Spec Sheet

Single Phase Soft Starters
Page 5
HVAC
Tons
Compressor
RLA
Before
In-Rush
After
SureStart
%
Reduction
1.59 481569
2.0 14 73 22 70
2.5 17 79 24 69
3.0 20 109 33 70
4.0 26 134 40 71
5.0 30 158 47 71
7.0 32 185 56 54
SureStart In-Rush Reduction
In-Rush Current
Motor in-rush occurs due to low resistance in motor
windings essentially acting like a short circuit. This
temporary short circuit causes an immediate spike in
current and simultaneous drop in supply Voltage. Voltage
drops for air conditioning compressors are often 15%
or more which is 3-4 times greater than what most
electrical power distributors prefer. The more frequently
the compressor starts, the more noticeable the problem
becomes. For most homes in the US, air conditioners
usually start at a rate of 6-10 starts per hour.
To add to the problem, the standard of living in the
US has increased over time where air conditioning
has become a basic want for most homeowners. As
government regulations tighten, calling for higher
effi ciency air conditioning, compressor manufacturers
have responded with higher effi ciency motors in the
compressors. The downside to improved effi ciency
is that in-rush current generally goes up as motor
effi ciency increases. Also, the larger home size results
in larger air conditioning units, and ultimately result in
higher in-rush currents. Figure 1 is a graph of typical
in-rush current for air conditioners in the US along with
the SureStart reduced in-rush current.
Measuring In-Rush
The effect of these electrical spikes can be observed
by watching the lights fl icker; however, this does not
help quantify the problem. In order to properly measure
in-rush current, the right tool is needed for the job.
In this case, that tool is an oscilloscope which has the
ability to observe electrical signals with great detail.
Oscilloscopes are used by engineers in a laboratory
environment to study the magnitude and shape of
signals at specifi c points in time. An oscilloscope is an
expensive device that is more powerful than what is
needed by most service technicians for diagnostic use
in the fi eld. Instead technicians carry a handheld device
called a digital multi-meter (DMM) that is capable of
basic current, voltage, and resistance measurements.
Some DMMs are also able to capture maximum
amperage or current value as well. It is likely that a
technician will use a DMM when trying to capture the
in-rush current of compressor at start-up. This can be
accomplished before and after adding hard start kit
to observe the reduced in-rush value. Unfortunately,
it is likely this maximum value does not refl ect the
true instantaneous spike in current. The DMM is only
capable of recording current at defi ned time intervals.
If the time interval of the event is too fast the DMM
will have a portion of the interval where it recorded
very low values. Low values will shift the average
down making it appear less signifi cant. The faster the
event the less likely the DMM is to display accurate
results. Furthermore, the DMM is limited on the type
of signal that it can detect. If the signal is smooth and
characteristic of an ideal sine waveform; then it is
likely to be okay. However, if the signal is a complicated
waveform like measuring with a SureStart in the circuit;
then it won’t be accurate. This is an important concept
since the wrong tool used for measurement may lead to
false interpretation therefore not actually solving the
problem. For example, hard start kits provide a start
boost to the compressor which reduces the start time.
It doesn’t lower the actual spike in current instead just
makes it less noticeable by speeding up the time. A
DMM might show that the spike is reduced; however, it
is due to a longer period of lower running current value
offsetting the average.
Figure 1: Normal 5.0 Ton LRA vs SureStart
Time (ms)
200
60
40
80
100
120
140
160
180
0
Current (A)
0 150
300 450
Normal LRA
SureStart
60 - 70%
Reduction