February  1997                                                                                                                                   Volume 3   Issue 3

When Does Motor “Soft Starting” Make Sense?

    When a contactor is used to start a 3-phase motor, all three voltages are applied to the motor simultaneously, and in full.  And because the motor’s rotor is at rest, the initial current is 5 to 7 times the full load current.  This abrupt application of power is called a “hard” start, and results in the motor going from zero rpm to full operating speed in minimum time.  The duration of the current surge ranges from a few seconds to a few minutes, depending on how long the motor takes to come up to speed.  The lower the motor torque and the greater the load inertia, the longer the acceleration time will be. 


        Motor-Start Current Profiles

1. Typical Hard Start
2. Current-Limited Soft Start
3. Ramp-Type Soft Start


    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.


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 starting will not reduce overall motor heating, it does insure that the starting-induced rise in winding temperature will happen more gradually.


    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.

    Today, however, when people talk about soft start, they usually mean the kind of programmable soft start that solid-state soft starters make possible.  These devices utilize silicon controlled rectifiers (SCRs), thyristors, or other power semiconductors to control the voltage and current supplied to the motor during the starting process.  They are available in 3-phase and single-phase models, in voltages from 115 to 575 volts (a few even higher), and for motors up to 700 hp.  Different models exhibit different characteristics, but two primary modes of operation are the current limit and current ramp modes.  Referring to the graph once again, curve 2 illustrates the first mode and curve 3 the second.

Current Limit Mode

    The objective here is to limit the maximum starting current to a fixed preset level and maintain that level until the motor comes up to speed.  The current level is selected by the user to provide a suitable balance between load on the electrical system, starting torque, and acceleration time.  The value shown in the graph (300% of run current) is typical.  The start provided in this mode is obviously softer than a straight connection to the 3-phase line, but is still electrically and mechanically abrupt.

Current Ramp Mode

    The ramp mode of start control provides a much more gradual start than the current limit mode.  Here, the initial voltage and current is a user-selected low value that gradually rises to a user-selected maximum — which is then held until the motor reaches full speed.  This approach makes any drop in feeder voltage happen more gradually.  It also eliminates the mechanical shock to the drivetrain and load that occurs when a motor suddenly produces high torque. 

    Finally, while it is not possible to use a solid-state soft start unit and an Adjustable Speed Drive (ASD) on the same motor, be aware that most ASDs are, themselves, able to provide a relatively soft start.


    Contrary to persistent rumors, soft starting will not save you any money on your plant’s electric bill.  There are two reasons for this.  First, starting a motor slowly still takes about the same amount of energy as starting it quickly.  Second, even though the surge kVA during a hard start may be 5 to 7 times the running kVA, the period of high current is almost always much shorter than the 15-minute response time of the electric utility’s demand meter.


    While soft starting won’t reduce your electric bill, it still makes sense in many industrial situations.  If hard starts are causing mechanical problems with the driven equipment or drivetrain, or if hard starts are causing electrical problems, solid-state soft starting is worth considering.  For further information, or to discuss a specific motor-starting situation, call Ron Estabrooks or Mike Proud at 368-5010 (toll free).