PF1352 USER MANUAL REV A 12/21/15
Operation into these types of loads usually causes little interaction with the AC power
supply. The main concern with a linear load is the inrush current associated with it. Most
heating elements and resistors have little or no inrush concerns and usually do not
present any problem for the power source. Inductive and capacitive loads may present
a special problem based on their construction and the way in which they are energized.
Motors and tungsten filament lamps also present some special start-up concerns. The
following is intended to give the end user some insight into applying the AC source to
these types of loads.
3.9 DRIVING REACTIVE LOADS
Capacitors and inductors are reactive in nature. If the load is applied during the peak of
the AC cycle there may be a considerable inrush of current several magnitudes larger
than the steady state current. This current is only limited by any series resistance that
may be present in the load circuit. Under the right conditions, this could trip the overload
protection circuits in the power source. Certain transformers and solenoids (inductance)
present the same problem.
Several methods can be used to prevent tripping the protection circuits in the power
source. One common method is to insert a limiting impedance in series with the load.
This could be a fixed resistor or NTC (negative temperature coefficient) thermistor. Also,
zero crossing switching can be employed. The most obvious way to prevent a high in
rush current is to apply the load with the voltage set to zero (or some low value) and
energize the load slowly by turning up the voltage.
3.10 DRIVING
LAMPS
Tungsten filament lamps, when cold, present a very low resistance. Once they are
energized, their resistance quickly climbs to a steady state value. This characteristic
must be accounted for when driving tungsten filament lamps. The same methods for
driving reactive loads can be applied to tungsten.
3.11 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 inrush current that usually occurs over the course
of one or two cycles of the AC waveform. The model PF1352's 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 current for the range in use.
