AC Drives and Soft Starter (7)

Application Differences

With the knowledge of VFD and soft starter principles of operation and motor performance with each, application differences can be reviewed. With the list of applications being very similar, the general application parameters will be covered along with several application examples.
Motor speed is a parameter where a VFD has an advantage over soft starters. First, and most obvious, is where the speed of the motor needs to be varied from 0 to line frequency and sometimes higher than line frequency. The soft starter applies line voltage and frequency; therefore, the operating speed is fixed.

The second speed-related advantage to which an inverter relates is processes that require a constant speed. If a fixed frequency is applied to a motor, the actual speed of that motor is not precisely regulated by the input frequency. The output speed is actually regulated by the load applied to the motor. So if a process requires very tight speed regulation, the frequency applied to the motor must be changed in relation to the load that is applied. With the use of feedback to the VFD this can be accomplished. Again the soft starter only applies line frequency so any speed regulation is not possible.
On applications where acceleration time needs to be consistent, an inverter should be used. This is due to the fact that acceleration time for a soft starter 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 that is needed, then a soft starter is a good candidate for the application. (Note: some soft starters use feedback, such as tachometers. These units can provide timed acceleration with varying loads. It should be noted that current during feedback acceleration could reach the same level as starting at full voltage – 600 – 800 percent of full load).
With regard to stopping, a VFD will bring the motor to a rest in a specified time. This may be built into an inverter or may require a dynamic braking optional function for high inertia and overhauling type loads. The soft starter with a soft stop feature can only extend the stopping time, and just like acceleration, the stopping time is dependent on the load. If stopping time and stopping characteristics are not critical then a soft stop may fit the application.
Some specially designed soft starters can also provide braking. These are designed to reduce stopping time where coast to rest is very long. If the load is not a pure inertia and can vary, the stopping time will also vary.
Where limiting current is the prime reason for not starting at full voltage, the first method to be considered today is usually soft starters. This is due to the cost differential between a soft starter and a VFD at the ampere ratings that current limiting becomes a factor. In most instances the soft starter is an appropriate choice.
There are applications where the additional cost of an inverter is appropriate. These cases are where the motor cannot provide sufficient torque to start the load with the ampere limitations imposed by the distribution system.
Table 1 shows the motor torque provided at various levels of soft starter current limit. Unlike soft starters, drives can accelerate a motor to full speed at full load torque with line current that does not exceed the full load amperes of the motor. Keep in mind that the power into the VFD is equal to the power out plus the losses. Therefore, for those loads that require higher torque than the soft starter can provide with the limits imposed by the distribution system, an inverter may be the required solution.
If starting torque is a concern when selecting a drive or starter, keep in mind the drastic difference in the amount of torque that can be developed for a given amount of line current. The drive has a much higher torque per ampere ratio.