Both the VFD and the soft starter can be used up to 11KV.
If you are using a soft starter, it pays to look at the high slip performance of the motor, especially the Locked Rotor Current and the Locked Rotor Torque.
As you reduce the start current, you reduce the start torque by the current reduction squared, so a modern high efficiency induction motor with a very high locked Rotor Current, and a low Locked Rotor Torque will require a higher start current than an older motor with a higher Locked Rotor Torque and a lower Locked Rotor Current. - The motor selection is very important for good results.
With a VFD, the motor always operates under low slip conditions, so the high slip characteristics are not relevent.
The advantage of a VFD are :
[ul]
[li]always runs under low slip conditions so the start current is low.[/li]
[li]can run at speeds other than line speed. - slow down when full speed is not needed. [/li]
[/ul]
The disadvantages of a VFD are :
[ul]
[li]Higher power loss - typically 2.5 - 3% losses in the VFD always while running[/li]
[li]High input harmonics, or higher losses if harmonic mitigation is fitted. - perhaps as high as 6% if harmonically mitigated. [/li]
[li]EMC and EDM problems due to output waveform. [/li]
[li]Higher capital costs and shorter life expectancy compared with Soft Starter. [/li]
[li]Potential insulation damage over time[/li]
[/ul]
The advantages of Soft starter are :
[ul]
[li]Much higher running efficiency and can be bypassed to virtually eliminate all losses. - lower running cost at full speed. [/li]
[li]Lower Capital Costs [/li]
[li]Typically, longer life expectancy [/li]
[/ul]
The disadvantages of Soft starter are :
[ul]
[li]Higher starting current[/li]
[li]Dependent on high slip motor characteristics.[/li]
[/ul]
There are many tales about the energy that is saved on all motors with a VFD. The motor efficiency does not improve with a VFD compared to a soft starter, but the energy wasted in the driven load can sometimes be reduced by slowing the load down. In many cases, because the work done is reduced, the motor runs for longer.
There is a genuine case for each technology and it depends what the application problems are as to what is the best solution.
For steep curve pumps with variable flows, there is energy to be saved by slowing the pump for low demand.
For flat curve pumps. slowing the pump reduces the discharge pressure, so it will always run at a fixed speed with varying flow. No energy to be saved, just 6% additional energy to be wasted.
Mark Empson
Advanced Motor Control Ltd