Technical Information
64
Precision Induction Heating
A
MBRELL
®
Doc#801-9526-00.doc
In practice, the recommended method of setting the workhead transformer tap is to begin
with the highest tap available and see if the needs of the application can be met. This
must be done with the actual load in the coil. If the desired setpoint current cannot be
reached, the “Decrease tap” message is displayed in the status page during the heat
interval, and remembered as the last message in the memory page. The tap must then be
incrementally decreased until regulation is achieved. Doing this yields the highest system
efficiency, requires the least from the water cooling system and mains current and
minimizes the peak currents and voltages required for regulation. A similar “Increase
tap” message is available should the need arise.
5.1.2
Power Measurement
Typically, the front panel power display is calculated using signals derived from the
mains input voltage and current and correction factors relating to efficiency of the RF
conversion process. The EASYHEAT series samples information from the mains input as
well, but also from the RF output. The value displayed represents an estimate of terminal
power. The displayed value cannot be directly related to mains input current (and the
circuit breaker limit) without taking into account power module efficiency and power
factor. Both efficiency and power factor change with operating conditions.
While power factor can be removed from measurement concern by using an external
measurement device that can distinguish between real and reactive power, efficiency is
more problematic as it requires knowledge of the RF output power.
RF output power is particularly difficult to measure in the EASYHEAT series as RF
currents can have a very high peak to average ratio, or crest factor. Conventional true
RMS meters will have difficulty with this waveform at EASYHEAT frequencies and the
difficulty will be compounded by the harmonics visible at the chassis terminals. Voltage
waveforms will present the same problem.
The EASYHEAT normally runs at a frequency slightly offset from resonance to improve
circuit efficiency, eliminating the ability to obtain accurate power measurements by
simply multiplying RMS voltage with RMS current. A user planning to measure output
power would require a high speed multiplying scope or similar product to give usable
information. The measurement instrument must be much faster that one would expect, to
accommodate harmonics. The delays on the voltage and current probes must be equalized
prior to multiplication, and have a dynamic range able to accurately measure in the face
of a high peak to average ratio.
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