23 Questions
What is the resistivity of copper?
1.72 x 10-8 Ω-m
If a shunt generator running at 1000 rpm has an emf of 100 V, what will be the emf when the speed increases to 1200 rpm?
140 V
What is the rate dissipation in watts for the x-ray tube requiring a current of 7 mA at a voltage of 60 kV?
550 W
If a copper wire has a resistance of 50 Ω at 10oC, what must be the maximum operating temperature for the resistance to increase by at most 10 percent?
35.54 oC
In a single-phase transformer rated 7.2 kVA with primary winding of 800 turns, what are the currents carried by the two windings when delivering its rated kVA at rated voltages?
7 A, 70 A
What is the duty cycle α if the load current varies linearly between 11 A and 17 A?
0.636
What is the loss of energy caused by hysteresis in 1 hour in 50 kg of iron?
138456 J
What is the DC current in the transmission line for two six-pulse converters rated at 1000 MW, +- 200 kV?
2300 A
Determine the generated per-unit voltage required to produce full-load current under short-circuit conditions for a single-phase, 10-kVA, 200-V generator with internal impedance Zg of 2Ω.
0.5 pu
What is the emf of eight coils in series when each coil has an induced emf of 20 V rms and there is a 30 degrees electrical phase displacement between them?
69.16 V
For a step-down chopper with a dc source voltage of 230 V, what is the power delivered to a load of R = 10 Ω with a duty cycle of 40%?
3.08 kW
What is the maximum uncertainty in the measurement of w?
± 9%
What is the resistance of the transmission-line cable?
Increased by 2%
What is the deflection in radians for a current of 2A in the moving iron ammeter?
$1$
What is the multiplying power of a 100 Ω shunt with a galvanometer having a full-scale current of 10 mA?
13
What is the emf of the test cell in the DC potentiometer setup?
1.11 V
What is the expression for the inductance of the moving iron ammeter?
$L = 2+5\theta-\theta^2$ uH
What is the change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8 A and a field current of 1.8 A?
63.63%
What is the induced armature voltage for an 8-pole DC generator running at 250 rpm, with a flux per pole of 0.06 Wb and a simplex wave wound armature containing 32 coils of 6 turns each?
384 V
In a single-phase, two-wire transmission line made up of round conductors separated by 40 cm, what is the equivalent diameter of a fictitious hollow, thin-walled conductor having the same inductance as the original line?
0.636 cm: 24.21 mH
When a DC shunt machine takes 4 A at 250 V, the efficiency is calculated as 81.7%. What is the calculated efficiency when it operates as a motor taking 40 A at 250 V, with armature reaction weakening the field by 4%?
85.2%
A step-down chopper has an input dc voltage of 220 V and R=10 Ω in series with L=65 mH. What is the main purpose of the chopper in this circuit?
Step down the voltage
For a 15 KW, 220 V shunt motor with an armature resistance of 0.25 Ω, what is the rated line current and how many poles does it have if it runs at 100 rpm?
Rated line current: 2.2 A, Poles: 4
Study Notes
- Copper resistivity: 1.72 x 10-8 Ω-m
- Shunt generator at 1000 rpm: generated emf of 100 V
- Shunt generator at 1200 rpm: emf will be 120 V
- X-ray tube: current of 7 mA at 60 kV
- Rate dissipation in watts: 330 W
- Copper wire resistance at 10oC: 50 Ω
- Maximum operation temperature for a 10% increase in resistance: 35.54 oC
- Single-phase transformer: rated 7.2 kVA, 1.2k V/120 V
- Transformer currents at rated kVA and voltages: 7 A, 70 A
- 220 V, 15 kW motor: rated line current of 2.2 A
- Change in field flux for speed of 1600 rpm: 44.44%
- 8-pole DC generator running at 250 rpm: induced armature voltage of 285 V
- Single-phase transmission line: 15 km long, round conductors 0.8 cm diameter
- Equivalent diameter of fictitious conductor with same inductance: 0.235 cm, 23.91 mH
- DC shunt machine efficiency when taking 40 A at 250 V: 85.2%
- Step-down chopper with input dc voltage of 220 V: duty cycle α = 0.576
- Loss of energy caused by hysteresis in 1 hour in 50 kg of iron: 145386 J
- Bipolar HVDC transmission system converters rated at 1000 MW, ±200 kV: dc current in the transmission line is 2400 A
- Single-phase, 10-kVA, 200-V generator: internal impedance Zg of 2Ω, per-unit voltage required for full-load current under short-circuit conditions: 0.6 pu
- Alternator winding with eight coils in series with 20 V induced emf in each: total emf of eight coils in series is 69.16 V
- Step-down chopper power delivered to load of R =10 Ω with duty cycle of 40%: 3.08 kW
- Reverse recovery time of a diode: storage charge of 152 us
- Inductance of moving iron ammeter expressed as L = 2+5θ-θ2 uH, deflection for a current of 2A: 3 radians
- DC potentiometer with emf balance at 680 mm for test cell: emf of test cell is 1.43 V
- Maximum uncertainty in measurement of w with variables x=80, y=20, z=50: ±5%
- Galvanometer with full-scale current of 10 mA and resistance of 1000 Ω: multiplying power of 100 Ω shunt is 11
- Transmission-line cable with 19 strands of copper conductors, each 1.5 mm diameter increased by 2% in length: resistance of the cable needs to be calculated.
Test your knowledge of electrical engineering calculations with these challenging questions. Calculate duty cycle and energy loss caused by hysteresis in iron when subjected to cyclic magnetic changes. Also, solve problems related to six pulse converters used in bipolar HVDC transmission systems.
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