Armature Winding
Part of Generators and Motors
The armature winding is the current-generating or current-carrying coil in a generator or motor — its design determines voltage, current capacity, and electrical efficiency.
Why This Matters
The armature winding converts mechanical rotation into electrical current (in a generator) or electrical current into mechanical torque (in a motor). Everything else in the machine — field magnets, shaft, bearings, commutator — exists to support the armature winding’s function. Getting the winding right determines whether your machine produces the designed voltage, handles the required current without overheating, and operates reliably for years.
Armature winding is a skill. It requires precise coil counting, careful slot filling, proper insulation, and connection according to a winding diagram. Done correctly, it takes a day or two for a medium machine. Done incorrectly, the machine either doesn’t work or fails within hours.
Understanding armature winding principles lets you design windings for specific voltage and current requirements, rewind damaged machines to restore them to service, and diagnose winding faults (shorts, opens, grounds) methodically.
Winding Types
Lap winding: Each coil connects to adjacent commutator segments. The winding laps around the armature sequentially. Number of parallel paths = number of poles. Lap winding provides higher current at lower voltage — suitable for low-voltage, high-current generators.
Wave winding: Coils connect to commutator segments approximately 2 pole pitches apart. Only 2 parallel paths regardless of pole number. Wave winding gives higher voltage at lower current — suitable for high-voltage, lower-current output.
For a 2-pole machine: Lap and wave windings are equivalent (both have 2 parallel paths). The distinction matters for 4+ pole machines.
Chorded (short-pitch) winding: Coil spans slightly less than one pole pitch. Reduces harmonics in the output waveform, improving voltage quality. Standard in well-designed machines.
Coil Parameters
Number of armature conductors (Z): Z = slots × conductors per slot. More conductors = higher voltage but more difficult construction.
EMF equation for a DC generator: E = (P × Φ × Z × N) / (60 × A)
Where:
- P = number of poles
- Φ = flux per pole (webers)
- Z = total armature conductors
- N = speed (RPM)
- A = number of parallel paths (A = P for lap winding)
Wire sizing: Choose wire gauge so current density stays below 4–5 A/mm² for natural air cooling, or up to 6 A/mm² with forced air. Higher current density increases temperature; excessive temperature destroys insulation.
Example: For a 12 V, 10 A (120 W) generator with 4-pole lap winding at 1,500 RPM, and assuming Φ = 0.005 Wb: Z = (E × 60 × A) / (P × Φ × N) = (12 × 60 × 4) / (4 × 0.005 × 1500) = 2880 / 30 = 96 conductors. With 24 slots and 4 conductors per slot: 24 × 4 = 96. Current per path = 10/4 = 2.5 A. Wire: 0.5 mm² or larger.
Slot Insulation
Insulation serves three functions: preventing phase-to-phase shorts in AC machines, preventing ground faults (winding to core), and preventing coil-to-coil shorts within a slot.
Slot liner: A U-shaped sleeve of insulating material placed in each slot before winding. Materials:
- Varnished cambric (traditional) — woven cotton treated with oil varnish
- Fish paper (press board) — stiff, good mechanical protection
- Aramid paper (Nomex) — modern, best temperature rating, hard to source in rebuilding scenarios
- Oiled cotton or linen — acceptable insulating capacity for modest voltage
Coil insulation: Wind magnet wire (enameled copper) with a continuous enamel coating on each conductor. For higher voltage (>100 V), add cotton or silk overwrap on each coil.
Slot wedge: After filling the slot, press in a wedge of insulating material (wood, plastic, or fiber) to hold coils in place and prevent them from flying out under centrifugal force at high speed.
Impregnation: After winding is complete, impregnate with varnish (shellac, linseed oil varnish, or modern epoxy impregnating varnish). Impregnation fills voids in the winding, bonds conductors together, excludes moisture, and dramatically improves heat conduction from winding to core.
Impregnation procedure:
- Pre-heat completed armature to 100–120°C to drive out moisture (2 hours in oven)
- Dip in varnish at room temperature; allow to soak 30 minutes
- Drain and cure at 130°C for 4–8 hours
- Repeat dipping and curing 2–3 times for best protection
Winding Procedure
Step 1: Prepare the core. Clean slots, install slot liners. Mark slots and number them.
Step 2: Wind coils. Wind coils on a coil form matched to the slot pitch. Each coil consists of a set number of turns. Use a hand-cranked coil winder with a counter for consistency.
Step 3: Insert coils. Start at slot 1 and slot 1+pitch. Press coil sides firmly into slots without damaging insulation. Use wooden or hard plastic slot tools — never metal (scratches enamel).
Step 4: Connect coils. For a lap winding, connect each coil end to the next commutator segment. For correct polarity, follow the winding diagram precisely. Test for continuity and resistance after each group of connections.
Step 5: Balance. Ensure coils are seated evenly. Balance the armature by removing material from heavy spots or adding solder to light spots on commutator end.
Testing the Completed Winding
Resistance test: Measure resistance across each commutator bar pair. Values should be equal within 5% for all pairs. High resistance indicates a poor connection; zero resistance indicates a short.
Ground test: Apply 100–500 V DC (from a hand-cranked magneto or battery stack) between each commutator bar and the shaft. Should read very high resistance (>1 MΩ). Any low reading indicates insulation failure.
Bar-to-bar test: Rotate armature one segment at a time, measuring resistance across adjacent commutator bars. Equal readings confirm symmetric winding.
Running test: Connect to correct excitation, spin at rated speed, and measure output voltage. Compare against design calculation. Voltage within 5% indicates a successful winding.