Input Amps VFD - Overload condition

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W@ttson

W@ttson

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Hello,

One of the benefits Flux Vector Drives (variable frequency drives) is that the normal 6x of FLA or so inrush current for motors is limited. A typical value that the inrush is limited to is 110% of FLA.

Looking at the Caterpillar gen sizer program, they have an option to list how the motor is started and a VFD is available. However, the there is a field that allows to mark what percentage the input current is limited to. The default is 110%. Suppose the motor at start needs to move a high inertia load, and maintain 150% FLT for 60 seconds, assume the drive is properly sized and can provide this, what would the input current to the drive look like? Would it still limit the input current to a maximum of 110%, or will it trip out and this setting will need to be adjusted for this application?

In other words, depending on the torque producing current, magnetizing current, and the Pythagorean theorem there will be some sort of output current draw proportional to the 150% FLT, but what about the input?
 
Jraef

Jraef

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The input current is going to be lower by virtue of the input PF being held constant at .95, but the output current going to the motor is going to be INCLUSIVE of the reactive current, but still limited to that 150% value. So regardless of what the motor is actually DOING with all of the 150% current, the value remains constant on the input side at output amps x input PF. So if a VFD is rated for 300A output and the output current is going to be limited to 450A (150%) of that for one minute, during that one minute the input current will be 450 x .95, so 427.5A. The thing is though, ther RATED INPUT CURRENT of the drive is always lower than the rated OUTPUT current. But drives are SOLD based on the rated OUTPUT current, so determining the worst case scenario of the current draw during a temprary overload condition should be consistent.
 
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W@ttson

W@ttson

Senior Member
Location
USA
Jraef said:
The input current is going to be lower by virtue of the input PF being held constant at .95, but the output current going to the motor is going to be INCLUSIVE of the reactive current, but still limited to that 150% value. So regardless of what the motor is actually DOING with all of the 150% current, the value remains constant on the input side at output amps x input PF. So if a VFD is rated for 300A output and the output current is going to be limited to 450A (150%) of that for one minute, during that one minute the input current will be 450 x .95, so 427.5A. The thing is though, ther RATED INPUT CURRENT of the drive is always lower than the rated OUTPUT current. But drives are SOLD based on the rated OUTPUT current, so determining the worst case scenario of the current draw during a temprary overload condition should be consistent.
Click to expand...

Thank you for the reply Jraef. I guess there was no way that it was possible to get power for free by having the drive magically limit the input current to 110% of the FLA during this time. So it seems like I need to select that the input current limit for the sake of sizing the generator needs to be set to 150% instead of the 110%. (By the way I like the new avatar)

SpecSizer 150Percent.JPG
 
Besoeker

Besoeker

Senior Member
Location
UK
Jraef said:
The input current is going to be lower by virtue of the input PF being held constant at .95, but the output current going to the motor is going to be INCLUSIVE of the reactive current, but still limited to that 150% value. So regardless of what the motor is actually DOING with all of the 150% current, the value remains constant on the input side at output amps x input PF. So if a VFD is rated for 300A output and the output current is going to be limited to 450A (150%) of that for one minute, during that one minute the input current will be 450 x .95, so 427.5A. The thing is though, ther RATED INPUT CURRENT of the drive is always lower than the rated OUTPUT current. But drives are SOLD based on the rated OUTPUT current, so determining the worst case scenario of the current draw during a temprary overload condition should be consistent.
Click to expand...
It's slightly more involved than that. The input voltage is fixed. The output voltage is variable.
Imagine going from standstill - after all, that's where you are at starting.
The output power is zero - power is torque times speed as you know.

P=Tω
ω=0, P=0, regardless of T.

So you could have 150% T, 150% Io and no P.
Iin would be supplying just losses.
 
Besoeker

Besoeker

Senior Member
Location
UK
spraymax6 said:
Thank you for the reply Jraef. I guess there was no way that it was possible to get power for free by having the drive magically limit the input current to 110% of the FLA during this time. So it seems like I need to select that the input current limit for the sake of sizing the generator needs to be set to 150% instead of the 110%. (By the way I like the new avatar)
Click to expand...

Most VFDs have a diode bridge at the front end. You can't use that to control the input current. You'd need an active front end, usually IGBT based. Maybe that's what you have........
 
Jraef

Jraef

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You can limit the output current to 110% and in most cases, the drive WILL be able to accelerate the motor, given enough time. The ENERGY it takes to accelerate a load is a fixed amount, but ENERGY is power across time; more power = less time, less power = more time. I have accelerated a 1250HP oil pipeline motor with a VFD set at 100% motor FLA as the current limit. It took almost 2 hours to get it to full speed, but since the motor was not exceeding nameplate FLC, that was not harming the motor (we had separate cooling for it). At 110% output current, I would expect you can likely run a Heavy Duty / Constant Torque drive (rated for 150% for 60 seconds) for 5 minutes or so before the drive over-temperature would kick in. Things such as carrier frequency and ambient temperature can affect that though, so be careful.

But a Normal Duty / Variable Torque rated drive can only handle 110% for about 30-60 seconds (depending on manufacturer), so be EXTRA careful with that.
 
Jraef

Jraef

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Staff member
Location
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Besoeker said:
It's slightly more involved than that. The input voltage is fixed. The output voltage is variable.
Imagine going from standstill - after all, that's where you are at starting.
The output power is zero - power is torque times speed as you know.

P=Tω
ω=0, P=0, regardless of T.

So you could have 150% T, 150% Io and no P.
Iin would be supplying just losses.
Click to expand...
Yes, it IS more complicated in terms of the TORQUE produced by that current, but current is current. If the output current is limited to 150%, the input current it limited too. That output current is doing less WORK at the lower speeds, which fits your description of the Iin supplying mostly losses, but it is STILL limited by the output controller to a constant kVA as seen by the input side.
 
Besoeker

Besoeker

Senior Member
Location
UK
Jraef said:
Yes, it IS more complicated in terms of the TORQUE produced by that current, but current is current. If the output current is limited to 150%, the input current it limited too. That output current is doing less WORK at the lower speeds, which fits your description of the Iin supplying mostly losses, but it is STILL limited by the output controller to a constant kVA as seen by the input side.
Click to expand...
Disagree.
The output current can remain constant during acceleration from ω=0 to ωmax.
Obviously the Iin does not.

Butterbot?
 
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