Three-phase motor questions
These questions on three-phase motors are from a Japanese examination for electricians (for second-class or junior electricians, in Japanese Denki Kouji-shi Dai-ni-shu Shiken, 電気工事士第二種試験). This exam needs to be passed to get a licence to do any kind of electrical work. All the questions on the written test are multiple-choice with four choices. The answers to the questions are at the bottom of the page.
Question 1
A three-phase induction motor has rated voltage $V$ [volts] and rated current $I$ [amps]. If it runs continuously for a fixed time $t$ [hours] at the rated values, its power consumption is $W$ kW$\cdot$h. What is the power factor of the motor?
- $\displaystyle{W\over\sqrt3VIt}\times10^5$
- $\displaystyle{W\over3VIt}\times10^5$
- $\displaystyle{\sqrt3VI\over Wt}\times10^5$
- $\displaystyle{3VI\over Wt}\times10^5$
This is question 5 from the 2016 first written exam.
Question 2
In the three phase circuit shown in the diagram, the current I (amps) is.
- 8.3
- 11.6
- 14.3
- 20.0
Question 3
In the three-phase circuit below, with source voltage E volts, the circuit is broken at the cross. In this case, the equation for the current flowing between a and c, I amps, is.
- $\displaystyle {E\over 2R}$
- $\displaystyle {E\over \sqrt3R}$
- $\displaystyle {E\over R}$
- $\displaystyle {3E\over 2R}$
This is question 5 from the 2015 first written exam.
Question 4
In the three-phase circuit below, if each supply line has a resistance of 0.15 Ω, and the line current is 10 A, what is the electrical loss in the circuit?
- 15
- 26
- 30
- 45
This is question 6 from the 2015 first written exam.
Question 5
In the three-phase circuit shown in the diagram, when a three-phase alternating voltage is applied, twenty amps flows in each line. The line voltage $E$ in volts is:
- 120
- 173
- 208
- 240
This is question 4 from the 2014 second written exam.
Question 6
What is the total power consumption of the illustrated three-phase circuit in kilowatts?
- 2.4
- 4.8
- 7.2
- 9.6
This is question 5 from the 2014 first written exam.
Question 7
In a three-phase circuit like the one in the diagram, if the resistance of each line is $r$, and the line current is $I$ amps, the equation for the power loss in watts is
- $\displaystyle \sqrt3 I r$
- $\displaystyle \sqrt3 I^2 r$
- $\displaystyle 3 I r$
- $\displaystyle 3 I^2 r$
This is question 7 from the 2013 second written exam.
Question 8
In the 200 volt three-phase circuit in the diagram, the resistor from c to o is cut. Which of these are the correct voltages before and after the cut?
-
Before: 116V
After: 100V -
Before: 116V
After: 116V -
Before: 100V
After: 116V -
Before: 100V
After: 100V
This is question 5 from the 2013 first written exam.
Answers
- 1: イ
- 2: ロ
- 3: ニ
- 4: ニ
- 5: ハ
- 6: ハ
- 7: ニ
- 8: イ