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**Unformatted text preview: **QUESTION A1: A DC generator has the following parameters: Full load current 120 amps
Open Circuit Voltage 100 V
Rated speed 1000 rpm
Armature resistance 0.10 ohms
Shunt ﬁeld winding resistance 80 ohms
Rated ﬁeld current 1.0 amp The no—load saturation curve at 1000 rpm is shown in the attached curve. To answer the
following sub—questions, you may write directly on the graph sheet when appropriate, but
must clearly indicate all pertinent points and the speciﬁc sub-question (a, b, c, or d) to
which they pertain. a. If the machine is operated as a separately-excited dc generator at 1000 1pm
with rated ﬁeld current, determine the terminal voltage at full load. Neglect
the effect of armature reaction [3 marks]. V, 2 EA — [ARA =100V — (120A)(O.IOQ) = 88V b. If the machine is operated as a separately-excited dc generator at 1000 1pm
with rated ﬁeld current, and given that the armature reaction at full load is
equivalent to 0.06 amps ﬁeld current, ﬁnd the terminal voltage [5 marks]. 1.an = 0.06/1
1;, = [f _ 1N,” :100 — 0.06 = 0.94/1 At [f of 0.94A, E A is read from the curve to be 98V VT 2 EA — (AR,1 = 98V — (120A)(O.10Q) = 86V c. If the machine is operated as a self—excited generator (shunt field) with a
rheostat (variable resistor) connected in series with the shunt ﬁeld winding,
and the rheostat is adjusted to give a no-load terminal voltage of 100V,
determine the full—load terminal voltage graphically if there is no armature
reaction [5 marks]. As determined previously the [ARA drop at full load is 12V. Moving to the right from the point where the induced voltage EA line equals the terminal voltage (ﬁeld voltage) line,
the difference is 12 voltswhen the ﬁeld current is 0.82 amps. VI is read at this ﬁeld current to be 82V d. [f the machine is operated as a self-excited generator (shunt ﬁeld) with a
rheostat (variable resistor) connected in series with the shunt ﬁeld winding,
and the rheostat is adjusted to give a no—load terminal voltage of 100V,
determine the full—load terminal voltage graphically with an armature reaction
equivalent to 0.06 amps ﬁeld current [7 marks], An [ARA drop at full load of l2V is still required. In addition, the ﬁeld current must be
adjusted to an effective value which accounts for the armature reaction. A triangle is
created above the terminal voltage (ﬁeld voltage) line with a vertical height of 12 volts
and a horizontal width of 0.06 amps. This triangle is moved between the induced voltage
EA line and the terminal voltage (ﬁeld voltage) line. The point where the trianglejust ﬁts
is when the actual ﬁeld current is 0.78 amps. Vt is read at this ﬁeld current to be 78V.
Note that the actual terminal voltage is read from the point of the line corresponding to
the actual ﬁeld current, but that the induced voltage EA (90V) occurs at the effective ﬁeld
current of 0.72A which is 0.06A below the actual ﬁeld current. 120 HJI'T
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Field Current (Amps) ...

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