AC Drives

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Thread Starter

Jeremy Pollard

Hey dudes and dudettes. If you had a 60 HP DC drive and wanted to replace it with an AC drive, what would your approach be if any?
Motor mechanics is also an issue re shaft size and gear boxes, but with that outstanding, can/would you replace the existing drive with an AC drive?
Application is a vertical crane which can be under load of 1200 pounds plus the weight of the steel itself.
Thx in advance for your insights.

Cheers from:

Jeremy Pollard, CET
The Crazy Canuckian!
Integration, Automation, and Training

On The Web - http://www.tsuonline.com
PLCopen North America - [email protected] www.PLCopen.org

8 Vine Crescent, Barrie, Ontario L4N 2B3
705.739.7155
 
Dude,
Like, 1200 pounds is not so bodacious a load for any gnarly crane. Your selections of drives can have, like, many levels [depending on your
application]. You could use an AC motor and an across the line starter, or you might want to use a variable frequency drive, but it we are talking
ultimate drive . . .well now, then we want an AC vector drive with load float, micro positioning, torque proving, oh, and yes, break proving . . . Cool.

[email protected]
 
T
My approach would be to only work with a vector drive manufacturer that has the special software features and application experience for "safe" gravity applications i.e. heavy experience in modern AC drive based vertical/gravity apps. These are not apps for anyone other than the experts. I was engineering/service manager for a drive mfg., and we had our vector drives on Oil Rig Cranes. You don't want to know the horror stories about the mistakes the OEM made (pwm noise problems, holding brake coordination, long
motor cables etc.) because they tried to do it all themselves.
 
B

Bob Peterson

I'd look very carefully before switching it out.

DC motors can put out full torque at zero RPM. AC motors require a vector drive to do this. Also, DC motors tend to be more rugged and can typically take more abuse then AC motors can.

You may also find that the DC drive/motor combination allows for a lot more starting torque then you will ever get out of a 60 HP AC motor/vector combination. You may find that to get comparable performance will require a larger HP AC motor (because people tend to take advantage of the abuse factor available with DC motors).

Other issues might include the need for a regen type AC drive. A 4 quadrant DC drive is relatively cost effective compared to regen type AC drives.

Bob Peterson
 
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Jeremy Pollard

Hey Terence - like the whole crane itself weighs like 3000 lbs or so. so like it still prtetty heavy eh! We could be on That 70's show couldnt we:)) Seriously - I'm really concerned with vertical UP direction and the torque. Not being too mechanical.... well you know. Crane is chain driven as well as a gearbox for torque multiplication but not sure of the startup torque, and holding capabilities when the crane is
stopped - yep there's a brake but./...

Thx for your feedback mate.

Cheers from:

Jeremy Pollard, CET
The Crazy Canuckian!
Integration, Automation, and Training

On The Web - http://www.tsuonline.com
PLCopen North America - [email protected] www.PLCopen.org

8 Vine Crescent, Barrie, Ontario L4N 2B3
705.739.7155
 
First start with the original crane manufacturer. If they are not interested then next best bet is a Drive manufacturer. You will need to provide as much data you can on the crane to them so they can select the matching AC Drive and motor for the application and controls. They may already have experience in providing such drives to crane vendors. Go with their recommendations even if it is non-drive solution.

You have to handle the motor switchout mechanics.

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D

Donald Pittendrigh

Hi All

We have replaced DC motors on several container cranes with AC motors, the main reason being the cost and service overhead of the DC motors. I am
not quite sure of the quantity or nature of the abuse factor and ruggedness of the DC motors referred to below, but I do know that DC motors have brushes that wear out, and null points that need to be set priodically, these issue don't arise on AC motors. DC motors are becoming truly rare on new machines for these reasons. I can also recall not so long ago, a client who swapped the motor heater supply and field supply around when reconnecting the motor, the consequences are obvious, but this would have been all but impossible to do on an AC motor.

We have had Siemens Simovert Masterdrives tested by the South African standards authority SABS and have certified that these drives are capable of generating in excess of 100% rated motor torque at 0 RPM. What has to be overated if this is to be done regularly or for extended periods of
time, is the size of the motor and its cooling mechanism, otherwise you will burn it out.

At the end of the day HP is a measure of the power required to move the machine and the flavour of the power supply does not change this, although the efficiency of the motors and gearboxes often obscures the real power required, and if these elements change, so might the required power.

Regen drives are an intelligent choice for hoisting motors on cranes as for emergency stops you will acheive faster and better controlled braking. For the travel functions where the safety aspect is not quite such an emotional issue, it would be an overkill, however most clients use 4 quadrant drives all round as it saves having to purchase spares for 2 different drives. In areas where the power supply is bad, it is better to keep away from 4 quadrant drives as they like to blow fuses when the power supply dips during regen, this problem is usually solvable but not cheaply. This is also a common problem on the wharfside cranes that I work on as there are usually several cranes at the end of a rather long supply cable, all regening and accelerating out of synch.

I am not sure what motion of the crane the originator of the thread was enquiring about, this somehow got forgotten with all the dudes and things which is a language I don't always comprehend.

Regards
Donald Pittendrigh
 
You mentioned that you want to replace a DC drive with an AC drive. If you want similar performance, load float, micro position, and what not, you'll have to get an AC vector drive.
A brake on a crane is really only a parking brake. But mechanical engineering, especially cranes, is really out of my league. You might want to contact one of the crane manufactures.

Good luck.
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B
One of the issues with AC drives is poor response at very low speeds. This might have been fully overcome, I am not sure. I would recommend a fully vector control with encoder feedback.

A bigger issue for a crane would be regenerative loads while you are lowering and/or holding the load. The DC drives would regen the current
back into the AC line. The AC drives would bleed off this energy into a resistor bank, which is very wasteful. This may have been overcome, but
check the cost of AC line regen.

This may be one of those apps where you should stay with a DC drive.

My experience is two or three years old, so maybe things have changed...

Bill Sturm
 
If you want regen, check csi controllers by toshiba for your 60 hp motor. Also, vector drives have near dc. torque characteristics,
however, motor cooling may be an issue at lower RPM and you may require external fans for cooling.
 
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Jeremy Pollard

Hey guys and gals. It seems clear that when it comes to changing out 20 drives that include mechanics etc, and that an AC drive MAY do the job at current horsepowers, it seems prudent to stick with a DC drive as suggested by many of you.
Thats just what we'll do:)
I thank you all for your very valuable feedback. It is appreciated.

Thx Ken and Jennifer - keep up the good work eh!

Cheers from:

Jeremy Pollard, CET
The Crazy Canuckian!
Integration, Automation, and Training

On The Web - http://www.tsuonline.com
PLCopen North America - [email protected] www.PLCopen.org

8 Vine Crescent, Barrie, Ontario L4N 2B3
705.739.7155
 
R

Randy Erickson

Hi there jeremy;
big randy here. well I do not claim to be a wizard on such things but I have recently started playing with VFDs and find that they are realativly easy to use inplace of any DC motor that I have messed with. The price is a very large factor in making your choice.

three phase motors are cheap in comparison to any DC motor out there but the VFDs are a whole other deal if you have to buy new there is not much diff. total per total. The best thing I could suggest to you is to go shopping on the net. I have found that there are a few good companys out there but it really come down to shopping and how hungry they are the day you ask them the questions ya know.

Try punching in VFDs to your browser and you'll get at least 20 good chances to find what you arelooking for. I like "dealers Electric" and VFD.com but I would not recomend that you go blindly into this venture as there are some real differences it the two animals and they do not always directly interchange. Thanks for listening to me yamer Big Randy.
 
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Jeremy Pollard

Thx Big Guy. Appreciated. I have found similar issues and I think that for an existing app sticking with the DC drives might be the best choice. On a new install however, AC would be the ONLY way to go. Retrofitting would be too big of a headache.
Thx agin for your insight

Cheers from:

Jeremy Pollard, CET
The Crazy Canuckian!
Integration, Automation, and Training

On The Web - http://www.tsuonline.com
PLCopen North America - [email protected] www.PLCopen.org

8 Vine Crescent, Barrie, Ontario L4N 2B3
705.739.7155
 
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