Types of Generators ACCORDING TO EXCITATION

Types of Generators ACCORDING TO EXCITATION

Separately Excited DC Generator A dc generator whose field magnet winding is supplied from an independent external d.c. source (e.g., a battery etc.)

Separately Excited DC Generator It may be noted that separately excited d.c. generators are rarely used in practice. The d.c. generators are normally of self excited type.

Self Excited DC Generator A d.c. generator whose field magnet winding is supplied current from the output of the generator itself

Types of Self Excited DC Generator Shunt Wound Series Wound Compound Wound

Presence of Residual Magnetism The field is in the form of an inductor, hence, it can store energy as an electro-magnetic field. This field will not all disappear once the generator is turned off, some will remain, as residual magnetism or flux.

Presence of Residual Magnetism Due to the residual flux in the field, it will enable to armature to develop the residual voltage, which causes a small current to flow through the field windings.

Presence of Residual Magnetism As the generated voltage rises, the field current also rises, which in turn causes more flux to be developed, and still a larger voltage. This process will continue until the voltage reach its proper value.

Presence of Residual Magnetism Should the field loses its residual flux, the field is connected to a separate DC source in order for it to produce small amount of flux, this method is called flashing the field.

Shunt Wound DC Generator The field windings are connected across or in parallel with the armature. They have the full voltage of the generator applied to them.

Shunt Wound DC Generator The shunt field is usually constructed of many turns of fine wire. The shunt field is a constant flux field, therefore, this type of generator is of the constant voltage type.

Equivalent Circuit of Shunt Generator i a =i sh + i L V sh =V t =i sh R sh E g =V t + i a R a P g =E g i a P d =V t i L Where: E g - generated voltage i L - line current V t - terminal voltage R sh - shunt field resistance V sh - shunt field voltage R a - armature winding resistance i sh - shunt field current P g - power generated / developed i a - armature winding current P d - power delivered

Series Wound DC Generator The field windings are connected in series with the armature winding. The current passing through the series field depends upon the load, thus, the flux produced is variable.

Series Wound DC Generator They consist of relatively few turns of thick wire or strips.

Series Wound DC Generator Due to the variable flux, the voltage generated by this type of generators are also variable. Such generators are rarely used except for special purposes, ie. Boosters.

Equivalent Circuit of Series Generator R s i a =i s =i L E g =V t + i a R a + i s R s P g =E g i a P d =V t i L Where: E g - generated voltage R s - series field resistance V t - terminal voltage R a - armature winding resistance i s - series field current P g - power generated / developed i a - armature winding current P d - power delivered i L - line current

Compound Wound DC Generator Combines both the series field and shunt field. a) Short shunt b) long shunt Can be connected as (a) short shunt or as (b) long shunt.

Types of Compound Wound Generator according to flux I. Cumulative Compound Generator the series field is aiding the shunt field to supply power and lighting loads. 1. Under Compounded the full load terminal voltage is less than the no-load voltage. It is used when the load is located near from it.

Types of Compound Wound Generator according to flux 2. Flat Compounded the full-load terminal voltage is the same as the no-load voltage. It is used when the load is at a medium distance from it. 3. Over Compounded the full-load terminal voltage is greater than the no-load voltage. It is used when the load is far from it.

Types of Compound Wound Generator according to flux II. Differential Compound Generator the series field flux opposes the shunt field flux. It is used in electric welding.

Equivalent Circuit of Long Shunt Compound Generator i a =i sh + i L i s =i a R s V sh =i sh R sh E g =V t + i a (R a + R s ) P g =E g i a P d =V t i L

Equivalent Circuit of Short Shunt Compound Generator i a =i sh + i L R s i s =i L V sh =i sh R sh E g =V t + i a R a + i s R s P g =E g i a P d =V t i L

Brush Contact Drop It is the voltage drop over the brush contact resistance when current passes from commutator segments to brushes and finally to the external load. Its value depends on the amount of current and the value of contact resistance.

Brush Contact Drop This drop is usually small and includes brushes of both polarities. However, in practice, the brush contact drop is assumed to have following constant values for all loads. 0.5 V for metal-graphite brushes. 2.0 V for carbon brushes.

Other Reminders The following may be included in the design of DC Machines: Interpole/ Commutating pole winding used to correct the objectionable commutation effects of armature reaction. This winding is permanently connected in series with the armature.

Other Reminders Compensating winding used for the purpose of neutralizing the effect of armature reaction in the zones outside the influence of the interpoles. This winding is also connected in series with the armature.

Other Reminders Diverter a low resistance shunt connected directly across the series field of a compound generator for the purpose of adjusting the degree of compounding.

Problems An 8-pole d.c. shunt generator with 778 wave-connected armature conductors and running at 500 r.p.m. supplies a load of 12.5 Ω resistance at 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 e.m.f. and the flux per pole. Answer: 21 A, 255.04 V, 9.83 mwb

Problems A short-shunt compound generator delivers a load current of 30 A at 220 V, and has armature, series-field and shuntfield resistances of 0.05 Ω, 0.30 Ω and 200 Ω respectively. Calculate the induced e.m.f. and the armature current. Allow 1.0 V per brush for contact drop. Answer: 232.56 V, 31.145 A

Problems The following information is given for a 300-kW, 600-V, long-shunt compound generator : Shunt field resistance = 75 Ω, armature resistance including brush resistance = 0.03 Ω, commutating field winding resistance = 0.011 Ω, series field resistance = 0.012 Ω, divertor resistance = 0.036 Ω. When the machine is delivering full load, calculate the voltage and power generated by the armature. Answer: 625.4 V, 317.7 kw

Problems A separately excited d.c. generator, when running at 1200 r.p.m. supplies 200 A at 125 V to a circuit of constant resistance. What will be the voltage when the speed is dropped to 1000 r.p.m. and the field current is reduced to 80%? Armature resistance, 0.04 Ω and total drop at brushes, 2 V. Answer: 90V

Voltage Regulation It is the percentage rise in the terminal voltage of the generator when the generator load is removed. %VR = V NL V FL V FL 100 Where V NL = no-load terminal voltage V FL =full-load terminal voltage

Problems The voltage of a 100-kw 250-volt shunt generator rises to 260 volts when the load is removed. What full-load current does the machine deliver, and what is its per cent regulation? Answer: 400 A, 4%

Problems A 25-kw 230-volt shunt generator has a regulation of 8.7 percent. What will be the terminal voltage of the generator at no load? Answer: 250 V

Problems If the change in voltage is assumed to be uniform between no-load and full-load kilowatts, calculate the kilowatt output of the generator when the terminal voltages are 240 and 235 volts. Answer: 12.5kW, 18.75kW