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University of Anbar College of Engineering Dept. of Electrical Engineering Dr. Settar S. Keream
CHAPTER 8
GENERATOR OPERATION
In generator operation, a dc machine is driven by a prime mover and supplies an electrical load. This chapter explains the operating characteristics of dc generator and the affect them.
8 .1 The Voltage Equation
In generator operation, we are interested in the voltage supplied at the output terminals. From KVL and the information of the previous chapters, the terminal voltage of a dc generator can be written: V =EA – (∑IR +Vb) (8.1a) = KenФr - (∑IR +Vb) (8.1b) = EAoc - (∑IR +Vb+ ∆E) (8.1c) = KenФm - (∑IR +Vb+ ∆E) (8.1d) Where EA = KenФr is the induced emf in the armature, and Фr is the actual (i.e. resultant) flux per pole; EAoc = KenФm is the induced emf on open circuit (no armature current), and Фm is the flux per pole due to the main field. Фr may be somewhat less than Фm due to the demagnetizing effect of armature reaction (chapter 5); ∆E represents the corresponding reaction in induced emf (∆E =EAoc – EA, section 5.3). The difference between the induced emf EA and the terminal voltage V is the sum of series resistive drops ∑IR (in the armature, series field wdg, commutating wdg, and compensating wdg) and the brush contact drop Vb. Eqn. 8 .1d tells us that the terminal voltage is determined primarily by the speed n and the main field flux Фm, with some reduction due to series voltage drops and armature reaction.
8.2 Speed of Rotation
The speed is set at the prime mover, not the generator itself. Of course the generator is a mechanical load on the prime mover, and hence affects its operation :as the electrical load on the generator increases, the armature current IA increases thus increasing the developed torque Td=)K IA Фr); if the prime mover torque does not increase to balance the increase in Td, it will slow down (reducing EA, hence V, hence IA, hence Td). However, in many applications, the prime mover is equipped with automatic control that maintains the speed almost constant (eg governor :as speed begins to fall, the governor enlarges the steam openings to the turbine).
8.3 Field Excitation
The main flux Фm is determined by the field mmf through the magnetization carves, or we say that EAoc is determined by the field excitation current through the OCC; see section 4. 3. The shunt field excitation may be controlled by adding the variable resistance in series with the shunt field wdgs, and the series field excitation may be controlled by means of a small variable resistor (diverter)in parallel with the series field wdgs, see fig. 8. 2.
University of Anbar College of Engineering Dept. of Electrical Engineering Dr. Settar S. Keream
8.4 Voltage Drops
The series resistance drops ∑IR and armature reaction drop ∆E increase with load (why?); the brush constant drop Vb is practically constant over the normal working range of IA. The total drop is generally small (small wdg resistance, and small demagnetizing effect of armature reaction). For simplicity, we shall use the symbol ∆V for the total drop: ∆V =∑IR+Vb+∆E (8.2) So that equs. 8 .1c and 8.1d become: V =EAoc - ∆V ( 8 .3a) = KenФm- ∆V (8.3b)
8. 5 Definitions
We shall need the terms and concepts defined below in our description of generator operation and the factors that affect it.
8.6 External Characteristic
The external characteristic of a dc generator is the curve relating terminal voltage V and terminal current I (i.e. load current). The curve shows now voltage change with load. For a simple source circuit composed of constant emf E in series with constant internal resistance- see fig. 8 .3a, the external characteristic is a straight line with negative slope, fig. 8 .3b: V = E – IR 0 (8.4) Comparing eqn. 8 .4 with eqn. 8 .1d we see that the external characteristic of a dc generator will be different from that of fig. 8 .3b. We shall study the external characteristics of dc generators in section 8. 3 - 8. 6. If a load resistance R 1 is connected across the terminals of the simple source of fig. 3 .3a, the terminal voltage and current will be: Tpm
Fig. (8.1) Generator operation
Vt It wr
DC GENERATOR
Fig. (8.2) DC machine with
field control resistance
V R
I
G
Ro E
V
E
I
v 1 v 2 O I 1 I 2
R 1
R 2
(a) circuit diagram (b) external characteristic
Fig. (8.3) Simple voltage source
University of Anbar College of Engineering Dept. of Electrical Engineering Dr. Settar S. Keream
8. 10 Separately Excited Generator
If the field current of a separately excited generator is kept constant, Фm and EAoc will be constant. The external characteristic is then as shown in fig. 8. 5 :V is less than EAoc due to the armature circuit resistive drop IARA (linear with current), the demagnetizing effect of armature reaction ∆E (nonlinear function of IA), and the brush contact drop Vb (constant-not shown in figure). The curvature of the characteristic comes from ∆E. Compare with the simple source of fig. 8. 3.
8. 11 Shunt Generator
The external characteristic of the shunt generator, fig. 8 .6, is similar to that of the separately excited generator, but has an additional drop due to shunt field weakening: IA↑ → ∆V↑ → V↓ → If↓ → Фm↓ → EAoc↓ To explain this process in more detail, we first draw the OCC as in fig. 8 .7; note that the OCC is obtained with the shunt field wdg disconnected from the armature and fed from a separate source. Next, we draw the V-I characteristic for the shunt field resistance. V =Rf If ( 8. 7 ) On the same graph. Now, for any field current If, the point (If, EAoc) must lie on the OCC, and the point (lf, V) must lie on the Rf-line. At no-load the terminal current I is zero so that IA is equal to 1f which is small, so that we may neglect the drop ∆V; thus, the terminal voltage V is equal to the induced emf EAoc. This condition is satisfied only at the point of intersection of the Rf-line with the OCC; therefore, at no-load, we have: If = Ifo, EAoc =E 0 , V =V 0 , with V 0 = E 0 ( 8 .8) Consider next the generator on load. IA has increased so that the drop ∆V is now large enough to make V less than EAoc, eqn. 8 .3a. Operation has to shift from the point of intersection :(If, EAoc( moves down the OCC, while (lf, V) moves down the Rf-line. If will take up a position at which the difference between EAoc and V is equal to the drop ∆V:
Fig. (8.7) Open Circuit Characteristics (OCC)
of shunt excited DC generator.
V
EAoc O I I
ΔE
IARA
V
Vf If V
I
(a) (b)
Fig. (8.5) Separately excited DC
generator
V
EAoc O I Imax
ΔE
IARA
V
If V
I
IA
Isc
Drop due to weakening of
shunt field
(a) (b)
Fig. (8.6) Shunt excited DC generator
University of Anbar College of Engineering Dept. of Electrical Engineering Dr. Settar S. Keream If = If1, EAoc =E 1 , V =V 1 , with V 1 = E 1 - ∆V 1 ( 8 .9) Comparing V 1 and V 0 = )E 0 ), we see that the difference between them is the drop ∆V 1 plus an additional drop (E 0 - E 1 ) due to the reduction of the induced emf EAoc from E 0 to E 1 corresponding to the reduction of the field current If from If0 to If1 as was stated at the beginning of this section. At each value of load current, I, the field current If moves to a position that makes the difference between the OCC and the Rf-line equal to the drop ∆V that corresponds to that load current or, more precisely, to the armature current IA. If you study the OCC and the Rf-line carefully, you will see that there is certain If at which the difference between them is maximum load current, Imax in fig. 8 .6; this is called the breakdown point. The short circuit current of the shunt generator is inherently limited :at SC the terminal voltage V is zero so that If =0; the emf is Eres (induced by the residual flux alone) which is very small. The resulting armature current is therefore small.
8.12 Voltage Build-Up
The preceding discussion helps us understand how the voltage of the shunt generator builds up. Assume that there is no load on the generator, and that there is an open switch in the field circuit so that If=0, EAoc=Eres, and IA= 0. If the switch is now closed, Eres is applied to Rf, and a small current If flows causing EAoc to climb up the OCC. But this increased value of EAoc is again applied to Rf and will increase If, which in turn increases EAoc some more. The process continues with (If, EAoc) climbing up the OCC and (If, V) climbing up the Rf-line until the two points coincide at the intersection point. We say that the shunt generator voltage V has ‘build up’ to V 0 ; what stops the build-up process from continuing indefinitely is the curvature of the OCC, i.e. saturation. (Exercise :how does KVL apply to the circuit during build up?). The process of voltage builds up requires the following conditions to succeed:
(1) There must be residual flux to start the process. A new generator, or one that has not been used for
a long time, must be magnetized first. This is done by applying a separate dc source (for example
a battery) to the field wdg for a short time; it is called ‘flashing the field’.
Figure 1.5 Voltage build-up Figure 1.6 field resistance control
