Different types of Starter for Motor

Different types of Starter for Motor

1)DC
-TWO point starter

A two-point starter is used for starting dc motor which has the problem of over-speeding due to loss of load from its shaft.

-Three point starter

A three-point starter is used for starting a dc shunt or compound motor. The coil of the hold-on electromagnet is connected is series with the shunt-field coil.

- Four point starter

The 4 point starter like in the case of a 3 point starter also acts as a protective device that helps in safeguarding the armature of the shunt or compound excited DC motor against the high starting current produced in the absence of back emf at starting. The 4 point starter has a lot of constructional and functional similarity to a three point starter, but this special device has an additional point and a coil in its construction. This naturally brings about some difference in its functionality, though the basic operational characteristic remains the same. The basic difference in circuit of 4 point starter as compared to 3 point starter is that the holding coil is removed from the shunt field current and is connected directly across the line with current limiting resistance in series. 

2)AC
- Direct on line (DOL) starter

The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.

- Star-Delta starter

Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed.

- Auto transformer starter

An auto-transformer starter makes it possible to start squirrel-cage induction motors with reduced starting current, as the voltage across the motor is reduced during starting.In contrast to the star-delta connection, only three motor leads and terminals are required.

- Semi-Automatic Star Delta Starter

 In semi-automatic starter Timer is not required for changing star to delta contactor. In this change-over take place as we unhold start button.

- Automatic Star Delta Starter

In automatic starter Timer is required for changing star to delta contactor. In this change-over take place after getting signal from timer. This type of starter use for higher ratting motor. Because it take more time to reach normal ratting of motor current.

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Air Insulated Switch-gear VS Gas Insulated Switch-gear

Air Insulated Switch-gear VS Gas Insulated Switch-gear

Gas Insulated Switch-Gear

Air Insulated Switch-Gear

Limitations of Air Insulated Switchgears (AISs)

• Large dimensions due to statutory clearances & poor dielectric strength of air
• Insulation deterioration with ambient conditions and susceptibility to pollutants
• Wastage of space above
• Life of steel structures
• Seismic instability
• Large planning and execution time
• Grounding-mat is essential for containing touch and step potentials
• Hot line washing and regular maintenance of the substation is essential, requires more spares inventory and man-power

Advantages of GISs over AISs

• Compact space-saving design
• Minimal operating cost
• Minimal weight by lightweight construction
• Safe encapsulation
• Environmental compatibility
• Economical transport
• Reliability
• Smooth and efficient installation and commissioning

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Why we are use 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV this type of ratio. Why can’t we use other voltage ratio like 54KV / 99KV etc?

Why we are use 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV this type of ratio. Why can’t we use other voltage ratio like 54KV / 99KV etc?

When an alternator generates voltage, we always use a multiple of 1.11 because for a pure sine wave the FORM FACTOR is the  ratio of rms value of voltage or current with the avg. value of voltage or current and for pure sine wave rms value of current is Imax/root '2' and avg. value is 2Imax/pie and which comes out to be 1.1;

We can't have a combination of other then a multiple of 1.11*.
So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).

Also it provides us the best economic construction of step up and step down transformers.


* In the case of a Square Wave ie. a digital wave, the RMS and the average value are equal; therefore, the form factor is 1.

 

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Different type of motors...

Different type of motors.....

Squirrel Cage Motor

 

Electric Motor

An Electric motor is a machine which converts electric energy into mechanical energy. Its action is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left-hand Rule and whose magnitude is given by F = BIl Newton.

Types of AC Motors

Classification Based On Principle of Operation:

(a) Synchronous Motors.

1. Plain

2. Super

(b) Asynchronous Motors.

1. Induction Motors:

(a) Squirrel Cage

(b) Slip-Ring (external resistance).

2. Commutator Motors:

(a) Series

(b) Compensated

(c) Shunt

(d) Repulsion

(e) Repulsion-start induction

(f) Repulsion induction

Classification Based On Type of Current:

1. Single Phase

2. Three Phase

Classification Based On Speed of Operation:

1. Constant Speed.

2. Variable Speed.

3. Adjustable Speed.

Classification Based On Structural Features:

1. Open

2. Enclosed

3. Semi-enclosed

4. Ventilated

5. Pipe-ventilated

6. Riveted frame-eye etc. 

Types of DC Motor

Most common DC motor types are-

1. Permanent-magnet motors
2. Brushed DC Motor

a.       DC shunt-wound motor

b.      DC series-wound motor

c.       DC compound motor

                                                              i.      Cumulative compound

                                                            ii.      Differentially compounded

d.      Permanent magnet DC motor

e.       Separately excited

3. Brushless DC Motor
4. Coreless or ironless DC motors
5. Printed armature or pancake DC motors
6. Universal motors

 

 

 

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How to choose transformer rating?

 How to choose transformer rating?

When an installation is to be supplied directly from a MV/LV transformer and the maximum apparent-power loading of the installation has been determined, a suitable rating for the transformer can be decided, taking into account the following considerations:

• The possibility of improving the power factor of the installation
• Anticipated extensions to the installation
• Installation constraints (e.g. temperature)
• Standard transformer ratings.

3-phase transformer

The nominal full-load current In on the LV side of a 3-phase transformer is given by:



where:

• P_a = kVA rating of the transformer
• U = phase-to-phase voltage at no-load in volts (237 V or 410 V)
• I_n is in amperes

Single-phase transformer

For a single-phase transformer:



where

• V = voltage between LV terminals at no-load (in volts)

Simplified equation for 400 V (3-phase load)

• I_n = kVA x 1.4

The IEC standard for power transformers is IEC 60076.

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Electrical Switchgear Protection

Definition of Switchgear

A switchgear or electrical switchgear is a generic term which includes all the switching devices associated with mainly power system protection. It also includes all devices associated with control, metering and regulating of electrical power system. Assembly of such devices in a logical manner forms a switchgear. This is very basic definition of switchgear.

Switchgear and Protection

Switchgear

 We all familiar with low voltage switches and re-wirable fuses in our home. The switch is used to manually open and close the electrical circuit in our home and electrical fuse is used to protect our household electrical circuit from over  current  and short circuit faults. In same way every electrical circuit including high voltage electrical power system needs switching and protective devices. But in high voltage and extra high voltage system, these switching and protective scheme becomes complicated one for high fault  current  interruption in safe and secure way. In addition to that from commercial point of view every electrical power system needs measuring, control and regulating arrangement. Collectively the whole system is called switchgear and protection of power system. The electrical switchgear have been developing in various forms.

Switchgear protection plays a vital role in modern power system network, right from generation through transmission to distribution end. The  current  interruption device or switching device is called circuit breaker in switchgear protection system. The circuit breaker can be operated manually as when required and it is also operated during over  current  and short circuit or any other faults in the system by sensing the abnormality of system. The circuit breaker senses the faulty condition of system through protection relay and this relay is again actuated by faulty signal normally comes from current transformer or voltage transformer.

A switchgear has to perform the function of carrying, making and breaking the normal load current  like a switch and it has to perform the function of clearing the fault  in addition to that it also has provision of metering and regulating the various parameters of electrical power system. Thus the switchgear includes circuit breaker, current transformer, voltage transformer, protection relay, measuring instrument, electrical switch,electrical fuse, miniature circuit breaker, lightening arrestor or surge arrestor, electrical isolator and other associated equipment.

Switchgear Panels

 

 

Electric switchgear is necessary at every switching point in the electrical power system. There are various voltage levels and hence various fault levels between the generating stations and load centers. Therefore various types of switchgear assembly are required depending upon different voltage levels of the system.
 

Besides the power system network, electrical switchgear is  also required in industrial works, industrial projects, domestic and commercial buildings.

 

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Differences between E.M.F and Voltage (P.d)

Differences between E.M.F and Voltage (P.d)

The following are the difference between E.M.F and P.D.

• The name E.M.F at first sight implies that it is a force that causes current to flow.But this is not correct because it is not a forced but energy supplied to charge some active device such as battery.


• E.M.F maintains p.d. while p.d. cause current to flow.


• When we say that E.M.F of a device (e.g., a cell) is 3V it means that the device supplies energy of 3 joules to each coulomb of charge. When we say that a p.d. between point A and B of a circuit (suppose point A is at higher potential) is 3V, it means that each coulomb of charge will give up an energy of 3 joule in moving from A to B.

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