Advantages of Solid-State Reduced Voltage Starting Versus
Across-the-Line, Auto-transformer and Wye-Delta Starting
Copyright © 2003 Francis J. Martino
The smooth acceleration of the motor and the programming
provided by the solid-state reduced voltage starter will have the
Reduces pressure transients, water hammer, and mechanical stress
Reduces cavitation during start-up.
Reduction of mechanical stress on pump shaft.
Extends motor life.
Current limit will limit torque when mechanical restriction occurs in
pump or pump system.
Stall protection and overcurrent protection when mechanical jam occurs
thus protecting against cavitation and loss of bearing lubrication.
Undercurrent protection to protect against cavitation caused by loss
Torque and/or accel time may be increased on cold and damp days when
the air is heavy.
Reduces belt slip, torn splices and belt stretch.
With an auto-transformer starter, a conveyor that is normally started
with no load may have insufficient starting torque if required to start
when loaded, causing an extended inrush time and tripped circuit breakers.
A solid-state reduced voltage starter with dual starting ramp profiles
may be programmed for both loaded and unloaded conditions.
Reduction of mechanical stress on couplings, gear boxes and belts.
Eliminate contact replacement on high cycling applications.
Two-speed motor starting capabilities.
Two or more motors may be started with one starter if the motors have
similar types of load and acceleration times, etc.
A dual ramp profile will allow jog in addition to run.
May be used on NEMA Design C motors.
Motor-Generators and Distributed Generation
Limits current to allow starting on a motor-generator set in the event of
power outage. When using a heavily loaded generator that supplies
supplemental facility power for energy cost savings, a solid-state
reduced voltage starter may be modified for use as a transfer switch to
start the motor on utility power and then transfer the load to the motor-
generator set. Thus the generator will accept the motor load without the
necessity of having to supply motor inrush. Transfer time will be
controlled to ensure that the generator power is not applied into the back
EMF generated by the collapsing field of the motor.
Reduction of Voltage Flicker
Addition of new loads to an existing distribution system could overload
the system so that there will be an increased voltage drop due to inrush
currents upon starting motor loads. A reduced voltage starter will reduce
the voltage drop on start-ups.
A 3% voltage drop will cause a discernible flicker in lighting. A 6% drop
will cause an irritable flicker.  Computers, instrumentation and
communications equipment may be affected by voltage drop even though
the drop is not enough to cause a discernible flicker in lighting. 
Compressors - Air and Refrigeration
Centrifugal, Reciprocating and Rotary Screw
Varying loads from one start to another i.e., increase in load due to
temperature, bearing condition, or unloader operation, may cause a torque
requirement that is greater than the 33% starting torque of a wye-delta
starter. Inrush current will then be sustained until the timer switches to
the delta configuration. Overheating of the motor will take place during
the first step of starting, reducing motor lifetime. The solid-state
reduced voltage starter will always ramp to meet the required torque
levels, accelerate the increased load, and maintain a low inrush.
Starting with liquid in the compressor will give liquid surges as the
compressor motor is brought to full voltage. The complaint valve will be
strained and will ultimately need maintenance. A solid-state starter will
give a smoother and softer start, eliminating both the initial and
transitional surges of a wye-delta starter. There will be less mechanical
stress to the compressor, compressor pistons and complaint valve.
A brief decel ramp will eliminate negative torque, reducing system shock.
 John A. Kay, Richard H. Paes, J. George Seggewiss, Robert G. Ellis,
"Methods for the Control of Large Medium-Voltage Motors: Application
Considerations and Guidelines," IEEE Transactions on Industry
Applications, Vol. 36, No. 6 November/December 2000, pg 1689.
 For a discussion on flicker, see IEEE Std 519-1992 section 10.5.
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