D.C. Machines: Armature Winding

Armature Winding

• Armature Winding is classified into two types: Lap Winding and Wave Winding

Lap Winding

• Used in machines designed for low voltage and high current
• Armatures are constructed with large wire due to high current
• Examples: used in starter motors of almost all automobiles
• Windings are connected in parallel, which adds the current capacity of each winding and provides a higher operating current
• Number of parallel paths, A = P, where P is the number of poles

Wave Winding

• Used in machines designed for high voltage and low current
• Windings are connected in series, which adds the voltage of each winding but maintains the same current capacity
• Examples: used in small generators
• Number of parallel paths, A = 2

Generated EMF or EMF Equation of a Generator

• The equation for generated EMF (Eg) is based on the following variables:
  • Φ = flux/pole in Weber
  • Z = total number of armature conductors
  • P = number of generator poles
  • A = number of parallel paths in armature
  • N = armature rotation in revolutions per minute (r.p.m)
  • E = e.m.f induced in any parallel path in armature
• The average e.m.f generated per conductor is dΦ/dt volts
• The flux cut per conductor in one revolution is dΦ = ΦP wb
• The time for one revolution is dt = 60/N sec
• According to Faraday's Law of Electromagnetic Induction, the e.m.f generated per conductor is dΦ/dt = ΦPN/60 volts

D.C. Machines

• Speed below critical speed affects the machine's performance.

Effects of Change of Speed

• The shunt field resistance circuit has a greater resistance than the critical resistance.

Total Losses in a D.C. Machine

• Total losses in a D.C. machine consist of three types: copper losses, iron losses, and mechanical losses.

Copper Losses

• Copper losses occur due to current passing through the windings.
• There are three types of copper losses:
  • Armature copper losses (30 to 40% of full load losses) = Ia²Ra, where Ra is the armature resistance and Ia is the armature current.
  • Shunt field copper losses (20 to 30% of full load losses) = Ish²Rsh, where Rsh is the shunt field resistance and Ish is the shunt field current.
  • Series field copper losses = Ise²Rse, where Rse is the series field resistance and Ise is the series field current.
• Losses due to brush contact resistance are usually included in armature copper losses.

Mechanical Losses

• Mechanical losses account for 10 to 20% of full load losses.
• There are two types of mechanical losses:
  • Friction losses, which occur due to bearings.
  • Windage losses, which occur due to the air gap between the armature and pole shoe.