EE8301 - Electrical Machines - I - Unit-5 Two marks

Subject Name: Electrical Machines - I

Subject Code: EE8301

B.E. EEE Dept - semester 03

Anna University Regulation 2017 B.E. EEE Dept 3^rd semester EE8301(Electrical Machines - I) 2 marks for 1st unit is provided below. EE8301 all units 2 marks, Electrical Machines - I all units notes & syllabus are listed down for students to utilize it for scoring maximum marks with our study materials.

1.State Fleming's left hand rule.

Answer

The rule states that, 'Outstretch the three fingers of the left hand namely the first finger, middle finger and thumb such that they are mutually perpendicular to each other. Now point the first finger in the direction of magnetic field and the middle finger in the direction of the current then the thumb gives the direction of the force experienced by the conductor'.

2.How to reverse the direction of rotation of d.c. motor ?

Answer

To reverse direction of rotation of d.c. motor, either direction of main field produced by the field winding is reversed or direction of the current passing through the armature is reversed. The direction of the main field can be reversed by changing the direction of current passing through the field winding, which is possible by interchanging the polarities of supply which is given to the field winding.

3.What is back e.m.f. ?

Answer

After a motoring action, there exists a generating action. There is an induced e.m.f. in the rotating armature conductors according to Faraday's law of electromagnetic induction. According to the Lenz's law, this induced e.m.f. in the armature always acts in the opposite direction of the supply voltage. This e.m.f. tries to set up a current through the armature which is in the opposite direction to that, which supply voltage is forcing through the conductor. So as this e.m.f. always opposes the supply voltage, it is called back e.m.f..

4.How does back e.m.f. in a d.c. motor makes the motor self regulating ? or State the significance of back e.m.f.

Answer

The back e.m.f. is proportional to speed. When load is suddenly put onto the motor, motor tries to slow down. So speed of the motor reduces due to which back em.f. also decreases. So the net voltage across the armature (V — Eb) increases and motor draws more armature current hence the torque on the armature increases. The increase in the torque is just sufficient to satisfy increased load demand. When load on the motor is decreased, the speed of the motor tries to increase. Hence back e.m.f. increases. This causes (V — Eb) to reduce which eventually reduces the current drawn by the armature. The motor speed stops increasing when the armature current is just enough to produce the less torque required by the new load. So back e.m.f. regulates the flow of armature current making the motor self regulating.

5.Define speed regulation of d.c motor.

Answer

The speed regulation for a d.c. motor is defined as the ratio of change in speed corresponding to no load and full load condition to speed corresponding to full load Mathematically it is expressed as, % Speed regulation = ( N(no load) - N(full load) / N(full load) ) x 100

6.State the important d.c. motor characteristics.

Answer

i) Torque-Armature current characteristics (T Vs Iₐ) : The graph showing the relationship between the torque and the armature current is called a torque-armature current characteristic. These are also called electrical characteristics. ii) Speed-Armature current characteristics (N Vs Iₐ) : The graph showing the relationship between the speed and armature current characteristics. iii) Speed-Torque characteristics (N Vs T ) : The graph showing the relationship between the speed and the torque of the motor is called speed-torque characteristics of the motor. These are also called mechanical characteristics.

7.Why is a d.c. series motor used to start heavy loads ? or Why d.c. series motor is never started on no load ?

Answer

In case of a d.c. series motor, ϕ∝Iₐ and on no load as Iₐ is small hence flux produced is also very small. According to speed equation, N ∝ 1/ϕ as Eb is almost constant. So on very light load or no load as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed. Hence a d.c. series motor is used to start heavy loads and never used on no load. Many times a flywheel is used with d.c. series motor to avoid no load starting condition.

8.What is the need for starter in a d.c. motor ?

Answer

At the starting instant the speed of the motor is zero, (N = 0). As speed is zero, there cannot be any back e.m.f. as Eb ∝ N and N is zero at start. Eb at start = 0 From voltage equation, at start V = Iₐ Rₐ ie. Iₐ = V/Rₐ As Eb =0 Generally motor is switched on with normal voltage and as armature resistance is very small, the armature current at start is very high. Such high current drawn by the armature at start is highly objectionable hence the starter is required to limit the high starting current in case of d.c. motors.

9.Mention the factor affecting the speed of d.c. motor.

Answer

The factors affecting the speed of a d.c. motor are, 1. The flux ϕ 2. The voltage across the armature 3. The applied voltage V

10.List the different methods of speed control of d.c. shunt motor.

Answer

1. Changing the flux ϕ by controlling the current through the field winding called flux control methods. 2. Changing the armature path resistance which in turn changes the voltage applied across the armature called rheostatic control. 3. Changing the applied voltage called voltage control method.

11.State the methods of speed control in d.c. series motor.

Answer

1. Rheostatic control : Controlling the armature voltage. 2. Applied voltage control : Controlling the voltage applied. 3. Flux control : In this, the various ways of controlling flux are, i) Flux divertor ii) Armature divertor iii) Tapped field iv) Series-Parallel grouping of field coils.

12.How does 4-point starter differ from 3-point starter?

Answer

In three point, NVC is in series with the field winding while in four point starter NVC is connected independently across the supply through the fourth terminal called 'N' in addition to the 'L', 'F' and 'A'. Hence any change in the field current does not affect the performance of the NVC. Thus it is ensured that NVC always produce a force which is enough to hold the handle in 'RUN' position, against force of the spring, under all the operating conditions.

13.When you will say the motor is running at base speed ?

Answer

The speed with which motor runs when its armature is excited at a rated voltage and the field winding carries maximum rated current, is called base speed of the motor. It is the rated speed of the motor which is mentioned on the name plate of the machine.

14.Why d.c. series motor is not suitable for belt driven loads ?

Answer

For belt driven loads, there is possibility of breaking of a belt causing no load condition for the series motor. But on no load, d.c. series motor tries to run at dangerously high speed and may get damaged. To avoid such situation, d.c. series motor is not suitable for belt driven loads.

15.Write down the applications of d.c. series motor.

Answer

The applications of d.c. series motor are, cranes, hoists, elevators, trolleys, conveyors and electrical locomotives.

16.Specify the techniques used to control the speed of DC shunt motor for below and above the rated speed ?

Answer

For speed control of d.c. shunt motor below rated speed, rheostatic control method is used in which voltage across the armature is controlled. While for the speed control above rated speed, the flux control method is used in which current through field winding is controlled.

17.Why DC series motor is suited for traction applications ?

Answer

The series motor has characteristics that it produces high starting torque and it can not be started on no load. It has capability of handling overloads. Its speed control is easy. All these features are well suited for the electric traction hence series motor is used for traction applications.

18.What is meant by plugging ?

Answer

Plugging is the method of braking the motors electrically. In this method, the direction of motor is suddenly reversed by reversing the armature connections of the motor. Thus motor tries to accelerate in other direction after coming to rest. An auxiliary device is used to cut off the supply as soon as motor comes to rest.

19.List various method of starting D.C motor.

Answer

The methods of starting d.c. motor are, i) Three point starter ii) Four point starter.

20.What is meant by dynamic braking in D.C motor?

Answer

This is the method used to brake the motor and stop. In this method, the armature is simply disconnected from the supply and connected to a resistance, keeping field connected to the supply. All the kinetic energy of the moving electrical and dissipated in the resistance connected. By varying armature gets converted to value of this resistance, braking torque can be controlled.

21.Mention the effects of differential compounding and cumulatively compound on the performance of D.C. compound motor.

Answer

In cumulative compound motor, shunt field provides the definite flux and series flux helps shunt field flux to increase flux level as motor is loaded. Thus this motor is capable of producing large torque at low speeds and without any risk of high speeds on no load. In differential compound type, as load increases, series flux increases and opposes the shunt flux which reduces the overall flux. Thus motor runs at higher speed as N∝(1/ϕ). Thus speed increases as load increases and this property is dangerous. Hence differential compound motor is generally not used in practice.

22.Explain why Swinburne's test cannot be performed on d.c. series motor.

Answer

The Swinburne's test is a no load test. On no load, d.c series motor runs at dangerously high speed and may get damaged. Hence no load condition can not be achieved on d.c. series motor for testing purpose. Thus the Swinburne's test can not be performed on d.c. series motor.

Important links:-

EE8301 - Electrical Machines - I - Unit-1 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-2 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-3 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-4 Two marks (Click here)
EE8301 - Electrical Machines - I - Unit-5 Two marks (Click here)
EE8301 - Electrical Machines - I - All unit notes (Click here)

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