VFD and vector duty vs inverter duty motors with or with encoders
VFD and vector duty vs inverter duty motors with or with encoders
(OP)
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.
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.





RE: VFD and vector duty vs inverter duty motors with or with encoders
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.
RE: VFD and vector duty vs inverter duty motors with or with encoders
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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RE: VFD and vector duty vs inverter duty motors with or with encoders
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.
RE: VFD and vector duty vs inverter duty motors with or with encoders
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!
RE: VFD and vector duty vs inverter duty motors with or with encoders
The gear itself doesn't actually spin backwards though does it?
RE: VFD and vector duty vs inverter duty motors with or with encoders
Not sure if what I am asking makes sense, but I hope you get the idea.
RE: VFD and vector duty vs inverter duty motors with or with encoders
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.
RE: VFD and vector duty vs inverter duty motors with or with encoders
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.
RE: VFD and vector duty vs inverter duty motors with or with encoders
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies