New VFD's on motors

[Jraef]

... Now if downtime because of a failed motor is not worth it, you should definitely put on the correct motor, some instances that down time costs more then equipment repairs do.

That's the most important issue in my opinion. I'm in the camp of "use it until you lose it" with regard to applying VFDs to older motors, but with respectful deference to that issue.

I did a project at a glass plant to optimize the furnace blowers using VFDs. They were 1,000HP 460V motors and my competitor told them they did not need to replace the motors when adding the drives, they would "probably" last for years. I simply asked a series of relevant questions.

"What is the consequence of losing one of those motors, and how long will it take to get it repaired?". They responded that the furnace had two blowers so the 2nd one would allow them to continue operation, just at reduced capacity. But it also turned out there was NOBODY left in northern California capable of handling that big of a motor, it would need to be sent to So Cal, so it would likely be a week minimum turnaround even on a rush basis.

Question 2: "Do you have a spare motor?". No, we used to, but when we added that 2nd blower we used that motor because the lead time was too long, then the replacement spare has been left out of the budget for the last 5 years."

Question 3: "So if that one motor goes down, you have the other blower still working to keep the glass from solidifying, but with lost production capacity and no further redundancy if that SECOND motor goes down in less than a week from the first one then?". They thought about it and said yes, that was correct.

Final question: "What is the cost of lost production and having to get the solidified glass out of the furnace if that second blower fails while waiting for the repair on the first?"

Approximately $700,000 per day in lost production revenue, likely $500,000 or more to clean out the furnace if it ever lost heat for more than a few hours.

I suggested they buy one new inverter duty motor so that if/when one of the others failed, they could swap it out quickly with the new one, then repair the failed ones to the same level, eventually cycling through the old designs and winding up with more reliability. I won. Even though I also convinced them to put in load reactors, motor #1 failed in 6 months, motor #2 failed 3 years later (the one that had been the spare, so it wasn't as old), I was the hero. So now all three are upgraded to "inverter duty". They have not had another motor failure since 2006.

 

[kwired]

That's the most important issue in my opinion. I'm in the camp of "use it until you lose it" with regard to applying VFDs to older motors, but with respectful deference to that issue.

I did a project at a glass plant to optimize the furnace blowers using VFDs. They were 1,000HP 460V motors and my competitor told them they did not need to replace the motors when adding the drives, they would "probably" last for years. I simply asked a series of relevant questions.

"What is the consequence of losing one of those motors, and how long will it take to get it repaired?". They responded that the furnace had two blowers so the 2nd one would allow them to continue operation, just at reduced capacity. But it also turned out there was NOBODY left in northern California capable of handling that big of a motor, it would need to be sent to So Cal, so it would likely be a week minimum turnaround even on a rush basis.

Question 2: "Do you have a spare motor?". No, we used to, but when we added that 2nd blower we used that motor because the lead time was too long, then the replacement spare has been left out of the budget for the last 5 years."

Question 3: "So if that one motor goes down, you have the other blower still working to keep the glass from solidifying, but with lost production capacity and no further redundancy if that SECOND motor goes down in less than a week from the first one then?". They thought about it and said yes, that was correct.

Final question: "What is the cost of lost production and having to get the solidified glass out of the furnace if that second blower fails while waiting for the repair on the first?"

Approximately $700,000 per day in lost production revenue, likely $500,000 or more to clean out the furnace if it ever lost heat for more than a few hours.

I suggested they buy one new inverter duty motor so that if/when one of the others failed, they could swap it out quickly with the new one, then repair the failed ones to the same level, eventually cycling through the old designs and winding up with more reliability. I won. Even though I also convinced them to put in load reactors, motor #1 failed in 6 months, motor #2 failed 3 years later (the one that had been the spare, so it wasn't as old), I was the hero. So now all three are upgraded to "inverter duty". They have not had another motor failure since 2006.

I bet you never received a check for the potential $700,000 per day or the $500,000 to clean up that they might have saved either

 

[Jraef]

I bet you never received a check for the potential $700,000 per day or the $500,000 to clean up that they might have saved either

Nope. But I did get a bunch of future business, some of which was very lucrative. Whenever I was within 10% of the low bidder, they gave it to me.

I lost one huge project for them on upgrading some compressors to a lowballer who had no idea what he was getting into and screwed it up royally. They called me later to come in and fix it all, I priced it to make some money on it and although they paid me, the exec who had made the bad decision in the first place commented on how I had raped them and the people below him were too afraid to give me another project after that.

 

[Besoeker]

Like the others I have seen many old motors retrofitted with VFDs without any issues.

I've seen many that have. Some in just weeks.

 

[JoeStillman]

I've seen many that have. Some in just weeks.

Me too.

It will be best for the motors to have the drives nearby. Longer cable runs are bad. You might also consider VFD cable, just to be sure you did everything you could to save the old motors. It could help prevent pitting in the bearings due to zero-sequence induction. Another option for that is a grounding ring, but I don't like them because its just one more thing to maintain.

Finally, I wouldn't hook a drive up to a non-inverter-duty motor without reactors on the drive output.

 

[iwire]

I've seen many that have. Some in just weeks.

I don't doubt that in the least but more do not die. So unless downtime is prohibitory costly the most economical move is to add the VFDs and change the motors if they go bad. The idea of not adding VFDs, or swapping every motor out that gets a VFD because the motor might expire is crazy to me.

VFDs are being added to old motors all the time in the places I work, typically to HVAC equipment.

 

[iwire]

Finally, I wouldn't hook a drive up to a non-inverter-duty motor without reactors on the drive output.

Done without reactors successfully very often.

I suspect you are spending other people's money.

 

[JoeStillman]

Done without reactors successfully very often.

I suspect you are spending other people's money.

It's cheap insurance.

 

[kwired]

I don't doubt that in the least but more do not die. So unless downtime is prohibitory costly the most economical move is to add the VFDs and change the motors if they go bad. The idea of not adding VFDs, or swapping every motor out that gets a VFD because the motor might expire is crazy to me.

VFDs are being added to old motors all the time in the places I work, typically to HVAC equipment.

What percentage of them are 208/230 volt compared to 460 volt?

Also with HVAC equipment the drive is probably right at the equipment so the distance to the motor is minimal.

480 volts and long distance (like as little as 50 feet) between drive and motor will have more failures then most other combinations if you don't at least install line reactors on the drive output because of the higher voltage spikes the motor will see. Most failures I have seen were because of 480 volts system coupled with longer distances to the motor.

 

[iwire]

What percentage of them are 208/230 volt compared to 460 volt?

Also with HVAC equipment the drive is probably right at the equipment so the distance to the motor is minimal.

480 volts and long distance (like as little as 50 feet) between drive and motor will have more failures then most other combinations if you don't at least install line reactors on the drive output because of the higher voltage spikes the motor will see. Most failures I have seen were because of 480 volts system coupled with longer distances to the motor.

So for non-critical applications would you make it a policy to just assume the motor will die and change it when it is still operational?

Moderator Don has been adding VFDs to old motors in industrial locations for many years. He may be able to answer your questions. As I recall he is not seeing failures and I don't think they are using reactors. They are also reusing the standard conductors, not VFD cable. He is on a ship in Alaska right now so I would not expect him to join in right away.



My own feeling is most of the people pushing for VFD motors, reactors and VFD cable are those who stand to make money on the sale of those products. Or the belt and suspenders crowd.


I was involved with some large VFD motor problems but that was bearing issues on VFD rated motors not insulation failure.

 

[Besoeker]

Me too.

It will be best for the motors to have the drives nearby. Longer cable runs are bad. You might also consider VFD cable, just to be sure you did everything you could to save the old motors. It could help prevent pitting in the bearings due to zero-sequence induction. Another option for that is a grounding ring, but I don't like them because its just one more thing to maintain.

Finally, I wouldn't hook a drive up to a non-inverter-duty motor without reactors on the drive output.

A couple of points.
It's common with drives (and other electrical equipment) that I deal with to be located all together in a switch room.
The location of the motor and driven equipment obviously determines the length of cable run and we will take the appropriate mitigating measures. I'm dealing with one at the moment where the cable run is about 350 metres - a bit over 1100 feet. That will get a sine filter on the output.

The second point, one I've made previously, I've heard of bearing patting and have seen pictures of the damage. It isn't something I've come across. But we use steel wire armoured cable which keeps the conductors more less symmetrical.

 

[kwired]

So for non-critical applications would you make it a policy to just assume the motor will die and change it when it is still operational?

Moderator Don has been adding VFDs to old motors in industrial locations for many years. He may be able to answer your questions. As I recall he is not seeing failures and I don't think they are using reactors. They are also reusing the standard conductors, not VFD cable. He is on a ship in Alaska right now so I would not expect him to join in right away.



My own feeling is most of the people pushing for VFD motors, reactors and VFD cable are those who stand to make money on the sale of those products. Or the belt and suspenders crowd.


I was involved with some large VFD motor problems but that was bearing issues on VFD rated motors not insulation failure.

I make it policy to assume all motors will die eventually. Most of them die from misuse or improper protection though.

I have put many older motors on VFD's without changing anything except replacing the old controller with the VFD. Most of the time there has been no problem with losing the motor. The times I have lost motors in relatively short time has always been 480 volts, and long runs between drive and motor. And some of those didn't survive long after replacing the motor with one rated for VFD either, but did once installing line reactors.

One other instance I do recall was a high pressure pump that was typically run at only about 50% speed most of the time. Had that motor rewound maybe three times before we bought one rated for VFD with an external cooling fan. This motor is loaded nearly to to the max no matter what speed is selected, and at 50% speed does not get much cooling from an integral fan, plus is inside a housing with limited air flow. Old motor died almost annually and about right on the annual date, was almost predictable. New one has been there for 7-8 years or maybe even more now. Careful selection of overload protection of the external cooling fan (and interlocking it with the drive) has caught bad bearings a couple times in the cooling fan, otherwise who knows, it likely fails due to lack of cooling.

 

[zbang]

It will be best for the motors to have the drives nearby. Longer cable runs are bad.

I've read and heard this many times, but don't get the physics of it. Sufficiently long runs could get standing waves along them, depending on how square the pulses actually are. I know what an inductor/reactor will do to the waveform, but would also expect a long line to shave some of the high harmonics off the waves. Or, do we get more high harmonics and a spike at the leading edge of each pulse? (I'll probably spend a couple of hours poking around google about this.)


Jraef- was that the O-I plant in Oakland? (Or is there another glass plant in northern CA?) I read about a glass plant in NJ that went to their own co-gen because of less-than-reliable utility power.

 

[iwire]

I make it policy to assume all motors will die eventually.

I like that.

Most of them die from misuse or improper protection though.

I agree that correctly selected, installed and operated motors seem to last a good long time. Bearings will need replacement before winding failure many times. At least that is what I have seen in my very small corner of a very large profession.

I have put many older motors on VFD's without changing anything except replacing the old controller with the VFD. Most of the time there has been no problem with losing the motor. The times I have lost motors in relatively short time has always been 480 volts,

It makes sense.

and long runs between drive and motor. And some of those didn't survive long after replacing the motor with one rated for VFD either, but did once installing line reactors.

Honestly I don't know enough about rectors to comment further. I did get to sit in at a large meeting of many electrical engineers battling with Cutler Hammer that reactors where not the fix all solution Cutler Hammer was pushing for. This was over the bearing issue I mentioned above.

One other instance I do recall was a high pressure pump that was typically run at only about 50% speed most of the time. Had that motor rewound maybe three times before we bought one rated for VFD with an external cooling fan. This motor is loaded nearly to to the max no matter what speed is selected, and at 50% speed does not get much cooling from an integral fan, plus is inside a housing with limited air flow.

Yes I can see that being an issue, however reduced cooling is a well known issue and should have been considered when deciding to run it at half speed.

 

[GoldDigger]

I've read and heard this many times, but don't get the physics of it. Sufficiently long runs could get standing waves along them, depending on how square the pulses actually are. I know what an inductor/reactor will do to the waveform, but would also expect a long line to shave some of the high harmonics off the waves. Or, do we get more high harmonics and a spike at the leading edge of each pulse? (I'll probably spend a couple of hours poking around google about this....

When you have a transmission line (looking at the characteristic impedance rather than the bulk inductance and capacitance separately) you can look at reflections that occur at impedance changes.
With the large wire size of non coaxial conductors the characteristic impedance will be low compared to the motor load reactance. That means that any impulse (sharp step in voltage) will be reflected back in the same polarity as the incoming impulse (since no current will flow at what we approximate to be an open circuit termination.) That means that as a first approximation the step voltage at the motor will be twice that of the input at the other end of the cable.
This voltage will persist until the reflection has had time to be absorbed or re-reflected at the VFD end of the cable. Hence the longer the cable the longer the voltage spike will persist.
The source end at the VFD looks like a short circuit compared to the characteristic impedance, so the reflection from that end will have opposite polarity and will try to cancel out the double voltage when it gets back to the motor end.
You can also look at the process in terms of harmonics (not of 60Hz but of the switching frequency) which can be very high harmonics indeed. And then look at standing waves at those frequencies. But to the extent that the wires can be considered as a transmission line, the attenuation over distance of those high harmonics will be small.
An RF coax cable can pass 1000 MHz without too much trouble....


Tapatalk!

 

[petersonra]

Yes, those are still fine. But they WERE listed with MCP mag-only breakers too, up until January of this year (I think it was).

why the change?

the information I have from AB still shows the 523 line as being able to use 140M MCP.

 

[steve66]

No, it is not that simple, your initial plan is the only correct one, replace the starters with feeder breakers. The VFDs will not be listed to have starters as the OCPD. If you read the manuals, they will list either fuses or thermal-mag breakers, maybe both, but not mag-only breakers, even if in a starter with a thermal OL relay. The only valid use of a mag-only breaker is when it is part of a starter that has a thermal OL that is directly feeding a single motor.

How about mag only breakers ahead of fuses, suggested by the Vfd mfg, then the VFD?

Any better?

Well, that does give me second thoughts. The VFD's have their own circuit breaker on the input. So that should take care of any of the VFD listing concerns, or any concerns about following the instructions.

So the MCP and overload combination is still only protecting the same wire they were always protecting before. True, the load on the end of that wire is now a VFD and motor, not just a VFD.

I'm not seeing any real reason why this wouldn't be just as safe as the original installation.

 

[Jraef]

why the change?

the information I have from AB still shows the 523 line as being able to use 140M MCP.

Well, it's confusing because of part numbers, I agree. I had to study the data sheets for a while to figure it out myself, so you are not alone. I'll see if I can help you get to what you need to know.

• 140M is the prefix used for the IEC style Manual Motor Starter type devices that have magnetic trips and adjustable thermal trips. Rockwell calls these "Motor Protection Circuit Breakers (MPCB). THESE are what are being listed in the protection charts for the PF520 series drives. Below 60HP @460V, those devices look like this:
  
  At 60HP 460V and above, they look like regular molded case circuit breakers, the difference is that the thermal trips are adjustable because technically, they are an overload relay.
  
• 140M was the same prefix used in the part numbers of the old Molded Case Magnetic-Only breakers (officially called IT for "Instantaneous Trip" in the NEC), which are called "MCPs" for Motor Circuit Protector, the type use in factory built combination starters where there is ALSO going to be a separate overload relay. They look the same as the MPCBs at first glance, but notice that they do NOT have the extra adjustment dials for the thermal trip settings.
  

So technically, your statement "AB still shows the 523 line as being able to use 140M MCP" is incorrect, because those charts do NOT actually use the term "MCP", you are assuming such because of the confusing way they are (were) numbered.

With the change in the A-B circuit breaker product line, the Molded Case versions which are Mag-Only (IT) for use in factory built combo starters are now called 140MG, which, although not as clearly delineated as I would have liked, is at least slightly less confusing.

So from here on out, the 140M is ONLY referring to the Motor protection Circuit Breakers, "MPCBs", and NOT the "MCPs".

 

[Jraef]

Well, that does give me second thoughts. The VFD's have their own circuit breaker on the input. So that should take care of any of the VFD listing concerns, or any concerns about following the instructions.

So the MCP and overload combination is still only protecting the same wire they were always protecting before. True, the load on the end of that wire is now a VFD and motor, not just a VFD.

I'm not seeing any real reason why this wouldn't be just as safe as the original installation.

Maybe it is "just as safe", but technically, it will not be in compliance with the NEC. A motor starter, although it performs the same FUNCTION as a feeder, is not a feeder in the strict sense of the NEC definitions.

 

[kwired]

I like that.




I agree that correctly selected, installed and operated motors seem to last a good long time. Bearings will need replacement before winding failure many times. At least that is what I have seen in my very small corner of a very large profession.





It makes sense.




Honestly I don't know enough about rectors to comment further. I did get to sit in at a large meeting of many electrical engineers battling with Cutler Hammer that reactors where not the fix all solution Cutler Hammer was pushing for. This was over the bearing issue I mentioned above.





Yes I can see that being an issue, however reduced cooling is a well known issue and should have been considered when deciding to run it at half speed.

Reactors are not going to remove all the spikes on the drive output, but they do smooth out the wave farily significantly.

The motor that was running hard at half speed - I had no idea what the conditions were going to be going into that one. I was asked if I could slow that motor down as it was driving a high pressure pump for injecting liquid product into a dryer that runs at very high pressure and the mist hitting the heat in the dryer evaporates the water and leaves fine powder free falling inside the rest of the dryer enclosure. They were spending a lot of $$ on maintenance parts for the high pressure pump and was suggested that many of those type of pumps were now being run on VFD's and saves on some maintenance costs. Way too many variables in this process, they can run faster raw product but need higher heat, they can change nozzles, which also effects flow rate and heat needed, final product texture and moisture levels are better for some products at different rates, temps, pressure...but turns out what seems to work best for most products is usually running between 30-45 Hz.

The manager of that plant who is now passed away, impressed me a lot though. This guy was a wizard at formulas for products and knowing just what to do to make changes to the product - like knowing if we use a specific temperature, we need this particular feed rate, to get the desired change to product. And this man never attended any college - only a high school graduate. It was a dairy products plant and he knew a lot about the food processing industry. He started working in a dairy plant when he was still in school, and went a long way in that field.


Sorry to get a little off topic here - I respected that guy a lot and do miss him.