Repeat Drive Failures on Overhead cranes 1

[dyno40]

I have two A.C. overhead cranes on the same runway (same power pickup). They have been in service for a number of years. The cranes are feed by a 600 volts 3 ph. supply. All axis are controlled by VFD or Vector drives.
Over the last year, I have had to replace the Bridge drives on both cranes twice. Yesterday, the drive failed a third time on one of the cranes.
The bridge drives is the only one affected. I never had problems with the hoists or trolleys. The drives are both baldor. One is controlled by an analog pendant station. the other a Cattron remote control system.

The control board is usually the one that goes on the drive. The faults I have retreived from the drives are Bus overvoltage, HW desaturation and Low init voltage.

The latest drive that failed had only been in service for approximately 2 months.It was a new drive, not only repaired.

The cranes receive moderate daily use with a few (2) near capacity lifts.

I have not done any line monitoring as I feel that any line problems would also be felt by the other drives.

Does anyone have any ideas as to the cause of such failures?

Regards

 

[aolalde]

In my opinion the engineering for VFD application is still experimental. The potential of applications is very ambitious but still lots of secondary problems have arisen. Since motors insulation failures, bearings damaged by capacitive discharges to power lines contamination.
Certainly we have to pay for innovation.

 

[GordS]

Since it is the control board that is failing, is there a chance of inducing harmful voltages on the control wiring due to proximity to power conductors?

 

[Skogsgurra]

Think about it. Never lost hoist or trolley? But bridge motor.

I immediatley think that you have a contact problem with the wires/bars supplying the crane. Hoist and trolley usually are supplied via cables, the bridge via naked wires and carbon brushes. A VFD does not like an unsteady grid (like bad contact produces) when working, and it results in all sorts of problems.

Why no problems with hoist and trolley? Their power is derived from the bridge power, isn't it?

Think again. Bridge usually stands still when hoisting and probably also when moving the trolley. At least the bridge isn't moving full speed in those cases.

The fact that you have got this during the last years indicate that something has happened to the wires or the contact brushes. Do a line monitoriing (onboard the bridge) and you will see what happens. Or try to inspect the brushes while running the whole stretch several times at full speed - but take care! It is a bit risky.

 

[Barry1961]

The bridge would have the most inertia which could be generating some very high bus voltages when you try to stop.
If the accel and decell are set to tight it can cause high bus voltage.
If your external braking resitor is bad it could cause high bus voltage.
If you have a operator that is trying to plug stop and the drive parameters to prevent this are not set correctly this could cause high bus voltage.

If I were to take a wild guess I would look at the resistor first to see if the years and heat have corroded the connections.

Barry1961

 

[peebee]

Re "I have not done any line monitoring as I feel that any line problems would also be felt by the other drives."

Seems like a big assumption to me. One of the first things I'd recommend is checking your power qualtiy, particularly your voltage level.

 

[DickDV]

skogsgurra's point about sliding bar/brush contacts is likely close to the source of the problem. I have seen a goodly number of drive failures due to intermittent contacts on the slide bars.

If the bridge travel is not too long, I would suggest a festooning system for power rather than slide bars but this is often impractical.

Given the slide bars, you will need to do some very thorough transient suppression engineering likely involving a transformer or line reactors between the slide contacts and the drive input. You may also need to add external MOV protection with much higher ratings than the drive has internally (if it has any).

A really great way to destroy a crane drive is to be lowering a load with heavy regen and be moving horizontally at the same time while encountering an intermittent open on the slide bars. You are talking pulse voltages many times higher than normal with large energy levels to complicate things further.

Crane drives are always challenging applications. Now you see why!

 

[mc5w]

You might want to consider the following:

1. A second set of runway brushes in parallel with the first set. This will help prevent bumps in power continuity at runway conductor joints and so forth.

2. Each drive should have its own input inductance or universal harmonic filter which is better than an inductance.

3. For high inertia loads, punch presses, and similar demanding applications that drive needs to be oversized and then derated. Same can be said about your braking resistor.

4. I encountered and instance where the neutral point of the motor was accidentally grounded. The culprit was a conduit box connector that was contacting a ring terminal in the motor junction box. Moving the ring terminal and taping it fixed the problem.

 

[dyno40]

Thanks you all for your feedback.

A few notes.
In regards to the power pickup, The crane is fitted with a dual pickup system. Even so, I did do some monitoring and was unable to find any irregularities.

Filtering is done with inductors. I have never used harmonic filters, are they effective and are they practical on a 3 phase system?

The braking resistor suggestion is very interesting.Upon further investigation, I discovered that the internal resistor on one of the failed drive had indeed blown. I have installed a larger external resistor on the drive in question.

Thanks Again .

 

[Barry1961]

I am very surprised the Baldor drive could stop a bridge crane with only the internal resistor more than one time.

I have never used the Baldor VFD's but have used many of the servos. They have a good website with a good bit of tech info on it. They should have a chart on resistor sizing by percent braking you need.

Does the hoist rely on the internal resistor also?

Barry1961

 

[dyno40]

Reply to Barry1961

I am also very surprised. The latest drive lasted approximately two months.
All the hoist drives have external resistors. Bridge and trolleys are the only ones without it.
I am still not sure why the control boards went. Hopefully Baldor will have something to add.

 

[mc5w]

You could also have a grounding problem. In the fall of 1993 my employer had problem with a bridge crane in a building that he had bought earlier in the year. The hoist ropes had a nasty tingle voltage when the hoist motor was running. The culprit was a fire damage cleanup crew that had sent excess spray paint all over including the rails and wheels that supported the trolley. Destroying 1.5 wire wheels and 0.75 electric drills and my back for the day fixed the problem.

You should also consider installing an insulated green wire in the supply conduit and using that to bond the runway rails. You also need to bond the conduit for extra conductivity. The green wire should be the same size as the hot wires because of the distance that cranes usually run. You can also get devices from Erico for bonding around rail joints.

In 2005 National Electrical Code 610.61 you also have to have a redundant grounding conductor such as a runway conductor or festoon conductor in case wheel continuity fails. You still need to maintain the wheels and rails as a 2nd grounding path. Evidentally, other people had the same problem that I did or you have.

 

[castera]

HW Desaturation generally points to the supply being removed, then reapplied before the drive bus decays, and so could point to a supply problem. Do you have line contactors in the supply to the drive, if so check the contact faces, i have experienced some wierd kind of pitting on a baldor drive which gave problems.
I have had problems with a baldor drive tripping on HW desaturation and bus overvolts when the crane driver has driven the motion into the crane buffers.
Have the drive perameters been set correctly after the first failure.
Is the drive set to coast to a stop or ramp down.
Is the drive set to plug reverse.
Are you using DC braking.
Reduce/increase the ramp down time whilst monitoring the bus volts (On the drive front, not with a meter) and a tong ammeter on the resistor cable to ensure that current is flowing to the resitor and that the bus volts are not rising to high.... (Best to increase first then slowly decrease the ramp down / deceleration time until the bus volts start to rise, then make sure that the regen circuit is clamping the bus volts via the resistor by monitoring the resistor current).

Fit a bigger regen resistor, its most likely that the muppet who priced the job has scrimped on the regen resistor.

 

[jraef]

dyno40,
I also like the reistor issues as sympomatic of the root cause. I have done plenty of bridge crane drives with sliding bar bus systems and have never experienced that as the cause of a problem like this. I agree with you that if it was, the other drives would be affected because they will get their power from the bridge, not from a separate source.

I have on the other hand seen all the things that Barry1961 mentioned above cause bridge drives to blow. If I am not mistaken, Baldor (from the old Sweo design) are one of the drives that uses the DC bus to power the control boards, as opposed to having a separate SMPS for that. This leaves the boards vulnerable to stress from repeated DC bus overvoltage, a condition that you may likely have. I too think that someone messed up by not using an external resistor on the bridge drive. That one has the most mass to stop; the bridge itself, plus the trolley, the hoist and the load! It is usually the trolley drive that does not always need an external resistor, but I always used them just in case.

"Venditori de oleum-vipera non vigere excordis populi"

 

[IASMike]

From my experience with cranes there are two major problems that account for most intermittant problems. They are trolley shoes and ground faults. If I were you I would make sure that there are no intermittant ground faults on the systems, and check the trolley shoes.