Describe the methods of improving commutation., Electrical Engineering

Assignment Help:

There are two practical ways of improving commutation i.e. of making current reversal in the short - circuited coil as sparkless as possible. These methods are known as (i) resistance commutation and (ii) e.m.f. commutation (which is done with the help of either brush lead or interpoles, usually the later).

 

Resistance Commutation

 

     This method of improving commutation consists of replacing low - resistance Cu brushes by comparatively high - resistance carbon brushes.

 

                   It is seen that when current I from coil C reaches the commutator segment b, it has two parallel paths open to it. The first part is straight from bar 'b' to the brush and the parallel path is vis the short - circuited coil B to bar a and then to the brush. If the Cu brushes (which have low contact resistance) are used, then there is no inducement for the current to follow the second longer path, it would preferably follow the first path. But when carbon brushes having high resistance are used, then current I coming from C will prefer to pass through the second path because (i) the resistance r1 of the first path will increase due to the diminishing area of contact of bar 'b' with the brush and because (ii) resistance r2 of second path will decrease due to rapidly increasing contact area of bar a with the brush.

 

 Hence, carbon brushes have, usually, replaced Cu brushes. However, it should be clearly understood that the main cause of sparking commutation is the self - induced e.m.f. ( i.e. reactance voltage), so brushes alone do not give a sparkless commutation; though they do help in obtaining it.

 

                   The additional advantage of carbon brushes are that (i) they are to some degree self - lubricating and polish the commutator and (ii) when sparking occur, they would damage the commutator less than when Cu brushes are used.

 

          But some of their minor disadvantage are : (i) Due to their high contact resistance (which is beneficial to sparkless commutation) a loss of approx. 2 volts is caused. Hence, they are not much suitable for small machines where this voltage form an appreciable percentage loss,  (ii) Owing to this large loss, the commutator has to be made some what larger than with Cu brushes in order to dissipate heat efficiently without greater rise of temperature, (iii) because of their lower current density (about 7-8 A/cm2 as compared to 25-30 A/cm2 for Cu brushes) they need larger brushes holders.

 

E.M.E. Commutation

 

 In this method, arrangement is made to neutralize the reactance voltage by producing a reversing e.m.f. in the short - circuited coil under commutation. This reversing e.m.f. as the same shows, is an e.m.f. in opposite to the reactance voltage and if its value


Related Discussions:- Describe the methods of improving commutation.

Electrons, Ask question #Minimum 100 words accepted how does the charge pla...

Ask question #Minimum 100 words accepted how does the charge plate relate to the movement of electrons

Evaluate the current flowing in the direction from b to a, Q A beam contain...

Q A beam containing two types of charged particles is moving from A to B. Particles of type I with charge +3q, and those of type II with charge -2q (where -q is the charge of an el

Define the term assembler directives, Define the term assembler directives....

Define the term assembler directives. An assembler directive is a statement to provide direction to the assembler to perform the function of assembly process. The assembler di

Equivalent circuit of a field-effect transistor, Q. The circuit shown in Fi...

Q. The circuit shown in Figure is the equivalent circuit of a field-effect transistor (FET) amplifier stage. (a) Determine the y-parameters. (b) For values of µ = g m /g d >

Example of binary subtraction - positive number , Example  of binary  sub...

Example  of binary  subtraction - positive number Case 1.   Positive  number is more than negative number ( i e, answer is positive)   Example   subtract  4510 ( 1011012

Scale-space pyramids, Scale-space pyramids   Write an m-file "scale_s...

Scale-space pyramids   Write an m-file "scale_space.m" which computes scale space representation of an input image for a given scale "sigma" and displays them. (i) 4 scale

A battery consists of five cells in series, A battery having of five cells ...

A battery having of five cells in series. Each cell is 2.5V and internal resistance is 0.05 Ω. A battery is linked to the load resistance of 15Ω. Verify: i.  Total e.m.f

Explain in detail the operation of 8255 in mode 1, Explain in detail the op...

Explain in detail the operation of 8255 in mode1 taking suitable example. In mode1, Ports A and B are programmed as input or output ports and Port C is used for handshaking.

Personal safety from power, Personal  safety Power supplies can be very...

Personal  safety Power supplies can be very dangerous. This is specifically true of high voltage circuits, but anything over 12 V must be treated as lethal potentially. A po

Connection of shunt capacitors at 11 kv substation, Connection of Shunt Cap...

Connection of Shunt Capacitors at 11 kV Substations 11 kV automatic switched capacitor banks are installed on bus of 33 or 66 kV substations. This has the facility that the re

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd