EIAT and NEIEP Test Motor Controls 2 — Questions and Answers
Question 1: What is the primary function of a Variable Frequency Drive (VFD)?
- To convert single-phase power to three-phase for motor use
- To control motor speed by varying the frequency and voltage of the power supply (Correct answer)
- To provide backup power to a motor during a power outage
- To protect the motor from ground faults and short circuits
Correct answer: To control motor speed by varying the frequency and voltage of the power supply
A VFD controls motor speed and torque by varying the frequency and voltage of the electrical power supplied to the motor.
Question 2: In a reversing motor starter, what prevents both forward and reverse contactors from energizing simultaneously?
- A time-delay relay set for a 30-second interval
- Mechanical and electrical interlocking between the two contactors (Correct answer)
- A thermal overload relay sized for both directions of rotation
- A double-pole selector switch on the control panel face
Correct answer: Mechanical and electrical interlocking between the two contactors
Both mechanical interlocks (physical blocks between contactors) and electrical interlocks (NC auxiliary contacts in each other's coil circuit) prevent simultaneous energization that would cause a phase-to-phase short circuit.
Question 3: A reduced voltage starter (soft starter) is primarily used to:
- Permanently reduce the operating voltage of the motor to save energy
- Limit inrush current and mechanical shock during motor starting (Correct answer)
- Step up the voltage for high-horsepower motor applications
- Provide variable speed control throughout the motor's full operating range
Correct answer: Limit inrush current and mechanical shock during motor starting
Reduced voltage starters limit the high inrush current and mechanical torque shock that occur when a large motor is started directly across the line.
Question 4: What does a time-delay relay accomplish in a motor control circuit?
- It permanently opens the circuit after a preset time has elapsed
- It introduces a timed interval before allowing a control action to occur (Correct answer)
- It measures the elapsed running time of the motor for maintenance logs
- It delays power to prevent voltage spikes at motor startup
Correct answer: It introduces a timed interval before allowing a control action to occur
A time-delay relay waits for a preset time interval before opening or closing its contacts, which is useful for sequencing starters or controlling transition timing.
Question 5: Which of the following best describes a contactor as used in motor control?
- A manually operated disconnect for isolating the motor from the supply
- An electromagnetically operated switch designed for making and breaking high-current load circuits (Correct answer)
- A solid-state device that regulates motor current through phase angle control
- A mechanical governor that limits motor speed under varying load conditions
Correct answer: An electromagnetically operated switch designed for making and breaking high-current load circuits
A contactor is an electromagnetically actuated switch designed to repeatedly make and break motor load currents under the control of a low-power coil signal.
Question 6: In a two-speed motor control circuit using a single stator winding (consequent pole method), what determines the two speeds?
- Varying the supply voltage between two preset levels
- Reconnecting the stator windings to change the effective number of magnetic poles (Correct answer)
- Using two separate motors coupled to a common shaft
- Adjusting the rotor resistance through an external rheostat
Correct answer: Reconnecting the stator windings to change the effective number of magnetic poles
A consequent pole motor changes speed by reconnecting the stator windings to alter the effective number of poles, since synchronous speed equals 120 times frequency divided by the number of poles.
Question 7: Why is it important that control components be connected in the secondary (control) circuit rather than directly in the primary (load) circuit?
- Control components are rated for higher voltages than load-side components
- Using lower voltage in the control circuit improves safety and allows smaller wire gauges (Correct answer)
- Primary circuit components are immune to overload conditions
- Control wiring must always operate at the same voltage as the motor nameplate rating
Correct answer: Using lower voltage in the control circuit improves safety and allows smaller wire gauges
Isolating control components in a lower-voltage secondary circuit (typically 120V) reduces shock hazard for workers and allows the use of smaller, less expensive control conductors.
EIAT and NEIEP Test Motor Controls 2
What is the primary function of a Variable Frequency Drive (VFD)?