## 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

#### Illustrate bandstop filter, Q. The dual situations is shown in Figure , in ...

Q. The dual situations is shown in Figure , in which a high-pass and a low-pass filter are connected in parallel to produce a bandstop filter. With ω CLP = 10 CHP = 50, ω 0

#### Describe in brief how email system works, Describe in brief how email syste...

Describe in brief how email system works Following is a simple description of how email system works: -  Sender composes a message (and attaches a file if required) and send

D flipflop

Vee vcc

#### Transfer function, how can i do a question concerning a transfer function

how can i do a question concerning a transfer function

#### Junctions and diodes, An intrinsic semiconductor at room temperature has fr...

An intrinsic semiconductor at room temperature has free electrons

#### Automatic voltage booster, Automatic Voltage Booster The functioning o...

Automatic Voltage Booster The functioning of Automatic Voltage Booster (AVB) is same to in which of the series capacitor. Automatic Voltage Booster is an on-load tap changer.

#### Issues for installation of meters, Issues for Installation of Meters T...

Issues for Installation of Meters The subsequent should also be ensured: a) Appropriate crimping device should be used for crimping the lugs. Thimbles should be of appro

#### Calculate the length of the wire required to get resistance, An  electric  ...

An  electric  heater  element  is  made  of  Nichrome  wire  having  resistivity  equal  to 100 ×10 -8 ohm-metre. The diameter of the wire is 0.3 mm. Calculate the length of the w

Assignment