Motor Start on VFD 4

[wbd]

Hello, I have reviewed information on VFD's and understand how they function with a load. I either missed it or can't find it but how does a VFD affect motor start? Is the starting curve the same for a motor on a VFD as it is for a standard across the line starter?

Thanks in advance.

 

[LionelHutz]

No, it is not.

Think of it as the VFD operating the motor at full speed all the time.

 

[wbd]

Ok, but the motor has to get rolling first so there will be a high current input to break the motor free and start rotating. So it is not operating at full speed all the time. It has to get started from a standstill which is what I am asking.

 

[LionelHutz]

Motor full speed is defined by the frequency and number of poles = 120xf/P. The VFD changes the frequency which results in the motor always running at full speed regardless of the full speed rpm changing as the VFD output frequency changes.

 

[waross]

Look at a motor torque speed curve.
Current and torque is mostly dependent on the slip speed or frequency.
That is the difference between the speed associated with the applied frequency and the motor actual speed.
Example. 60 Hz applied to a 4 pole motor, synchronous speed = 1800 RPM.
Motor full load speed, 1760 RPM.
Slip speed, 1800 - 1760 = 40 RPM.
So depending on the load, a motor may be started with less than full load current and torque.
Consider a fan or centrifugal pump:
With a slowly rising frequency, the motor may accelerate at less than full load torque and current.
Full load torque and current may only be reached as the load closely approaches rated speed.
The actual results will depend on the load torque/speed demands and the speed at which the frequency is ramped up.
A VFD may accelerate a motor faster than DOL starting with less current.
Consider:
DOL; Starting torque about 150%, Starting current about 600%.
VFD, Best case starting; Breakdown torque about 200%, breakdown current about 200%.
Reading the torque/speed curve:
The part of the curve of interest when using a VFD is the part between 100% and the breakdown torque.
First relabel the curve:
For a 1760 RPM motor, 1800 RPM will become zero RPM.
Full load torque will become 1800 - 1760 = 40 RPM.
This is the slip frequency/speed.
At breakdown torque the slip speed will be about 80 RPM to 100 RPM depending on the individual motor.
Now read the curve backwards.
At zero slip, the torque will be zero.
Think of a 1760 RPM motor turning at 1800 RPM with 60 Hz applied.
As the applied frequency/speed is increased, the slip is increased and as the slip is increased, the torque and current are increased.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wbd]

Thank you for the great explanation, Waross!

 

[itsmoked]

I liked LH 'always full speed' too!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

I liked LH 'always full speed' too!

Took a minute for that to sink in, but I like it too.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[davidbeach]

Yep, just have to recognize that sometimes full speed is zero rpm. Then full speed is a bit more than zero, and then a bit more than that, etc.

 

[waross]

I don't see any contradiction between Lionel's explanation and mine.
He said it with a lot fewer words.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Yes! Bill. That is frequently the case.

Your responses have a lot in them, rather like a fine wine. One must 'taste' them, savoring them over time.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

I just love that emoticon, Keith!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]