Vfd Line Side Overload Protection

[bodoo23]

Does a vfd protect itself from line side overload ? I know it has built-in overload and short circuit protection for load side. I also know manufacturer recommend fuses to protect vfd line side from short circuit. But no one talks about line side overload. How to protect diodes and other power electronic components from overload ?

 

[pturmel]

They aren't directly connected to the motor. They charge the DC bus. The manufacturer sizes them to tolerate the initial bus charge inrush, or regulates them to avoid the inrush. The line side simply isn't subject to overload reflected from the motor the way you seem to think.


You sometimes do want to protect them from incoming power disturbances--surge protection, etc.

 

[PreLC]

Does a vfd protect itself from line side overload ? I know it has built-in overload and short circuit protection for load side. I also know manufacturer recommend fuses to protect vfd line side from short circuit. But no one talks about line side overload. How to protect diodes and other power electronic components from overload ?

I have not specced/designed a circuit for VFDs, but for heaters and SCR-Power controllers, one of the ways is to have a monitoring relay look at the input. Monitoring relays can look at phase orders being correctly wired up, accurate power coming in, etc.
Put this monitoring relay as a driver for a HV Line relay, and we are now able to protect against bad input issues.

 

[rupej]

Does a vfd protect itself from line side overload ? I know it has built-in overload and short circuit protection for load side. I also know manufacturer recommend fuses to protect vfd line side from short circuit. But no one talks about line side overload. How to protect diodes and other power electronic components from overload ?

It's unnecessary to protect against overloads on the line side. If there is an overcurrent event that affects the line side but not the load side, that means that the VFD has some internal failure, most likely a short circuit. This would be a near instantaneous rise in current- which is what the line side fusing/CBs are designed to protect against. The VFD is probably trash at that point also.

 

[bodoo23]

Thanks for appreciated answers. As you know, each electrical device has its own thermal limit so I can't understand why overload protection is not required for vfd front end.
I know motor power usage affects vfd's input power drawn by power grid. Overload protection built-in drive for motor may protect vfd input side as well. but when we don't use line reactor at the line side, pf of vfd is small so can't there be a case where motor is not overloaded (close to overload) but vfd line side is overloaded with a small amount ?

 

[pturmel]

No. Bridge rectification power factor improves with load.

 

[I_Automation]

I went for a PLC engineer job interview. As part of it their CAD guy brought in a schematic and had me identify components on it and what they did.
I pointed out he was using manual overloads as protection before the VFD's and that the inrush current to charge the DC bus could easily exceed the OLR rating, that OLR's are used after the VFD and fuses or circuit breakers are used before.
He felt his toes were stepped on & I didn't get the job - the owner was sitting there at the time.

 

[rupej]

As you know, each electrical device has its own thermal limit so I can't understand why overload protection is not required for vfd front end.

What else do you put overloads on?

 

[bodoo23]

I was reading rockwell pf 755 drive manual and saw that drive also has a built-in function for line side overload. I guess drive also handles this by itself.
Thanks for your answers.

 

[jraef]

How would one have an overload on the line side without there being an overload on the load side?

 

[I_Automation]

How would one have an overload on the line side without there being an overload on the load side?

"Because the VFD has it internal" - Bubba, 2013 (Also why the VFD is wired right off the disconnect not fused power)



Plus as I mentioned, an overload rated for the motor would easily be overloaded & trip by the amount of power going INTO a VFD. I only spec circuit breakers with line reactors before, with overloads (and load reactors if needed) after.

 

[PreLC]

How would one have an overload on the line side without there being an overload on the load side?

I guess OP meant overvolt/undervolt not really overload. Technically if a vfd is exposed to substantially higher than rated voltage, would it short(I mean if we reach substantially high voltages, everything shorts, but my question would be around substantially, but not cosmically high voltages)?
If so, if we want to protect the VFD before failure, is it possible to put a monitoring relay on the line side?

 

[Saffa]

"Because the VFD has it internal" - Bubba, 2013 (Also why the VFD is wired right off the disconnect not fused power)



Plus as I mentioned, an overload rated for the motor would easily be overloaded & trip by the amount of power going INTO a VFD. I only spec circuit breakers with line reactors before, with overloads (and load reactors if needed) after.

Is this a US thing, putting overloads on the VSD motor side? I can't see the point... all the drives I have ever worked with have their own internal thermal model for most motor types, the drive will flag a motor overload alarm.

 

[Gene Bond]

Is this a US thing, putting overloads on the VSD motor side? I can't see the point... all the drives I have ever worked with have their own internal thermal model for most motor types, the drive will flag a motor overload alarm.

The only time we use overload relays on the output of a vfd is when there are multiple motors, such as some fan systems, long conveyors and such. Otherwise, one of the motors could be overloaded, as long as the other is underloaded, and melt...

As far as protecting the input, the failure is normally short circuit, thus fuses for minimizing the explosion. If it is a continuous overload, the heatsink over-temp sensor will take care of it.

 

[bodoo23]

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.