Control of 6 to 25 motor through VFDs for lifting load 3

[Skyba]

Hi everyone,
I am a student working for a company and I have an interesting task. The project consists to lift different heavy loads (15 to 50 tons) with the help of 6 to 25 motors. The amount of motors depends on mass and geometry of the load, and it should be easy for any operator to add/remove a motor.

I have outlined a plan for the electric configuration :

http://img404.imageshack.us/img404/7163/ohzy.jpg

- Required individual motor power is less than 1/2 HP (200-300 watt per motor)
- Motors are tri-phase asynchronous. All motors are the same model.
I am using one VFD for 5 motors because :
- it seems to be the cheaper solution (VFD can control up to 5 motors according to constructor)
- i do not require exact speed/torque regulation (lifting time is 15 min, over 2 m)

However,
Since the motors are working together to lift the load through chain hoists, and the load needs to be lifted on a horizontal plane, it is important that one motor does not lift faster than the others. I have read that the charge would distribute naturally because of "slipping" of the motors, given a high slipping frequency.
If not, the inclinometer would jump in. If it detects an uneven lifting, the PLC would stop the system, and the operator would level back the motors by opening switches on the motors that are "ahead". This should only be an exceptional situation.

And now with my question : Is my outlined draft a good (and affordable) design for sharing the load between the motors ? Should I interconnect my VFD's ? Do I even need inclinometer and PLC ? I am quite new at this and I am shurely missing some points here.

Thank you in advance for your answers ! Every help is greatly appreciated.

Basile

 

[SceneryDriver]

In entertainment, we do this type of lifting all the time. We use these:

http://www.cm-et.com/ElectricChainHoists/LodestarD8

We use multiple hoists to lift large, complex truss structures carrying lights, speakers, etc... for concerts. Very few of these hoists have any type of VFD or encoder. They're run across the line; my industry has very few issues, as they all run within a few% of the same speed. If one motor is a a little slow/fast, we stop the lift and bump that motor up/down, and then continue the lift.

This is a small show:

http://blog.avvegas.com/wp-content/uploads/2009/05/truss-pre-show-copy1.jpg

We use controllers like this:

http://www.skjonberg.com/manual.html#cs800lx

Use the toggle switches to select the motor(s) and direction(s) and push the RUN button to engage the hoist(s). We're pretty old-school, and judge height using 100' tape measures taped to the truss at several locations; bump the motors up and down to get things level. More and more, the entertainment industry is using load cells on many/all hoists to ensure loads are evenly distributed and that nothing is structurally overloaded.

I guess what I'm saying is, this is more or less a solved problem. I'd think twice about reinventing the wheel.


SceneryDriver

 

[Skyba]

My boss and I are pretty sure that the system would pay out, because the practical manual lifting time of one ring is about 6 hours as opposed to the 15 mins with the motors. Plus we do not need 12 persons to operate the PLC. Set up time and storage will remain the same since the motor would only be a small addition to the existing system.
The only question remaining is, do I need to convince my boss to pay the difference for one VLC for each motor. A lot has been said on this thread about it. As the manner seems quite complex and the opinions differ on wheter one VFD for 5 motors is sufficient, I think I will have to do some further research before taking a shot. Most of all, I need a tutor that would be willing to help me on the "practical details" like cabling, etc. I will keep you informed if anything is decided ! Thanks again for the help =)

 

[LionelHutz]

OK, what is your reason for wanting to use the VFDs?

 

[Skyba]

For having a smooth ramp-up, for being able to control the output speed (there might be some situation where we would want to slow down the lift) and controlling the torque as far as possible.

 

[Skyba]

EDIT : "controlling" -> leveling out the torque. AND it has to stay as simple as possible : We should be able to plug motors in and out without changing any setting in the PLC or VLC configuration (The operation of setting up the lifting system should quasi be a no-brainer to avoid human errors on this matter).

 

[Skyba]

@ SceneryDriver : I have just seen you comment now, soz for not responding xD.
Well this is an interesting feedback. I had not thought of that possibility. But it is a different kettle of fish - a tank can weight up to 50 tons. We also have around 30x 5t hoists lying around in the plant and the additional motors cost arount $400 each, much less than any 5t electric hoist. But this is interesting : How much do the motors vary in speed ? I assume that the control is open loop and without any torque coordination.

AND I have some new info =). I have made some calculations and the tank inclination does not make a huge difference on the individual loads. For an inclination of 1%, on a tank of 30t, 8m diameter, 8m height and 7 jacks, the forces vary from 3.12e+004 N for the most charged jack to 3.01e+004 N for the least loaded. For 5% inclination, (which is huge!) the forces are respectively 3.35e+004 N and 2.8e+004 N. We could therefore neglige positive feedback on the motors and the system should be stable.

I'd conclude from there that we dont a close loop, what imports at this point is that no single hoist gets afront of behind of the horizontal level.

I have just come to an idea. What about using a short, heavy duty, medium elasticity material (like rope) between each hoist and its tank portion ? The elasticity could naturally balance out the load ! If you see where I am getting at... What do you think, any idea of material ?

Basile

 

[waross]

That elasticity is already present in the motor slip. If one motor is more heavily loaded it will slip a little more and run at a few RPM less than the others until the load is reduced. Stay with approved lifting rigging in good condition.
Have you considered fewer but more powerful hoists?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skyba]

@ waross : Yes, but the motors will be oversized around 3 times because we do not have smaller motors available. That means the maximum load a motor can carry is 3 times more than what the hoist can support. If we want to upsize the hoists, not only we'd have to buy new ones, but we´d have to reinforce the jacks, too...

 

[iop995]

I think it's much better to use more smaller hoists/motor than fewer with bigger motor. And plus, more jackets assure a much better load balance and good redundancy degree in case of failure. As waross said, elasticity is in slip motors already so no need for additional parts.Small load variation at tank inclination lead to not use such info in a loop contol. May use a rougly inclination detector as emergency alarm only, when tank inclination reach dangerous value (maybe above 10%) and no one operator see such situation or jacket base/terrain slightly collapse.

 

[SceneryDriver]

Basile,
There is no closed loop control with most chain hoists. They run open loop across the line. Since they are all running from the same supply, they all run (more or less) at the same speed. Three-phase versions are more consistent across their load range than are the single-phase versions, but you can easily get within a few percent. The largest chain hoist I have ever seen used in entertainment was 3-ton capacity. It was basically a 1-ton motor with a heavier frame, and triple reeved for a 3:1 mechanical advantage.

Chain hoists also exist these days with VFD's built right in. You feed them power and control (typically Ethernet) and you have variable speed for an off-the-shelf item. I'm not sure you really need variable speed, but I can see the utility when inching a 50T tank up and down. Perhaps one REALLY large VFD feeding all the motors, if you decide to install motors on your existing hoists. Each motor would need individual overload protection, etc.

Be careful with your VFD selection; I've used different brands with wildly differing success. Mitsubishi brand VFD's were by far the easiest to use, and most able to smoothly control the load. I always ran the Mitsi VFD's in V/Hz mode and had no problems. Let the VFD control the motor(s') brake through a relay. Mitsi has their A700 series drives that actually have a PLC built in; you may not need any other programmable controller at all.

As always, the devil's in the details. Be careful - you're putting 50T over peoples' heads. Get competent assistance if you're unsure. This is not a project to guess.


SceneryDriver

 

[Skyba]

elasticity is in slip

Even with an 4x oversized motor ? The nominal torque is 17 N.m and the rated motor torque is 55 to 84 N.m (I have min/max speed req). And no, i cannot get a smaller motor, I'm in Peru and 0.37 kW is the smallest the manufactor has

For the VFDs : Thanks for the heads up. What about the constructor´s own VFD, the SEW Movitrac LTP B ?

 

[waross]

I'm in Peru and 0.37 kW is the smallest the manufactor has

Half voltage will give you 1/4 torque.
A change from delta connection to wye connection, if feasible, will give you 1/3 torque.

Bill
--------------------
"Why not the best?"
Jimmy Carter