Choosing motors for crane application

[Denimkd]

Hi to all,
I´m new in this forum so please be gentle with me
I have a little problem, so if you can help me with my choise.
I have to choose motors for crane aplication. Old motors are slipring motors. I will put drive for control too.
1. If I choose squrrel cage motors, should I predict to be 30% bigger motors then slipring? I know that Tm/Tn of squerrel cage motors are much smaller then Tm/Tn on slipring motors.I don´t know how to calculate Tstart, and wich motors have biger Tstart.
2. If I choose to keep the slipring motors, and to shortcircuit the rotor (to put out the resistors), how then is going to be the torque of the motor? Is it going to be smaller then beffore, and how much? How can I calculate this?

Can someone advice me what to do?
Thanks

 

[Skogsgurra]

DC motors with thyristor drives are among the best to use in lifting applications. Gantry cranes use them extensively. Have a look att ABB's solutons.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Denimkd]

Hi, Skogsgurra
Thanks for your response, but DC motors are not among my wishes.
If you didn´t notice, I said that I already have AC slipring motors. DC motors maybe are better for speed control, but they are much more expensive to buy them and after to maintainced them.

So, my question was: should I choose AC squirrel cage motors with drives, or to keep old slipring motors (just to add drives on them)?

Thanks

 

[wayne440]

My advice is to strongly consider Gunnar's suggestion. While cost is certainly a factor, if the crane doesn't work well after your changes, you (or your replacement) will hear about it often, and eventually spend the money to fix it properly.

 

[tygerdawg]

Gearmotors are one of the best value/cost bargains on the planet. Applied properly, they will be very reliable and last a long time. Easy/inexpensive to replace if ever needed. Take a look at SEW Eurodrive, they have an online sizing calculator for rail crane-type applications. There are many, many other gearmotor manufacturers out there also, each with their engineering applications manuals of how to fit their products to the application.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[Skogsgurra]

Denimkd

We do not have much to go on. What kind of crane application is it? Is holding torque important? Is precision lowering/positioning important? Or is it more important to lift/dump loads? What power are we talking about? What speed ranges?

You asked about advice. And also told us that you are new here and wanted us to be gentle. That, we always are. But we also expect some kind of readyness to listen and discuss - not just saying that 'I didn't ask for that'. Then we perhaps will not be so extremely gentle any more.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Denimkd]

Gunnar, with all do respect,
I didn´t meen to insult you. I´m very sory if I did that.
All I wanted to say was that, a DC motors are out of question. I wanted from someone to explain to me the difference between AC slipring and squirrel cage motors.

The motors are for overhead crane 13t in metal industry. Speeds:
Hoist=16m/min, motor=55kW
Gantry=63m/min, motor=7,5kW
Long Travel=125m/min, motor=45kW
The crane was working until now, but because the motors are very old, we would like to make revamping.

This is everything that I have from data. If you know something to explain to me about torques for above mensioned motors, I´ll be very pleased to hear you.

I apologise ones more,

Best regards

 

[jraef]

Actually take a step further back and tell us if it is the hoist or the travel motor!

Certainly what you are asking can be done either way. In general though applying a VFD to a WRIM with a shorted rotor is not a task for a novice, it is fraught with potential pitfalls both from a design and operational standpoint. So what is being conveyed here is that if you are going to have to replace the motor anyway, you should not so quickly dismiss the DC option, especially if it is the hoist motor. You can consider PMDC if the maintenance is an issue for you.

DC motors and drives have a very proven track record on hpoisting applications and are very forgiving from a deign standpoint because they are inherently capable of the most difficult of the tasks, that being full torque at zero speed when you release the brakes while holding a load. That said, I actually am a big fan of AC motors and drives, even for this, if done right. But I am far from a novice on them and when I first got into it, I learned at the feet of a person who had already made all the mistakes and learned from them. So although I haven't made most of those mistakes, I know from his experience just how bad they can be. I also know that, especially with hoists, the VFD makes all the difference as well. Some are great, but despite the claims from nearly every VFD manufacturer of being able to handle hoisting applications, I wouldn't stand under the load when the brakes are released with the majority of them.

Just to show you how possible it is, these are some AC squirrel cage induction hoist motors and drives I have done at Boeing.

Welcome to the forum (and don't let Gunnar scare you, he's really a cuddly puppy dog but he sometimes growls at the mail man until he gets to know them).

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

Ok, that is a standard overhead crane. Not much to it.

A slip ring motor always gives you better control than a squirrel cage motor. The reason is that you can change the speed torque curve by changing the resistor.

With full resistance you can have a smooth, yet strong motor for acceleration and also counter-braking. That is not possible with a standard induction motor.

If you want to use a standard induction motor, you will need a VFD and an encoder on the motor. You will also need to use a VFD with a built-in hoist application. It takes lots of trial-and-error before you get good performance from VFD that doesn't have that built in.



Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Jeff. I am not! I am a grumpy ol' man!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Denimkd]

Hi,

I said that I´m new in this forum, but I didn´t said that I´m new in engineering. I have worked on PLC and SCADA programming,
and I have also worked with drives with squirrel cage motors, but not on crane systems.
So, I know how can I controled etc, but I´m not sure about how powerfull are squirrel cage motors compared with slipring motors.
I know that slipring motors have better characteristics than squirrel cage motors.
For example:
Slipring motor: 55kW, n=1000 r/min,Tmax/Tn=4
Squirreld cage motor:55kW, n=1000 r/min, Tmax/Tn=2,8

You probably don´t understand me. This is my question:
If I change the motors with new ones with same power size, will the squirrel cage motors provide me enough torque to pull up the load? Or I should pick up bigger motors?

And jraef about yours ´´I wouldn't stand under the load when the brakes are released´´, I think that ABB has a Crane Control SW that has load memory function that gives
much better safety, don´t you think?

Thanks again, and have a nice weekend, even you Gunnar))

Best regards

 

[Skogsgurra]

You don't learn quickly - do you?

I have told you that a WRIM will work better than a squirrel cage motor. You have a WRIM now. Then do not change to a squirrel cage. I'm sorry. Can't be any clearer than that.

Stil, you say that we do not understand you. What do you want? Tell us what answer you want and we will give it to you.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[badservo]

Denimkd, jraef and Skogsgurra are very active members here and I think they are just trying to save you from making the wrong choice. I don't know much about cranes, but I would use there advice. If you search around here you may find another thread about crane motors or hoist controls that explain more details.

 

[Skogsgurra]

I was just trying to live up to my reputation

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Don't worry, you are succeeding admirably.


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If we learn from our mistakes I'm getting a great education!

 

[Denimkd]

Thanks guys for your response,
clearly I can´t get an answer that I´m looking for.
Never mind thanks for your contribution in this.

Best regards

 

[LionelHutz]

If I change the motors with new ones with same power size, will the squirrel cage motors provide me enough torque to pull up the load?

Probably not, but it is possible they could work.

Or I should pick up bigger motors?

Most likely you'll need bigger motors.

You really haven't provided enough info. You need to determine how much torque you need. Try looking at the weight you are lifting, the drum diameter and the gear reductions to calculate the required static shaft torque just to hold the load. You can use the expected acceleration rate of the load to determine the acceleration torque. The torque you need is the sum of these.

Also, keep in mind that a drive must be capable of the current which is required to produce the higher torque (the 2.8 times torque you listed). It's pretty common to hear that a drive and motor combination are capable of running at breakdown torque, but that isn't true if the drive can't produce the current required by the motor to produce the breakdown torque.

 

[Denimkd]

Hi LionelHutz,
thanks for your answer. That is what I need. How can I calculate the Tmax? For ex. Tmax=Tconst+Tacc, now:
Tconst=...?
Tacc=...?
These equations I was looking for.
If you know these equations, ok, if you don´t I will find them on internet.

Thanks again, clear and simple,

Best regards

 

[LionelHutz]

Thinking about it more, I believe it's even simpler. First calculate the accelerating force required in Newtons (1N = 1kg.m/s^2). Then, calculate the a torque at the drum shaft - multiply the Newtons by the drum radius to get the torque in Nm. Finally, multiply by any gearbox ratios to get the required motor shaft torque. This is the minimum required so add a safety factor, likely something like 2x the minimum.

 

[BrianE22]

If you're looking at acceleration torque then you need the "reflected" inertia (use gearbox ratio squared).