VFD Application

randy

Supporting Member
Join Date
Apr 2002
Posts
158
We just install a PowerFlex VFD for a conveyor application. The motor is coupled to a gearbox with a ratio 7:1 and the motor is a 3/4 hp. The speed range which we have sit is from 30Hz to 60Hz.

My question is with the range set at 30Hz, half the speed, will that affect to motor performamce? Is there VFD rated motor for this application?

Thanks

rmonroe
 
I am not an expert, but experienced

As long as the motor is able to keep itself cool, it should be okay.

I usually don't run into problems until I try to run a motor at 10Hz or less for extended periods. At extremely low speeds running in V/Hz mode, a standard induction motor may try to stall and can overheat from the reduced airflow.

Keep an eye on the drive current display and make sure it stays below the motor rated FLA, make sure the motor stays adequately cooled, and you will probably be just fine.
 
At half speed and full torque, both an Open Drip Protected (ODP) motor and a Totally Enclosed Fan Cooled (TEFC) motor will be okay.

Next time when you do a job like this, pick a higher ratio gearbox (in this case around 10/1 and run the motor from 84Hz down to 42Hz. The speed regulation will be better, the motor will run cooler, and the starting torque will be higher.
 
DickDV said:
At half speed and full torque, both an Open Drip Protected (ODP) motor and a Totally Enclosed Fan Cooled (TEFC) motor will be okay.

Next time when you do a job like this, pick a higher ratio gearbox (in this case around 10/1 and run the motor from 84Hz down to 42Hz. The speed regulation will be better, the motor will run cooler, and the starting torque will be higher.
I 2nd the above.
We have had issues when we dropped to 15Hz range with the motor overheating and going into an OL Fault on the PowerFlex drive.
I do agree with DickDV, that's the way to go if you know ahead of time the application. Gear reducers aren't that expensive if you bump the ratio up a bit. Peace of mind anyways.
 
DickDV said:
At half speed and full torque, both an Open Drip Protected (ODP) motor and a Totally Enclosed Fan Cooled (TEFC) motor will be okay.

Next time when you do a job like this, pick a higher ratio gearbox (in this case around 10/1 and run the motor from 84Hz down to 42Hz. The speed regulation will be better, the motor will run cooler, and the starting torque will be higher.

Dick - can you clarify a bit on "the starting torque will be higher" please? I have not succeeded in getting my mind to automatically deal with the concept of torque (it gets twisted up and I gotta think to unravel it) So that is why I ask

The torque demand of the conveyer will be constant (not really true - depends on what is on conveyer and how much but let us assume)
and assuming same motor different geabox
By increasing the gearbox reduction
The motor torque demand will be reduced
-- so how can starting torque increase??

Would a more accurate statement be "the new starting torque demanded by load (at the motor shaft) will be a smaller fraction of available motor starting torque?

I like the idea of increasing the motor speed range to 40 to 80Hz by going to the greater reduction gearbox. Makes total sense to me - did not have to do much mind unraveling.

Dan Bentler
 
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Please bear in mind that going beyong 60Hz also has its own problems... problems in motor and possibly problems in Gear Reducer.

Problems in motor includes. Lesser Motor output torque and wear and higher wear and tear for the motor. bearing and such...

Problems in gear reducer is watch out for the rated speed...

we have seen our clients go beyond 60Hz but VERY SELDOM go beyond 70Hz but then again we have a client that went to 90Hz (Of course, we tell them that it is not recommended because their motor is only rated at 60Hz) In Most Applications the Drives can take it but its mostly the mechanical that will pose problems...
 
I'm sorry danieluy but you are almost entirely mistaken. As to the motor, with the exception of two pole (3600rpm) motors, going to 90hz is not going to be any bearing issue. The same bearings are used in two pole motors as are used in four and six pole models.

The torque from the motor does drop off but never more than the increase in gear ratio so the load always sees at least the same torque.

It is true that some gearboxes are not rated over 1750rpm but that is the exception.

As to available starting torque, leitmotif, because of the higher gear ratio, the available torque from the motor/gear system is increased for the same input amps to the motor. You are correct that the load may not need the extra but it is available if needed.
 
DickDV said:
As to available starting torque, leitmotif, because of the higher gear ratio, the available torque from the motor/gear system is increased for the same input amps to the motor. You are correct that the load may not need the extra but it is available if needed.


The starting torque of a motor is determined by the torque rating of the motor and the torque setting percentage in the VFD. Changing the ratio doesn't increase motor torque, it changes the torque of the output shaft of the gearbox. There are several ratings that you can find when you look at the motor specs. It doesn't matter what kind of drive you have hooked up to the motor because when you run through a VFD you loose the Stall torque (starting torque). I have seen some apps that require a large amount of starting torque but need the control of a VFD. We have several motors that start at full line voltage and switch to a VFD once it is running.

As far as motor selection goes, if the motor will run on a VFD, the it should be rated "Inverter Duty".
 
Kev77 said:
We have several motors that start at full line voltage and switch to a VFD once it is running. QUOTE]

Kev tell me more about this ie what the motors drive. From what I have seen so far starting a motor on VFD is always preferable because (assuming you ramp speed up) it is easier on the motor and drive components.
ASSUMING motor and VFD combination can meet the breakout torque of the load.

Dan Bentler
 
Kev77 said:
It doesn't matter what kind of drive you have hooked up to the motor because when you run through a VFD you loose the Stall torque (starting torque).

This has not been my experence. Most drives will give you 150% of full load amps for 30 seconds when sized 1 to 1 (1hp to 1hp , 50 hp to 50hp, ect) If you over size by 2 (1hp motor to 2 hp drive, 50hp motor to 100hp drive ) then you can get the 225% torque that you would get from an across the line start. And you can make it gentile, and avoid the 600% inrush. In fact I have seen just the opposite. I was called out to a remote pump site (in the boonies) at the end of a power line. The pump owner had just installed a larger pump, and could not get it to start. After looking over the situation I discovered that the power drooped so low during starting that the pump could not come up to speed. Forunately I had a VFD of the proper size, and tried it out. The motor started and ran perfectly. The last I heard it's still running that way (about 6 years).

Kev77 said:
I have seen some apps that require a large amount of starting torque but need the control of a VFD. We have several motors that start at full line voltage and switch to a VFD once it is running.

This sounds expensive and complicated.

Kev77 said:
As far as motor selection goes, if the motor will run on a VFD, the it should be rated "Inverter Duty".
I would agree with this if the application excedes any of the following items.
1. More than 70 to 1 speed range
2. "T" leads longer than 70 to 300 feet (shorter on small motors longer on larger motors)
3. I sold inverter duty motors.
 
DickDV said:
As to the motor, with the exception of two pole (3600rpm) motors, going to 90hz is not going to be any bearing issue. The same bearings are used in two pole motors as are used in four and six pole models.

ah yes, then good for you because in our country there are many underrated motors :( that's why i recommend that the motors should be checked.

Starting Torque: Not all drives are created equal. Some have 150% and some have 200% and i have also seen only 125% torque. So you need to check your suppliers.

higher end drives can can be configurable. as to boost the starting torque. some have "Power curves"... some can help u vary your V/F curve. refer to your manuals regarding this.

Yes, upgrading to higher drives mostly solves the torque problems. it is what we recommend also when all else fails.
 
We have a bin that is about 30'H X 30' L X 15' D and it is full of saw dust. There is a 2'dia X 15' long auger that traverses across the 30' length. The motor is connected to a gearbox of some ratio (don't remember). Several years ago they went from a DC motor and drive to a AC Motor and ABB VFD. Same HP (50hp) from what I am told. The bin never ran the same. If the bin was over 10' full the motor would not run. They tried changing pulleys and other cheap stuff with no luck. When I started working on it, the drive would keep kicking out and we tried everything to get it to run (ramp rates,torque settings, ect). I called Baldor and was going to get a price for a 75HP motor setup and the tech on the phone is the one who told me to remove the VFD because it limits the starting torque. With nothing to lose I changed the VFD for a starter and now it has no problems. Didn't make sense at the time but it has been running for several months with no flaws. Baldor could have sold me $13,000 but didn't. Now the bin runs at about a 28' level.


Just remembered, the motor is 50HP and the VFD is a 65HP ABB
 
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Fellas! Please look at a motor torque-speed characteristic curve. In North America that would usually be a NEMA Design B curve. Note that the starting (locked rotor) torque on these typical motors is about 160% of full load. That's the starting torque that you will get with a magnetic starter. Note also that the current required to get that torque is between six and eight times FLA.

Now note on the same torque-speed curve the breakdown torque which is the peak torque the motor can deliver. It occurs just as the motor is overloaded to the point that it becomes magnetically dis-synchronous. Note that the torque at that point is 220% and the current required to get this torque is around 240%.

The beauty of a properly engineered drive/motor system is that you can start right at that peak current instead of at the (usually) lower locked rotor current. In the case of the NEMA Design B motor, that's an increase of 60%. And you can do this with a drop of current from 600% to only 240% of nameplate.

Now, the key to this is the "properly engineered" part. A drive with the same hp rating as the motor will not do this, not the heavy-duty version nor the normal duty version. You have to size the drive to provide the required AMPS, not HP. When this is done, you can actually start with 60% more starting torque than across-the-line.

The examples cited above where the VFD-driven motor didn't have the torque to start the load are simply examples of improperly designed systems. I'm not trying to trash anybody's work here but simply trying to give you the facts so this kind of trouble can be avoided in the future.

As to the comment about all motors on VFD's being "inverter duty", the problem is that the term "inverter duty" means different things to each different motor manufacturer. What is clear to me is that all motors on VFD's running 400VAC and higher need to be insulation class F minimum. Yes, that's all motors. Further, while many are not and get away with it, all motors on inverters running 400VAC and higher should also be MG1 Part 31 endorsed or equivalent. Yes, that's all motors too. Beyond this, the application has a large impact on motor selection. If you are dealing with a centrifugal pump or fan with a speed range of 4/1, an Open Drip or TEFC motor is plenty good enough as long as it is Insulation Class F and MG1 Part 31 rated. On the other hand, if you are dealing with a constant torque load like a conveyor, auger, or many industrial machines where the full torque speed range takes the motor below 20hz or so, you need to deal with the motor thermal issues at slow speeds and heavy loads. You may need to chose a TENV motor if under 30hp or an auxiliary cooled motor if larger than that.

Finally, leitmotif, did I make myself clear about the available starting torque when the gear ratio is increased? If not, let me know and I'll try again using some specific numbers.
 

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