Specifying torque rating of a VFD

[rockman7892]

I know that when you spec a motor its output torque curve has to be greater than the torque curve of the driven load. The best way to do this is the look at both the speed vs. torque curves for the motor and load, and make sure that the motor curve is above that of the load for all speeds.

When specifying the torque rating of a VFD what steps should be taken to verify that the torque output of the VFD will be able to drive both the motor and load? Is there a torque curve for VFD's that can be compared to the motor and load curves as mentioned above?

The reason I ask, is because someone told me that a particular VFD was oversized to drive a particular motor and load. I am now looking to replace this VFD with another model and wanted to see how to correctly size it.

 

[dpc]

VFDs produce current, motors produce torque from that current.

Motor driven loads roughly fall into a couple of broad categories - variable torque such as fans and pumps, then "constant torque" such as conveyors.

Variable torque loads are generally not a problem. Constant torque loads require more consideration. The drive must be able to provide the motor's peak current requirements over the required time period.

Forget the VFD's "horsepower" rating and focus on the continuous and overload current capabilities.

 

[DickDV]

dpc has given you most excellent advice. Drive "variable torque" and "constant torque" ratings have nothing to do with torque. They only tell you the short term (1 minute) overload rating: VT is 110% and CT is 150%.

Look at your load torque requirements, translate that into continuous current short term current requirements and then size your drive to meet or exceed those amp requirements.

As dpc has said, forget hp and size by continuous and short term overload amps.

 

[rockman7892]

Does the VFD usually specify the continuous and short term overload amps for specifying purposes?

 

[ScumPunk]

Yes,
Comparing different manufacturers, identical hp/kW ratings doesn't mean identical current ratings. In my experience, however, all manufacturers give continuous and overload current rating. This may be be given for VT and CT also. Allow me to give an example:
VFD rated as 11kW (CT) / 15kW (VT)
Inom (plate): 26A (CT); 32A (VT) - %ages given below are based on these numbers
Continuous: 110% (CT); 100% (VT) - these are averages
Overload: 200% for 3secs + 150% for 57secs per 5 mins (CT); 150% for 3secs + 110% for 57secs per 5 mins (VT)
Max. Ambient: 50degC (CT); 40degC (VT)

Hope this helps,
Cheers,
Mort

 

[dpc]

Yes, but they don't all provide exactly the same data for the overload condition. That would be too easy. Some give it at 125%, some at 150%, etc.

 

[rockman7892]

The application I am looking at is a roller mill. Would a roller mill be considered a constant tourque or variable tourque motor/application.

Do motors say weather or not they are rated for constant or variable toruqe.

I'm assuming the continuous ratings of a VFD are how much current they can output continuously for an infinie amount of time and the overload values are only the current values that apply for the specified amount of time. So for something like a conveyor that is a continous load should I be more concerned with the continuous rating of the vfd?

For a roller mill what is the best way to figure out what type of overload time I need? Should this be based on how the motor is operated and its starting characteristics?

 

[dpc]

rockman,

I'm not sure I'm seeing much effort on your part to try to gain knowledge on all these topics other than by firing off question after question to this forum.

Any basic text on motors would cover their speed vs torque characteristics.

For the roller mill, you should check with the supplier and ask for a speed vs torque curve.

 

[ScumPunk]

dpc,
You said:
"they don't all provide exactly the same data for the overload condition. That would be too easy. Some give it at 125%, some at 150%, etc. "
Check, point taken.
I'm not sure if this is true for other drives, but the example I gave is capable of an average of 110%. The example times for 150% and 200% are just that, the I^2t counter will respond proportionally to any overload %. In reality you can get more than the specified time at 200% for example, the figures given are a minimum. I've seen a few competitors products which seem to work the same. Some seem to go by the IGBT temperature, meaning that if you use extra cooling, you can get more overload. This doesn't protect the DC caps from excess ripple, however, which is one of the protections given by the I^2t software.
Cheers,
Mort.
(Probably off topic, but it's interesting)

 

[dpc]

ScumPunk,

I'm not sure if there is some standard rating method for the overload capability, but there is a pretty wide variation in the capabilities admitted to by the various vendors, even for the same size and type of drive.

You're right, I'm sure a lot of factors go into to determining the ratings, including variables like ambient temperature.

When comparing drive specs, it's hard to know how much the ratings can be extrapolated and what type of de-rating is required as the current is increased.

 

[ScumPunk]

Hi, dpc,
We spend a great deal of time during development working out / confirming by test various deratings for different situations. The problem is that all of this information can't be included in the basic documentation, but it is available via the support website and/or Customer Support lines.
Our quoted specs are given at absolute max. "everything" - max. ambient still air, max. cable length (screened), max. supply voltage, I/O load, etc. This means that in the real world, it would be possible to exceed these ratings by some margin. We "suspect" that some manufacturers do not follow suit, and this gives rise to the situation you describe. A customer thinks they are comparing apples with apples, when in reality they are looking at aubergines and mangoes
Cheers,
Mort

 

[rockman7892]

Dpc

I apoligize if I come across as just firing off questions looking for quick answers, I've got alot on my plate lately. I'm a fairly new engineer a couple of years removed from school with not much field experience. I'm at a plant which when I was hired had a senior electrical engineer whom I reported to and recieved guidance from. When he left a year ago they did not replace him, mostly because I have been working really hard and have been able to fill his shoes for the most part.

However like I said with not much experience there are situations that come up that I'm not familiar with the people look to me for answers for. I'm also doubling roles as an on staff electrical engineer for day to day operations as well as the electrical project engineer for a new plant we are building On-Site, so as you can see I have been throw to the wolves and am seeing alot of new situations, which are sometimes difficult but I enjoy learning from them. Alot of times I need to provide answers for certain situations, and to get me started I have been very thankful for the support that this formn has given. I can assure you that I do my best to research all of the topics, but sometimes I need help and dont have a lot of time to get answers.

This is not meant as a guilt trip of any sort, just wanted to fill you in on where I am coming from and explain the number of posts from me as well as Thank everyone for the information and support they have given.

 

[dpc]

rockman,

I'm sure we're all busy and overloaded with work. But why should other engineers on this forum provide free engineering support to your company?

We are all this forum to help each other with specific issues, locate specific information and share experiences.

We all get stuck on certain problems and questions from time to time and can use help from people with more experience in a particular area.

But I don't expect anyone here to do my Google searches for me, or to take the place of the Standard Handbook of Electrical Engineering.

I'm sure you'll continue to get answers to your questions here. I'm just suggesting you spend a little more time doing your own research before posting questions.

 

[edison123]

I agree with dpc that OP raising a lot of questions and shoot off further questions from the free answers given here.

And for all the answers he has received, he is yet to thank even a single poster for their time and knowledge, which are given free.

rockman, you should take some time out to thank the people who help you here for free, by pressing the left blue link at the end of their posts and give them a star and mojo.