Modeling A VFD Soft Start in ETAP

[ThePunisher]

I would like to request for some advice and experienc from power system and VFD folks. I am modeling a 4kV motor connected to a VFD in ETAP. The VFD is having a soft start feature. The program has options to either model the soft start as "Current Control" or "Voltage Control" or "Current Limit". I disregarded "current limit" since this is more of a soft starter than a VFD.

I don't have any commissioning data for the VFD at the moment and I may assume the VFD soft start feature. I am assuming that the VFD can start the motor in such a way the it will start on a 0~60Hz steps assuming V/Hz is maintained along the starting path. What I am thinking is to consider 0Hz~60Hz in 5Hz steps and calculate the equivalent speed by Nr=120*F/(no. of poles)and convert the calculated Nr in terms of synchronous speed (%Ns) and create a curve that is %Ns vs. %FLA. I will assume %FLA in proportion of %Ns up to 100%FLA at 100% Torque (at rated slip).

I am thinking that provided V/Hz is maintained at different frequency steps, the breakdown torque is being shifted and hence there is enough torque to turn the load from rest. Is this a close assumption to a VFD soft start?

I appreciate any suggestions.

 

[LionelHutz]

You idea is somewhat correct but you'd have to do the modeling on much smaller steps, maybe 1Hz but more likely you will need to use something like 0.1Hz steps.

I will assume %FLA in proportion of %Ns up to 100%FLA at 100% Torque (at rated slip).

You have to calculate the required torque to accelerate through each speed step and then you can will find the current required to produce that torque.

The VFD will likely allow the frequency ramp to be over ridden with the current limit. In other words, the motor acceleration will be slown down if the current limit is reached. So, you could model for worst case and use something like 100% or 150% current limit, or another value you chose.

 

[ters]

I’m not trying to hijack this thread, but I’m wondering if someone could explain the purpose and need for such detailed modeling. Why that is needed given that VFDs can be set to provide a very soft start which causes no voltage drop and similar disturbance to the system typical for DOL and RVSS.

On the same project where I had a long lasting saga with medium voltage RV soft starters (discussed here for months, thank you again LioneHuthz), we also had three 1 MW medium voltage VFDs, driving the same sort of load as the motors with soft starters did - large axial fans. And while the best we could achieve with the soft starter was to start turning the motor + the load at 300% FLA (turning, but would never reach the full speed at only 300% FLA), VFDs start the same type of load under the same conditions (inlet damper closed) with only ~ 30% of FLA and at initial 20% of V and Hz. Then the 30% FLA current remains more or less constant throughout acceleration until V and Hz at constant ratio increase from 20 to 100%, which takes about 3 min.

 

[jraef]

ters,
While your experience is that of an "ideal" scenario for starting with a VFD and is very common, it by no means is a guaranteed result. Everything is load dependent, so unless you already KNOW that the VFD will start it without going over FLC, then the only way to be sure it's possible is to model it. I have seen a machine that, even with a VFD, still had to have 200% current at some point in the start sequence to get the load accelerated (vertical SAG mill). It's by no means common, but it happens.


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