IEC Motors and Motor Controls 4 — Questions and Answers
Question 1: A motor disconnecting means must be within sight of the motor and its driven machinery. 'Within sight' is defined by the NEC as visible and not more than:
- 25 feet
- 50 feet (Correct answer)
- 75 feet
- 100 feet
Correct answer: 50 feet
Per NEC 430.102, 'within sight' means visible and not more than 50 feet distant from the motor and driven machinery.
Question 2: What is plugging as a method of motor braking?
- Removing power and allowing the motor to coast to a stop
- Reversing the phase sequence of the motor while it is running to create braking torque (Correct answer)
- Applying DC to the stator after disconnecting AC power
- Using a mechanical friction brake
Correct answer: Reversing the phase sequence of the motor while it is running to create braking torque
Plugging reverses the phase sequence applied to the motor while it is still rotating, creating a reverse rotating field that produces braking torque.
Question 3: Which of the following best describes a split-phase motor?
- A three-phase motor operating on single-phase power
- A single-phase motor with two windings (main and auxiliary) displaced in time and space to create a rotating field (Correct answer)
- A DC motor with a split commutator
- A motor that splits its output between two shafts
Correct answer: A single-phase motor with two windings (main and auxiliary) displaced in time and space to create a rotating field
Split-phase motors use a main running winding and an auxiliary starting winding with different impedances to create phase displacement and a rotating magnetic field.
Question 4: What is the primary advantage of a capacitor-start motor over a standard split-phase motor?
- Higher running efficiency
- Higher starting torque due to greater phase displacement between windings (Correct answer)
- Lower noise operation
- No need for a starting winding
Correct answer: Higher starting torque due to greater phase displacement between windings
The capacitor in the starting circuit creates nearly 90° phase displacement between the main and auxiliary winding currents, producing significantly higher starting torque.
Question 5: In a motor control ladder diagram, the symbol for a normally open (NO) contact that is controlled by a pushbutton is:
- A horizontal line with a diagonal arrow through it
- Two parallel vertical lines
- Two vertical lines with a diagonal line between them representing the actuator (Correct answer)
- A circle with a line through it
Correct answer: Two vertical lines with a diagonal line between them representing the actuator
In ladder diagrams, a normally open pushbutton contact is represented by two vertical lines (the contact symbol) with a slanted line representing the actuator arm.
Question 6: The maximum rating of a dual-element (time-delay) fuse used for motor branch circuit short-circuit protection per NEC 430.52 for a Design B motor is:
- 100% of motor FLC
- 150% of motor FLC
- 175% of motor FLC (Correct answer)
- 225% of motor FLC
Correct answer: 175% of motor FLC
Per NEC Table 430.52, the maximum rating for dual-element time-delay fuses protecting NEMA Design B motors is 175% of the motor's full-load current.
Question 7: A motor controller's SCCR (Short Circuit Current Rating) must be equal to or greater than:
- The motor's full-load current
- The motor's locked rotor current
- The available fault current at the point of installation (Correct answer)
- The branch circuit breaker rating
Correct answer: The available fault current at the point of installation
The SCCR of a motor controller must be equal to or greater than the maximum available fault current at the terminals of the controller.
IEC Motors and Motor Controls 4
A motor disconnecting means must be within sight of the motor and its driven machinery. 'Within sight' is defined by the NEC as visible and not more than: