Soft starts usually vary voltage with SCR’s
A key difference between drives and soft starts:
Drives can continuously vary the frequency and voltage supplied to the motor. Soft starts vary only the voltage supplied to the motor, and usually only when ramping a motor up and down.
What does this mean?
Soft starts may vary the speed of the motor during startup and ramp down but this is done by reducing the motor voltage. On this note, the soft start is also called a reduced voltage starter. Drives do the same but also have the option to control motor speed by varying the voltage frequency instead of the voltage. Motor speed is directly related to its supply voltage frequency.
Inherently, a drive or a soft start reduces the inrush current that every motor is subject to when starting across the line. From this perspective, either a drive or a soft start will probably prolong the life of a motor -specifically if compared to an across the line starter. This is a general statement and like most general statements, there are some conditions. Specifically for drives, this general statement usually applies to inverter duty rated motors which can withstand the continuous high frequency switching (PWM) of the drive. Otherwise, there is a risk in applying a drive to a motor. It might heat up the winding insulation and ultimately break it down.
The soft start topology is usually a single stage SCR based switching scheme with a bypass. The bypass takes over for operation at full speed (diagram above). The SCR’s fire for gradually longer parts of the AC voltage cycle until the entire AC wave is passed through to the motor. At this point, operation is handed over to a bypass.
VFD’s vary voltage and frequency with the 3 stage design.
The drive topology has 3 stages (diagram above), with a rectifier taking in line supply and then a DC bus capacitor that stores and buffers the DC energy within the . The final stage is the inverter which is usually made up of IGBT’s at the motor supply side of the drive. The IGBT’s can continuously operate at varied gating frequencies to produce a variable frequency supply to the motor.
When choosing between drives and soft starts, some key differences to consider are:
Key differentiators
| VFD | Soft Start | Meaning | |
| Speed Variability | Continuously variable speed throughout operation | Initial ramp up of voltage/speed. Subsequently pegged to line frequency. | Drives can save energy if the load does not need to run at full speed. Soft starts do not save energy in full speed operation. Then again, some applications are designed to operate in full speed operation. These loads will not benefit from a drive from this perspective. |
| Control features | More control features: Features that take effect during operation at regulated speeds. | Less control features as speed is not regulated besides during startup and ramp down. | |
| Application Torque | Constant torque supported- i.e. high torque at low speed.
Examples: Screw compressors, conveyors. |
Variable torque applications- lower torque at low speed.
Examples: Centrifugal pumps, fans. |
Some loads require a high amount of torque when starting. A VFD is applied on these applications |
| Main features | Reduce inrush and continuously vary frequency and motor speed. Energy savings during operation. | Reduce initial inrush. Energy savings during ramps, no energy savings after ramp up and ramp down. |
