Millwright Certification Electrical Systems and Controls 3

Question 1

What is the purpose of a phase sequence relay in a three-phase motor installation?

Accepted Answer

To prevent motor startup when the phase rotation is incorrect, protecting equipment from reverse rotation

Answer

To regulate the output voltage delivered to the motor terminals

Answer

To continuously monitor and log each phase's operating temperature

Answer

To automatically balance load current equally across all three phases

Question 2

When using a digital multimeter to measure voltage in a 480V three-phase industrial circuit, which setting must be selected?

Accepted Answer

AC Volts (ACV)

Answer

DC Volts (DCV)

Answer

Resistance (Ω)

Answer

DC Current (DCA)

Question 3

What does NEC stand for, and what is its role in electrical work?

Accepted Answer

National Electrical Code — the standard for safe electrical installation and wiring in the US

Answer

National Energy Corporation — federal regulations governing power plant operations

Answer

Network Equipment Certification — standards for industrial data networking

Answer

National Equipment Code — OSHA standard for machinery guarding requirements

Question 4

Which protective device monitors all three phases of a motor's supply current and disconnects the motor if a phase is lost or severely unbalanced?

Accepted Answer

Phase loss and unbalance relay

Answer

Ground fault circuit interrupter (GFCI)

Answer

Thermal cutout bimetallic switch

Answer

Single-element time-delay fuse

Question 5

What is the primary function of a contactor in a motor control circuit?

Accepted Answer

To electromechanically make and break the high-current power circuit to the motor

Answer

To sense winding overtemperature and open the circuit to protect insulation

Answer

To convert single-phase utility power into balanced three-phase power for the motor

Answer

To provide stepless variable voltage to the motor for smooth speed control

Question 6

What is the standard control circuit voltage used in most US industrial motor control centers (MCCs)?

Accepted Answer

120V AC

Answer

480V AC

Answer

240V AC

Answer

24V DC

Question 7

When a motor nameplate shows a service factor (SF) of 1.15, what does this indicate to a millwright?

Accepted Answer

The motor can operate continuously at 15% above its nameplate horsepower rating

Answer

The motor can safely start up to 1.15 times per hour under full load

Answer

The motor's minimum safe operating voltage is 15% below its nameplate voltage

Answer

The motor's insulation is rated for temperatures 15% above the standard class rating