Armature Winding: D.C. Machines Quiz and Flashcards

Study Notes

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

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

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

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

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