Module 1 Assessment: Plasma Arc Technology

Assessment ID

U5M1-Assessment

Assessment Type

Formative Quiz (Knowledge Check)

Total Questions

11

Passing Score

8/11 (73%)

Duration

20 minutes

Instructions

Answer each question based on the content covered in Module 1 slides and activities. Select the best answer for each multiple-choice question, or answer short-response questions in 2–3 sentences.

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Question 1: Plasma State & Ionization

What is plasma, and what makes it different from a regular gas?

Plasma is a gas that has been heated to very high temperature

Plasma is an ionized gas containing free electrons and positive ions, making it electrically conductive

Plasma is a liquid that conducts electricity

Plasma is a solid with metallic properties

Submit

Explanation: Plasma is defined by the presence of free charge carriers (electrons and ions), not just high temperature. A gas can be hot without being ionized; a plasma must be ionized.

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Question 2: Arc Column Temperature

The temperature in the arc column of a plasma cutting system is approximately:

3,000 K (similar to the melting point of steel)

8,000 K (similar to the surface of the sun)

15,000–25,000 K (much hotter than the surface of the sun)

100,000 K (not achievable with current technology)

Submit

Explanation: Plasma cutting arcs operate at 15,000–25,000 K. This extreme temperature is necessary to ionize the gas and melt the workpiece rapidly.

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Question 3: Nozzle Function

Why is the nozzle orifice so small (typically 2–3 mm diameter)?

A small orifice reduces the amount of gas consumed, saving money

A small orifice constricts the arc, increasing current density and temperature, and accelerates the gas to high velocity

A small orifice is easier to manufacture than a large one

A small orifice prevents the arc from spreading to the sides

Submit

Explanation: The small orifice is the critical design feature that constricts and focuses the arc. Without constriction, the arc would be unstable and would expand uncontrollably.

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Question 4: Electrode Material

Tungsten is the preferred electrode material in plasma cutting systems because:

Tungsten is the cheapest metal

Tungsten is the most abundant metal

Tungsten has the highest melting point of any metal and excellent thermionic emission properties

Tungsten is easier to machine than other metals

Submit

Explanation: Tungsten's high melting point (3,422°C) and ability to emit electrons readily (thermionic emission) make it the only practical choice for the extreme arc environment.

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Question 5: Arc Transfer Modes

In a transferred arc plasma cutting system, the current path is:

Electrode → Gas → Nozzle → Workpiece

Electrode → Ionized gas → Workpiece → Ground clamp → Power supply

Workpiece → Gas → Electrode → Power supply

Ground clamp → Electrode → Gas → Nozzle → Workpiece

Submit

Explanation: In transferred arc cutting, the workpiece is directly in the circuit (the anode), and current flows from the electrode (cathode) through the plasma to the workpiece and back through the ground clamp.

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Question 6: Arc Stability & Lorentz Force

An arc that wanders or drifts from the center of the nozzle is likely caused by:

Excessive gas pressure pushing the arc sideways

The Lorentz force on the current-carrying arc column, pushing it away from the axis; the nozzle constriction should overcome this, but if consumables are worn, stability is lost

The electrode being off-center in the electrode holder

The workpiece surface being uneven

Submit

Explanation: The self-induced magnetic field around a current-carrying conductor (the Lorentz force) naturally destabilizes the arc. The nozzle walls physically contain the arc, but worn or damaged nozzles lose this effect.

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Question 7: Swirl Ring Function

The swirl ring is designed with helical grooves. What is the primary purpose of these grooves?

To reduce the weight of the component

To direct the incoming gas in a spiral (swirl) pattern, which stabilizes the arc and cools the electrode

To increase the surface area for heat dissipation

To create turbulence that cools the plasma jet

Submit

Explanation: The helical grooves create controlled swirl motion in the gas flow. This swirl resists the destabilizing Lorentz force and helps keep the arc centered on the electrode.

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Question 8: Consumable Lifespan

Which consumable component typically has the shortest lifespan in a plasma cutting system?

The nozzle (typically 50–150 hours)

The electrode (typically 200–500 hours)

The swirl ring (typically 200–500 hours)

The shield cup (typically 500–1000 hours)

Submit

Explanation: The nozzle experiences the most extreme thermal and mechanical stress. Its orifice erodes and enlarges with use, requiring replacement before other components.

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Question 9: High-Frequency Arc Initiation

Why do modern plasma systems use high-frequency (HF) voltage for arc initiation instead of contact start (electrode touching the workpiece)?

HF is cheaper than contact start circuitry

HF produces a cooler arc than contact start

HF reliably ionizes the air gap without electrode contact, protecting the electrode from erosion and enabling fast, repeatable initiation

HF produces a more stable arc during cutting

Submit

Explanation: HF voltage ionizes the gas between the electrode and workpiece, creating a pilot arc without mechanical contact. Contact start erodes the electrode rapidly and is unreliable in production.

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Question 10: High-Frequency Safety

What is the primary safety concern with high-frequency plasma cutting systems?

HF voltage increases the risk of thermal burns

HF voltage is a shock hazard, and HF circuits generate electromagnetic interference (EMI) that can affect nearby equipment if not properly shielded and grounded

HF causes the nozzle to overheat

HF reduces the thermal conductivity of the consumables

Submit

Explanation: The 4–6 kV HF voltage and rapid switching at 4–6 kHz create both electrical hazard (operator shock) and EMI hazard (interference with computers, sensors, communications).

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Question 11: Power Supply Constant Current Regulation

A constant current power supply maintains a fixed output current regardless of load changes. What is the advantage of this for plasma cutting?

It reduces the operating cost

As the electrode moves closer to or farther from the workpiece, the voltage adjusts automatically to maintain a stable, predictable arc without operator intervention

It makes the electrode last longer

It eliminates the need for a ground clamp

Submit

Explanation: Constant current regulation is self-regulating. If the arc length changes, the voltage adjusts to keep current constant, maintaining arc stability. This is essential for mechanized and CNC cutting.