Skip to main content

Characteristics of Separately Excited D.C. Generators

The characteristics is separately excited d.c. generator are divided into two types,
1) Magnetization   and         2) Load characteristics.
1.1 Magnetization or Open Circuit Characteristics
        The arrangement to obtain this characteristics is shown in the Fig. 1.
Fig. 1  Obtaining O.C.C. of separately excited generator
       The rheostat as a potential driver is used to control the field current and the flux. It is varied from zero and is measured on ammeter connected.
       Eo  = (ΦPNZ)/(60A)
       As If is varied, then Φ change and hence induced e.m.f. Eo  also varies. It is measured on voltmeter connected across armature. No Load is connected to machine, hence characteristics are also called no load characteristics which is graph of Eo  against field current If as shown in the Fig. 2. As If increases, flux Φ increases and Eo increases. After point A, saturation occurs when Φ becomes constant and hence Eo  saturates.
Fig. 2  Open circuit characteristics
1.2 Load Saturation Curve
       This is the graph of terminal voltage Vt against field current If. When generator is loaded, armature current Ia flows and armature reaction exists. Due to this, terminal voltage Vt is less than the no load rated voltage. On no load, current Ia is zero and armature reaction is absent. Hence less number of ampere turns are required to produce rated voltage Eo .
       These ampere-turns are equal to OB as shown in the Fig. 3. On load, to produce same voltage more field ampere-turns are required due to demagnetizating effect of armature reaction. These are equal to BC as shown in the Fig. 3. Similarly there is drop Ia Ra across armature resistance. Hence terminal voltage V = E - Ia Ra . This graph OR is also shown in the Fig. 3. The triangle PQR is called drop reaction triangle. Thus OP is no load saturation curve, OQ is the graph of generated voltage on load and OR is the graph of terminal voltage on load.
Fig. 3  Load saturation curves
1.3 Internal and External Characteristics External
       Let be the no load rated voltage which drops to E due to armature reaction on load and further drops to Vt due to armature resistance drop Ia Ra on load.
The graph of Vt against load current IL is called external characteristics while the graph of E against load current IL is called internal characteristics. These are shown in the Fig. 4. for separately excited d.c.generator. The graphs are to be plotted for constant field current. In case of separately excited d.c. generator induced e.m.f. is totally dependent on flux Φ i.e. field current If . Hence to have control over the field current, in case of separately excited d.c. generators field regulator is necessary.
Fig. 4  Load characteristics
Labels:

Comments

Post a Comment

Comment Policy
We’re eager to see your comment. However, Please Keep in mind that all comments are moderated manually by our human reviewers according to our comment policy, and all the links are nofollow. Using Keywords in the name field area is forbidden. Let’s enjoy a personal and evocative conversation.

Popular posts from this blog

Transformer multiple choice questions part 1

Hello Engineer's Q.[1] A transformer transforms (a) frequency (b) voltage (c) current (d) voltage and current Ans : D Q.[2] Which of the following is not a basic element of a transformer ? (a) core (b) primary winding (c) secondary winding (d) mutual flux. Ans : D Q.[3] In an ideal transformer, (a) windings have no resistance (b) core has no losses (c) core has infinite permeability (d) all of the above. Ans : D Q.[4] The main purpose of using core in a transformer is to (a) decrease iron losses (b) prevent eddy current loss (c) eliminate magnetic hysteresis (d) decrease reluctance of the common magnetic circuit. Ans :D Q.[5] Transformer cores are laminated in order to (a) simplify its construction (b) minimize eddy current loss (c) reduce cost (d) reduce hysteresis loss. Ans : B Q.[6] A transformer having 1000 primary turns is connected to a 250-V a.c. supply. For a secondary voltage of 400 V, the number of secondary turns should be (a) 1600 (b) 250 (c) 400 (d) 1250 A...
4 comments
Read more

Condition for Maximum Power Developed In Synchronous Motor

The value of δ for which the mechanical power developed is maximum can be obtained as, Note : Thus when R a is negligible, θ = 90 o for maximum power developed. The corresponding torque is called pull out torque. 1.1 The Value of Maximum Power Developed        The value of maximum power developed can be obtained by substituting θ = δ in the equation of P m .        When R a is negligible,     θ = 90 o  and cos (θ) = 0 hence, . . .               R a = Z s cosθ   and X s = Z s sinθ        Substituting   cosθ = R a /Z s in equation (6b) we get,         Solving the above quadratic in E b we get,        As E b is completely dependent on excitation, the equation (8) gives the excitation limits for a...
Post a Comment
Read more

Effect of Slip on Rotor Parameters : Part2

Effect of Slip on Rotor Parameters 2. Effect of Slip on Magnitude of Rotor Induced E.M.F        We have seen that when rotor is standstill, s  = 1, relative speed is maximum and maximum e.m.f. gets induced in the rotor. Let this e.m.f. be,                 E 2 = Rotor induced e.m.f. per phase on standstill condition         As rotor gains speed, the relative speed between rotor and rotating magnetic field decreases and hence induced e.m.f. in rotor also decreases as it is proportional to the relative speed N s - N. Let this e.m.f. be,                E 2r = Rotor induced e.m.f. per phase in running condition  Now        E 2r α N s while E 2r α N s - N        Di...
Post a Comment
Read more