MNL940-677-000
REV - B
3/20/2017
12
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 a typical AC in-rush current that usually occurs over the course of one or less cycles of the
AC waveform. The latter is a consequence of where in the AC cycle that a load switch closes.
The model 1200'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 its 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 current crest factor cause distortion of the AC voltage waveform.
The amount of distortion incurred is dependent 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 will exhibit similar distortion for this reason.
Figure 3-3 below give an example of a non-linear load. In this case the load current is said to be
discontinuous as it does not flow over the full cycle of the output waveforms. This action can
cause perturbations on the output waveform. This may show up as ringing or clipping. Ringing
can be reduced by adding a small resistive “pre-load” of about 10 to 20 watts across the output
terminals.
VAC IN
NON - LINEAR CIRCUIT EXAMPLE
VAC IN
V OUT DC
VOUT UNFILTERD
INPUT CURRENT
FIGURE 3-3 NON-LINEAR LOAD EXAMPLES
