VFD Application

[leitmotif]

Fellas!
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.

No Dick I am still a little fogged. maybe terminology
Let me provide numbers

Conveyer starting (breakout) torque 500 ft lb
Conveyer running torque 200 ft lb
Initial gear reduction 10:1

Motor starting torque demand 50 ft lb
Motor running torque demand 20 ft lb

Motor size HP = speed x torque divided by 5252
= 1800 x 20 x 12 divided by 5252
= 20 ruffly

NEW GEARBOX reduction 100:1
Motor starting torque demand 5 ft lb
Motor running torque demand 2 ft lb


Conveyer is a constant torque load (ASSUME constant loading)
If I put in 100:1 gearbox and keep 20 HP motor
I have a 10:1 reserve capacity for conveyer overloads and starting. With a VFD I will have plenty of motor torque to start and will still have proper size motor that when ramped up to regain conveyer speed should still have enough torque and HP to drive conveyer

So this is somewhat same for sawdust bunker. In his case keep the same screw and drive motor, new VFD with better starting characteristics, deeper reduction gearbox, drive motor above baseline to regain product output.

I dont get to setup systems from drawing board so I gotta think about things.
Do I have it?

Dan Bentler

 

[Kev77]

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.

I don't know about you, but there are not very many times that I walk in an find a "properly designed system". I guess if it was designed correctly I would not need to do anything to it. As I stated above the motor was 50 HP and the VFD was 65HP. None of which I installed. If you look at some of the AB drive like the Powerflex 70 series there is a standard HP rating and a heavy duty rating. If you are installing a 10 motor, they AB recommends a 15HP drive. I was looking at a 75HP drive to help correct the problem but removing the drive did the same thing and saved $7500.00.

 

[DickDV]

OK, let's look at leitmotif's conveyor using the data he gave us. With a 10/1 gearbox, the breakaway starting torque on the gearbox input shaft is 50ft-lbs and the continuous running torque is 20ft-lbs. We are going to assume that full speed on the conveyor occurs at 60hz on the motor.

Since a four pole NEMA B motor produces about 3 ft-lbs torque per hp, it looks like a 7.5hp motor should cover the continuous torque at 22.5ft-lbs. Checking the locked rotor torque at 160%, it calculates to 360ft-lbs which is not enough to start the load across the line. A 10hp motor provides 30ft-lbs continuous x 1.6 = 480ft-lbs which isn't enough either. For across-the-line starting, a 15hp motor would be needed simply to start the load.

Now, let's do this with an inverter. The motor is still a 22.5ft-lb motor continuous but we can start at peak torque instead of locked rotor torque. That would be 22.5 x 2.2 = 495ft-lbs. Not quite enough so a 10hp motor would be required.

Let's go back to the 7.5hp motor but now change the gearbox ratio to 12/1. The motor would now have to go to 72hz to get the conveyor to full speed. Now, the continuous torque on the output of the gearbox is 22.5ft-lbs x 12 = 300ft-lbs at and below 60hz which is more than enough. The torque fall off above 60hz at the same ratio as the overspeed so at 72hz, the torque would be 300 x 60/72 = 250ft-lbs, still plenty to run the conveyor.

Across-the-line starting is not possible now due to the overspeed so, with an inverter, we can start at peak torque. At the gearbox output, that would be 22.5 x 2.2 x 12 = 660ft-lbs, more than enough to start the conveyor. So, the 7.5hp motor is good to go.

We have to be careful to size the drive to provide the peak current for at least one minute. A NEMA B 7.5hp motor at 460V will be nameplated around 10amps FLA. The peak current will be 10 x 2.4 = 24amps for one minute. That would be a 16amp continuous drive with a 150% overload rating which is probably a 10hp drive.

So, we have done with a 7.5hp motor/10hp drive/12to1 gear what would have required a 15hp motor across the line or on a drive limited to 60hz.

Further, let's just say that leitmotif's conveyor must operate over a 5 to 1 speed range. With a 10/1 gearbox and 60hz max, that would be 12 to 60hz on the motor---too much reduction for a TEFC motor. On the other hand, with a 12/1 gearbox and 72hz max, that would be 15hz to 72hz on the motor which is right at the usual rating for TEFC motors.

Bottom line, using overspeed got you a smaller, cheaper motor, a 10hp instead of of a 15hp drive, and a TEFC motor instead of an inverter-duty fan-cooled motor.

A bargain, I would say!

 

[Kev77]

What am I missing here? In the setup I had 50hp motor and 65hp VFD, why could we not get the motor running? The day I pulled the drive and installed the across the line starter I put a fluke power monitor on it to monitor and the amps. Once the motor was going it didn't even draw 40 amps and rated amps was over 60. I have heard all the numbers before and that is why I looked at a bigger setup. I was told that even if the drive has the available current it is not applied the same way and you will trip on over current before you could apply the same amount of torque as what is supplied with an across the line starter. It might have worked if I had a 100hp VFD.

 

[DickDV]

Kev77, I don't have enough details of your application to know exactly what went wrong but here is what we can know. Assuming that 50hp motor was a NEMA B motor and had a base speed of 1800rpm, then its continuous available running torque is 150ft-lbs and its starting torque across-the-line is 160% of that or 240ft-lbs. We also know that that level of torque reliably started your machine.

Using that 240ft-lb number and dividing it by 220% gives us 109ft-lbs which would have to be the motor's continuous torque rating to get 240ft-lbs peak. A 40hp motor will give us 120ft-lbs continuous so that would be enough motor. A 40hp motor will have an FLA around 48amps so 2.4 times that would be 115amps. You would need to chose a drive with a one-minute rating of at least 115amps. That would likely be a 60 or maybe a 75hp drive. All of this is assuming that the existing power train to your machine provides full machine speed at 60hz on the motor.

Now, let's change the power train so the motor needs 90hz to get full speed on the machine or, if that is too difficult, simply choose a six pole 1200rpm motor instead of the four pole 1800rpm version.

Based on your data above about running amps, 40amps continuous on a 50hp motor is about 60% load or 150 x .6 = 90ft-lbs. That would be a 30hp four-pole motor fully loaded. That motor will give us 90 x 2.2 = 198 peak ft-lbs torque but when passed thru the higher power train ratio will calculate to 198 x 1.5 = 297ft-lb at the load. That far exceeds the 240ft-lbs we know will start the load so this job can be done with a 30hp motor. The drive will have to be sized to produce short-term peak amps matching the motor's peak torque requirements and that will probably be a 50hp drive.

If I was doing the application analysis on this machine, that's how I would do it. And, yes, I would expect it to work just fine.