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AC Drive
Guidelines
There are several choices
consumers have when starting their motors. Two of these choices are
variable frequency drives and soft start drives, which both have
similar characteristics. In the product literature written by
manufacturers, they seem to be the same. Even the list of
applications appears to be the same. Our Three Phase Electricity
article will give you the basic technical information to understand
the differences.
Variable Speed Drives
The variable speed drive works by taking an AC line voltage and
converting it to a DC voltage. The DC voltage is then inverted back
to a pulsed DC, whose RMS value simulates an AC voltage. Most
variable speed drives of today work by using pulse width modulation
to create the output sine wave. The conducting components used in
variable speed drives are diodes, SCR’s, transistors and IGBT’s. The
diagram below shows that the inverters have three distinct and
different sections to its power circuit.
The first section uses a diode to convert the AC line voltage to DC.
In the second section, a filtering of the DC is done with a
capacitor to supply the inverter bridge with a stable DC power
source, and with 10 horsepower and larger drives, a DC link choke is
usually used. In the final state, a transistor or IGBT bridge is
used to deliver a pulse width modulated DC voltage to the motor.
The control of the inverter and the user-programmed settings
determine the frequency output of the inverter. During acceleration,
the frequency will vary according to a pre-determined algorithm such
as linear ramp or
s-curve from minimum or 0 Hz up to commanded
speed. The drive can also be programmed to skip over certain
frequencies that may cause mechanical vibration.
Soft Start Drives
The Soft Start Drive works on a different premise. The soft
starter principle is when the starter adjusts voltage applied to the
motor during start-up. By adjusting voltage during start-up, the
current drawn by the motor and the torque produced by the motor can
be reduced and controlled.
By using six SCRs in a back-to-back configuration, the soft starter
is able to regulate the voltage applied to the motor during starting
from 0 volts up to line voltage. Unlike the VFD, line frequency is
always applied to the motor, only the voltage changes. Feedback from
the motor to the circuit controlling the SCR firing is required to
stabilize motor acceleration. Timing when to turn on the SCR’s is the key to controlling the voltage output. During
the starting sequence, the logic of the soft starter determines when
to turn on the SCRs. It does not turn them on at the point when the
voltage goes from negative to positive, but waits for some time
after. This is known as "phasing back" the SCRs. The SCRs are turned
on and are set by what is called either initial torque, initial
current or current limit setting.
Application Differences
Motor speed is what variable frequency drives have over
soft start drives. First, and most obvious, is when the speed of
the motor needs to be varied from 0 to line frequency and sometimes
higher. The soft start drives apply line voltage and frequency and,
therefore, the operating speed is fixed.
Variable frequency drives have advantages when constant speed is
required. If a fixed frequency is applied to a motor, the actual
speed of that motor is not regulated by the input frequency. The
output speed is actually regulated by the load applied to the motor.
If an application requires very tight speed regulation, the
frequency applied to the motor must be changed in relation to the
load that is applied. This can be accomplished with the use of
feedback to the variable frequency drive, Again, remember soft
starter only applies frequency, so any speed regulation is not
possible.
On applications where acceleration time needs to be consistent, a
variable frequency drive should be used. The reason is that soft
starter acceleration time is more dependent on the load than the
selected ramp time. If acceleration time is not an issue and
controlling the torque or current is what is needed, then a soft
start drive is a good candidate for the application.
In an application where stopping is required, a variable frequency
drive will bring the motor to a rest in a specified time. This may
be built into a VFD or may require a dynamic braking optional
function for high inertia and overhauling type loads. The soft start
drive can only extend the stopping time. Just like acceleration, the
stopping time is dependent on the load. If stopping time and
stopping characteristics are not critical, then a soft start drive
may fit your application. Some soft start drives are designed to
provide braking. These are designed to reduce stopping time where
coast to rest is very long. When limiting current is the prime
reason for not starting at full voltage, a soft start drive should
be used. This is due to the cost difference between a
Soft Start Drive
and a
Variable Frequency Drive. A VFD is more expensive than a soft start drive.
The Three Phase Electrical Specialists are here to give you only
guidelines, and we recommend always talking to an AC drive
specialist such as
TEMCo Tower Electric
Company
in deciding what is the
best drive for your application. For all your AC drive needs call TEMCo at 510-490-2187 or toll free at
1-800-613-2290
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Soft Start Drives start motors at lower
voltages, slowly ramping up to operation voltage, which reduces wear
on mechanical gears, chains, sprockets, belts and gearboxes.
Soft Start Drives eliminate motor over-fluxing and allow
under-loaded or oversized motors to operate more efficiently.
Baldor Electric Company
Baldor is a U.S. based company and is a
leading manufacturer of industrial electric motors, drives, power
transmission products and generators.
Weg Electric Motor Company
Weg has been in business since
1961 and is a reputable manufacturer of soft start drives and
electric motors. They distribute high quality products
worldwide.
The leading manufacturers will offer quick nationwide and
international shipping from regional
warehouses.
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