Question 178Electricity/Electronics - 1st Asst/Chief70% to pass
In the illustrated motor controller, the motor fails to start. A voltmeter reading between 1 and 5 reads line voltage, while the voltmeter reading between 2 and 5 reads 0 VAC. What is most likely the problem? Illustration EL-0007
AI Explanation
The correct answer is A) an open stop switch contact (when not pushed in).
The reasoning is that with line voltage present between terminals 1 and 5, but 0 VAC between terminals 2 and 5, it indicates that the circuit between the stop switch and the main contactor "M" coil is open. This means the stop switch, when not pushed in (i.e., in the open position), is preventing the contactor coil from being energized, which would prevent the motor from starting.
The other options are incorrect because an open main contactor coil (B) would result in 0 VAC between all terminals, and an overload relay contact (C) or an open start switch (D) would not explain the specific voltage readings observed.
Related Questions
#176 Why are motor controllers seldom troubled by grounds? #177 As shown in the illustration, what is the purpose of the Time Delay (TR) coil in the circuit? Illustration EL-0104 #179 During its operation, loud buzzing and resultant welding of contacts of a magnetic relay may occur. Which of the following would be the likely cause? #180 If the contacts of a motor starter or controller fail to drop out when the 'stop' button is depressed, what could be the cause? #181 If the motor of the illustrated circuit fails to start and gives a loud hum when the start button is pushed, what is most likely the problem? Illustration EL-0007