V/f or Open Loop Vector

[2rlp]

Hi all,

Have to use 2 sets of VFDs. Each set caters to 4 nos of 3.7KW Induction motors. These 8 motors are situated at the 4 corners of a square platform that will be used to lift 10,000Kg material. The platform moves in the vertical direction thru a rack pinion arrangement at all 4 corners of the platform. As shown in the diagram attached, M1,2,3&4 will be driven by one 20KW Yaskawa A1000 VFD and M5,6,7&8 will be driven by another 20KW Yaskawa A1000 VFD. The brakes on the motors will be operated by combined Torque proving relay.

I need suggestions as to how would be the best mode of operation of such a system

Thanks

 

[AustralIan]

For multiple motor on the one VSD, you can only use V/f. The open loop vector algorithm cannot sense what is going on with multiple motors.

Also, for load sharing applications, you are better using one VSD per motor, with a master / slave torque arrangement. Otherwise, if one motor is slightly weaker than the other, you will have uneven loading.

 

[rupej]

I've never heard of using a single VFD to drive multiple motors simultaneously. It might work, sort of, but how do you detect a motor overload? Do you have fusing and a overload relay for each motor? Sounds like a bad idea.

 

[2rlp]

Yes each motor will be protected by a Thermal Overload Relay

 

[Geoff White]

Thats a pretty "interesting" solution to that problem. Good luck keeping the platform level unless you have the motors mechanically linked too.

 

[keithkyll]

Why wouldn't it be level?

 

[kamenges]

Originally posted by keithkyll:

Why wouldn't it be level?

2rlp didn't specifically say anything is mechanically linked together. Given that omission it is quite possible he has 8 motors each independently driving a pinion gear mated to four racks. At that point he would be trusting that at least the four motors being driven by one VFD would maintain absolute position with each other simply because they are driven from a single drive. He says they are induction motors so they will turn different speeds if loaded differently. Just due to manufacturing differences they will likely turn different speeds if they loaded the same.

This is why we tend to ask for as many details as possible in an initial post. It tends to limit speculation.

Keith

 

[jraef]

As AustralIan said, you have no choice in control methods once you go with multiple drives in one VFD, you are inherently limited to V/Hz (Scalar) mode. No point in discussing it otherwise. So because you cannot use Vector mode, you cannot implement torque proving. The fact that you recognize the need for torque proving means this is therefore NOT an application that is suitable for using multi-motor implementation, the only valid solution from a safety standpoint is to have a separate drive on each individual motor and a coordination system for them that monitors the platform and maintains the position. This is something that is done quite a bit on overhead crane system "carriers" with multiple hooks, I have done it for the carriers at Boeing used to move jets through the assembly line.

To be quite honest this is a highly complicated application and the fact that you asked these questions indicates that you are unlikely suited for the task.

 

[AustralIan]

Nice pic, jraef!

 

[2rlp]

Thank you all for the informative feedback. Initially, I too had suggested the use of 4 motors with one VFD per motor. The VFD would be in Close Loop Vector, with a droop feature. The client (whose principals are overseas) insists that they have been advised to use 8 motors and preferably only one VFD. Finally they managed to get an approval for the use of 2 VFDs. They were also inquiring if a soft starter could be used instead of a VFD.
Anyhow, will keep the forum informed, when this project is in the installation stage.

 

[jraef]

The people insisting on these other alternatives have no business making these decisions as they do no comprehend the potential risks and consequences of them. The soft starter idea is actually extremely dangerous and potentially fatal! There is a reason why you never see soft starters used on hoist motors. The failure mode of the power devices means the hoist could run away uncontrolled. Absolutely ludicrous, they are most likely talking to a salesman who has no idea what they are talking about but does not care, because he is not going to ever be standing underneath this thing.

When I did the work at Boeing, I had numerous salespeople bring ideas to me about different VFD technologies that they claimed were as good as what I was using. I gave them all the same challenge: I'll lift my 40 ton test weight, you stand underneath it when I release the brakes. If you are so confident in your drive technology, you will be willing to risk your life for it, because if I use it, OTHERS will be. Didn't get even one taker... (which was good, because of course Boeing would have never let me do it).

 

[2rlp]

jraef, I have used Impact 1336 on hoists and have traversed 150Tons across the hall with brakes released and the hoist holding at zero speed.

With regards to soft starter, what i think the client meant was akin to an ABB ASTAT system. They previously had some experience with those