Electrical Engineering 1==10 Q/A

 1:What are the different methods for the starting of a synchronous motor.

Starting methods: Synchronous motor can be started by the following two methods:

•By means of an auxiliary motor: 

The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.

By providing damper winding: 

Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.

2: Name the types of motors used in vacuum cleaners, phonographic appliances,vendingmachines,refrigerators, rolling mills, lathes, power factor improvement and cranes.

Following motors are used:

•Vacuum cleaners- Universal motor.

Phonographic appliances - Hysteresis motor

•Vending machines - Shaded pole motor.

•Refrigerators Capacitor split phase motors.

Rolling mills - Cumulative motors.

Lathes DC shunt motors.

•Power factor improvement - Synchronous motors.

3:Explain different losses in a transformer.

There are two types of losses occurring in transformer:

Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses.

Two types of iron losses are:

1. eddy current loss

2. Hysteresis loss. These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses. Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. 

4: Define Voltage types?

1. Extra Low: Normally not exceeding

50 Volts between conductors, and not exceeding 30 Volts A.C. or 50 Volts D.C. between any conductor and earth.

2. Low: Normally exceeding extra low voltage but not exceeding 250 Volts, whether between conductors or between any conductor and earth.

3. Medium: Normally exceeding 250 Volts but not exceeding 650 Volts, whether between conductors or between any conductor and earth.

4. High Normally exceeding 650 Volts between phases and earth.

5: What is Tunnel Diode?

A Tunnel diode is a heavily doped p-n junction diode in which the electric current decreases as the voltage increases.

In tunnel diode, electric current is caused by "Tunneling". The tunnel diode is used as a very fast switching device in computers. It is also used in high-frequency oscillators and amplifiers.

6:What is the difference between phase and line voltage?

Line voltage (line-to-line voltage) in a polyphase system is the voltage between two given phases.

On the other hand, Phase voltage is the voltage between the given phase and neutral. Note that neutral is available in star connection but not delta connection.

7: What is difference between terminal voltage and generated voltage?

Generated Voltage is the voltage developed across the terminals of the Generator without any load connected. Suppose a Generator is rotating at synchronous speed so its winding will link the changing magnetic flux and because of this an emf will be developed across the terminal as Faraday's Law of Electromagnetic Induction suggests. So this generated voltage is often called Excitation voltage.

But now suppose you connect a load across the terminals of the Generator. In this case current will start flowing and the Voltage across the load will now be

V = E-IZ where E is the Excitation voltage, I the current and Z the impedance of the Generator winding. Here V is the terminal voltage which load face. Thus you see how Generated voltage and load terminal voltage is related. 

8: What is the main difference between voltage, EMF and a potential difference?

Let's see the following experiment. A container 'A' is kept at a height. Another container 'B' is kept at lower height. A is connected to B using a tube with valve 'C'. A is filled with water.

When C is opened water from A flows to B.

Here the force of gravity makes the water flow. This is EMF. The force responsible for flow.

Gravity acts differently as per height difference. This is nothing but potential difference. It is the same as voltage.

The amount of water flowing is nothing but current.

The valve C is nothing but a switch.

The length, bends and restrictions in flow of water in the pipe is nothing but resistance.

9:How does a battery produce its potential difference (voltage)?

A battery stores energy chemically.

You have a part of the battery with an excess of electrons, and another part with a lack of electrons.

The excess electrons want to go to the other side (because like charges repel), and that generates a voltage.

If you complete the circuit and let the electrons go to the other side, the difference in number of electrons will go down and the remaining ones will want to move to the other side less (lower voltage).

Eventually the electrons are balanced, and the battery is dead, because it no longer has a potential difference

10:What is the main difference between negative terminal and ground terminal of voltage source?

The ground terminal in a circuit is just a reference for absolute potentials at different nodes. It really does not matter if you call the potential at _any_ node as ground. So, if you wish you can call the positive terminal of a voltage source as ground. All the circuit analysis will still hold good. Defining a ground terminal will in no way affect the physical properties of the circuit.

The negative (or positive) terminal of a voltage source is relevant in the following sense. When you connect the two terminals of the voltage source to two nodes of the circuit, then there is a change in the physical properties of the circuit - potential & current. You are adding potential between the two nodes. So the negative terminal along with the positive terminal tells us the stepping up of the potential in that mesh.