BEHLMAN BL10000 Series, User'S Manual And Technical Reference Manual

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3.9 OPERATIONAL CONSIDERATIONS (continued)
DRIVING MOTORS
Driving an AC motor presents a special problem. Most motors require a starting current that is several times
higher than the running current. This current may last for a few cycles to several seconds depending on the
construction and mechanical load on the motor. This current is sometimes referred to as the motor’s “locked
rotor” current. This current is not to be confused with the in rush current that usually occurs over the course of
one or two cycles of the AC waveform.
The BL series fold-back current limiting can be an advantage when starting motors. During the starting period,
the motor will attempt to draw excessive power from the power source. The fold-back circuit will reduce the
output voltage in order to maintain the maximum output current for the range in use. During this time the current
supplied to the motor will remain sinusoidal, this allows the motor to start rotating. Once the motor reaches it’s
normal operating speed, it generates the required “back EMF” and the supply current drops off to the nominal
“run” current for the motor.
DRIVING NON-LINEAR LOADS
Loads utilizing rectifiers and SCRs interact with the AC power source and have a profound effect on the
distortion of the output waveform. Consider the use of a bridge rectifier followed by a capacitive filter, the input
current to this type of circuit is drawn in large “gulps” whenever the voltage across the capacitor falls below the
peak of the input waveform. This current is limited only by the series impedance present in the wiring and
capacitor . The impedance of large electrolytic capacitors is very small. This action causes a current waveform
with a peak value that may be several times the RMS value. This ratio of peak current to RMS current is known
as “ Crest Factor”. High values of crest factor cause distortion of the AC voltage waveform.
The amount of distortion incurred is dependant on many factors and is beyond the scope of this manual. It
should be noted that this type of load may cause the output waveform to exhibit “flat topping” . This should not
be associated with a defect of the power source. Most “real world” electric distribution systems exhibit this
distortion for this reason
.
OUTPUT NOISE
Because the BL series uses a high frequency PWM conversion technique, a certain amount of output noise or
ripple is to be expected. The amount of noise present on the output voltage waveform from this unit varies
somewhat with the load. Maximum noise levels are present when there is no load applied. In any event, the
amount of noise present should not constitute a problem for properly designed equipment. If the devices being
tested are disabled by the noise present on the output waveform, then serious consideration should be given
to the design of the unit being tested as it may not pass the European EMI tests.
In special cases where the output noise is objectionable, an external line filter can be added to the output of
the unit. Please note that most line filters are not intended to be used at 400Hz. If the noise level is interfering
with low level measurements a linear type AC source should be considered. For more information on linear
sources, contact Behlman Sales.
T10 & T10D UNITS
Adding the external transformer chassis will affect the regulation specifications of the power supply when
measured at the transformer secondary. This is due to the winding resistance of the transformer and will vary
depending on which transformer is used. Higher voltage transformers tend to exaggerate this effect. An internal
“boost” circuit as been added to the control chassis to minimize this drop in voltage versus load current. The
boost circuit works very similar to the “Line Drop Compensation” provided on the control chassis ( see operating
instructions).