found this online:
Cut speed and nominal voltage have an important and sometimes counterintuitive relationship in plasma cutting. Here’s how cut speed affects the required nominal voltage:
The Core Relationship: Speed Affects Voltage
There is a direct relationship between cut speed and arc voltage: as speed increases, voltage drops, and as speed decreases, voltage rises 5.
Why This Happens
The relationship works through the interaction of two effects:
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Arc lag / deflection: At higher cut speeds, the plasma arc trails behind the torch (arc lag). This effectively shortens the electrical path, which lowers the arc voltage. At slower speeds, the arc runs more vertically through the material, producing a higher voltage for the same physical height 1.
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THC response: Because the THC maintains a target voltage, changes in speed cause the THC to adjust torch height to compensate — which can create problems (see below).
Practical Consequences
When you slow down:
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The voltage-to-height ratio changes — the same physical height produces a higher voltage reading 1.
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If your nominal voltage was set for a faster speed, the THC will drop the torch closer to the plate to bring the voltage back down to target. This can cause the torch to get dangerously close or even collide with the material 2.
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This is why THC voltage control is often locked out (frozen) when cutting small holes at reduced speeds (typically ~60% of contour speed) — otherwise the THC would drive the torch too low 3.
When you speed up:
- The arc voltage drops due to arc lag, so the THC will raise the torch to bring voltage back up to the nominal target, potentially causing too high a cut height and poor cut quality.
How to Set Nominal Voltage for a Given Speed
Because the voltage-to-height relationship shifts with speed, nominal voltage should be determined at the actual cut speed you plan to use. The recommended method from Langmuir Systems is:
> Cut an 8–10" line at the desired cut speed and amperage for a given material, monitor the live cutting voltage, and enter that voltage as your Nominal Voltage 4.
Summary
| Speed Change | Effect on Arc Voltage (at same height) | THC Response | Resulting Cut Height |
|---|---|---|---|
| Faster | Voltage drops (arc lag) | THC raises torch | Higher cut height |
| Slower | Voltage rises (vertical arc) | THC lowers torch | Lower cut height |
Key takeaway: If you change your cut speed significantly, you should re-determine your nominal voltage with a line test at that speed. Using a nominal voltage set for one speed while cutting at a very different speed will cause the THC to maintain the wrong torch height, degrading cut quality or risking collisions.