Commutation
Currents induced in the armature conductors of a DC Generator are alternating. To make their flow unidirectional in the external circuit we use a Commutator. It is commonly used in direct current (DC) motors and generators to maintain continuous rotation or electrical flow. Commutation is achieved using a commutator, which is a split ring that switches the direction of current in the coil as it rotates. This process by which current in the short-circuited coil is reversed while it crosses the M.N.A. is called Commutation. The period during which coil remains short-circuited is known as the commutation period Tc.
Tc = Wb-Wm/V second
Wb = brush width in cm
Wm = width of mica in cm
V = peripheral velocity of commutator segment in cm/second
Methods Of Improving Commutation
Resistance commutation
E.M.F commutation.
Resistance commutation
This method of improving commutation consists of replacing low-resistance carbon brushes with comparatively high-resistance carbon brushes.
E.M.F commutation
E.M.F commutation method arrangement is made in such a way as to neutralize the reactance voltage by producing a reversing e.m.f in the short-circuited coil under commutation. As the name shows is an e.m.f in opposition to the reactance voltage and if its value is made equal to the latter, it will completely wipe it off. The reversing e.m.f may be produced by
Using Interpoles
Interpol is small as compared to the main fixed pole, these are small poles fixed to the yoke and spaced in between the main poles. Interpole wound with comparatively few heavy gauge copper wire turns and connected in series with the armature so that they carry full armature current. In the generator, their polarity is the same as that of the main pole ahead in the direction of rotation.
Interpoles induce an e.m.f in the coil which is reversing e.m.f is called commutating or reversing e.m.f. The commutating e.m.f neutralizes the reactance e.m.f thereby making the commutation sparkless. Using inter poles, sparkless commutation can be obtained.
Interpoles are used to neutralize the cross-magnetizing effect of the armature reaction. Brushes are not to be shifted from the original position.
Another way to improve Commutation
Use High-Quality Commutators: The commutator is a crucial component in DC machines. Using high-quality commutators made of durable materials can reduce arcing, sparking, and wear. Proper maintenance, such as regular cleaning and resurfacing, can also help.
Select Proper Brush Materials: Brushes play a significant role in commutation. Choose the right brush materials, such as carbon or graphite, that have good electrical conductivity and wear resistance. The brushes should also be designed to maintain proper contact with the commutator.
Optimize Brush Pressure: Adjust the brush pressure to ensure proper contact between the brushes and the commutator. Too much pressure can cause excessive friction and wear, while too little pressure can result in poor electrical contact.
Reduce Commutator Segments: Reducing the number of commutator segments can help improve commutation by minimizing the chances of sparking and arcing between segments. However, this approach may not be suitable for all applications, as it can reduce the precision of the motor or generator.
Implement Electronic Commutation: In some cases, replacing mechanical commutation with electronic commutation can provide more precise control and reduce wear. This is commonly done in brushless DC motors (BLDC) and stepper motors, where Hall effect sensors or encoders provide feedback for precise commutation.
Monitor and Adjust Timing: Regularly monitor the timing of commutation, which refers to when the brushes switch the current direction on the commutator. Adjusting the timing can help optimize performance and reduce sparking.
Maintain Cleanliness: Keep the commutator and brushes clean from dust, debris, and contaminants. Dirty commutators can cause poor electrical contact and increase wear.
Use Commutation Aids: Commutation aids like mica or other insulating materials can be used to reduce sparking and improve the transition of current between commutator segments.
Proper Cooling and Lubrication: Ensure that the commutator and brushes are adequately cooled to prevent overheating, which can lead to poor commutation. Proper lubrication of moving parts can also reduce friction and wear.
Regular Maintenance: Implement a proactive maintenance schedule that includes cleaning, inspection, and replacement of brushes, commutators, and other relevant components as needed.
Motor Design Optimization: When designing DC motors or generators, consider factors like the size and shape of the commutator, the type of windings used, and the magnetic field strength to optimize commutation performance.
Improving commutation in electrical systems is essential for ensuring efficient and reliable operation. The specific methods used will depend on the type of machine and its application, so it’s important to consider the unique requirements of your system.
