14
INV 2500 SERIES MANUAL REV 1 1/13
SECTION 3
OPERATION
3.3
OPERATIONAL CONSIDERATIONS
POWERING REACTIVE LOADS
( continued )
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 in the form of
a fixed resistor or NTC (negative temperature coefficient) thermistor. Also, zero crossing switching can
be employed. Most commercial DC power supplies present a capacitive in-rush characteristic. Line
filters with large shunt capacitance may also cause considerable in rush current “spikes”. If not
accounted for these loads can trip protective circuits in the inverter.
POWERING LAMPS
Tungsten filaments lamps, when cold, present a very low resistance. Once they are energized, their
resistance quickly climbs to it’s steady state value. This characteristic must be accounted for when
driving tungsten filament lamps. Like incandescent lamps, certain Ni-Chrome based heaters will
present a 2:1 or 3:1 in rush when cold. The same methods for limiting in rush current for reactive loads
can be applied to tungsten or Ni-Chrome base elements.
POWERING MOTORS
Driving an AC motor presents a special problem. Most motors require a starting current that can be
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 model 2500'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 inverter. The fold back circuit
will reduce the output voltage in order to maintain the maximum output current. 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” to reduce the supply
current to the nominal “run” value for the motor.
POWERING NON-LINEAR LOADS
Loads utilizing rectifiers and SCRs interact with the AC power source and have a significant effect
on the distortion of the output waveform. Consider the use of a bridge rectifier followed by a capacitive
filter, the current waveform associated with this circuit is illustrated below in figure 3-3. 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. The capacitor charging current is limited only by the series
impedance present in the wiring and capacitor .Because the impedance of large electrolytic capacitors
is very small this action causes a current wave form with a peak value 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 inverter. Most “real world” electric distribution
systems exhibit this distortion for this reason.
