Motor Controls Flashcards

A star-delta (wye-delta) starter is used primarily to:

Answer: Reduce starting current by initially connecting windings in wye

In star-delta starting, windings are first connected in wye (reducing voltage by 1/√3), then switched to delta for full-speed operation.

What is 'plugging' in motor control?

Answer: Applying reverse phase voltage to quickly stop or reverse a running motor

Plugging applies reverse-phase power to a running motor to produce rapid braking, creating high braking torque.

In a motor control ladder diagram, what does an X inside a circle symbol typically represent?

Answer: A pilot light

A circle with an X (or letter) inside typically represents a pilot light or indicator lamp in ladder logic diagrams.

What is the primary purpose of a time-delay relay (TDR) in a reduced-voltage starter?

Answer: To allow the motor to accelerate before switching to full voltage

The TDR provides a timed delay so the motor can accelerate to near full speed before the circuit transitions from reduced to full voltage.

Which enclosure type is rated for outdoor use and protection against rain, sleet, and snow according to NEMA standards?

Answer: NEMA 4

NEMA 4 enclosures are weatherproof and protect against rain, sleet, snow, windblown dust, and hose-directed water.

In a two-wire control circuit, what is a key characteristic that differentiates it from a three-wire circuit?

Answer: It uses a maintained-contact device and will restart automatically after a power loss

Two-wire circuits use maintained-contact devices (like float switches) and restart automatically when power is restored, unlike three-wire circuits with low-voltage protection.

What is dynamic braking as applied to a DC motor?

Answer: Disconnecting the armature and connecting it to a resistor to dissipate kinetic energy

Dynamic braking disconnects the armature from supply and connects it across a resistor; the motor acts as a generator, converting kinetic energy to heat.