First, what size of variable frequency drive (VFD) is being approached? When we speak about costs, remember that cost is quite different between a 10 MW VFD and a 7.5 kW VFD. Performances vary as well, in per unit basis and absolute value (kW).
There are also different types of VFDs for different types of motors, that is synchronous or induction for example. Therefore, evaluating case by case makes sense and other considerations along with the above mentioned also needs to be carried out. Here are some examples.
- There are multiple motors that must start with the VFD to reduce the starting current. In this case, one should consider the number of motors that can start in succession, using the same VFD.
- Check with the VFD manufacturer whether they offer a simplified VFD for starting only. For starting only, the losses aren’t of greater importance. Preferably you may want the VFD to come with bypass switch already incorporated to it.
- As it comes to soft starters, those vary only voltage, never forget that these soft starters reduce the torque of the motor to the ratio of voltage to the power of 2.2 (yes 2.2, not 2), so you will want to evaluate if the motor will accelerate the load with the desired voltage, and if so, without overheating. Also remember that some loads are more demanding than others. Check for torque requirements and load inertia. If the motor can accelerate, then a soft start can be used. If not, a VFD can be one solution.
- When the process requires, or the output of production varies, the application of a VFD makes sense.