AFA Drive Tripping

Upgrade for the fourth time? Sooner or later you'll get there....

What are the particulars of this conveyor? How much weight on the belt at any given time? Horizontal or sloped? Slider or roller bed? What sort of product? HP of motor? Gear reduction ratio? Etc.?

Weight is about 2200tons/hr. The belt is slopped about 70 to 80 metres long.The Gear ratio is 23.7/ 1. Motor speed is 1490RPM. The motor power is 220Kw. We want to upgrade to 300kw. The product we convey is copper ore.

What is belt width? Is it designed for the initial torque to start in loaded condition? Is it fitted with fluid coupling, if not, you can try it for reducing start up torque.

There are industry standard formulae available for calculating motor power for sloped conveyor belts, you can find them on the web for free. But here's the caveat, likely the thing your ME missed. Those calculators are all based on RUNNING power, and they all assume that you will start the belt unloaded. To calculate the starting torque for a fully loaded belt, you must calculate the total weight on the length of belt at the worst case scenario, determine the starting torque requirements AND acceleration time, then size your motor and VFD accordingly.

So for example let's say you need X torque running full speed, but to start fully loaded, you need 2X torque and it will take 100 seconds at that torque to accelerate to full speed. Your VFD, assuming vector control, can make your motor deliver 2X torue, but for only 60 seconds. You would need to double the size of your motor and drive.

To be blunt though, most people consider the added cost and loss of energy efficiency to be unworthy of the expense on something as small as a 70-80m belt, so the just tell their workers to grab a shovel.

But how much weight is on the belt at a given time? Is the slope up versus down, and how many degrees (or vertical rise)? Belt width? Pulley diameter or actual belt speed? (These are part of the full particulars.)

There is a feature called "speed search" in the VFD. Please enable it.

Speed search has nothing to do with a stalling motor.

Quote "Torque Limit and motor over load"

Obviously you have a torque problem. Have you measured the required torque to move the belt? If not you can take an actual torque wrench an turn the belt til it moves. Then you will know the requirements. How did you determine this at the start of the project before you upgraded the system?

Is the power outage an instantaneous outage or a time delayed outage?

It'd be good to know what your starting & running current is - no load on conveyor & then again with a load. If the motor / drive is undersized, well then you have the first part of the answer. (Will the loaded conveyor start with motor contactor in lieu of a VFD?) There is function or 2 in the VFD setup called torque boost, application type (pump, fan, etc.) or equal - read up and figure you will need all the boost / starting current the drive will provide to re-start a loaded conveyor. There are also some cases where high starting loads are addressed (in part) by driving the VFD to 90 or 120 Hz (max. speed) in combination with a commensurate change in gear reducer ratio so actual speed range is not changed. I doubt accel time has much to do with your trouble but play with it anyway - I'd guess something around 5-10 seconds should be ok, but try it out maybe even 0 seconds - if the load (ore) isn't moving, you havn't lost anything by trying.

The book is your friend, painful as it may be to read. Danfoss is a pretty good VFD mfg., try their tech support. There is little you can do in the setup to blow the thing up.

"for some reason it trips out due to power outage or some protection, on restarting the belt fails to run and drive trips on Torque Limit and motor over load.We have tried to upgrade both motor and VSD twice."

Do you mean you have changed twice or just tried to? What did you start with and what's the history - has it ever worked correctly? Why does it trip in the first place - what is the drive message?

How do you get the conveyor restarted then, by unloading the conveyor by other means?

Your best chance of starting is to try a low speed for a time before accelerating to normal speed. then you do not have accelerating load as well as stiction load. If you have a starting period, I would start at something like 0.5Hz for 1 s then move to 5 Hz for 60s, this may alleviate the torque calc errors on fluxing this motor. Check for a current balance whilst you are doing this.

At this point, I'd suggest you need reassess the design, if it runs the loaded conveyor ok but won't start, can't you change the gearing in between to give higher starting torque and then run over frequency to achieve top speed, for example? I'd recommend you get advice and just trying changing things without knowing what is needed or what benefit the changes may make is too much guesswork. Speak to the conveyor supplier and get professional VFD advice.

A conveyor belt that trips whilst fully loaded and then won't start, has either incorrect settings or is under sized for the application. You have not supplied enough information, either on the trip faults, settings or Danfoss type to get specific advice or assistance.

First you need to ensure you have the correct settings for maximum output, after that you are looking at a further power upgrade preferably based on running knowledge and some calculations.

That conveyor fails to start when trips on load. NB. If the belt is running with 2200tons per hour, there is no problem. The problem comes when it is loaded and you want to start it with load on. If this happens, and the belt is fully loaded, it is very difficult to run it from stand still. The message on the drive is torque limit and the drive does not ramp up speed. After this the other message that pops up is motor overload which is sensed by the electronic thermal relay. If we start without load no problem.

A Danfoss Technician came to assist but it never yielded much despite several changes he made on the Drive settings.

I found out the total length of the belt is 228metres and 1100mm wide. It is installed over a distance of 113metres.

Torque limit = the load is greater than the drive is prepared to supply. Game over if no further adjustment to the drive is possible (torque boost, application, possibly others). Man the shovels as previously suggested.

It clearly appears as under designed drive. It is not designed to take start-up torque in loaded condition. So redesign it. Please remember that always gearbox is rated higher than motor. Just changing electric motor with higher capacity without looking at gearbox may not be the right solution. Till such time you change your drives take following precautions:

1. Never stop conveyor in loaded condition (unless emergency). Let it fully empty by stopping feed in advance.

2. While starting, let empty conveyor reach to full speed, then only to be loaded by starting upstream equipments.

3. To start in loaded condition create provision to apply additional torque manually (safety should be in mind always). For this remove coupling guard, spirally wrapp few rounds of flat nylon cloth belt on coupling flange, while pulling it in the direction of rotation start motor. This little additional manual torque may be helpful to start. Once started, run till conveyor become empty (without feeding any material), then stop and put back coupling gaurd and restart in empty condition.

I like your advice. We will give it a try.

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Sounds like the drive is not happy with incoming power,or something? fix this problem then decide about shovels or a reversing unload vs sizing for fully loaded starts. Is this a running set up, or new install start up?

This is a running unit. we started with a 132kw motor with a fluid coupling,then we went upto 160kw. The problem however was that we were blowing the fuseable plug on the fluid coupling. So we went to 185KW with a variable drive after removing the fluid coupling. We are now sitting on 220kw motor with a variable speed drive and still get those problem when belt is fully loaded. I think we need to go through the process of sizing.