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Tutorial #01
EMT 113/4: Electrical Engineering Technology
TUTORIAL #1
Transformers, DC Machines,
and AC Machines
EMT 113/4
ELECTRICAL ENGINEERING
TECHNOLOGY
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Tutorial #01
EMT 113/4: Electrical Engineering Technology
1. Consider a single phase 2400/240V transformer in Figure 1. The primary is connected to
a 22000 V source and the secondary is connected to a load impedance of 236.9 Ω.
Ω
Figure 1
Determine the following
a) The secondary output current and voltage
b) The load impedance as seen from the primary side
c) The input and output apparent powers.
2. A 250 kVA, 11 000V/400V, 50Hz single-phase transformer has 80 turns on the
secondary. Assume the transformer is ideal. Calculate:
a) The values of the primary and secondary currents
b) The number of primary turns
c) The maximum value of flux, Фm.
3. A 10 kVA 2400/240V 60Hz transformer was tested with the following results: Power
input during short circuit test is 340W, power input during open circuit test is 168 W.
Determine the efficiency of the transformer at full load. Given the load power factor is
0.8.
4. A 1: 10 step-up transformer is placed at the generator end of the transmission line and a
10:1 step-down transformer is placed at the load end of the line as shown in Figure 2.
Determine the load voltage and the transmission line losses.
Figure 2
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Tutorial #01
EMT 113/4: Electrical Engineering Technology
5. A 1000-VA, 230/115-V, transformer has been tested to determine its equivalent
circuit. The results of the tests are taken from the primary side as in Table 1.
Open-circuit test
Voc = 230 V
Ioc = 0.45A
Poc = 30W
Short circuit test
Vsc = 19.1 V
Isc = 8.7 A
Psc = 42.3 W
Table 1
(i)
Find and sketch the equivalent circuit of the transformer referred to
low-voltage side.
(ii)
The transformer’s efficiency at rated conditions and 0.8 PF lagging.
Given Pcu= 10.6 W and Pcore = 32.0 W.
6. A 13.2-kV single-phase generator supplies power to a load through a transmission
line. The load’s impedance is Z load  500  36.87 , and the transmission line’s
impedance is Z line  60  53.1 . Transformers may be assumed to be ideal.
Figure 3
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Tutorial #01
EMT 113/4: Electrical Engineering Technology
a) If the generator is directly connected to the load (Figure 3(a)),
(i)
(ii)
What is the ratio of the load voltage to the generated voltage?
What are the transmission losses of the system?
b) If a 1:10 step-up transformer is placed at the output of the generator and a 10:1
transformer is placed at the load end of the transmission line (Figure 3(b)),
(i)
(ii)
What is the new ratio of the load voltage to the generated voltage?
What are the transmission losses of the system now?
7. The secondary winding of a transformer has a terminal voltage of v s (t )  282.8 sin
377t V. The turn’s ratio of the transformer is 100:200. If the secondary current of the
transformer is i s (t )  7.07 sin (377t - 36.87 0 )A . Given the impedances of this
transformer referred to the primary side are;
Determine the following:
i.
ii.
The primary current of this transformer.
The transformer’s voltage regulation and efficiency
8. The armature winding of a dc motor has 320 conductors, only 70 % of which lie directly
under the poles, where the flux density B = 1.1 T. The armature diameter is 26 cm and its
length is 18 cm. The conductor current is 12 A. Determine:
a) The total force created by the conductor.
b) The torque developed.
9. The armature of a 120V dc motor has a resistance of 1.5 Ω and takes 4A when
operating at full load. Calculate:
a) The counter EMF produced by the armature.
b) The power developed by the armature.
c) Calculate the speed of the motor if the armature winding of the four poles
dc motor has 360 conductors and 0.01 Wb flux per pole.
10. A shunt generator has a field resistance of 60 Ω. When the generator delivers 60kW the
terminal voltage is 120 V, while the generated EMF is 133 V. Determine:
i)
ii)
The armature circuit resistance
The generated EMF when the output is 20kW and the terminal voltage is 135 V.
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Tutorial #01
EMT 113/4: Electrical Engineering Technology
11. A 100 kW, 230 V, shunt generator has Ra = 0.05Ω and Rf = 57.5Ω. If the generator
operates at rated voltage, calculate the induced voltage at full load. Neglect the brush
contact drop, Vbrush.
12. A 220V, 7.5 kW series motor is mechanically coupled to a fan. When running at 400 rpm,
the motor draws 30A from the mains (220 V). The torque required by the fan is
proportional to the square of the speed. Neglect armature reaction and rotational loss. Also
assume the magnetization characteristic of the motor to be linear.
Determine
(i)
The power delivered to the fan.
(ii)
The torque developed by the motor.
(iii)
The external resistance (Rext) to be added in series to the armature circuit to
reduce the fan speed to 200 rpm.
(iv)
The power delivered to the fan at the speed of 200 rpm.
13. A 208-V, two-pole, 60Hz, Y-connected wound motor induction motor is rated at
15 hp. Its equivalent circuit components are,
R1 = 0.200 Ohm
X1 = 0.410 Ohm
Pmech = 250 W
R2 = 0.120 Ohm
X2 = 0.410 Ohm
Pmisc = 0 W
XM = 15.0 Ohm
Pcore = 180 W
For the slip of 0.05 find,
a)
b)
c)
d)
e)
f)
g)
h)
The line current IL
The stator copper lost PSCL
The air gap power PAG
The power converted from electrical to mechanical form Pconv
The induced torque,  ind
The load torque,  load
The overall machine efficiency
The motor speed in revolutions per minute and radian per second.
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