Electrical Systems and Controls Flashcards
7 cards from real Millwright Certification practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Electrical Systems and Controls flashcards as text
What is the purpose of a phase sequence relay in a three-phase motor installation?
Answer: To prevent motor startup when the phase rotation is incorrect, protecting equipment from reverse rotation
A phase sequence relay detects reversed phase rotation and inhibits motor startup, protecting equipment such as pumps and compressors that would be damaged by running backward.
When using a digital multimeter to measure voltage in a 480V three-phase industrial circuit, which setting must be selected?
Answer: AC Volts (ACV)
Industrial power systems supply alternating current, so the multimeter must be set to AC Volts (ACV) to obtain an accurate voltage reading.
What does NEC stand for, and what is its role in electrical work?
Answer: National Electrical Code — the standard for safe electrical installation and wiring in the US
The National Electrical Code (NEC), published by the NFPA as NFPA 70, establishes the minimum safety requirements for electrical wiring and installation throughout the United States.
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?
Answer: Phase loss and unbalance relay
A phase loss/unbalance relay continuously monitors current in each phase and trips when a phase is lost or when current imbalance exceeds a set threshold, preventing single-phasing damage.
What is the primary function of a contactor in a motor control circuit?
Answer: To electromechanically make and break the high-current power circuit to the motor
A contactor is a heavy-duty electromagnetic switching device designed to repeatedly make and break the high currents required by industrial motors without damage to its contacts.
What is the standard control circuit voltage used in most US industrial motor control centers (MCCs)?
Answer: 120V AC
US industrial MCCs typically use 120V AC for control circuits, stepped down from 480V by a control transformer, as this voltage is standardized for most US control components and safer for technicians.
When a motor nameplate shows a service factor (SF) of 1.15, what does this indicate to a millwright?
Answer: The motor can operate continuously at 15% above its nameplate horsepower rating
A service factor of 1.15 means the motor can sustain a continuous load of 15% above its rated horsepower without exceeding its thermal limits under standard conditions.