What is Commutation : Working Principle, Effects on DC Machines

In our daily life, the usage of DC machines for our day to day needs has become a common thing. DC machine is an energy conversion device that makes electro-mechanical conversions. There are two types of DC machines- the DC motors and the DC generators. DC motors convert DC electrical power into mechanical motion whereas the DC generators convert the mechanical motion into DC power. But the catch is, the current generated in a DC generator is an AC but the output of the generator is DC!! In the same way, the principle of the motor is applicable when the current in the coil alternates, but the power applied to a DC motor is DC!! Then how are these machines running? The answer to this wonder is the small device named “Commutator”.

What is Commutation?

Commutation in DC machines is the process by which the reversal of current takes place. In DC generator this process is used to convert the induced AC in the conductors to a DC output. In DC motors commutation is used to reverse the directions of DC current before being applied to the coils of the motor.

How Does the Commutation Process Take Place?

The device called Commutator helps in this process. Let’s look at the functioning of a DC motor to understand the commutation process. The basic principle on which a motor works is electromagnetic induction. When current is passed through a conductor it produces magnetic field lines around it. We also know that when a magnetic north and magnetic south faces each other, magnetic lines of force move from North Pole magnet to South Pole magnet as shown in the figure below.

When the conductor with a magnetic field induced around it, is placed in the path of these magnetic lines of force, it blocks their path. So these magnetic lines try to remove this obstacle by either moving it upwards or downwards depending upon the direction of current in the conductor. This gives rise to motor effect.

When an Electromagnetic coil is placed in between two magnetic with north facing south of another magnet, the magnetic lines moves the coil upwards when current is in one direction and downwards when the current in the coil is in reverse direction. This creates the rotatory motion of the coil. To change the direction of current in the coil, two half-moon shaped metals are attached to each end of the coil called Commutator. Metal brushes are placed with one end attached to the battery and the other end connected to the commutators.

Commutation in DC Machine

Each Armature coil contains two commutators attached at its end. For the transformation of current, the Commutator segments and brushes should maintain a continuously moving contact. To get larger output values more than one coil is used in DC machines. So, instead of one pair, we have a number of pairs of Commutator segments.

The coil is short-circuited for a very short period of time with the help of brushes. This period is known as commutation period. Let us consider a DC motor in which the width of the Commutator bars is equal to the width of the brushes. Let the current flowing through the conductor be Ia. Let a, b, c be the Commutator segments of the motor. The current reversal in the coil .i.e. commutation process can be understood by the below steps.

Position-1

Let the Armature starts rotating, then the brush moves over the commutator segments. Let the first position of the brush commutator contact be at segment b as shown above. As the width of the commutator is equal to the width of the brush, in the above position the total areas of commutator and brush are in contact with each other. The total current conducted by the commutator segment into the brush at this position will be 2Ia.

Position-2

Now the armature rotates towards the right and the brush comes in contact with the bar a. At this position, the total conducted current will be 2Ia, but the current in the coil changes. Here the current flows through two paths A and B. 3/4th of the 2Ia comes from the coil B and remaining 1/4th comes from coil A.When KCL is applied at the segment a and b, the current through the coil B is reduced to Ia/2 and the current drawn through segment a is Ia/2.

Position-3

At this position half of the brush, a surface is in contact with segment a and the other half is with segment b. As the total current drawn trough brush is 2Ia, current Ia is drawn through coil A and Ia is drawn through coil B. Using KCL we can observe that the current in coil B will be zero.

Position-4

In this position, one-fourth of the brush surface will be in contact with segment b and three fourth with segment a. Here the current drawn through coil B is – Ia/2. Here we can observe that the current in coil B is reversed.

Position-5

At this position, the brush is in full contact with segment a and the current from coil B is Ia but is reverse direction to the current direction of position 1.Thus commutation process is completed for segment b.

Effects of Commutation

The computation is called Ideal commutation when the reversal of current is completed by the end of the commutation period. If the current reversal is completed during the commutation period, sparking occurs at the contact of brushes and overheating occurs damaging the surface of the commutator. This defect is called Poorly commutated Machine.

To prevent this type of defects there are three types of methods for improving commutation.

• Resistance commutation.
• EMF commutation.
• Compensating winding.

Resistance Commutation

To tackle the problem of poor commutation Resistance commutation method is applied. In this method, copper brushes of lower resistance are replaced with carbon brushes of higher resistance. Resistance increases with the decreasing area of cross-section. So, the resistance of the trailing commutator segment increases as the brush moves towards the leading segment. Hence, the leading segment is most favored for the current path and large current takes the path provided by the leading segment to reach the brush. This can be well understood by looking at our figure below.

In the figure above the current from coil 3 can take two paths. Path 1 from coil 3 into coil 2 and segment b. Path 2 from short-circuited coil 2 then coil 1 and segment a. When copper brushes are used current will take the path 1 due to lower resistance offered by the path. But when carbon brushes are used, the current prefers the Path 2 because as the area of contact between brush and segment decreases the resistance increases. This stops the early reversal of current and prevents sparking in the DC machine.

EMF Commutation

Induction property of the coil is one of the reasons for the slow reversal of current during commutation process. This problem can be tackled by neutralizing the reactance voltage produced by the coil by producing the reverse e.m.f in the short circuit coil during the commutation period. This EMF commutation is also known as Voltage commutation.

This can be done in two methods.

• By Brush Shifting method.
• By Using commutating poles.

In brush shifting method, brushes are shifted forward for DC generator and backward in DC motor. This establishes a flux in the neutral zone. As the commutating coil is cutting the flux, a small voltage is induced. As brush position has to be shifted for every variation in load, this method is rarely preferred.

In the second method, commutating poles are used. These are the small magnetic poles placed between main poles mounted to the stator of the machine. These are attached in series connection with the armature. As load current causes back e.m.f. , these commutating poles neutralizes the position of the magnetic field.

Without these commutating poles, the commutator slots would not stay aligned with ideal portions of the magnetic field as magnetic field position changes due to back e.m.f. During the commutation period, these commutating poles induce an e.m.f in the short circuit coil which opposes the reactance voltage and gives spark-less commutation.

The polarity of commutating poles is the same as the main pole situated next to it for the generator whereas the polarity of commutating poles is opposite to the main poles in the motor.

Learning about the commutator we found that this small device plays a significant role in the proper working of DC machines. Not only as a current converter but also for the safe functioning of machines without damage due to sparks, commutators are very useful devices. But with increasing development in technology, commutators are being replaced with new technology. Can you name the new technique that replaced commutators in recent days?