Compensating Winding
When generators need to supply a heavy current but have weak magnetic fields excitation, they need an auxiliary winding called a Compensating Winding. This winding helps to balance out the Cross magnetising effect of the armature reaction. It also helps to keep the magnetic field evenly distributed under the main poles.
The Compensating Winding is an auxiliary winding that is placed in the slots on the main pole faces. It is connected in series with the armature winding in a way that makes the current flow in the opposite direction to the adjacent armature conductors.
Using a Compensating Winding can make the generator more expensive and cause more copper loss, but it also makes the generator more resilient to sudden changes in the load it is powering. This type of winding is often used in generators and motors for Ward Leonard Sets, especially in steel mills.
Derivation of Ampere Turns per Pole For Compensating Winding
Let “Z” will number of armature conductors, “P” will be number of poles and “I” will be armature current per conductor.
Number of Armature Conductors per Pole = \;\frac{Z}{P}\;\\\;\\Number of Armature Turns Per Pole =\;\frac{Z}{2P}\\\;\\Number of Armature conductor per pole face = \frac{Z}{2P}\times\frac{\textrm{Pole arc}}{\textrm{Pole Pitch}}\\\;\\Number of Ampere turns per Pole = \frac{ZI}{2P}\times\frac{\textrm{Pole arc}}{\textrm{Pole Pitch}}
Equalizing Rings
In the lap winding of a multipolar machine, the armature circuit comprises as many parallel paths as the number of poles. Due to wear and tear in the bearings and other factors, the air gaps in a generator may become uneven, causing varying levels of flux in different poles. As a result, the voltages of the different parallel paths become unequal, leading to the flow of circulating currents even at no load condition. If these currents are too strong, some brushes will have to carry more electrical current than they were designed. This can result in sparking. To reduce the burden on the brushes from these circulating currents, points on the armature that have the same electrical potential are connected together using copper bars called equalizer rings.
The equalizer rings provide a low resistance path for the circulating currents to flow through. This means that the circulating current caused by small differences in voltage between different parts of the machine will go through the equalizer rings instead of going through the brushes. This reduces sparking.
Equalizer rings are only used in lap winding machines. The Satisfactory results are obtained by connecting about every third coil to an equalizer ring. In order to distribute the connections to the equalizer rings equally, the number of coils per pole must be divisible by the connection pitch.
Maximum number of Equalizer rings is equal to the number of conductor under one pair of poles. Hence, number of rings is = \frac{\textrm{Number of Conductors}}{\textrm{Number of pair of Poles}}
Read Also
- Methods of Improving commutation – Resistance and EMF Commutation
- Commutation in DC Machines | Generator
- Calculation of Demagnetizing and cross magnetizing ampere turns
- Armature Reaction | Armature Reaction in a DC Generator