Vfd Line Side Overload Protection

[goghie]

VFD will trip if motor current is higher than it can output. As simple as that. You need to protect drive input side according to the manufacturer specs, not according to output. On there side you need to select drive according to motor current and type of load.

 

[jraef]

I actually meant overload condition for vfd input bridge. I will try to explain what I meant (sorry for my english since I am not a native speaker I sometimes can't explain what I think well.
Let's think a 10KW motor running with vfd at the rated voltage (480V) . Drive output current approximately be about 15A (mot. eff.= 0.9 and p.f.=0.9)
If the KVA to be reflected to vfd input side will be about 13.7 KVA and the current be about 16.5A (vfd eff. =0.9 & p.f. =0.9). I know the displacement power factor is high in vfd's but the distortion p.f also depends on using line filter. In this case I consider vfd is used without line filter and input current contains high level of harmonics.
I know it is a forced example but it seems to me vfd input may be overloaded while the output is not. Please let me know if my calculation is wrong.

When the VFD components are selected, the designers take all of that into consideration. If a VFD is rated for 10A output, the rectifier section will be based on the worst case scenario that it will see under the rated load of the unit. So you cannot by design overload the rectifier unless there is either an overload on the output side, which the VFD is monitoring, or a failure of a component on the input side, which will be a different issue than an "overload" condition. So no, there is no "overload" monitoring of the input rectifier, there is no need for that.

The caveat, as possibly expressed by bodoo23 above in referencing a "755 manual", is that Line Regenerative ACTIVE FRONT END (AFE) drives do need to monitor the front-end current, because current can go both directions. But a standard diode bridge front-end drive does not.

By the way, the input current of a VFD is typically LOWER than the output current rating. The output devices see the Power Factor of the motor, the input rectifier does not.

 

[pturmel]

Please let me know if my calculation is wrong.

You aren't wrong, per se, but VFD manufacturers build the necessary tolerance into their units' bridge rectifiers. For the rated range of input voltage and distortion. Otherwise they'd have absurdly high failure rates in the field. Protect the input per the manufacturer's recommendation.

 

[Gene Bond]

As I stated, if there is excessive current on the input, the heatsink will overheat, and the drive will trip, or if enough current, the input fuses/cb will blow.

What these guys are saying is correct. Heck, we have 1 drive's DC bus powering multiple drives in special applications. Just follow the manual on input protection to minimize the fire when the diodes on the input fail. Typically (these days), the capacitors or fans are the first to fail, and customers just replace the drive.

 

[bodoo23]

Thank you all for your appreciated replies. The case is clear for me now.