AC hp needed to replace DC bridge crane drive?

[eeinpa]

Howdy, folks. Hope you can help me out as much as in the past...

I'm working on plans to convert several bridge cranes (only the bridge motions are involved here) from DC motors to AC with VFDs. I've had suggestions that I need to significantly increase total horsepower over the DC. I understand that torque/speed curves of DC and AC differ substantially above base speed, but in an application where speed of operation is not the primary goal, does this really matter? In fact the original speed was 300 fpm unloaded, which we can probably halve without complaints.

The plan is to replace one DC motor (e.g. 25 hp) and its gearbox, jackshaft, and bearings (all friction, of course!) with TWO AC inline gearmotors directly driving pinions on the crane wheels. Motors would be something like the SEW Eurodrive R series. The gearmotors will have DC brakes for static holding, and I'd use drive dynamic braking for ordinary braking.

I'm interested in hearing of any experiences doing similar conversions, how you sized the motors and how it has worked out. Thanks!

 

[DickDV]

AC horsepower should equal DC hp in this application. Size your gearing so the motors are at 90hz when the travel speed is at maximum. This will give you better starting, better speed control and better motor cooling.

 

[HCBFlash]

I doubt that you would have enough torque at a speed low enough to account for the speed reduction of a gearbox. If you were to substantially reduce the spool diameter on the hoist you'd begin to help that situation.

If "bridge motions" only means travel motors, were the DC gearmotors direct to the drive wheels hub pinions? If so, your new direct drives - direct to the same pinions - operating at very low speeds, likely will not give you nearly as smooth a motion.

If there is any drive linkage, such as cross-shaft with chain drives at the ends you could try to gain some reduction ratio back there. Giving up speed reduction and the higher motor speed ratio, and adding the torque loading to the motor/drive, and attempting to synchronize the achieved ramp-up / ramp-down will introduce mis-tracking factors you will have to deal with and to some degree be able to tolerate. VFDs w/ induction motors is not a close approximation of stepper motors!

You may well find yourself having to look carefully at motor cooling as well, depending on the duty factor. One way or another torque is generated by the magnetism that comes from (motor winding)coil current, and that current is the stuff that heats those windings. Work with a motor shop or manufacturer that you really trust, and be sure you order motors that will take the heating AND the VFDs waveform without overstressing the insulation system.

I tell you this in the light of having "inherited" a very simple, [COLOR=red yellow]very loosely coupled[/color] (mechanically) gantry system which had been attempted to be converted to something like you're speaking of, without even changing the 30:1 gearmotors, and put back to "like we found it" because of difficulties (especially a lack of knowledge and experience with machinery and VSDs), and so I've been denied the opportunity (permission) to do it sucessfully.

I wish you the best of luck, but if the cost of failure is potentially high - and don't forget the very substantial safety concerns of overhead cranes - take things in small steps and prove each before moving on to the next.

 

[eeinpa]

Dick, I appreciate the suggestion of the 90Hz setup. It sounds like you have been down this road before!

To better explain for HCBFlash, "bridge motion" means moving the bridge--only. The original configuration (one of them) has a single DC motor driving a reduction gearbox, which drives a jackshaft supported by friction pillow blocks. The ends of the jackshaft are pinions driving the wheels.

Our intent is to remove the motor, gearbox and jackshaft, and place one gearmotor on each end. The gearmotors will drive very short shafts holding the pinions that drive the crane wheels (it is impractical to drive the wheels directly from the gearmotors).

Since the original DC motor full speed was ~500rpm, we would expect the new gear reduction would be at least ~3x the old, to have the AC motors at ~1800rpm. This is really not a problem for something like the SEW Eurodrive gearmotors mentioned. As Dick suggested, perhaps we want the motors running even faster at full speed.

Virtually every modern crane I have seen uses two gearmotors instead of a jackshaft, so tracking can't be too much of an issue. I have not seen encoders used on bridge drives for general purpose use: anyone wish to comment on whether they are needed? I tend to think not. Yes, I understand the slip may differ between motors. On the other hand, the wheel sizes may differ!

The SEW Eurodrive motors are designed for VFD use, since they sell their own line of drives. Since the crane is not used continuously, problems with duty cycle should be minimal.

 

[waross]

Sizing for 150% speed will help the cooling. If the motors may be operated for significant periods at very slow speeds you may want to consider adding cooling fans.
I would compare the torque curves of the DC and AC drives.
respectfully

 

[eeinpa]

I appreciate the suggestion to compare curves for the motors. Unfortunately, the DC motors are very old, from a company no longer in business. From past experience, I would be fairly surprised if I can get any curves on them. Perhaps I will try calling EC&M, who have actually been very helpful with ancient motor data (from other companies!) before.

 

[DickDV]

If I were facing this project, (of course, I'm not!) I would be very reluctant to remove the existing gearbox and jackshaft. As mentioned above by others, you introduce all kinds of skewing problems without the end-to-end mechanical link.

Unless there is some heavy influence pushing you towards independent end drives, I would replace the 500rpm DC motor with a new AC gearmotor with an output ratio producing 500rpm when the motor is turning at 90Hz. If the motor is four-pole, that would be 2700/500 = 5.4. I would use a TEFC motor as the duty cycle pretty much protects the motor against overheating at low speeds.

Check that the gearbox can tolerate 2700rpm on the input shaft.

 

[sreid]

Will this application require full vector drives so that low speed jogging can be done?

 

[DickDV]

Sensorless vector should be good enough for precise slow-speed torque development and tight speed regulation

 

[eeinpa]

Everyone: I appreciate the dialog. It is very helpful to bounce ideas off others with an appreciation for the problems.

1) Why am I doing this? The existing systems are entirely oil-lubricated, waste-packed bearings. The central gearbox, plus at least 4 support bearings for the jackshaft, plus two bearings on each pinion at the crane wheels. They are worn out, they leak oil and make a mess, and repairing them will cost plenty and require significant downtime. Furthermore, the jackshaft and all the related garbage (including a large electromagnetic brake) weigh a lot. Our structural consultant feels he can increase the capacity rating of the crane bridge, but any mass we can remove from the dead load is rather helpful.

2) Will having two "independent" drives cause problems? Virtually every modern crane I see has two independent gearmotors, one on each end truck (some small bridges retain the single motor and jackshaft). They don't seem to have problems with this, and generally do NOT have encoders on the drives. In fact we have several small (~5 ton) cranes in our shop which drive both gearmotors off just one VFD, and they seem to work fine. One can argue that new cranes will have nicely matched wheel diameters, and so should have minimal tracking problems. BUT, if we have wheel mismatch problems, shouldn't the existing jackshaft arrangement be causing all kinds of problems??? Hmmm. It doesn't seem to be. I can hardly see how the two gearmotors would be any worse, and potentially I could tweak drive settings to accommodate any wheel diameter difference... I guess I could also add limit switches on both rails, allowing the bridge to be "reset" to a home (square) position.

3) Slow speed positioning? These are general purpose cranes, not intended for any kind of "precise" positioning. As mentioned, similar cranes do NOT seem to use feedback devices. One of the more popular lines of drives for small cranes is by

http://www.peinfo.com.

They have vector drives for hoisting applications, but suggest their non-vector drives for bridge motion.

I notice the largest "stock" cranes from Shaw Box (

http://www.shaw-boxhoists.com/)

are 20-25 ton capacity and seem to be using two 10hp gearmotors, so I think I am in the ballpark with 25hp to 30hp total for this older and heavier bridge.

 

[Simmers]

eeinpa

How did your project turn out?

CMAA standards would be helpful . . .

http://www.mhia.org/

This is a challenging issue. Crane engineering companies could be helpful too.