Starting time of a motor with VFD in ETAP 2

[tonyjony]

Hello everybody. I'm trying to run a motor starting analysis in ETAP. I think that with a VFD, power factor should be aprox. 0.95 and current needed shouldn't exceed 150% FLA, but I can't acomplish that with ETAP. I get "Failed to Start" if I limit the current beneath 350% using torque or current control. Anycase, PF is the motor model locked rotor PF.

Any idea?

Thanks in advance!

PS: I'm from Argentina, so english it isn't my native language!

 

[dpc]

I'm not sure ETAP is capable of really analyzing transient starting parameters of a motor fed from VFD.

Power factor at the motor and power factor at the drive input may be very different.

What are you trying to determine?

 

[tonyjony]

Our costumer is asking for a starting analysis in ETAP. They said they need the starting time, but I think we'll need to do it manually, or use another software. Do you have any suggestion?

 

[tonyjony]

A workmate just told me this: You must create new motor model curves with constant current & PF and the torque curve you want to start the motor.

 

[dpc]

I'm not aware of any program that will (easily) do this for a VFD. There are too many variables. Maybe someone else will have a candidate.

Your workaround is probably a good approximation.

With a VFD, the starting time can be nearly whatever they want.

 

[jraef]

I can't speak for ETAP, but I used to be able to do it with SKM Power Tools TMS module. It had a canned VFD starting analysis routine.

But basically, I have never seen a VFD that would not be capable of starting a load that it was capable of running. The starting time can essentially be infinite if necessary. It's not like a Soft Starter or Autotransformer where your current increase is significantly consuming the thermal model of the motor without providing much accelerating torque. Don't forget, 150% current on a VFD is not the same as 150% current on a current controlled start model (i.e. Reactor, Autoxfmr or RVSS), because you are creating a lot more torque at 150% current with the VFD by keeping the V/Hz ration constant, essentially Locked Rotor Torque.

So if ETAP doesn't have a VFD model, try modeling it as if it is a 550% Current Limit Start from a soft starter to get the starting time. But remember that will NOT then be an accurate model for Voltage Drop etc.


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[LionelHutz]

You are modeling a constant frequency start, not a V/Hz start.

Pick the current you do not want to exceed, somewhere between 100% and 150%. Then, pick the motor torque off the motor curves for that current. Use those 2 values to create new "flat line" motor curves with the same current and torque for every speed. Run the simulation using the current you picked. That should get you a reasonable simulation of the minimum starting time.

The maximum starting time can be whatever you want up to the limit of the VFD ramp time setting.

 

[Marke]

I have never seen a VFD that would not be capable of starting a load that it was capable of running

I have!! some loads have very high breakaway torque such as Mono pumps when dry etc. Some VFDs just can not provide the torque at zero shaft speed.

Best regards,

Mark Empson
L M Photonics Ltd

 

[jraef]

I have!! some loads have very high breakaway torque such as Mono pumps when dry etc. Some VFDs just can not provide the torque at zero shaft speed.

LOL Mark, you just weren't using a big enough VFD!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376