An induction furnace melts metal by inducing eddy currents in the charge material. The power supply delivers kilowatts; the melt absorbs energy. But how do you decide when the melt is done? A time-based controller runs the power supply for a preset duration — 45 minutes, 60 minutes, whatever the recipe calls for — and assumes the energy input was sufficient. A kWh-based controller integrates instantaneous power over time and terminates the melt when a preset energy total has been delivered, regardless of how long it took. In a foundry where utility voltage can sag 5% during the afternoon shift when the whole industrial park is running at peak demand, these two strategies produce different metal.
The Case Against Time-Based Melting
A time-based melting recipe assumes three things hold constant: input power, charge material characteristics, and furnace thermal losses. In practice, none of them do. Line voltage at the furnace busbar can vary ±5–10% over the course of a day depending on grid loading and on-site transformer tap positions. Because induction heating power scales roughly with the square of the applied voltage, a 5% voltage sag translates to approximately a 10% reduction in delivered power. A 60-minute time-based cycle that delivers the required energy at nominal line voltage will under-deliver by roughly 10% during a voltage sag — the melt reaches the target temperature but may not have received the full homogenization energy needed for alloy dissolution and slag separation.
The second failure mode is charge material variability. If the scrap mix contains slightly more rust, oil, or moisture than expected, part of the input energy goes into vaporizing water and burning off contaminants rather than heating and melting metal. A time-based controller cannot detect this — it simply runs the timer to zero and signals cycle complete, regardless of whether the actual energy delivered to the metal reached the target.
How kWh-Based Control Closes the Loop
A kWh-based controller continuously measures instantaneous power — derived from voltage and current sensors on the furnace bus — and integrates it into cumulative energy delivered. The recipe specifies a target energy in kWh per batch. When the accumulated energy reaches the setpoint, the controller terminates the power delivery, regardless of elapsed time. Controllers like the LMK Thermosafe BF400 power controller use burst-firing modulation at up to 25 A to deliver precise energy packets into the induction coil, enabling per-cycle energy resolution fine enough to detect a 0.1 kWh deviation in a 500 kWh melt.
The operational benefit is batch-to-batch consistency independent of utility voltage. On a day with nominal line voltage, the melt might complete in 52 minutes. On a day with a 7% voltage sag, the same melt takes 58 minutes — but the total energy delivered to the charge is identical. The metallurgical result — tap temperature, alloy homogeneity, slag condition — is the same whether the batch took 52 or 58 minutes. For foundries producing certified alloys where chemistry and mechanical properties are tied to melt energy history, this consistency directly reduces heats rejected at the spectrometer.
Energy-per-Kilogram: The Metric That Matters
Neither time nor kWh alone captures melt efficiency. The metric that foundry operators track is specific energy consumption — kWh per kilogram of metal tapped. A well-tuned induction melting system running at nominal conditions consumes roughly 550–650 kWh per metric ton for ferrous alloys and 400–500 kWh per metric ton for aluminum, with the variation coming from furnace size, lining condition, and charge preheat. A kWh-based controller makes this metric directly measurable and controllable: by logging cumulative energy per heat against charge weight from the crane scale, the foundry builds a running database of actual energy efficiency, enabling operators to detect deteriorating furnace lining (rising kWh/ton) or improving scrap quality (falling kWh/ton) weeks before those trends would be visible in melt temperature or cycle time alone.
For foundries operating multiple furnaces on a common bus, also consider electrical power quality monitoring and power meters to track total demand and avoid peak-demand charges from simultaneous high-power melt cycles.
When Time-Based Control Is Still Acceptable
Time-based control remains common — and perfectly adequate — in small foundries melting a consistent scrap mix from a single source, with a stable grid connection and a furnace maintained on a strict relining schedule. In these conditions, the variables that kWh-based control corrects for are tightly bounded, and the additional cost of energy metering hardware and integration may not pay back. But if any two of the following three are true — variable scrap quality, fluctuating line voltage, or customer specification requiring melt energy traceability — kWh-based control transitions from an upgrade to a necessity. Temperature control integration is the logical next step; see our temperature controller selection for closed-loop melt temperature management options.
