Control of 6 to 25 motor through VFDs for lifting load
Control of 6 to 25 motor through VFDs for lifting load
(OP)
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
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





RE: Control of 6 to 25 motor through VFDs for lifting load
Normally this would have brakes all over the place. If power fails what stops it all from crashing back down.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Control of 6 to 25 motor through VFDs for lifting load
I could see you having trouble with starting these lifts though. I could see you having difficulty with the winches all taking up the load if you're using a different number of winches for different sizes of loads. Since some winches have been off, they'll be starting will different lengths of cables so you may be requiring some motors/winches to slip at locked rotor conditions until the other motors/winches catch up.
With the system you're looking at, the next logical step would be to build the system so the inclinometers and PLC can control the speeds of the different VFDs to automatically level the load. Depending on the VFD, you would do torque control to automatically load all winches to ready for the lift and then speed control to keep the load level.
RE: Control of 6 to 25 motor through VFDs for lifting load
The system functions with a chain hoist for each motor. It would help lift up tank rings of 8 to 15m diameter, as depicted on http://www.in.all.biz/img/in/catalog/256202.jpeg
@ keith : The chain hoists have a mechanical locking which prevents the load from falling down if no torque is applied.
@ Lionel : The hoists are linked to the motor with a chain gear - unlike a winch, the chain hast no "start" or "end".
That would require a VFD for each motor, right ? Is it worth the cost ? An closed loop should not be necessary if the mass has a good distribution over the motors. The inclinometer here is only used as a "unbalance detector" that would shut down all motors. The operator would then switch to manual mode and choose which motors to activate. The inclinometer may not be necessary though.
My hope is to get your opinion on the ability of this system to load a mass evenly without any torque / speen control. We would like to build an easy but safe system.
Thanks !
Basile
RE: Control of 6 to 25 motor through VFDs for lifting load
1/2HP drives are pretty cheap and you'd save the cost (and panel space) of the overloads and fusing as well, since the VFD-per-motor solution would handle that.
SceneryDriver
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
Thanks for your post.
If i sum up, for each group of 5 motors we have :
In a 5 VFD configuration
5* $250 for a 2.2kW drive
= TOTAL $1250
Is it more flexible ? To get a good load distribution, we would need to connect the VFD to between each other to inter-regulate Torque and Speed, as suggested LionelHutz. The system overall would be more complex, woudn't it ?
In a 1 VFD configuration
5* $30 Thermal Overload Relay
5* $20 manual switch
$350 for a 2.2kW drive
= TOTAL $600
This seems to me the fastest setup. As showed on my initial drawing, we could just simply connect 5 motors to each VFD casing, making them easy to plug/unplug. However, I do not know how good the load share is if we have a 3 VFD - 15 motors situation, for example.
Does anyone have any experience in using that many motors to share a load ? If the system is reliable I might also get rid of the inclinometer/PLC...
RE: Control of 6 to 25 motor through VFDs for lifting load
The only way to deal with this would be to hope that it doesn't happen or that the loads would carry close to equal loads anyway... Or to update the whole configuration.
Note : Today, the tanks are lifted manually, with one person pulling the chain hoist at each jack
RE: Control of 6 to 25 motor through VFDs for lifting load
I would want 1 VFD per motor unless I can talk with people who have used similar systems and know from their feedback that manual intervention is a rare occurrance. I would also put enough PLC to VFD connections to allow individual control of each motor. Then I could try it first without any levelling controls first and this leaves the system flexible enough that I could implement closed loop level control if necessary.
It would be silly to have ~most~ of the hardware installed to eliminate the manual intervention and not do it if necessary. Just saying.
A ring like that might be able to be leveled with 5 motors on common VFD control if each VFD motor group is together in one area of the ring.
Here is something that hasn't been defined. Say you have all 5 motors lifting a part of the ring. How many are really needed to lift the load? I hope you can see where this question is leading. The load might not be equally supported unless you control each winch.
RE: Control of 6 to 25 motor through VFDs for lifting load
Or perhaps this begs the question, what is it that you perceive you will gain by doing it that way?
"Will work for (the memory of) salami"
RE: Control of 6 to 25 motor through VFDs for lifting load
Yes, I lean towards that too, given your comments. Unless someone on the forum knows for a fact that controlling 20 motors with 4 VFD doesn't need manual correction.
That's the case. But even then, I guess interconnecting the VFD's would't bring much, because every VFD is in V/f mode. Torque or speed control works only for 1 motor per VFD.
That's a good question. If I buy motors that can barely lift their supposed share, then motors that are "ahead" would slow down by "slipping" under the excessive weight. But as poited out Compositepro, if motors are too slow, they would not be able to catch up.
If we do that, then some motors would be strong enough to lift a large part of some smaller tank's mass. The problem is that the hoists too are a limiting factor, because they have a maximum load of 5t. We could buy stronger hoists, but since we have a big stock of those here, the price wouldnt not be worth it. I am trying to give a 2.5t nominal load on each jack, with a maximum load of 5t to account for wind and tilt.
To get back to the initial problem, I get more and more the impression by reading your comments that, when not sure, one should lean towards the safest option. The system is pricier and complexer though and I wish someone could tell from experience which works or not. I will try to make some calculations over the maximal acceptable tilt angle so that each post does not carry more than 5t. Maybe it turns out that even a 1% speed difference in the motors would not make huge difference in the load distribution...
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
@ iop995 :
Do you mean something like http://img845.imageshack.us/img845/2486/yyra.png, figure 1 ?
What is the principle behind your thought ? If I had to guess, I'd say the current (torque) distribution of one VFD would happen on the whole ring rather than only a section of it. Problem with Figure 2 being that the leveling out on the VFD level (as opposed to motor level of figure 1) is not possible because each VFD would only work in V/f mode and interconnecting them woudn´t help. Am I right? (I am an newbie in VFD and load sharing, i'm just making wild assumptions here
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
Using 4-20mA speed signal could introduce enough error between each VFD frequency that the distributed method might provide a more even lift.
RE: Control of 6 to 25 motor through VFDs for lifting load
Yes, I'll try to link the thermal overload to the emergency stop of all the VFD to shut down the system if any motor fails.
Here again, i'm walking in darkness. The PLC->VFD current plays a role too ?? Aaaww so many parameters !! How can a beginner possibly have knowledge of such a thing... Can you tell me a bit more ?
If you happen to know these items: I am thinking of using a "Siemens LOGO!" PLC, with 1 to 3 "3HP SEW Movitrac MC LTE B" or "MC LTP B" variator, along with a set of 0.5 HP "SEW Motoreductor R17 DRS71S4".
RE: Control of 6 to 25 motor through VFDs for lifting load
Some other considerations:
What is the motor cable distance from the planned VFD and motors? It is important to know that maximum cable length achievable with a VFD would be calculated as each motor length summed together. Too long for the drive rating and you will have problems.
Usually, with a VFD running a hoist application, the VFD will monitor the load on the motor to ensure the timing of the brake release is coordinated with the run command on the drive. This prevents over-current of the VFD trying to start into a motor that is already braked and also preventing the brake releasing before the VFD has generated the correct torque in the motor (I.e the load could drop). With multiple motors on hoists (that will have a brake), I struggle to see how this can be coordinated. How does the VFD know when each motor is ready? the first motor to release the brake will tell the drive to run but it may cause problems with the other motors.
I see some challenges with this setup.
RE: Control of 6 to 25 motor through VFDs for lifting load
Actually, I do not need any brakes, because the hoist has an auto locking mechanism =). And even without locking : The force that is transferred to the chain connected to the motor is solely used to lift or lower the tank. The weight of the tank is only and directly transferred to the hoist hook.
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
http://www.cm-et.com/ElectricChainHoists/LodestarD...
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...
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
RE: Control of 6 to 25 motor through VFDs for lifting load
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 =)
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
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
RE: Control of 6 to 25 motor through VFDs for lifting load
Have you considered fewer but more powerful hoists?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
RE: Control of 6 to 25 motor through VFDs for lifting load
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
RE: Control of 6 to 25 motor through VFDs for lifting load
For the VFDs : Thanks for the heads up. What about the constructor´s own VFD, the SEW Movitrac LTP B ?
RE: Control of 6 to 25 motor through VFDs for lifting load
A change from delta connection to wye connection, if feasible, will give you 1/3 torque.
Bill
--------------------
"Why not the best?"
Jimmy Carter