Starting time of a motor with VFD in ETAP
Starting time of a motor with VFD in ETAP
(OP)
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!
Any idea?
Thanks in advance!
PS: I'm from Argentina, so english it isn't my native language!





RE: Starting time of a motor with VFD in ETAP
Power factor at the motor and power factor at the drive input may be very different.
What are you trying to determine?
RE: Starting time of a motor with VFD in ETAP
RE: Starting time of a motor with VFD in ETAP
RE: Starting time of a motor with VFD in ETAP
Your workaround is probably a good approximation.
With a VFD, the starting time can be nearly whatever they want.
RE: Starting time of a motor with VFD in ETAP
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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RE: Starting time of a motor with VFD in ETAP
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.
RE: Starting time of a motor with VFD in ETAP
Best regards,
Mark Empson
L M Photonics Ltd
RE: Starting time of a motor with VFD in ETAP
"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: Eng-Tips.com Forum Policies