Methods Of Speed Control DC Machines

Electrical Engineering Objective On Electrical Machine's

Star to Delta snd Delta to Star Conversion Techniques......

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How does an alternator work?

How does an alternator work?

An alternator also called as AC generator is an electromechanical device which converts energy from mechanical domain to electrical domain. It is most widely used generator for generating the 3 phase power worldwide. When an AC generator is used along with other generators, its operation is in synchronism with them. Under this condition they are also called Synchronous generator.

Working Principle

Basic principle of operation of alternator is similar to that of DC generator i.e on the Faraday’s law of electromagnetic induction which states that

• Whenever the flux linking through a coil changes, an emf is induced in it.

• The amount of emf induced in the coil is directly proportional to the rate of change of flux and number of turns in it. Mathematically it can be summarized as

E= -N (dφ/dt) here (–)ve sign is due to opposing nature of emf induced (Lenz's law)

To satisfy Faraday’s law of electromagnetic induction, alternator should have 3 essential constructional requirements :

• A field system to produce the required working flux.

• A system of conductors or coils to which flux can link and emf can be induced.

• A relative motion between these two system.

Alternators having small voltage and power rating can be constructed in either way (rotating field type system or stationary field system).But alternators having larger voltage and power rating are invariably constructed with rotating field type system i.e. field winding is placed on rotor with DC supply provided to it and armature winding is placed on stator through which electrical output is withdrawn. Advantage for rotating field system are :

• Field system require about 2% of the power capacity of machine which can be easily supplied via slip rings and brushes.

• It is easier to provide the necessary insulation for stationary system of conductors.

• The insulation provided to the stationary system of conductors is not subjected to any kind of mechanical stress due to centrifugal action.

Working

The rotor of alternator is coupled with a prime mover mainly turbine which provides mechanical input to the rotor. Further rotor is also provided with DC excitation. Hence a synchronously rotating magnetic field is created in the air gap. This flux links to the stationary 3 phase armature conductors placed in stator part. As per Faraday’s law, an emf is induced in the conductors. This emf is used to drives the load connected to the output terminal of the alternator.
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Admin:- Bellapuri saikumar
                      ( sai saikumar jn)

Electrical machines interview questions set - 2

Can a DC series motor operate in AC?

A series motor is a special type of motor, which has been designed to operate on either AC or DC power. This type of motor is also called universal motor.

You know that the magnetic field of armature winding and field winding opposes each other, and a rotational motion is achieved. In AC power, the polarity of the input changes in a cycling process . But, the field magnetic polarity still opposes the armature magnetic polarity because the reversal affects( due to AC power) both of the armature and the field winding.

In AC power, these reversals take place continuously. So, when AC power is used to operate a DC series motor ,some special laminations and windings are provided with dc series motor to reduce the problems caused by eddy currents, hysteresis, and high reactance in case of AC operation.

Thus, a DC series motor can operate in AC with some special arrangement.

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What is the speed regulation of a dc motor?

Suppose a dc motor is running at no load. That means no conveyor belt or any other type of gear fitted with the shaft of the motor and the shaft is free to rotate. Now assume the load is put on the shaft by means of conveyor belt or gear etc. After applying the load there may be a change of speed of the motor. The term speed regulation refers to the change in speed of a motor with change in applied load.

The speed regulation is defined as the change in speed when the load on the motor is reduced from rated value to zero, expressed as percent of the rated load speed.

The speed at which the motor rotates when no load is applied to the rotor shaft is referred as no load speed.

The speed, at which the motor rotates when full rated load is applied to the motor shaft, is referred as full load speed.

Now speed regulation of dc motor is referred as the difference of no load speed and full load speed expressed as percentage of full load speed.
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Why starter is needed in DC motor?

In a dc motor there are two winding which acts simultaneously. Rotor winding is energised from a voltage source and the field winding provides the necessary magnetic field. But due to the interaction an another electromotive force is also generated which is opposing the main electromotive force given to the rotor winding. Actually, the effective resistance of a typical armature winding is in few ohms.

The ohm’s law states that, V = IR. If we think deeply we can visualize the necessity of a starter.

Initially, the entire voltage (suppose 230 volt) is applied to the armature. But the armature resistance is very small, suppose 2 ohm. Therefore, I = V/R that is the current through the armature is 230 / 2 ampere that is 115 ampere. Initially no such electromotive force is generated which can oppose the main voltage. So, this huge amount of current can destroy the armature winding as well as it is also harmful for the other sophisticated parts of that dc motor.
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To protect the machine from this huge initial current we need to attach a starter with a dc motor. A starter is nothing but a series resistance arrangement with some special security arrangement like low voltage protection and over voltage protection.

The huge initial current is controlled by the starter itself and after sometime the resistance is reduced by changing the starter taps and in the mean time the main electromotive force is already opposed. In this way, implementation of a starter can reduce the very high starting current.
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Specially, in the larger motor, the high starting current can damage the commutator or the windings. So, a starter is needed to insert a current limiting resistor until the motor gets up to a specific speed.
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What is tesla coil?

Tesla coil is named after Nikola Tesla who invented this. He invented this in the year of 1981. It is a high frequency, resonant transformer circuit. Tesla coil is an air core transformer. This transforms small value of ac voltage to very high voltages mainly of the order of 10⁵ volt. Tesla coil is capable of creating very strong electric field. Tesla performed a series of experiments with couple resonant circuits. Tesla coils are used to perform experiments regarding x-ray generation, electric lighting, wireless transmission of electrical energy. Commercially Tesla coils are also used. It is used in case of electrotherapy, wireless telegraphy, etc.

A Tesla coil operates in somewhat different manner than a conventional transformer. In a conventional transformer windings are tightly coupled whereas in a Tesla coil they are coupled loosely. This enables the coil energy to transfer from one circuit (resonating) to another.
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Tesla coil transformer uses a capacitor which gets connected to the coils of less number of turns (primary) to form a resonant circuit. The frequency of oscillation of this circuit varies form 20-100 KHz. This value is determined by the capacitance of the said capacitor and coil’s inductance value.
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While tuning primary coils resonant frequency care should be taken because this often causes steamers i.e. spark discharge into the air.
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.Admin:- Bellapuri saikumar

                  ( sai saikumar jn)