# VFD sizing for motor loads



## erics37 (May 7, 2009)

I just get one of these for any motor:


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## varmit (Apr 19, 2009)

Per code, the circuit to the drive is sized at 125% of the drive input rating. This is due to the inherent losses in the drive.

The drive is normally sized to match the motor HP. The motor HP (kw) selected for use often needs to be adjusted based on the driven load characteristics and the speeds that the load is driven. Example: If you have a high torque load, that will normally require a 10 HP motor to run at 100% base speed, might require a 25 HP motor to run continuously at 30% base speed. A highly cyclic load, such as a reciprocating machine, would usually require upsizing of the motor and drive for durability reasons.


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## JRaef (Mar 23, 2009)

The conductor sizing must be based on 125% (min.) of the VFD input current regardless of the motor size, because what if someone resized the motor? You can size them higher if you want to, some people do, but the NEC talks about minimums. 

The VFD should be sized for the motor nameplate FLC, not HP because HP is not a reliable indicator of current draw, and VFDs are designed around current draw. 

There are only rare occasions when a VFD needs to be over sized. What a VFD does is maintain the SAME amount of torque output (FLT) at theoretically any speed, the best can do so right down to zero speed. So if an ME has sized the motor correctly, all you have to do is size the VFD to the motor FLC. The only time you need to over size it might be if the ACCELERATING torque needs to be high for a brief time, such as a high friction load. But most of the time this involves having to have over sized the motor anyway, so if the VFD is sized to the motor, you are done. As the electrician motor sizing should not be your job, that's a mechanical function; torque and speed. 

There is an issue of "Constant Torque" or "Variable Torque" VFD sizing. This is essentially a marketing game by VFD mfrs to sell cheaper VFDs when they can get away with it on things that have variable torque loads such as centrifugal pumps and fans. It's only because they know that the overload capacity of the VFD is not going to be needed in those machines, so they drop down one size rating. If you don't know for sure, always assuming CT will ensure your VFD will work, it just may make it cost more compared to a VT drive. The VFDs are essentially identical though, just rated differently.


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## John Valdes (May 17, 2007)

JRaef said:


> The VFD should be sized for the motor nameplate FLC, not HP because HP is not a reliable indicator of current draw, and VFDs are designed around current draw.


I think ABB at one time rated drives in only current and not HP. I have seen some big screw ups sizing controls. I remember one I did several years ago.
I replaced a 60 HP DC @ 900 RPM, with a 75 HP AC Flux Vector* @ 1800 RPM.
I thought I was going to lose my job. Turns out the existing system was well over sized electrically and mechanically and thus saved my ass. Lucky me. A lesson I will never forget.

* Baldor series "H" control.


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## varmit (Apr 19, 2009)

Yep, I should have written "motor load" not "motor HP" to be technically correct. Almost all VFD's are rated in current (amps), but most also have a HP (kw) rating.


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## dronai (Apr 11, 2011)

1) If a VFD is not working properly, do you just replace it, or can you replace components ?

2) For controlling the VFD, is the input digital, analog dc, and can it even be 120V ?


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## varmit (Apr 19, 2009)

dronai said:


> 1) If a VFD is not working properly, do you just replace it, or can you replace components ?
> 
> 2) For controlling the VFD, is the input digital, analog dc, and can it even be 120V ?


1) Most current VFDs, other than about 200 HP and larger, have no field replaceable parts. Even in the larger sizes, about the only replacement parts are the control boards. The higher end (more expensive) drives can sometimes be returned to the manufacturer for repair. The less expensive drives are throw aways when they fail.

2) It can be any of the above- depending on brand, model and options. The basic on-off type inputs are usually 24 VDC powered from the drives internal power supply. The speed reference can be as simple as a potentiometer. However, many drives can be directly connected to a PLC network via a data highway (ethernet, rs-232, fiber optic or any other data transmission protocol). Many times, other than the power wiring, there are no "hardwire" connections other than an e-stop.


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## JRaef (Mar 23, 2009)

dronai said:


> 1) If a VFD is not working properly, do you just replace it, or can you replace components ?


Depends on the size. Generally, anything under maybe 25-30HP is throw away if there is a power problem, the power components are all bundled into single units called IPMs now (Intelligent Power Modules), a potted block that ranges from the size of 5 credit cards stacked up, to a deck of cards for drives up to that range. It contains all six diodes, the CD bus, 6 or 7 transistors, the gate firing circuit and sensors, all in one. If one of those components goes bad, its not worth replacing on smallish drives because they are soldered directly onto the other PC boards (larger ones are bolt connected). But you might be able to still swap out control boards. Once the VFD size gets over 100HP or so, you go back to having discrete components that can be more easily repaired. But as prices drop, it gets less and less worth it.



> 2) For controlling the VFD, is the input digital, analog dc, and can it even be 120V ?


 Depends on the VFD. American made drives like A-B's larger units can be ordered with 120V control interface, but a lot of the European and cheap Asian drives cannot, they never use 120V control anywhere else in the world.


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## JRaef (Mar 23, 2009)

varmit said:


> 1) Most current VFDs, other than about 200 HP and larger, have no field replaceable parts. Even in the larger sizes, about the only replacement parts are the control boards. ...


Would you mind terribly if I come by and check your trash can now and then for "bad" 50-150HP drives? :whistling2:

I know what you mean though, it's getting less and less worth it all the time.


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## varmit (Apr 19, 2009)

JRaef said:


> Would you mind terribly if I come by and check your trash can now and then for "bad" 50-150HP drives? :whistling2:
> 
> I know what you mean though, it's getting less and less worth it all the time.


As, I am sure that you have noticed, most drive manufacturers no longer will sell replacement parts, even for the drives that are repairable. Some of the mfr. have a "replacement exchange program" that can save you some money on a replacement. The smaller drives would not be cost effective to repair anyway.

At one time, I was fast at repairing A-B drives (1336 and 1352 especially), now, out of necessity, I have become a parts changer. One more useless skill taking up brain space.


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## 76nemo (Aug 13, 2008)

varmit said:


> As, I am sure that you have noticed, most drive manufacturers no longer will sell replacement parts, even for the drives that are repairable. Some of the mfr. have a "replacement exchange program" that can save you some money on a replacement. The smaller drives would not be cost effective to repair anyway.
> 
> At one time, I was fast at repairing A-B drives (1336 and 1352 especially), now, out of necessity, I have become a parts changer. One more useless skill taking up brain space.


 
At one time the 1336 and 1352's were our highest numbers in VFD repair. Nothing against the design itself, just the fact there were thousands and thousands of them out there.

From where I stood, the 1336 had some great reviews.......


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## varmit (Apr 19, 2009)

76nemo said:


> At one time the 1336 and 1352's were our highest numbers in VFD repair. Nothing against the design itself, just the fact there were thousands and thousands of them out there.
> 
> From where I stood, the 1336 had some great reviews.......


I also always thought that the 1336 drives were great drives. They seem to survive well in some brutal applications.There are still a lot of these in use. Sometimes I still encounter 1305 drives in use. 

The A-B 1352 drives never impressed me very much. Most of the drive was manufactured by Stromberg, of Finland. A plant where I did a lot of work, had four 200 HP 1352 drives. These seemed to average at least two drives requiring a major repair each year. The parts, for these, were insanely expensive. Of course, in that time period ( early 90s), these drives were about the best there was in the higher HP ratings. As I recall, these were also made in medium voltage (4160) models also.


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## JRaef (Mar 23, 2009)

varmit said:


> I also always thought that the 1336 drives were great drives. They seem to survive well in some brutal applications.There are still a lot of these in use. Sometimes I still encounter 1305 drives in use.
> 
> The A-B 1352 drives never impressed me very much. Most of the drive was manufactured by Stromberg, of Finland. A plant where I did a lot of work, had four 200 HP 1352 drives. These seemed to average at least two drives requiring a major repair each year. The parts, for these, were insanely expensive. Of course, in that time period ( early 90s), these drives were about the best there was in the higher HP ratings. As I recall, these were also made in medium voltage (4160) models also.


Yes, the 1352s were made by Stromberg (or at least the guts were). I worked for ABB right after the merger of Asea (who had just bought Stromberg) and Brown Boveri. We made it known to everyone that the A-B 1352 Low Voltage drive was the same as (what became) the ABB SamiStar drive and the 1352-MV was the ABB MegaStar. It didn't play as well as we had hoped though. 

What I learned from that was that people don't buy on technology so much as they buy on comfort with doing business with someone they trust. Here we were selling the same drive, never got a single order for them yet the A-B guys were kicking butt and taking names. I even helped Boeing engineer a huge project for jet engine Power Take Off test stands where we used 6000HP MegaStar MV drives and motors to simulate the jet engines. But when it came time to cut a PO, guess who got the order?

I'm not bitter though, they sign my paycheck now...


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## varmit (Apr 19, 2009)

JRaef said:


> Yes, the 1352s were made by Stromberg (or at least the guts were). I worked for ABB right after the merger of Asea (who had just bought Stromberg) and Brown Boveri. We made it known to everyone that the A-B 1352 Low Voltage drive was the same as (what became) the ABB SamiStar drive and the 1352-MV was the ABB MegaStar. It didn't play as well as we had hoped though.
> 
> What I learned from that was that people don't buy on technology so much as they buy on comfort with doing business with someone they trust. Here we were selling the same drive, never got a single order for them yet the A-B guys were kicking butt and taking names. I even helped Boeing engineer a huge project for jet engine Power Take Off test stands where we used 6000HP MegaStar MV drives and motors to simulate the jet engines. But when it came time to cut a PO, guess who got the order?
> 
> I'm not bitter though, they sign my paycheck now...


Trying to understand people is one of life's impossible challenges. I suppose that the drive marketing was a similar situation to the perception that a Buick is a superior car to a Chevrolet, even though they are made on the same assembly line with the same engines.


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