VFD ramp times effecting current
VFD ramp times effecting current
(OP)
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.
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.





RE: VFD ramp times effecting current
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RE: VFD ramp times effecting current
RE: VFD ramp times effecting current
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?
RE: VFD ramp times effecting current
RE: VFD ramp times effecting current
Trying to accelerate the load faster.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD ramp times effecting current
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
RE: VFD ramp times effecting current
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.
RE: VFD ramp times effecting current
CJC
http://www.vfdriveinfo.com
RE: VFD ramp times effecting current
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.
RE: VFD ramp times effecting current
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RE: VFD ramp times effecting current
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.