Current limiting with VFD

[DScott]

Hello:

I have an powerflex 400 22C-D142A103 in a compressor . It was driving a motor with FLA of 127. and NP hz of 89 I would like to use a motor with a FLA 159 NP hz 108, on the same drive. If I set A179 Current limit to 127 will the drive be able to handle the motor? Will the motor performance be similar to the smaller motor

Thanks:

Scott

 

[jtashaffer]

http://literature.rockwellautomation.com/idc/groups/literature/documents/td/22c-td001_-en-p.pdf

Page 8. Google is your friend

 

[DScott]

Thanks:

But I still have the question. If I set current limit to within the capacity of the drive, can I use the oversized motor?

 

[Bering C Sparky]

I am not an engineer so cant speak on the performance characteristics of doing this.
And dont have the time right now to look up code articles, but I would bet somewhere it is written that the drive must be sized to at least the motor FLA/HP/KW. And the feeder to the Drive must be 1.25% the input current rating of the power conversion device.

NEC is written so that everything in the circuit is sized accordingly and there is usually not alot of wiggle room to get around this code.

This would make since, what if someone comes behind you and recomissions the drive parameters using the motor nameplate values but does not consider the drive and possibly the feeder is undersized?
Silly but could happen.

Another factor is what is the existing feeder size going to the VFD and from the VFD to the motor?
If this is not rated for at least 1.25% of the 125hp motor FLA you are wanting to connect then you have your answer right off.

(Is your feeder 3/0 or bigger?)

If the feeder does not meet this speck then your answer would be NO according to 430 of NEC without even considering the VFD size is only 100hp.

If you are asking would this work if you had not a care in the world as to what the electrical code says, then the answer might be Maybe.

What part of Florida are you from?
I live about 1 hour north of Tampa.

BCS

 

[Lancie1]

I would bet somewhere it is written that the drive must be sized to at least the motor FLA/HP/KW.

It is okay to use a smaller drive with a larger motor, if you use the drive's electronic trip to limit the current to be no more than the drive's rating. The motor is okay as long as it has a nameplate rating in current, voltage, Hertz, and Horsepower to handle AT LEAST the largest current and voltage that the drive will allow to pass though its electronic output. In other words, it is a case of the drive being the weakest link, so the motor can be larger (but not smaller) than the drive rating.

This is a similar philosophy to many electrical code situations, where the item with the smallest current-handling capability limits the power that can be applied to any other item in the same circuit.\

If I set A179 Current limit to 127 will the drive be able to handle the motor?

Looking up your PowerFlex 400 model 22C-D142A103 in the PowerFlex 400 User Manual, Appendix 1, page A-2, this drive is rated for a maximum output of 100 Horsepower, and 142 Amps at 45 C, 128 Amps at 50 C, with a voltage input range of 340 to 528 volts AC and a current input of up to 150 Amps.

The important question is whether or not the motor will get enough power to run the compressor at all stages.

Depending on your average ambient temperature, you could set the A179 current output limit as high as 142 Amps, but certainly as much as 128 Amps. If you also change your PowerFlex drive Maximum Hertz setting from 89 up to 108 (depending on maximum speed allowed by compressor), that should allow your motor to run with enough power and speed to drive the compressor. You might have to set the maximum HZ setting lower than 108 to prevent the drive from faulting on current limit (or bumping against the current limit).

If your branch circuit protection is an existing 200 Amp circuit breaker, and your drive current limit is set for no more than 142 Amps, then you should be okay with code requirements.

 

[Bering C Sparky]

so the motor can be larger (but not smaller) than the drive rating.

I would have to take your word on that one.

The NEC does address smaller motors on larger drives. There is plenty written in the NEC on this.

We use smaller Motors on larger drives all the time, mainly when you have 2 or more motors connected to a single drive but it is perfectly fine to put say a 5 hp motor on 10 hp drive. No problem at all.
NEC simply states that the feeder conductors to the Drive must be rated 1.25% the drives input current capacity.

I have never seen anything in the code that says you can hook a motor up to a feeder and drive that is rated smaller than the Motor FLA just because you prommise to set a parameter in the drive to limit the current so the feeder will not burn.

There are a few exceptions in the code where feeders can be smaller than FLA rating of a motor but they have to do more with duty cycle and not the fact that a under-sized drive has been put in the circuit.

I am not saying this would not work, only that the NEC does not execept this as a correct installation.

If anyone knows of a code artical that says otherwise I would be interested in reading it.
I have not been keeping up with alot of the code change the past few years so who knows it might exist but I would not put big money on that bet.

BCS

 

[Lancie1]

I have never seen anything in the code that says you can hook a motor up to a feeder and drive that is rated smaller than the Motor FLA just because you prommise to set a parameter in the drive to limit the current so the feeder will not burn.

You don't have to make any promises. The drive serves like a fuse. VFDs are designed so that the Current Limit parameter can never be set higher than the drive maximum output current rating applied over some time period. The drive can only produce so many Amps, no matter if the motor is rated for more than that.

If the drive reaches its limit, it will either go into fault mode and shut off, or as many drives are programmed to do, simply hold the motor speed (output Hertz) at the point where the current flowing to the motor is equal to the drive's maximum rating. In either case, the motor current rating can never be exceeded, so all is well.

The more serious problem can come if the motor is smaller than the drive rating. In that case the drive current limit MUST be carefully set for a much lower number than its Maximum, or the motor will be burned up.

The NEC is lacking in its treatment of VFDS. The closest thing for what you are talking about is probably NEC Article 480.83(A)(1). That says that motor "controllers, other than inverse time circuit breakers and molded case switches, shall have horsepower ratings at the application voltage not lower than the horsepower rating of the motor."

The VFD running at its Current Limit Setting, will always have a horsepower rating, at the variable voltage and speed being applied, that is not lower than the horsepower rating of a larger motor AT THAT VOLTAGE AND SPEED.

The rating of the motor feeder cables may be too small (according to the NEC) for the larger motor.

 

[Bering C Sparky]

The rating of the motor feeder cables may be too small (according to the NEC) for the larger motor.

This above is my entire point and will be the straw that broke the camels back for pulling this off.

If the feeder is sized appropriately then all is well, have at it with any size drive you want to install, as long as the over current and overload protection are not larger than allowed all will be well.

But if the conductors are not rated for 1.25 FLA of the motor being connected then you have an illegal installation.

Installing the smaller drive if the conductors are sized right would be no different than installing a smaller overload module on a starter.
(Execpt the drive may limit the current and not just trip out like the OL would.

What you are saying make since and is sound as long as the feeder is rated large enough for the motor.

But if the feeder is not large enough then all bets are off.

This is kind of why I asked where the OP was from.
I can just see him trying to tell an inspector from Dade or Broward County "Well I put a Smaller Overload in the Circuit so its ok that the feeder is not rated for the Motor I connected, Right".
That would not go over very well.

BCS

 

[Bit_Bucket_07]

Hello:

I have an powerflex 400 22C-D142A103 in a compressor . It was driving a motor with FLA of 127. and NP hz of 89 I would like to use a motor with a FLA 159 NP hz 108, on the same drive. If I set A179 Current limit to 127 will the drive be able to handle the motor? Will the motor performance be similar to the smaller motor

Thanks:

Scott

Your drive is rated for 142 Amps. However, as previously mentioned, code requires adequate wiring for your motor circuit. Your motors are either 50 Hz or 60 Hz motors. I think that you are referring to a maximum Hz rating when you say that they are rated 89 and 108. The motor RPM, SF and LRC ratings are more significant, if you intend to do this.

So long as the branch circuit feeding the drive is adequate, the motors are of similar design, and you don't require full torque nor HP from the replacement motor, it would probably function adequately as an emergency replacement. Why do you want to do this?

 

[DScott]

Hello All;

Bering C, well we definitely travel some of the same roads. I used to live in NPR and Ocala, Now I live up near the state line.

The machine feeder is 400 Amps. This drive is fused for 200

The good news is my problem is solved. As of last night the lead time on the correct motor was 12+ weeks. As of this afternoon they had found one in production on an unsold machine. They are gonna have it pulled and to me on Monday, so it will be running with the correct motor by Tuesday morning.

Another piece of good news is I found RTO powerflex 400 at the shop. I should have some down time Friday to experiment with it

Thanks all. Your input has been great and helpful

 

[jraef]

So just for future reference, let's talk about current limiting with VFDs. When you set an artificial current limit on a VFD, it cannot just cease to allow the motor to draw all the current it wants, it has to "make" the motor want the current you are saying it can have. The only tool in the tool bag for the VFD is frequency. So what "current limit" really is with a VFD is a "current based override of your frequency command", or maybe a better term is "current prioritizing". So what happens is that if you tell the drive to run the motor at 89Hz but limit the current to 127A, when the current reaches 127A but the frequency has only accelerated that load to 63Hz at that point, it ceases accelerating the load any further and stays at that speed until you either lessen the load or lower the speed command. So basically if you had used the larger motor, you would have faced that reality.

 

[Bering C Sparky]

DScott- We have traveled same roads for sure, I live in Crystal River.

Also I did not realize that you were looking to do this installation as a temp fix just to get you by until the correct motor arrives.

My thinking was you want to leave this as a permanent installation.
So my bad on giving you such a hard time on conductor size.
Looks like you had that well covered though anyway with the 400 amp feeder.

Glad to here you have your correct motor on the way.

Have a good one and enjoy that Fla Sunshine, it will be some time before I see it again and where I am at right now looks nothing like Fla and there is little sunshine.

BCS

 

[DScott]

So just for future reference, let's talk about current limiting with VFDs. When you set an artificial current limit on a VFD, it cannot just cease to allow the motor to draw all the current it wants, it has to "make" the motor want the current you are saying it can have. The only tool in the tool bag for the VFD is frequency. So what "current limit" really is with a VFD is a "current based override of your frequency command", or maybe a better term is "current prioritizing". So what happens is that if you tell the drive to run the motor at 89Hz but limit the current to 127A, when the current reaches 127A but the frequency has only accelerated that load to 63Hz at that point, it ceases accelerating the load any further and stays at that speed until you either lessen the load or lower the speed command. So basically if you had used the larger motor, you would have faced that reality.

That was the next question. What happens when you limit current below motor NP. . This motor/drive was the second stage of a 2 stage machine. When we were hanger flying on this around the shop we thought the speed or torque would be limited too much and thought it would really cause problems. I think ultimately the drive current limit would have ended up as a smaller problem,

I do appreciate all the info and help. I'm trying to learn these as a tech with limited exposure to drives.
Thanks:

Scott

 

[Lancie1]

When we were hanger flying on this around the shop we thought the speed or torque would be limited too much and thought it would really cause problems.

The thing that might have saved you was that you knew the 2nd Stage Compressor could run on less horsepower than the larger motor. In other words, the compressor load was still the same old 100 HP (or whatever) as before. So the proposed larger motor only had to produce that much HP, not its rated HP. It is possible that it could have done that using slightly more than the original 127 Amps, out of the possible 142 Amps from the drive.

 

[Sherlock4me]

Lots of good reading here. I had a little time to kill and refreshing my way through NEC Article 430 seemed like a good way to do so, since Bearing C said he was too busy to dig 'em up at the time.. (Article 430- Motors, Motor Circuits, and controllers)

As to the 125% of the motor full load current for the minimum rating of motor circuit conductors.

"430.22 Single Motor. Conductors that supply a single motor used in a continuos duty application shall," (shall is THE powerful word in the NEC as per Article 90.5(A) and means mandatory) "have an ampacity of not less than 125 percent of the motor full-load current rating, as determined by 430.6(A)(1), or not less than specified in 430.22(A) through (G)" which addresses specialized motors.

Where do we get the current from for the above calculations. Nameplate or NEC charts?

For general motor applications, 430.6(A)(1) tells us to go to tables 430.248 for single phase and 430.250 for three phase motors, and find our full load current depending on our HP and system voltage. The full load current as determined by these tables "shall be used when determining the ampacity of conductors, the size and ratings of switches, branch circuit short circuit and ground fault protection," (most often breakers or fuses) as opposed to the actual current rating marked on the motor nameplate. The maximum overload device setting however, IS determined by the nameplate current rating as per 430.32(A)(1) which uses the service factor or temperature rise of the motor to determine this max setting percentage.

BUT when dealing with VFD's 430.6(c) says
• "If a variable-frequency drive controls the motor, use the maximum operating current that’s marked on the nameplate (motor or control). If that value isn’t on the nameplate, use 150% of the values found in the NEC tables." (430.248 and 430.250)

As to controllers: Lanci1 addressed 430.83(A)(1) and "at the application voltage" appear to be the magic words regarding the resulting HP...

And I agree that the code at present addresses older methods of controlling motors better than methods more prevalently used today. I suspect changes will come in Article 430 to address that sooner or later.

On that last note, I was quoting from a 2011 NEC. Depending on your local, different publication years may apply. (Va. tested us on 011 this year for license renewal and Colorado on 014 while the 08 was still the "law" on most military camps in Afghanistan when I left in 2012. (BS 7671) not withstanding.

http://www.jade1.com/jadecc/nec_code_adoption.php lists NEC code adoptions by publication and state.

"590.2(A) Except as specifically modified in this article, all other requirements of this code for permanent wiring shall apply to temporary wiring installations."

I saw no motor conductor allowances in the 590 Temporary Installation. (But I would not readily admit to all the temps I've done over the years to keep water flowing to homes and businesses, and to keep sewage out of the rivers.) All the above was for my own refreshing, just sharing it in case any one is interested; as I am a humble electrician.

Regards