VFD and vector duty vs inverter duty motors with or with encoders 3

[Vipul19]

I am designing a drive system that carries cement material through cutting heads on a tray and then returns for new material. I have established the gear ratio and motor size.

I need a 10HP motor that is capable of 155rpm to 1800rpm under constant torque. Based on this, I have been told that an inverter duty motor (15:1 CT) is sufficient. It makes sense to me, since I do not need to go any slower.

My Gear ratio is 49:1.

I am envisioning the following drive cycle:


1. We plan on signalling running the drive at around 50 - 60Hz for 2 – 2.5 sec. – This is the inital approach to the cutting heads
2. Then 5-10Hz for about 30sec – 40 secs – This is the cutting phase
3. Then 120 Hz for about 2-3secs. – This is the final departure from the cutting heads
4. Then 0Hz for about 5 secs – This is unloading the cut material
5. Then 120Hz for about 5 secs – This is the return of the cart for the next cycle
Then 0Hz for about 5-10secs – This is the wait for loading the new material.

My questions are:

1. What type of VFD to get
2. Do I need encoder? I don't want proximity sensors to determine each stage, but want to do it based on either time or position.

If I need an encoder, I am told that I need to go to a vector duty motor since the inverter duty does not have encoder option.

Thanks.

 

[ozmosis]

Some comments:
1) Regarding use or not of an encoder. It depends on the accuracy and repeatability of control. If you want position control then this depends on how accurate. Some VFD's provide reasonable accuracy open loop but the answer depends on your needs for accuracy.
2) Cement, cutting heads... and encoders often do not mix too well. If you do need encoders, make sure they are suitable for the environment.
3) What are you looking to stop the VFD at the end of each cycle? The encoder will show position but you may need some sort of switch to determine stop/start/reverse etc.
4) It may be me but I'm not sure what the difference is between a vector duty motor and an inverter duty motor. Most motors have an opton to mount an encoder.

 

[jraef]

As far as I know, the only difference in the motors is in the semantics, and even then only if the motor supplier offers a standard package with a built-in encoder. In other words if they offer both, they use the terms exclusively for one or the other. If they don't offer a built-in encoder product, they use either term to describe inverter duty motors. Neither of the terms is "officially" designated.

I agree with ozmosis, an encoder feedback should only be used when absolutely necessary, especially in that kind of application. Cement dust is highly corrosive (and abrasive of course), encoders that could survive for any reasonable amount of time there are going to be very expensive and I'm not sure from your description that you will need them.

Also as ozmosis mentioned, think about the braking aspects of this. You may need to use dynamic braking and you may even want to consider regenerative braking if you have a lot of inertia and high duty cycles to deal with. But that also depends on the gear box too; some are not made to handle negative torque.

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[DickDV]

If you buy a good quality sensorless vector drive, then you only will need an encoder if you need to hold speed very precisely at and right around zero speed or, you need to control speed accuracy while switching between no load and full load to within .2% or less. That would be less than 4 rpm with a typical motor. Less of a load switch will result in proportionally less error. And, of course, as has been mentioned, you would need an encoder if you are using it servo-style to cut to length or for positioning purposes.

Using overspeed like you are already planning to do is wise and very good utilization of the resources. Congrats on that!

In my experience, cutting concrete is not going to require the kind of speed accuracy that an encoder will give you so I would leave it off. As mentioned, they can be maintenance problems especially is the rough environment that you will have.

 

[Vipul19]

Thanks everyone for the input! I am learning a lot about the lingo as well as what will work best for my application.

I have asked to confirm that the gearbox quoted to me is capable of negative torque for braking and will pursue the non-encoder option. After reading the posts, it seems clear that it is not necessary.

The motor guy told me that they use the term vector duty when you need constant torque below around 3-4Hz, but I understand that there is not necessarily a clear definition. As was also mentioned, they do not have the encoder option with what they call "inverter-duty" motors.

I guess my next and final piece of the puzzle is the braking requirements and holding the cart in place while it loads/unloads.
I will see what I can find from the VFD suppliers, but will be back for more help as we progress.

Thanks again!

 

[Vipul19]

My gear supplier is not familiar with the term negative torque (should I be worried?). I assume that if the VFD is operating at 120Hz, and the motor is spinning at around 1760RPM and the gear is spinning at around 35 RPM with a torque of around 83 ft-lbs., the negative torque refers to the drive sending a signal to the motor to spin in reverse in order to slow down faster and overcome the inertia of the spinning motor faster as well.

The gear itself doesn't actually spin backwards though does it?

 

[Vipul19]

What happens at the 2 ends of each cycle? If the cart is moving fast with small torque (83 ft-lbs on max capcity gear of 1600 ft-lbs) and I have a sensor to indicate that it is near the end of travel. Then I can program to slow down and still keep the 83ft-lbs max. When it hits the end of the rail, if the drive is programed for the small torque, then does that mean that motor will be locked, pushing the cart against the rail stop? If the motor is only good for 15:1 ct, and I am looking at a torque of 1/15 to 1/20 of the rated capacity of the motor, will it still be ok at 0rpm?

Not sure if what I am asking makes sense, but I hope you get the idea.

 

[DickDV]

Negative torque occurs when the motor is trying to slow the load at a more rapid rate than the load naturally coasts down. Under those conditions, the load is pushing the motor thru the gearbox.

The mechanical people might recognize the term "back driven" You can also describe it to your gearbox agent as the load pushing the motor thru the gearbox. While the rotation is still forward, the torque is negative or in the reverse direction. Some gearboxes lock up under those conditions while others pass the torque thru smoothly.

 

[LionelHutz]

I doubt you need an encoder. You will likely have some other position sensors to know the ends of travel or even the aproach to the cutting heads. Just make sure you don't high speed the material into the cutting heads.

I would look for a VFD with the capability of installing a braking resistor.

Negative torque in this case basically means the output of the gearbox is trying to drive the input of the gear box.

There is no clear definition for the term inverter duty motor either. It's just a motor which the manufacturer has placed an "Inverter Duty" nameplate on.

 

[jraef]

An example of a gear box incapable of negative torque is a worm gear. If you apply force from the spinning load to the gear, it basically does not turn the worm, the force is absorbed in the power transmission components as deformation.

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