Electrical machines rely on armature windings to convert energy between electrical and mechanical forms. In DC machines, the armature winding sits on the rotor and interacts with the stationary magnetic field to induce an electromotive force (EMF) or to produce torque. Over time, engineers have developed different winding types—open, closed, Gramme-ring, drum, lap, wave, and frog-leg—each with distinct advantages, disadvantages, and applications. This article explores all major armature windings, their construction, working, key equations, pros and cons, and practical applications.
1. Classification of Armature Windings
The classification is primarily based on whether the winding closes on itself or requires external completion:
Open Coil Winding
- Does not close on itself.
- Requires an external connection to form a closed circuit.
- Rarely used in DC machines, but often applied in AC machines.
Closed Coil Winding
- Closes on itself—if you trace a path along the conductor, it eventually returns to the starting point.
- Used universally in DC machines for smooth commutation and reliable operation.
Closed coil windings are divided into two important types:
- Gramme-ring winding
- Drum winding
2. Gramme-Ring Winding
2.1 Construction
- A hollow iron cylinder (ring) as the armature core.
- Insulated copper wire wound spirally around the ring.
- Coil taps taken at equal intervals and connected to commutator segments.
A classic example is a 2-pole, 16-turn winding:
- Coils 1–8 form one path, and 9–16 form the other.
- The winding is continuous and closed, forming a two-circuit system.
2.2 Working
- By Fleming’s right-hand rule, conductors under the N-pole generate inward EMF, while those under the S-pole generate outward EMF.
- EMFs in each half are additive, so current flows in two parallel paths, splitting equally between them.
2.3 Advantages
- No crossing of conductors → easy to visualize working principle.
- Magnetic and electrical effects are similar to drum winding.
- Can be designed theoretically for multiple poles (2, 4, 6, 8).
2.4 Disadvantages
- Half the coil lies inside the core, wasting copper.
- Difficult and costly to wind, as conductors must pass through the ring center.
- Insulation and repairs are complicated.
- Induced EMF is only half compared to drum windings with the same number of coils.
- Requires a large air gap → stronger field excitation needed.
Conclusion: Gramme-ring winding is now obsolete, replaced by drum winding due to inefficiency.
3. Drum Winding
3.1 Construction
- Conductors are placed in slots on the armature surface.
- Ends are connected by front and back connectors.
- All armature copper is active in flux cutting.
3.2 Advantages
- Efficient: all conductors cut flux, generating EMF.
- Coils can be pre-formed and insulated, lowering cost.
- Allows fractional pitch winding for copper saving.
3.3 Single vs Double Layer
- Single layer winding: one coil side per slot → rarely used.
- Double layer winding: two coil sides per slot (one top, one bottom) → widely used for economy.
3.4 Fractional Pitch Winding
When coil span is slightly less than pole pitch:
- Reduces copper in end connections.
- Improves commutation.
- Minimizes mutual inductance.
4. Practical Features of Drum Windings
- Multiple coil sides per slot → saves space and reduces the number of slots.
Advantages of fewer slots:
- Stronger armature teeth.
- Reduced sparking (smaller voltage between commutator segments).
- Lower manufacturing cost.
Two major drum windings: Lap winding and Wave winding. A hybrid third type is the frog-leg winding.
5. Lap Winding
5.1 Construction
In lap winding, the finishing end of one coil is connected to the commutator segment adjacent to the starting end of the next coil. The winding “laps back” on itself. The number of parallel paths equals the number of poles (A = P).
5.2 Types
- Simplex Lap Winding: single winding; A = P.
- Duplex Lap Winding: two simplex windings connected alternately; A = 2P.
- Triplex Lap Winding: three simplex windings; A = 3P.
Hence lap winding is also called multiple or parallel winding, suited for large current at low voltage.
5.3 Progressive & Retrogressive
- If back pitch
Yb> front pitchYf→ winding progresses left to right → Progressive winding. - If
Yb<Yf→ right to left → Retrogressive winding.
5.4 Key Equations
- Coil pitch:
Yp ≈ Z / P(Z = number of conductors, P = poles) - Back pitch:
Yb = Yf ± 2m(m = 1 for simplex, 2 for duplex, 3 for triplex) - Average pitch:
Yav = (Yb + Yf) / 2 ≈ Z / P - Resultant pitch:
YR = 2(simplex),4(duplex),6(triplex) - Commutator pitch:
Yc = ± m - Number of parallel paths:
A = mP
5.5 Applications
- Electroplating generators.
- Traction motors.
- Low voltage, high current DC machines.
6. Wave Winding
6.1 Construction
In wave winding, the coil ends are connected to commutator segments separated by nearly two pole pitches. The number of parallel paths is always 2, irrespective of the number of poles. It produces higher voltage but handles less current.
6.2 Applications
- High-voltage, low-current machines.
- Lighting generators.
- Small DC motors.
7. Frog-Leg Winding
The frog-leg winding combines lap and wave windings:
- Lap portion provides multiple parallel paths (large current).
- Wave portion provides longer pitch (higher voltage).
- Used in large DC machines requiring both high voltage and high current.
8. Comparative Summary of Armature Windings
| Winding Type | Parallel Paths | Voltage/Current Suitability | Key Applications |
|---|---|---|---|
| Gramme-Ring | 2 | Low efficiency, obsolete | Early DC machines |
| Drum (Lap) | mP (multiple) | Low voltage, high current | Traction, plating generators |
| Drum (Wave) | 2 | High voltage, low current | Lighting, small motors |
| Frog-leg | Hybrid | Both high current & voltage | Large special-purpose DC machines |
9. Applications of Armature Windings
- Lap winding: heavy-duty applications (traction, rolling mills, plating).
- Wave winding: constant voltage applications (lighting, small motors).
- Frog-leg winding: hybrid applications where both voltage and current are high.
Conclusion
The study of armature windings in DC machines shows the evolution from simple Gramme-ring windings to more efficient drum windings (lap, wave, frog-leg). Gramme-ring windings hold historic importance but are now obsolete. Lap winding is best for low-voltage, high-current machines; wave winding suits high-voltage, low-current needs; and the frog-leg winding combines both benefits for large machines. Understanding these windings helps engineers and students select the right configuration for performance, cost, and reliability.