VFD Output Filter?

[synchro]

A dV/dt filter such as the ones at the link below from Hammond incorporate resistors that are shunted across a reactor to dampen oscillations that may occur when the VFD output is switching and then be conducted to the motor. And the reactor slows the risetimes of the switching waveforms to minimize problems with reflected voltages that can increase the peak voltage at the motor.

https://americas.hammondpowersolutions.com/en/products/control-automation/dv-dt-filters

 

[synchro]

There are four of these units at the facility and because of the configuration of the equipment, It is necessary to bring in a large crane to replace one of these motors. The one that went bad has the longest motor leads.

So do you fine folks think it is likely that an VFD output filter would have prevented this fault. Do you think all the VFDs feeding these motors should get VFD output filters?

It would appear prudent to at least put a dV/dt filter on the VFD that drives the motor that would need a large crane to replace it. The next in line could be the one with the longest run to the motor. Doing all four would be up to you.

 

[herding_cats]

(Please cite some sources for this.)

Don't know about that, I work on a printing press that regularly runs at under 15 Hz for hours on end and has a non-antique and non-inverter-rated motor (1 hp, iirc).

Then you have a really quality winding. I spent over 4 years designing ARC technology for an engineering company in Omaha NE. Classic 3-phase blower motors in rooftop hvac units will consistently burn up the windings running below 20Hz. Like I stated it’s not every motor. So many different variables. But most every modern non-inverter blower motor in RTUs will destroy themselves running below 20Hz.

 

[herding_cats]

Then you have a really quality winding. I spent over 4 years designing ARC technology for an engineering company in Omaha NE. Classic 3-phase blower motors in rooftop hvac units will consistently burn up the windings running below 20Hz. Like I stated it’s not every motor. So many different variables. But most every modern non-inverter blower motor in RTUs will destroy themselves running below 20Hz.

You won’t know if dropping below 20Hz will burn the winding, until you do it. So why try? You only get 1 smoke test.

 

[TwoBlocked]

.....

As for the second question, it's hard to say without knowing what "these motors" actually are.

25HP, 480V, 3-ph, 4-pole, induction motors driving axial fans.

 

[TwoBlocked]

Any insights about how long a motor lead run can be before an output filter is recommended?

 

[ptonsparky]

FWIU, the 20hz limit is largely due to the motors ability to cool itself is reduced by the lower speed of its fan.

 

[ptonsparky]

Any insights about how long a motor lead run can be before an output filter is recommended?

Look at the drive manual. They usually have recommendations. They may be CYA suggestions, but that is what you need.

 

[ptonsparky]

YMMV

 

[petersonra]

I am pretty sure of my diagnosis and remedy, but want to check with you fine folks in case I am missing something. A 25HP 3-ph 480VAC VFD driven motor driving an axial fan was tripping out the VFD on overcurrent. Meggerd fine, but had uneven resistance phase to phase. disconnected all the leads. Each lead read fine by itself, but some were shorted to others. Obviously an internal short, but why, and what to do about it.

You should read zero resistance from each lead to at least one other lead because each winding has a connection to two leads. Do you have a picture of the connection diagram you can post?

If you are getting an overload fault it is likely that you have an overload. Your drive probably has a keypad of some sort that will allow you to look at the output current. That would be where I would start. I don't believe that any kind of filter is going to solve an overload problem.

 

[winnie]

The 20Hz limit is not a hard wall, but a probabilistic thing.

Have a lightly loaded motor, with low dV/dt from the drive and good cooling? You can go all the way down to 0 Hz with no problem.

Have a heavily loaded motor in a confined space...

Jon

 

[TwoBlocked]

You should read zero resistance from each lead to at least one other lead because each winding has a connection to two leads. Do you have a picture of the connection diagram you can post?

If you are getting an overload fault it is likely that you have an overload. Your drive probably has a keypad of some sort that will allow you to look at the output current. That would be where I would start. I don't believe that any kind of filter is going to solve an overload problem.

It is a 12 lead motor wired high voltage delta. I disconnected every connection in the connection box. (Sounds like a country western song...) Each of the six windings had the same resistance, 0.27 ohms. Each winding showed high resistance to ground, as far as the megger would go at 1000 volts, like 2.2 gigaohms? But some windings showed low resistance to other windings, like 1/2 ohm. When I saw that, I knew I had winding to winding shorts.

Motor has been replaced and runs fine.

 

[TwoBlocked]

Look at the drive manual. They usually have recommendations. They may be CYA suggestions, but that is what you need.

Yes, but then there is real experience from real people.

 

[JoeStillman]

Any insights about how long a motor lead run can be before an output filter is recommended?

I think 100 feet is getting pretty close to needing filters at the motor terminals. I read somewhere they are best located at the load end of the output cables.

 

[zbang]

At first thought, putting the filter/reactor at the load end doesn't make as much sense as the wiring is still subjected to switching transients. Seems that lowering the slew rate at the source would be more beneficial.

 

[Jraef]

Any insights about how long a motor lead run can be before an output filter is recommended?

Depends on a LOT of factors., it's all over the map. Some manufacturers publish that data, others ignore it and leave it to the end user to find out on their own.

Here is a document from Rockwell that shows THEIR tested distances from Drive to Motor for each different VFD line they sell (or sold). If you pick out a single size, let's say 10HP 480V with 1200V winding insulation (non-inverter duty), you get the following from their tables:

PowerFlex 523... with nothing between the drive and motor, 40ft; add just a load reactor, 300ft; with a DV/DT filter, 1000ft.
PowerFlex 753... with nothing between the drive and motor, 40ft; add just a load reactor, 300ft; with a DV/DT filter, 1200ft.

But if you use minimum "NEMA MG-1 part 31 inverter rated" insulation of 1488V:
PF523... 300ft, 1000ft, 1000ft
PF753... 350ft, 1200ft and 1200ft

So note that the thing that makes the MOST difference is the motor insulation value. Once you step up to the 1488V insulation, the DV/DT filter makes no difference.

 

[TwoBlocked]

Depends on a LOT of factors., it's all over the map. Some manufacturers publish that data, others ignore it and leave it to the end user to find out on their own.

Here is a document from Rockwell that shows THEIR tested distances from Drive to Motor for each different VFD line they sell (or sold). If you pick out a single size, let's say 10HP 480V with 1200V winding insulation (non-inverter duty), you get the following from their tables:

PowerFlex 523... with nothing between the drive and motor, 40ft; add just a load reactor, 300ft; with a DV/DT filter, 1000ft.
PowerFlex 753... with nothing between the drive and motor, 40ft; add just a load reactor, 300ft; with a DV/DT filter, 1200ft.

But if you use minimum "NEMA MG-1 part 31 inverter rated" insulation of 1488V:
PF523... 300ft, 1000ft, 1000ft
PF753... 350ft, 1200ft and 1200ft

So note that the thing that makes the MOST difference is the motor insulation value. Once you step up to the 1488V insulation, the DV/DT filter makes no difference.

Ok... so what voltage is class F insulation rated for?

 

[zbang]

Class F is a temperature rating (https://www.engineeringtoolbox.com/nema-insulation-classes-d_734.html). You need to look farther than just the temperature. However https://www.se.com/us/en/faqs/FA238221/....

Motor rating plates explained- https://wiki.testguy.net/t/motor-nameplate-and-ratings-explained/66

 

[winnie]

Ok... so what voltage is class F insulation rated for?

It isn't.

Class F is an insulation temperature rating, not a voltage rating.

But the Inverter rated insulation seems to all carry the Class H temperature rating, so there is a hint.

-Jon

 

[petersonra]

It isn't.

Class F is an insulation temperature rating, not a voltage rating.

But the Inverter rated insulation seems to all carry the Class H temperature rating, so there is a hint.

-Jon

I think the hint is that the insulation used for class h windings just happens to be rated at a higher temperature and a higher voltage simultaneously.