DC-Gen-example.pdf - QUESTION A1 A DC generator has the...

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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 110—“g ‘ f? _;T 1‘ ,tr _ :1 1.; ., .. ; 100 fi " . 7 ’7 .2, " : H" :3" , g. z _. ,r_ _" it? 70 01"! 111‘ _.._. i’ a) 60 5 ," T i on g- . ~ .. .-l . E ,. .t,‘ 1' f 0' ' . § 50 -‘ 1?, f; _ i 40 4 i 20 ' _. 0 "TH?“ '.54- ~ swat"; ".HH +"i'~*‘“ T5" 0 0.10.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2131415 Field Current (Amps) ...
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