit during incipient fault.

The lower element contains a mercury switch mounted on a hinged type flap cate I the direct path of the flow of oil from the transformer to the conservator and is arranged to trip the circuit breaker in case of severe internal faults.

Buchholz Relay

Operation of Buchholz Relay

1.When an inci pant fault occurs inside the transformer, it generates the heat due to fault current.

2.This heat causes the decomposition of some transformer oil in the main tank.

3. Decomposition of transformer oil produces more than 70 % of hydrogen gas.

4.Due to lightweight of the hydrogen gas it tries to go into the conservator and in the process gets accumulated in the upper part of relay chamber. 

5. When a predetermined amount of gas get accumulated in the conservator it exert 

sufficient pressure on the float to cause it to tilt and close the contacts of mercury switch attached to it.

6.Closing of mercury switch completes the alarms circuit and gives an alarm. 

7. When a major fault occurs inside the transformer, an enormous amount of gas is generated in the main tank.

8. The oil in the main tank rushes towards the conservator via the Buchholz relay and

in doing so tilts the flap to close the contacts of mercury switch. 

9. This completes the trip circuit of the circuit breaker and circuit breaker disconnects the transformer by opening its contacts.

Advantages Of Buchholz Relay

➡️Inci pant faults are difficult to detect but with the help Buchholz relay it is possible to detect it at much earlier stage.

➡️Construction of Buchholz relay is much simpler.

Disadvantages Of Buchholz Relay

➡️Buchholz relays can only be used with oil-immersed transformer equipped with conservator tanks.

➡️Faults at connecting cable have to be provided with separate protection scheme since the device can detect only faults below oil level in the transformer.