PLT 2054 Electrical Machines Technology 1 DC GENERATOR

PLT 205/4 Electrical Machines Technology 1 DC GENERATOR By: Miss Syatirah Binti Mohd Noor syatirah@unimap. edu. my

Types of DC Machines DC machines are usually classified according to the way in which their fields are excited. 1. Separately-excited DC machine 2. Self-excited DC machine (a) Shunt DC machine (b) Series DC machine (c) Compound DC machine (i) Long shunt compound (ii) Short shunt compound

Generating an AC Voltage The voltage generated in any dc generator inherently alternating and only becomes dc after it has been rectified by the commutator.

Direct Current Generator

Generated emf or back emf or emf Equation of a DC Generator General form of generated emf, or E=CNΦ where Note: Φ = flux/pole (Wb) Z = total number of armature conductors = number of slots x number of conductor/slot P = number of poles A = number of parallel paths in armature N = armature rotation (rpm) E = emf induced in any parallel path in armature A = 2 (for wave winding) A = P (for lap winding)

Types of Generators • Separately-excited generators are those whose field magnetics are energized from an independent external source of dc current. Equivalent circuit of separately excited dc gen. Separately excited 2 -pole generator V = terminal (load) voltage E = emf induced Ia = armature current IL = load current Ra = armature circuit resistance Vf = field circuit voltage Rf = field circuit resistance If= field current V=E−I Ra Ia = I L Vf =I f Rf a

• Self excited generators are those whose field magnets are energized by the current produced by the generators them selves. (1) Shunt Generator. The field windings are connected across or parallel with the armature conductors and the full voltage of the generator across them. DC shunt generator V=E−Ia Ra Vf =V=IR sh sh Equivalent circuit of dc shunt generator where Ish = shunt field current Rsh = shunt field resistance

(2) Series Generator. In this case, the field windings are joined in series with the armature conductors. As they carry full load current, they consist of relatively few turns of thick wire or strip. Such generators are rarely used. V=E −I a(Ra+R) se where Rse = series field resistance Ise = series field current 26

(3) Compound Generator. It is a combination of a few series and a few shunt windings and can be either short or long-shunt. short-shunt compound dc generator or 27

Long-shunt compound dc generator Compound generator also refer as: 1. Cumulatively compounded generator, where both a shunt and a series field are present, and their effects are additive 2. Differentially compounded generator, where both a shunt and a series field are present, and their effects are subtractive.

An 8 -pole dc shunt generator with 778 wave connected armature conductors and running at 500 rpm, supplies a load of 12. 5 Ω resistance at a terminal voltage of 250 V. The armature resistance is 0. 24 Ω and the field resistance is 250 Ω. Find the armature current, the induced emf and the flux per pole.