Glossary Of Technical Terms; Rms Servo; Servo Boost - Elgar SmartWave SW5250A Operation Manual

SW 5250A•SW 3500A•SW 1750A
1.12

Glossary of Technical Terms

1.12.1

RMS Servo

An RMS Servo uses a feedback scheme to regulate the RMS value of the output
voltage or current by adjusting the RMS value of the reference input signal. For
example, if the output sags under load by 1%, the reference input is increased by 1%.
Thus, the net sag is zero and steady-state load regulation is greatly improved ("DC"
output impedance is very low). Typically, it takes several cycles to respond to new load
conditions.
In some cases, such as IEC-1000-3-3 flicker testing, it is undesirable to have such a low
"DC" output impedance. The servo should be bypassed. The SmartWave allows you to
turn the servo off by selecting "Low Frequency ON" in the System/External menu.
The programmed voltage accuracy of the SmartWave is approximately ±1% when the
servo is turned off, and the 312V range load regulation at 50 Hz is approximately -0.33
V/Amp (-0.11 V/Amp in the parallel mode). If tighter voltage accuracy is required, use
the Measurement menu to read the actual output voltage and adjust the program value
as necessary.
1.12.2

Servo Boost

Servo Boost is an enhancement to an RMS Servo, used to improve its low voltage
performance.
An RMS Servo uses an analog multiplier to adjust the reference input. Under certain
conditions, the multiplier range is insufficient to correct for the output sag. For example:
An amplifier has an output impedance of 1Ω at 2000 Hz. The load current is 10
Amps. This causes the output to sag by 10 volts. If the programmed output was
230 VRMS, the reference multiplier would have to multiply the reference by 1.045
to bring the output back to 230V.
However, if the output was programmed to be 10.02 VRMS, the multiplier value
would have to be 500 in order to bring the output back to 10.02V. This large
dynamic range is not practical in the analog domain.
To overcome this limitation at low programmed voltages, the reference signal going into
the multiplier is maintained at a minimum of 10% of its full scale value, and the multiplier
value is allowed to drop to a fraction of its "normal" value. This function is called Servo
Boost.
In the example above, to maintain 10.02 VRMS in a 300 VRMS full scale system,
the multiplier values are 0.25 at no load and 0.33 at 10A, a much smaller
dynamic range (vs. 500:1 in the example).
Operation Manual
General Description
1-13