Power can also be applied to a motor more gradually, resulting in what is
called a soft start. With soft start techniques the initial surge current is reduced, but the motor and load take longer to reach full speed.
What are the advantages of a soft start? In what situations should it be used? And does soft starting reduce the electric bill? This article attempts to answer these questions.
PROBLEMS CAUSED BY HARD STARTS
Electrical Problems
One might expect that a surge current 5 to 7 times normal could cause problems, and it often does. Such a surge may cause fuses to blow and breakers to trip, and can produce an excessive voltage drop on power feeders. This voltage drop can cause
• difficulty in starting the motor,
• lights to dim,
• other motors to stall, and
• sensitive equipment, such as computers, to malfunction.
Mechanical and Thermal Problems
A hard start also results in mechanical and thermal stresses within the motor, as well as mechanical stresses to the drivetrain and load. Particularly if the drivetrain has any looseness or play, repeated hard starts can result in excessive stress and wear. And although
soft starter will not reduce overall motor heating, it does insure that the starting-induced rise in winding temperature will happen more gradually.
SOFT START APPROACHES
Some techniques for soft-starting a motor have been around for many years. These include:
Autotransformer - Winding taps provide
lower voltage (and thus lower current) to the motor during starting.
• Part winding - Some motors have multiple windings that allow an incremented start.
• Series resistor or reactor - These devices limit the initial current, and are switched out at some point in the starting process.
• Wye-Start, Delta-Run - Motors capable of both Wye and Delta operation can be configured as Wye to start and Delta to run. Follow us on
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