motor load control

[rmarotta]

Gentlemen,
I have a vfd that I'd like to use on a machine that requires a certain range of power to move material.
Hopefully, this vfd will replace several components. (A reversing contactor, soft starter, and a non-functioning load detector.)
Variable speed is not a primary requirement, but can I get a control signal from the drive that will allow me to externally adjust for load on the motor within certain limits, ie. 20-40% of nameplate KW. (2HP)
As I understand it, sensing power used by the motor should reflect the true load, regardless of varying levels of voltage or amps.
Any simple ideas for how I can get contact closures at the minumum and maximum power settings would be appreciated.
Regards,
Ralph Marotta

 

[CJCPE]

Many VFDs provide contact closures for external use. Set-up configuration settings can be used to determine which operating parameter is used to control each contact and at what parameter level the contact closes. Motor power and motor torque are often two of the available selections. You should download or otherwise obtain specification sheets and manuals for VFDs that are available to you and select the one that best suits your needs. If you already have a VFD that doesn't give you what you need, see if you can get one that does.

 

[rmarotta]

Thanks for the response.
I have two drives actually. Both are Allen Bradley Cat.160 series. One is a preset model and the other is analog signal follower.
The only useful output configuration I see listed for either is one that signals when the motor is running above a preset current.
Won't the current change with varying voltage, even though the load has not changed?
Regards,
Ralph Marotta

 

[CJCPE]

Current is somewhat proportional to torque. If the voltage varies in proportion to frequency as it should, current will remain relatively constant if the torque remains constant as speed changes. However, current does not go to zero at zero torque. It might be anywhere between 25% and 50% of the full load value. It is difficult to use current to determine load torque.

Larger drives and more expensive small drives typically have more configuration options.

 

[jraef]

To answer your first question, yes a VFD will replace the reversing starter and the soft starter for sure. But I agree with CJCPE, you will not get accurate load control with that particular VFD, it was not intended for an advanced function such as that. Current feedback is much much too variable to mean much about the load condition. 2HP VFDs with that capability are not very expensive though, you may want to consider upgrading.

Despite all that, I am curious as to what type of material handling application you have that could take advantage of that feature without reducing speed. On constant torque applications, AC motors will only use as much power as the load demands of them, with or without a VFD. So even with kW sensing in a VFD, you have 2 choices of action;
1) Program kW limits at which the VFD will shut down, or use output relays to control something else that is controlling the load:
2) Automatically have the VFD change speed based on the loading. When you change speed, you are changing power. So if you want to maintain a power level setpoint, you can do that IF changing speed is a possibility. To do so you will also need the ability to do PI control at the very least. Some VFDs have that ability, some do not. If not, you will need an external PI or PID controller.


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[rmarotta]

Thanks for the input, jraef. You had it right in your 1) above. I guess I should have been more informative in my original post.
The motor is used to drive a suspended rake/conveyor, which spreads crushed ice inside a storage bin.
The load detector monitors power used by the rake to determine the proper amount of ice that is being moved during the leveling and bin loading/unloading process. The rake is raised and lowered above the ice as needed, using a hydraulic lift, in response to the load detector contacts. Too much power here could damage the rake or it's drive chain components, so speed change won't help.
Since there have been past problems with these controls, and I already had the drive, I thought I might simplify the circuit by installing it to replace the other discrete components.
The replacement load detector will probably cost at least as much as a new drive, so it looks like I need to find a different drive.....
I just thought there might be a simple way to set up the one I already have.
Thanks for the help guys.
Regards,
Ralph Marotta

 

[jraef]

What are you using as a load detector that is so expensive? Check out these aternatives, I have used both and they work great. I prefer the Emotron though, it's a smaller package and mounts on a DIN rail.

Emotron EL-FI M20 load monitor


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