# Motor runs slightly before brake opening



## smartatuls (Dec 12, 2017)

Hi,

We have a EOT crane which has 3 hoists (30T/60T/30T). The issue is when we give command for Hoist Up or Down the the motor runs slightly(Jerk) for few rotations then stops completely and after full opening of brake it starts running again. Not sure how this is happening. 
There are 2 Thruster brakes for each hoist, their opening and closing works together. The brake open/close command is given by drive output relay. The drive is ATV71 - Schneider which has inbuilt brake logic parameters.
There is a Brake release contact input used in drive parameters which enables the run command after acknowledging brake open. We bypassed this input still the issue is there. Also tried to change brake release current(Torque) at different settings but no change.

It has become serious issue since there was motor shaft breakage happened at this site in past.

We never faced such problem . This is something new. Please help
__________________
Thanks,
Smartatuls
Teaching Once is Learning Twice


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## paulengr (Oct 8, 2017)

smartatuls said:


> Hi,
> 
> We have a EOT crane which has 3 hoists (30T/60T/30T). The issue is when we give command for Hoist Up or Down the the motor runs slightly(Jerk) for few rotations then stops completely and after full opening of brake it starts running again. Not sure how this is happening.
> There are 2 Thruster brakes for each hoist, their opening and closing works together. The brake open/close command is given by drive output relay. The drive is ATV71 - Schneider which has inbuilt brake logic parameters.
> ...


It's torque check. It might go under different names but that's what it is. It's either normal or your brake thruster is worn out and needs rebuilding.

A lot of drives do a torque check where typically the drive increases torque just until the motor starts to move as detected by the encoder. This is basically load testing the motor. This is a safety test. If the current readings don't indicate a sufficient amount of torque is being generated the drive faults without releasing the brakes so loss of control with a failed motor never occurs. You will normally see a jerk as it takes all the slack out and maybe slip the tiniest amount since the moment the motor pulls through the brake, friction drops off rapidly (dynamic friction is about 1/3 of static) so the drive has to quickly back down.

Then it releases the brakes with the motors fluxed up with a zero speed command until the brakes have fully released as detected either by torque demand levelling off or limit switches on the brake thrusters before beginning movement. This second step prevents burning up the brakes.

During brake setting it reverses. The motors hold the load as the brakes set. Then it backs off the current slowly while monitoring for movement. If it slips most hoist algorithms fault but don't shut down...They slowly lower the load until everything stops moving.

There are lots of variations on this design and you have to read the real (programming) manual closely to find this out. That's at least with the newer drives such as the Altivar 900 series. I'm a certified startup tech for that one but I've worked on drives for over 20 years. Torque check started showing up and became common about 15 years ago when we started going to more advanced application specific drive algorithms.

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## JRaef (Mar 23, 2009)

In the ATV71 drive, I'm not sure if it has the same torque proving capability as others. But if they do, it is called "Brake Engage and 0" in their manual (bECd). The description implies it is doing something like this, but I'm not sure it is actually PROVING that the torque is there before releasing the brake, their description is a bit cagey on that. I've always had trouble with the "reverse French notation" style of descriptions in the Schneider manuals. Drives me nuts actually...

But even if it is capable, it is ONLY capable of that in full Flux Vector Control mode (FVC or FUC on their LED display). That then means you MUST have an encoder feedback (it says so in the manual). So without that, there is zero chance that it can be calculating and proving that you have the proper shaft torque at release, it is just guessing, and guessing wrong is what breaks shafts. So if you tried adjusting the brake torque but are not using FVC, that would explain why it seems to ignore your settings, it can't actually do that.

The only VFD I am aware of that can do FVC _and _Torque Proving without the use of a shaft encoder is the A-B Power Flex 755 drives. Even then, this task is something that should not be undertaken by novices and I personally would not stand under a load when the brake is released unless I knew there was a shaft encoder.


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## smartatuls (Dec 12, 2017)

JRaef said:


> In the ATV71 drive, I'm not sure if it has the same torque proving capability as others. But if they do, it is called "Brake Engage and 0" in their manual (bECd). The description implies it is doing something like this, but I'm not sure it is actually PROVING that the torque is there before releasing the brake, their description is a bit cagey on that. I've always had trouble with the "reverse French notation" style of descriptions in the Schneider manuals. Drives me nuts actually...
> 
> But even if it is capable, it is ONLY capable of that in full Flux Vector Control mode (FVC or FUC on their LED display). That then means you MUST have an encoder feedback (it says so in the manual). So without that, there is zero chance that it can be calculating and proving that you have the proper shaft torque at release, it is just guessing, and guessing wrong is what breaks shafts. So if you tried adjusting the brake torque but are not using FVC, that would explain why it seems to ignore your settings, it can't actually do that.
> 
> The only VFD I am aware of that can do FVC _and _Torque Proving without the use of a shaft encoder is the A-B Power Flex 755 drives. Even then, this task is something that should not be undertaken by novices and I personally would not stand under a load when the brake is released unless I knew there was a shaft encoder.


It is in close loop in FVC mode. Customer requirement is there should not be any movement untill complete brake release. But somehow it moves little bit during torque proving. Movement is kind of a jerk and stops then starts running continuously.


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## paulengr (Oct 8, 2017)

smartatuls said:


> It is in close loop in FVC mode. Customer requirement is there should not be any movement untill complete brake release. But somehow it moves little bit during torque proving. Movement is kind of a jerk and stops then starts running continuously.


Usually it doesn't "move"...it just rocks against the brake. Tout as a safety feature.

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## smartatuls (Dec 12, 2017)

Drive connected was ATV320U11N4C(1.1kW, 3 A) but it was running motor without load only and did not move with load(10 Tons) condition.

A higher rating drive ATV320U15N4C (1.5kW, 4.1 A) connected but same problem is coming. Site engineer increased NCR(motor rated current) and ITH(Thermal current) as 4.5 and 6 and auto tuned then motion was working with load and showing current as (3 A to 4.5 A)

When tried with other motion drive(Long Travel, 3kW ) temporarily with actual motor details entered and auto tuned then it was working fine showing current consumption as 2.6 A.

Not sure why it is not working with 1.1kW or 1.5kW drive as per motor name plate details and auto tuned parameters.


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## smartatuls (Dec 12, 2017)

smartatuls said:


> Drive connected was ATV320U11N4C(1.1kW, 3 A) but it was running motor without load only and did not move with load(10 Tons) condition.
> 
> A higher rating drive ATV320U15N4C (1.5kW, 4.1 A) connected but same problem is coming. Site engineer increased NCR(motor rated current) and ITH(Thermal current) as 4.5 and 6 and auto tuned then motion was working with load and showing current as (3 A to 4.5 A)
> 
> ...


Please ignore above reply. It was for other thread by mistake i copied to this thread.


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