In this article, I will give simplified explanation of the development of armature winding design, taking Lap winding as an example.Lap winding of armature is used in DC machines. “Lap” is a short form of overlapping winding. To explain further, I will take a winding development problem.
Develop a winding diagram for a 4 pole dc generator with 16 slots on the armature. The winding will be a simplex, double layer,progressive, lap winding.
Let us first know the specifications well. The above diagram shows a dc machine and I consider it as a generator. The armature rotates in the flux created by the four poles. Here are our observations:
- It is a four pole machine
- It has 16 slots on the armature surface
- Every slot has two conductors inserted
To connect the conductors in a particular fashion, to form coils so that they can deliver maximum power-output , is known as winding the armature. These coils rotate in the flux and emf is induced across them. In our case, the machine is a generator and mechanical input gets converted to electrical and the emf forces current into the load.
To simplify the process of winding, we assign numbers to the conductors that are inserted in the slots. As a convention, upper conductor is given an odd number and the lower one gets an even number.
To decide which conductors are to be connected, we first define some terms…
- The pole pitch: Number of conductors under every pole at any time.
- The back pitch: The number difference between the two conductors which are connected at the back of the armature.
- The front pitch: The number difference between the two conductors which are connected at the front of the armature.
Before we go through the process of winding development, here are some fundamentals:
- Two conductors when joined at the back form a coil. (e.g. 1 & 10)
- Every such coil thus has two conductors. These conductors follow one important rule. At any time, if one conductor of a coil is moving under the N pole (or S pole) the other conductor of the same coil must be moving under the S pole (or N pole).
- Any two consecutive coils (in lap winding) will overlap each other.
Using the above fundamentals, following formulas will emerge:
for progressive winding;
for progressive winding ;
Using the above formulas , we have Yb=9 and Yf=7.
Using the above formulas, following connection sequence table is generated: Front connections are shown by underscore (_) and back connections are shown by ^.
- Begin with conductor number 1. Remember to connect to the next conductor at the back by adding 9 (the back pitch).
- Front connections are obtained by subtracting 7 (the front pitch).
The above sequence table will guide us to form coils.
Brush positions and polarities: A very important part of the winding design is to decide the brush position and brush polarity. If we follow the steps, we can easily do the same.
- Decide the instantaneous current directions in individual conductors. This depends upon which pole they are moving under. The diagram above shows these directions.
- To decide the brush positions, we search that front connection which is connected to two conductors having the same current direction. Once these positions (segments) are located, we place the brushes there.
- If the currents are coming out of the generator, the brush will be +ve . If the currents enter the generator, the brush will be -ve.
Using the above rules, we position the brushes at segments as shown. The brush polarities are also decided.
Here ends the basic part of lap winding diagram development.