Frog Leg Winding, Drum Winding & Gramme Ring Winding (Advantages & Disadvantages)

What is a Frog Leg Winding?

A frog leg winding, or self-equalizing winding, combines a multiplex wave winding with a simplex lap winding in the same armature slots. The two sections use the same commutator, combining features of lap windings and wave windings.

The lap and wave sections are designed with the same number of parallel paths before their paths are connected at the commutator.

For a simplex lap section in a P-pole machine, the lap section supplies P parallel paths. A multiplex wave section of the required multiplicity also supplies P paths. Their combined frog-leg winding therefore has 2P parallel paths, the same count as a duplex lap winding.

Advantages of Using Frog Leg Winding

1. The series-parallel arrangement provides more paths than a simplex lap or simplex wave winding. It is used where a machine requires a balance of current capacity and voltage, rather than the extreme associated with either basic winding.
2. The wave section connects lap-winding points that should be at equal potential. These connections act as equalizers and limit circulating current if pole fluxes are unequal. Separate equalizer rings are therefore unnecessary, which is the main reason frog-leg windings have been used in medium and large DC machines.

What is a Drum Winding?

In a type of winding called drum winding, the active conductors lie in slots on the outer surface of a drum-shaped armature. Front and back end connections join the coil sides. Drum winding replaced ring winding because both sides of each coil can cut the main field flux.

Construction of Drum Winding

A drum winding may use a single-layer or double-layer slot arrangement.

Single Layer Drum Winding

A single-layer winding places one coil side in each armature slot. DC armatures use this arrangement less often than double-layer winding.

Double Layer Drum Winding

A double-layer winding places two coil sides in each slot, one in the top layer and one in the bottom layer. This symmetrical arrangement is common in DC armatures.

The diagram below shows the two layers in a slot.

Coils are formed and insulated before insertion. The insulation system, conductor covering, slot liner and impregnation material must match the machine’s voltage, temperature class and operating environment.

The conductors are shaped into coils and secured before they enter the armature slots. Their ends remain accessible for connection to the commutator bars.

A full-pitch coil spans about one pole pitch, placing its two active sides under poles of opposite polarity so their induced emfs add through the end connections.

Each junction between adjacent coils terminates on a commutator segment. In the diagram, odd numbers identify top-layer coil sides and even numbers identify bottom-layer coil sides.

Lap winding and wave winding are the two main drum-winding connections. Their end connections determine the number and arrangement of parallel paths.

Frog leg winding is another drum-winding arrangement. It places lap and wave sections in the same armature slots and connects both to the commutator.

Advantages of Drum Winding

1. Both coil sides lie in outer armature slots and cut the main magnetic flux. More of the copper is active, so the induced voltage is higher than in a comparable Gramme-ring armature winding.
2. Coils can be formed and insulated before they are inserted into the armature slots, which makes manufacture and replacement more practical.
3. The two sides of a full-pitch coil lie under opposite poles. Their emfs have the polarity needed to add through the end connection.
4. A short-pitch coil reduces the copper needed for end connections. It can also reduce mutual coupling and the reactance associated with each inductor, which can help commutation.
5. Fractional-pitch winding uses a coil span shorter than one pole pitch. It sacrifices some induced emf but may reduce end-connection copper and improve commutation. The selected fraction depends on the machine design rather than a universal eight-tenths value.
6. Placing several conductors or coil sides in one slot permits fewer, wider armature teeth than using one conductor per slot. Wider teeth are mechanically stronger and give more space for slot insulation.
7. Preformed coils and practical slot arrangements can reduce manufacturing and repair work compared with ring winding.

What is a Gramme Ring Winding?

A ring armature winding passes insulated wire around the outer and inner surfaces of a cylindrical iron ring.

The Gramme-ring type of armature winding is an early armature design. Its laminated iron core forms a hollow cylinder, and a continuous insulated winding passes around the ring.

The closed winding is tapped at intervals for connection to commutator segments. Between brush positions, groups of voltage-generating conductors form parallel paths.

The illustrated two-pole winding has two paths between the positive and negative brushes. Coils 1 to 6 form one path, and coils 7 to 12 form the other.

As the armature rotates clockwise, the outer conductors cut the field flux. In the drawing, induced emf and current point into the page under the N pole and out of the page under the S pole, as determined by Fleming’s right-hand rule.

For Fleming’s right-hand rule, hold the thumb, forefinger and middle finger mutually perpendicular. The forefinger indicates the magnetic field, the thumb indicates conductor motion and the middle finger indicates induced current.

When both paths are traced around the closed ring, their generated emfs have opposite directions. Across the brush terminals, however, each path supplies the same terminal polarity and its series emfs add from one brush to the other.

Because the illustrated winding has two parallel paths, the generated voltage of either path appears across the brushes, and each path carries half the total external current when the paths are balanced.

Advantages of Gramme Ring Winding

1. The closed-ring path is easy to trace because the conductors do not cross on the armature surface.
2. The basic ring arrangement can be adapted to different even pole counts, with the brush and commutator connections selected for that pole count.

Disadvantages of Gramme Ring Winding

1. The inner side of each turn links little useful field flux, so it has little voltage induced in it. Only the outer side is an effective active conductor.
2. For a comparable conductor length, the Gramme-ring armature winding uses only about half its conductors as active conductors, so it produces less emf than drum winding.
3. The inner portions act mainly as end connections, adding copper that contributes little to generated emf.
4. The winding passes through the rotor bore, which makes access, repair and shaft support more difficult.
5. Insulating and securing conductors on both the outer and inner surfaces is less practical than using formed coils in drum slots.
6. The hollow ring construction leaves less core area to carry flux and makes the rotor larger for a comparable output.

Because much of its copper is inactive and its rotor is difficult to support and manufacture, Gramme-ring winding has been replaced by drum winding in modern DC machines.