Frog leg winding is a combination of a multiplex wave and a simplex lap winding in the same slots. It retains the advantages of both lap and wave windings without their inherit disadvantages.
Both lap and wave windings have equal number of parallel paths and they are connected to the same commutator. The frog-leg winding have as many parallel paths as duplex lap winding because the simplex lap winding portion supplies P no. of parallel paths and the multiplex-wave section also provides P no. of parallel paths, then total being 2P no. of paths in parallel.
Advantages of Using Frog Leg Winding
- This winding has more no. of parallel paths and the current and the voltage rating is higher than that of lap or wave winding. These frog leg wound armatures are designed for the use with moderate current and moderate voltage.
- These windings are connected in series-parallel. Any wave element and the succeeding lap element connected on the commutator exactly two pole pitches apart in a series combination. This two commutator segments are exactly 360 electrical degrees apart and develop zero net voltage. Therefore, this lap-wave combination of a frog leg winding is fully equalized and eliminates the use of an equalizer. That is why most large DC machines use frog leg wound armatures.
This is the type of winding in which the conductors are placed in slots over the drum-shaped armature surface and connected to one another by front and back connections at coil ends. Drum winding is introduced mainly to overcome the shortcomings of ring type winding.
Construction of Drum Winding
This winding may be of single layer or double layer winding.
Single Layer Drum Winding
When we place only one conductor or one coil side in each armature slot, then it is called single layer winding. We rarely use this type of drum winding.
Double Layer Drum Winding
When we place, two conductors or two coil sides in each armature slot, then it is called double-layer winding. In this winding, we divide the slots into two layers, one top layer, and one bottom layer. This winding is usually used for economy purpose.
Here we mainly concentrate on double layer winding. The figure below shows a double-layer winding.
We usually dip the coils into some insulating compound such as asphaltum, and then they are dried before placing them into the slots. For very high-temperature operation, we also use mica, paper tape, fiberglass tape, silicon impregnated insulation.
We give the conductors proper shape and bind the coils together with cotton tape and then place on the armature slot. The end of the coils left bare for soldering later to the commutator bars. The span of the coils is made equal to the pole pitch to get the maximum emf induced in the coil. Usually, we house the forward conductor of a coil in the top layer of a slot and the return conductor in the bottom layer of a slot which is at a distance of approximately one pole pitch along the armature. We terminate the junction of two coils on a commutator segment. We number the coil sides placed at the top layer of the slots with odd integers, and the coil sides placed at the bottom layer of the slots with even integers.
There are mainly two types of drum winding – one is lap winding, and the other is wave winding. We can distinguish them from each other by the manner of end connections. Another type of drum winding is also there. It is called Frog leg winding. This winding consists of lap and wave winding housed on the same armature.
Advantages of Drum Winding
- Each winding, placed on the armature slots, surrounds the core and so that the entire length of the conductor, except the end connections, cut the main magnetic flux. Therefore the voltage induced in this type of armature winding is larger than the Gramme-ring winding.
- The coils, before placing on the armature slots, can be pre-formed and insulated. Hence cost can be reduced.
- The two sides of the coil placed under two different poles, one North Pole and another South Pole, hence the emf induced in them are always additive with the help of the end connection.
- Fractional pitch winding can be used in drum winding. The advantage of fractional pitch winding is that it gives substantial savings in the copper of end connections. Commutation is also improved because of the lesser mutual inductor between the coils.
- Fractional pitch winding: The span of the coils should be made equal to the pole pitch to get the maximum emf induced in the coil. However, it is possible to reduce the span of the coil as much as eight-tenths(8⁄10) of the pole pitch without much reduction in the induced emf. When it gets done, then the winding is called fractional-pitch winding.
- Because of several conductors are placed in a single slot, the nos. of the slot get reduced in the armature core, the armature core teeth become mechanically stronger. The lamination and the protection of coils are also improved.
- The manufacturing cost will be reduced in the drum type winding because here we have to construct fewer coils.
Gramme Ring Winding
Ring winding is the type of armature winding in which the wire is wound around the outer and inner surfaces alternately of a cylindrical or ring-shaped core.
The Gramme-ring type of armature winding is an old type of armature winding. In this winding, the armature consists of a hollow cylinder or ring made up of iron lamination. The core is wound with insulated wire spirally about the ring. The winding is continuous, and hence it is closed. We connect the coils between brushes in series. The figure shows Gramme-Ring type winding and its equivalent circuit. We can see that there is an equal number of voltage-generating conductors placed on each side of the armature. We take taps from the wire at regular intervals, and we connect them to the commutator segments. There are two paths between the positive and negative brushes, and we connect them in parallel. Here, coils 1 to 6 has formed one path and coils 7 to 12 formed the other path.
When the armature rotates in the clockwise direction, then the emf is induced in the conductors. The direction of induced emf and the direction of current will be inward in case of the conductors under N-pole by Fleming’s right-hand rule. In case of the conductors under S-pole, the direction of induced emf and the direction of current will be outward.
According to Fleming’s right hand rule, we held the right hand with the first finger, second finger and thumb at right angles to each other and if forefinger represents the direction of the line of force, the thumb points in the direction of motion or applied force, then second finger points in the direction of the induced current.
Thus the emf generated in two paths are in opposite direction as shown in above figure. The emf generated on each path is additive from bottom to top on each side. Since there are two parallel paths, the voltage per path is the generated voltage of the machine, and each path provides half of the current output in the external circuit.
Advantages of Gramme Ring Winding
- Operating principle of armature is simpler because there is no crossing of conductors in the winding.
- Same winding can be employed with 2, 4, 6 or 8 poles theoretically.
Disadvantages of Gramme Ring Winding
- The part of this winding located on the inner side of the iron ring cut very few lines of flux. Thus they have very little voltage induced in them. For this reason, it is not widely used.
- With the same no. of poles the same velocity of the armature winding the induced emf in Gramme-ring winding is half of the induced emf in the drum type winding.
- As the portion lying inside the inner ring acts only as connectors so, there is a wastage of copper.
- The repairs and maintenance are very costly.
- Insulation of winding is much difficult.
- Strong field excitation needed to produce the require flux because the construction requires a large air gap.
For this wide range of disadvantages nowadays Gramme-ring winding has been replaced by more efficient drum type of winding.