# Dynamic Braking



## KennyW (Aug 31, 2013)

You're trying to hold the load suspended at zero speed without the mechanical brake being applied? Normally the mechanical brake is sequenced to switch on as the vfd output reaches zero (usually some value greater than zero, around the slip frequency of the motor).

Does the motor have a shaft encoder? Is the brake controlled by the VFD?


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## GarryB (Jan 27, 2015)

Yea, they're holding the load with only the VFD and Braking resistor. They do have a relay controlling the motor mechanical brake, but it has been bypassed (looks like after the fact, but by the manufacturer) and that's the way it's been working for a couple of years. This problem has just started. I'm not a fan of using the dynamic braking to hold the load, but I've been told that it's normal...
I have a mind to remove the by-pass and control the electro-mechanical brake by the VFD, but if there is a problem with the DC Chopper or the braking resistor, I need to find that as well.


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## JRaef (Mar 23, 2009)

When using a VFD on a hoist motor, this issue is supposed to be considered VERY carefully. You must have one of two approaches: a hoist design that is inherently unable to be overhauled and drop the load, such as a worm gear, or a full blown Flux Vector VFD with a feature called "Torque Proving", which will require an encoder feedback feature on the motor that tells the VFD where the motor shaft is BEFORE you release the mechanical brake so that the Flux Vector algorithm can make the VFD put out "Full Torque and Zero Speed", kind of the holy grail of VFD capability. Assuming this is NOT a worm gear driven hoist (because if it was, it could not be creeping), this is the other issue in one form or another.

That VFD is capable of this, but requires a "PG Interface Card" option to do it (I think PG is short for Pulse Generator). If your drive has a PG card installed, then it may have been set up correctly, but something is now failing. If there is no PG card, the VFD was misapplied from the outset and the behavior you are experiencing is to be expected, and is by the way, not at ALL safe! You could kill someone.


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## GarryB (Jan 27, 2015)

JRaef said:


> When using a VFD on a hoist motor, this issue is supposed to be considered VERY carefully. You must have one of two approaches: a hoist design that is inherently unable to be overhauled and drop the load, such as a worm gear, or a full blown Flux Vector VFD with a feature called "Torque Proving", which will require an encoder feedback feature on the motor that tells the VFD where the motor shaft is BEFORE you release the mechanical brake so that the Flux Vector algorithm can make the VFD put out "Full Torque and Zero Speed", kind of the holy grail of VFD capability. Assuming this is NOT a worm gear driven hoist (because if it was, it could not be creeping), this is the other issue in one form or another.
> 
> That VFD is capable of this, but requires a "PG Interface Card" option to do it (I think PG is short for Pulse Generator). If your drive has a PG card installed, then it may have been set up correctly, but something is now failing. If there is no PG card, the VFD was misapplied from the outset and the behavior you are experiencing is to be expected, and is by the way, not at ALL safe! You could kill someone.


Understood and Agree, however this is not my design. The crane was manufactured by Oriental Crane Company in Busan Korea. The Hoist is a worm gear configuration on the gear box, but yes it does creep until the electro mechanical brake is applied. The VFD if Fuji as noted above and again was as designed by the manufacturer. No change in programming or application has been noted. I'm not sure if there's a PG card, but I didn't see anything in the options noted in the part number. The type is FRN3.7E1S-4J Frenic-Multi. I mis quoted the motor size earlier. The VFD is rated for 3.7kw but the actual motor is 2.2kw. This is a 3 ton overhead crane aboard an ocean going vessel...


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## JRaef (Mar 23, 2009)

GarryB said:


> Understood and Agree, however this is not my design. The crane was manufactured by Oriental Crane Company in Busan Korea. The Hoist is a worm gear configuration on the gear box, but yes it does creep until the electro mechanical brake is applied. The VFD if Fuji as noted above and again was as designed by the manufacturer. No change in programming or application has been noted. I'm not sure if there's a PG card, but I didn't see anything in the options noted in the part number. The type is FRN3.7E1S-4J Frenic-Multi. I mis quoted the motor size earlier. The VFD is rated for 3.7kw but the actual motor is 2.2kw. This is a 3 ton overhead crane aboard an ocean going vessel...


It's damned near impossible to make a self locking worm gear drive, as typically used on a hoist, overhaul like that. Are you sure the VFD is not keeping energy on the motor? The only way it can go from self-locking to allowing overhaul is if the gears are so worn that the coefficient of friction becomes so low that it no longer self-locks. That's an indicator that the gearbox is about to fail.


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## McClary’s Electrical (Feb 21, 2009)

I saw an 40 ton psi manufactured crane drop 40 tons of logs on a truck. The crane operator lifted too much weight, which would normally just trip the drive and the mechanical brakes would slam shut and hold the load, but this load would so heavy, the brakes slapped shut, but slipped. Dropping 40 tons of logs right on top of the truck.


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## GarryB (Jan 27, 2015)

"It's damned near impossible to make a self locking worm gear drive, as typically used on a hoist, overhaul like that. Are you sure the VFD is not keeping energy on the motor? The only way it can go from self-locking to allowing overhaul is if the gears are so worn that the coefficient of friction becomes so low that it no longer self-locks. That's an indicator that the gearbox is about to fail."

First, Thank you for your response.
Actually, I didn't get back up there to see if the VFD was putting power to the motor, that was an assumption on my part. During the initial testing without a load on it, the motor stopped and the VFD indicated 0 Hz, 0 Amps, but I didn't put a volt meter on it. The controller is on top of the crane and in a very difficult position, and during testing with a load, I only made the observation that the drum creeped down. I do know that when the electro-mechanical brake is applied on loss of control power, the drum ceased to creep down.


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## JRaef (Mar 23, 2009)

mcclary's electrical said:


> I saw an 40 ton psi manufactured crane drop 40 tons of logs on a truck. The crane operator lifted too much weight, which would normally just trip the drive and the mechanical brakes would slam shut and hold the load, but this load would so heavy, the brakes slapped shut, but slipped. Dropping 40 tons of logs right on top of the truck.


I was lifting 747s ad 767s with drives at Boeing. Each crane carrier had 4 x 40 ton hoists. Dropping a 747 is "unacceptable" to say the least. 


















As I recall, the mechanical guys told me that it takes roughly 4x the torque to STOP a dropping load than it does to HOLD it still in the first place.


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## Jhellwig (Jun 18, 2014)

Check to make sure that the dc brake amperage is high enough to hold the load. There might be some paramiters that also control when the mechanical brake comes on.


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## dronai (Apr 11, 2011)

What surprised me about working around these kind of cranes, is the speed they can move across the plant. 

We had to lockout the disconnect every time we worked on these huge furnaces, because the crane would come less than a foot across the top when passing by.

I thought that's lame, thinking they moved like a steam roller, until I saw it go by !! That thing was quiet and would sneak up on you when you were looking the other way. The operators never looked up at us.


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## JRaef (Mar 23, 2009)

On those cranes at Boeing, we could scoot along at 300 ft./minute. That's only about 3.5MPH, but when something that is 600ft long moves that fast, it's a little crazy. We had to test them to make sure they were capable of it. 

When they built the 777 facility, they had to build a test fixture that would break the wings off for FAA certification. The brilliant Civil Engineers goofed and made the top of the test fixture 1" higher than the lowest point of the crane carriers. I hit it at the full 300FPM, the boom shook the whole building sending people scrambling for the exits! I almost got thrown out of the carrier and because I was in a chair, I didn't have a safety harness on. I spend the rest of that day and most of the next doing paperwork and being interviewed by the safety people over this, which I think was worse than the physical pain of getting slammed around in the steel cage of the carrier cab.


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## dronai (Apr 11, 2011)

JRaef said:


> On those cranes at Boeing, we could scoot along at 300 ft./minute. That's only about 3.5MPH, but when something that is 600ft long moves that fast, it's a little crazy. We had to test them to make sure they were capable of it.
> 
> When they built the 777 facility, they had to build a test fixture that would break the wings off for FAA certification. The brilliant Civil Engineers goofed and made the top of the test fixture 1" higher than the lowest point of the crane carriers. I hit it at the full 300FPM, the boom shook the whole building sending people scrambling for the exits! I almost got thrown out of the carrier and because I was in a chair, I didn't have a safety harness on. I spend the rest of that day and most of the next doing paperwork and being interviewed by the safety people over this, which I think was worse than the physical pain of getting slammed around in the steel cage of the carrier cab.


 
That's crazy ! The cranes I was talking about are the typical ones you see in factories, not those monsters you showed.


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## Black Dog (Oct 16, 2011)

JRaef said:


> I was lifting 747s ad 767s with drives at Boeing. Each crane carrier had 4 x 40 ton hoists. Dropping a 747 is "unacceptable" to say the least.
> 
> 
> 
> ...


Man that's really boring work.....:laughing:



















Good man, I really miss the industrial side of the electrical trade...


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## JRaef (Mar 23, 2009)

Yea, I kind of miss it too. I'm mostly a desk jockey now. Although I still get involved in some cool stuff, it's not so much hands-on any more. I took an opportunity to help a technician tear down and rebuild a 500HP VFD 2 weeks ago at a mine in Arizona. He groused about it during the whole project, I was enjoying it but I had to play along with him about what a crappy job it was. He would have thought I was nuts...


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## GarryB (Jan 27, 2015)

Jhellwig said:


> Check to make sure that the dc brake amperage is high enough to hold the load. There might be some paramiters that also control when the mechanical brake comes on.


Any Idea what amperage I should be looking at? The motor current FL rating is 4.4a starting current is at 28.6... 
This crane, as mentioned above, has been modified from construction so the electro-mechanical brake only operates on loss of power (emergency stop, control power switched off, or loss of 480). It's set up so that the dynamic braking resistor is the only thing stopping the load during normal operations. To me that's scary because when overhauling the main engines, the cylinder liners are right at the max load this crane is rated for. 
My inclination at this point is to rewire the controls so the electro-mechanical brake is used when the hoist stops. The Fuji drive has a set of auxiliary contacts that can be programmed to use for this purpose, but was not used when it was manufactured.
The ship is supposed to come back in today, so I'm going to take a look to see if that auxiliary contact is being used for something else or not...
My main reason for the original question was to see if anyone knew if this was an indication of a potential failure in the drive or if the resistor could loose it's capacity to stop the motor.


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## hardworkingstiff (Jan 22, 2007)

JRaef said:


> He would have thought I was nuts...


Well .... ? .... :jester: :laughing:


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## dronai (Apr 11, 2011)

GarryB said:


> My inclination at this point is to rewire the controls so the electro-mechanical brake is used when the hoist stops. The Fuji drive has a set of auxiliary contacts that can be programmed to use for this purpose, but was not used when it was manufactured.
> The ship is supposed to come back in today, so I'm going to take a look to see if that auxiliary contact is being used for something else or not...
> My main reason for the original question was to see if anyone knew if this was an indication of a potential failure in the drive or if the resistor could loose it's capacity to stop the motor.


 
Is there any downside to wiring the controls like you want ? I think with the maxed out load it's a good idea.


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## oliquir (Jan 13, 2011)

just did one like that with a vfd (to replace contactor) and mechanical brake (is applied each time hoist stop), drive was an eaton and there was a macro exactly for that :blink:


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## GarryB (Jan 27, 2015)

dronai said:


> Is there any downside to wiring the controls like you want ? I think with the maxed out load it's a good idea.


Only downside is liability of modifying an overhead crane...
That part isn't my primary concern. This crane is used in maintenance and repair of the main engine of an ocean going tanker. If any of my modification put this thing out of commission while it's at sea, and they can't make necessary repairs to the main engine.... tugs are really expensive!


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## McClary’s Electrical (Feb 21, 2009)

GarryB said:


> Any Idea what amperage I should be looking at? The motor current FL rating is 4.4a starting current is at 28.6...
> This crane, as mentioned above, has been modified from construction so the electro-mechanical brake only operates on loss of power (emergency stop, control power switched off, or loss of 480). It's set up so that the dynamic braking resistor is the only thing stopping the load during normal operations. To me that's scary because when overhauling the main engines, the cylinder liners are right at the max load this crane is rated for.
> My inclination at this point is to rewire the controls so the electro-mechanical brake is used when the hoist stops. The Fuji drive has a set of auxiliary contacts that can be programmed to use for this purpose, but was not used when it was manufactured.
> The ship is supposed to come back in today, so I'm going to take a look to see if that auxiliary contact is being used for something else or not...
> My main reason for the original question was to see if anyone knew if this was an indication of a potential failure in the drive or if the resistor could loose it's capacity to stop the motor.


I'd be scared to do that. You're using the emergency brake for normal operation, which will greatly increase the maintenance needed on those brakes. They'll get out of adjustment, nobody will know it because the drive is helping to hold the load also, and the next power loss, the crane drops a load.


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## McClary’s Electrical (Feb 21, 2009)

mcclary's electrical said:


> I'd be scared to do that. You're using the emergency brake for normal operation, which will greatly increase the maintenance needed on those brakes. They'll get out of adjustment, nobody will know it because the drive is helping to hold the load also, and the next power loss, the crane drops a load.


 
Now, what I would be willing to do, is use a motor brake, and leave the emergency hoist brakes as they are. You can probably get a brake kit for the motor without even changing it.


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## GarryB (Jan 27, 2015)

mcclary's electrical said:


> I'd be scared to do that. You're using the emergency brake for normal operation, which will greatly increase the maintenance needed on those brakes. They'll get out of adjustment, nobody will know it because the drive is helping to hold the load also, and the next power loss, the crane drops a load.


That's why I'm hesitant to just make the mod and move on. Also I am curious if this is a sign of another failure about to happen, hence my original post...


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## dronai (Apr 11, 2011)

GarryB said:


> Only downside is liability of modifying an overhead crane...
> That part isn't my primary concern. This crane is used in maintenance and repair of the main engine of an ocean going tanker. If any of my modification put this thing out of commission while it's at sea, and they can't make necessary repairs to the main engine.... tugs are really expensive!


 
The guy I work part time for, has an Engineer, who designs, and stamps a drawing for the machine modifications we do. Too much liability as you said. It's your headache, and responsibility, but your not an Engineer. Let them know what you've come up with, and your solution. They may say, well it's worked like that for years, but the load is still at maximum.


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## GarryB (Jan 27, 2015)

dronai said:


> The guy I work part time for, has an Engineer, who designs, and stamps a drawing for the machine modifications we do. Too much liability as you said. It's your headache, and responsibility, but your not an Engineer. Let them know what you've come up with, and your solution. They may say, well it's worked like that for years, but the load is still at maximum.


I am a Marine Engineer, but I'm also just a contractor to the owners of the vessel. I've put the details to the Port Engineer in charge of the vessel for the modifications hoping he has some insight into who changed the original configuration and why, also if he want's me to put it back as original. Then it's on him, not me...


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## GEORGE D (Apr 2, 2009)

I'm still concerned as to how or why a worm gear setup is slipping, assuming it's not via the motor as jraef mentioned.


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## McClary’s Electrical (Feb 21, 2009)

GEORGE D said:


> I'm still concerned as to how or why a worm gear setup is slipping, assuming it's not via the motor as jraef mentioned.


I think it slipping proves gearbox wear. Wormgear, is the principal that landing gear on large tractor trailers work on. You crank them down until they hit the ground, and when you pull out from under them, they hold their own.


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