VFD ramp times effecting current 2

[pittengineer]

Is it true that the faster a VFD is ramped during a start then the more current will be drawn by the motor. I have heard this and both seen this in the field. My question now is why?

I'm thinking that for a faster ramp, the faster the VFD changes frequency fast compared to how fast the motor moves up to speed. This greater difference causes a greater diff in slip while starting and therfore this greater slip causes more current.

If the ramp time is slow the vfd changes frequency slowly compared to how fast the motor is accelerating to match speed and keeps the slip value lower. This lower slip difference during acceleration causes lower current.

 

[jraef]

Pretty much dead on. In fact, when you enable the current limiting feature of a VFD, that's essentially what it will do; sacrifice ramp time in order to keep current within the set limit.


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[LionelHutz]

Why - the motor must produce more torque to accelerate quicker and to produce more torque requires more current input.

 

[pittengineer]

Thanks for the response guys!

So is the current more related to the slip being greater during faster ramps, or the motor trying to accelerate quicker and producing more torque? Or are these two aspects one in the same?

 

[dpc]

With a vector drive, the slip can be held relatively constant as the motor accelerates, I believe.

 

[itsmoked]

F = ma
Trying to accelerate the load faster.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[CJCPE]

"So is the current more related to the slip being greater during faster ramps, or the motor trying to accelerate quicker and producing more torque? Or are these two aspects one in the same? "

The two are pretty much two sides of the same coin. If you set the ramp and solve F = ma for F, that is the torque the drive will try to produce.

If you set the slip, that is like setting the torque. If you solve F = ma for acceleration, that is the rate of acceleration that the drive will try to produce.

The difference is that if you can maintain a set torque and accept the rate of acceleration that you get, the drive should not get into an overcurrent trip problem.

If you set acceleration, the drive might need to protect itself with overcurrent trip.

CJC

http://www.vfdriveinfo.com

 

[pittengineer]

Is it true also that for a given load a larger motor will accelerate the load faster then a smaller motor at a set speed?

I would think so due to the fact that acceleration time is esentially the difference between motor torque and load torque. If the torque produced by a larger motor is greater over a given speed range then it will accelerate the load faster than a smaller motor.

 

[CJCPE]

It is true, a larger motor will accelerate the load faster. However, if the load inertia is small, it is possible that the increased rotor inertia of the larger motor will offset the increased torque.

CJC

http://www.vfdriveinfo.com

 

[rhatcher]

pittengineer, you are right in understanding that acceleration torque = motor torque - load torque. The difference is what provides acceleration.

And, CJCPE is right that in order to precisely caclulate the acceleration time then you will need to know the inertia (load torque) of the entire rotating assembly which includes the motor rotor.

 

[amptramp]

Nice white paper from AB on induction motor basics:

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/mot-wp001_-en-p.pdf

 

[LionelHutz]

The motor torque and the amount of slip are more or less directly related up to the breakdown torque.

If you are talking about bigger motors with a VFD then yes the bigger motor would likely be able to accelerate the load quicker but it would still depend on the VFD setup. Typically, a VFD is programmed to accelerate the motor with a fixed acceleration.