Motor inner parts

ANATOMY

Result if we connect d.c motor to a.c supply

What happens when a d.c motor is connected across an ac supply?

A) *.Since on ac supply, reactance will come in to play,the a.c supply will be offered impedance(not resistance) by the armature winding. consequently,with a.c supply,current will be much less.The motor will run but it would not carry the same load as it would on d.c. supply.
*.There would be more sparking at the brushes
*.Though motor armature is laminated as a rule, the field poles are not. consequently,eddy currents will cause the motor to heat up eventually burn on a.c. supply

REF relay

What is REF relay?

A) It is restricted earth fault relay. When the fault occurs very near to the neutral
point of the transformer, the voltage available to drive the earth circuit is very small,which may not be sufficient to activate the relay, unless the relay is set for a very low current.
Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

 

Difference between generator and alternator

State the difference between generator and alternator

Ans: Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction.
Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used

 

Ratings of synchronous generator and Alternator

 Why rating of Synchronous Generators and Alternators in MVA or KVA?

A: Normally the rating of synchronous generators or alternators will be in KVA or MVA instead of kW rating.
Electrical apparatus or machines are usually rated the load which it can carry without overheating and damaging to insulation. i.e, rating of the electrical machines are governed by the temperature rise caused by the internal loss of the machine. The copper loss in the armature depends on the strength of the armature current and core loss depends on voltage and these losses are independent of the power factor.
The reason for which the transformers and synchronous generators are rated in volt-amperes instead of watts is that manufacturer does not know at what power factor does these equipments are going to operate. On the other hand while manufacturing the motors manufacturer specifies the power factor at which the motor should be operate. This is the reason why the motor loads (including synchronous motors are rated in kw) are rated in wattage power.
In the case of alternator and transformers manufacturer

does not know the operating power factor. Hence they are rated in KVA or MVA

 

Auto transformer

Autotransformer:

An autotransformer is an electrical transformer in which there is one winding, a portion of which is common to both the primary and the secondary circuits. In other words, the primary and secondary coils have some or all windings in common.

An autotransformer is commonly used for the voltage conversion of local power line voltage to some other Voltage value needed for a particular piece of electrical equipment. Most often, this conversion is from 125 Volts to 250 Volts, or 250 Volts to 125 Volts.

Unlike an isolation transformer, an autotransformer uses common windings and offer no interference or disturbance isolation.

A given size autotransformer will support a load equal to its rated value whether it is connected in the 125 Volts to 250 Volts configuration, or in the 250 Volts to 125 Volts configuration.

These units are employed in custom designs or when converting industrial/military equipment between various operating voltage systems. Most often, this conversion is from 125 Volts to 250 Volts, or 250 Volts to 125 Volts.

Unlike an isolation transformer, it uses common windings and offer no interference or disturbance isolation. You add any suppression or filtering networks your system requires.

With a single tapped winding, an autotransformer is generally preferred to an isolation transformer, with two separate windings, for numerous reasons. It is much smaller and lighter than an isolation transformer. It also has better voltage stability and greater overload tolerance. It performs in much the same way as the electrical transformer that the electric utility uses to bring power to a building.

An autotransformer is a tapped winding transformer that changes the voltage available locally to the voltage required by a particular load. Thus, a load may operate anywhere around the world, as long as a transformer is available to convert the local voltage to the voltage it requires.

"Variac" is a trademark of General Radio (mid-20th century) for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage.
The term is often used to describe similar variable autotransformers made by other makers. An autotransformer is an efficient and quiet method for adjusting the voltage to incandescent lamps.

While lightweight and compact semiconductor light dimmers have replaced variacs in many applications such as theatrical lighting, these transformers are still used when an undistorted variable voltage sine wave is required.

Defination of reactive power

Defination of Reactive power :



Real power is that portion of apparent power,which is consumed resitive portion of the ckt. We can see d result of this consumed power,as heat,rotation,light etc.Reactive power is that portion of apparent power,which is not consumed by load. Reactive components store energy in positive half cycle, & release energy in negative half cycle. We can see this reactive power as magnetism in inductor & as voltage in capacitor.

Pole face of d.c machine

Why are the pole faces of dc machine kept curved i.e. circular???

 

et,we have used plane face pole. Distance between pole & rotor will minimum in the middle of pole, and maximum in the edge of the pole. So,magnetic fld intensity will vary to the conductor of armature. But,we need to provide a constant fld. To do this,we should make the gape betwn pole & rotor constant. As,rotor is circular,so pole should be circular.

Importance of flourescent coating in tube light

What will happen,if we don't use flourescence coating in tube light??

In tube light,marcury(Hg) gas is used. When,using choke coil, sufficient voltage is generated across tube light,this gas will be ionized, & emit ultra violate ray. This ultra violate ray struck the flourescence coating(f.c.). Now,f.c. will emit visible light (nature of emitted light depends on their bandgap).

*so,without f.c. ultra violet ray will emitted from light.

Different frequencies and different voltage levels

Why different frequencies and different voltage levels for electrical supply systems around the world??

Europe and most other countries in the world use a voltage which is twice that of the US. It is between 220 and 240 volts, whereas in Japan and in most of the Americas the voltage is between 100 and 127 volts.

The system of three-phase alternating current electrical generation and distribution was invented by a nineteenth century creative genius named Nicola Tesla. He made many careful calculations and measurements and found out that 60 Hz (Hertz, cycles per second) was the best frequency for alternating current (AC) power generating. He preferred 240 volts, which put him at odds with Thomas Edison, whose direct current (DC) systems were 110 volts. Perhaps Edison had a useful point in the safety factor of the lower voltage, but DC couldn't provide the power to a distance that AC could.

When the German company AEG built the first European generating facility, its engineers decided to fix the frequency at 50 Hz, because the number 60 didn't fit the metric standard unit sequence (1,2,5). At that time, AEG had a virtual monopoly and their standard spread to the rest of the continent. In Britain, differing frequencies proliferated, and only after World War II the 50-cycle standard was established. A big mistake, however.

Not only is 50 Hz 20% less effective in generation, it is 10-15% less efficient in transmission, it requires up to 30% larger windings and magnetic core materials in transformer construction. Electric motors are much less efficient at the lower frequency, and must also be made more robust to handle the electrical losses and the extra heat generated. Today, only a handful of countries (Antigua, Guyana, Peru, the Philippines, South Korea and the Leeward Islands) follow Tesla’s advice and use the 60 Hz frequency together with a voltage of 220-240 V.

Originally Europe was 110 V too, just like Japan and the US today. It has been deemed necessary to increase voltage to get more power with less losses and voltage drop from the same copper wire diameter. At the time the US also wanted to change but because of the cost involved to replace all electric appliances, they decided not to. At the time (50s-60s) the average US household already had a fridge, a washing-machine, etc., but not in Europe.

The end result is that now, the US seems not to have evolved from the 50s and 60s, and still copes with problems as light bulbs that burn out rather quickly when they are close to the transformer (too high a voltage), or just the other way round: not enough voltage at the end of the line (105 to 127 volt spread !).

Note that currently all new American buildings get in fact 230 volts split in two 115 between neutral and hot wire. Major appliances, such as ovens, are now connected to 230 volts. Americans who have European equipment, can connect it to these outlets

Difference between generator and alternator

Interview question:

State the difference between generator and alternator

Answer:
Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction.
Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load
whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

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EEE Interview Question

Noload power factor is very low

During No load why the power factor of the transformer is very low ?

■ Ans: Current flowing through the transformer consists of two components. Magnetizing current (Im) which is in quadrature (900) to the applied voltage and in phase current which is in phase to the applied voltage. During no load condition most of the excitation current drawn by the transformer from the primary winding is to magnetize the path. Hence excitation current drawn by the transformer during no load condition mostly consists of magnetizing component of current which is used to provide magnetic field in transformer circuits (Inductive nature). Therefore as the nature of the load is inductive, hence the power factor of transformer during no load condition will by in the order of 0.1 to 0.2