Looking for some worldly advice.

I need some of this forums expert advice. Rather than reposting a lot of information please see my initial post of “ArmorStart Controller can release brake with no voltage on motor” from February 13th, 2014, 07:45 AM.

Since that posting our Plant has chosen to continue to use the ArmorStart284G VFD to control the Hoist Motor. The reasoning is the Hoist is in an enclosed work Cell and plant policy states no people should be in Cell when Hoist is operational (lifting and lowering car bodies). Therefore, in their opinion, there is no threat to human life and it is not a safety issue.

I do not want to see the Lift fall again because it will cause lost production, damage equipment, and who knows maybe something breaks and sends a projectile flying that may cause an injury. While the chances of the Hoist falling again are slim, it did happen once, I’d like to prevent it from happening again. I know the current setup is flawed (the plant does not want to spend the money to install proper VFD) and I want to make it better if possible. The Lift / Hoist Motor is wired using cords with Quick Disconnects for the Motor power connection instead of hard-wiring. This quick and easy connection and disconnection of components is a selling feature in ArmorStart advertising. Quick and easy initial installment, replacement of cables and components is fast and easy - all reduces cost.

See attached pictures showing two locations where Motor cable can be easily disconnected. I was wondering if there is some kind of interlock that can be incorporated into the existing setup that would make it impossible to release brake when power to the Lift’s Motor has been disconnected. Also I was thinking /wondering if there might be some kind of a sleeve that could be secured or locked over the cable connection points to prevent cables from being easily unscrewed and disconnected? Has anyone ever see such Safety add-ons?


Black cable is Motor Power and yellow cord is Brake Power. This pic is at VFD.


Lift Motor power can be easily disconnected on floor level at VFD.


Picture of Lift Motor and motor pecker head.


In the last picture this is how the cable was disconnected when I lowered Hoist and it fell. This was the Backup Drive and it was not being used at the time. From the HMI the Backup Drive was accidently selected (an easy to do fat finger). When I selected “Lower Hoist” the brake released and with no power and therefore no torque on Motor the Lift fell to the ground. You can see the Main Drive in the background.

thank you

Do you have access to the program? Write some code to prevent Lower Hoist when there is no power? Not ideal but should cost next to nothing.

Use a voltage monitoring relay. It must be connected to the terminals in the motor terminal box - so a new cable is needed.
Then you must monitor that there is voltage when the VFD is on, and no voltage when the voltage is off.
The error has to be memorized and require a reset by the operator. No auto-reset.
Only when no error is detected (or memorized), is it ok to power the brake coil when the VFD starts.
There has to be a slight delay between the VFD starts and the brake coil activates. Without the delay, if the brake coil activated and the motor cable was off, the load on the hoist could create a regenerative voltage that could fool the logic to think that everything is ok.

Also, most VFDs has a load monitoring so that if there is no current, the VFD stops with a fault and the brake coil deactivates.
So I guess that this VFD does not have load monitoring or it is bypassed somehow.

JesperMP,
I am not sure what a Voltage Monitoring Relay is?

In my original posting back in from February 13th, 2014, I talked about installing a relay at the pecker head of the motor and then run my brake through its contacts so the motor would have to be energized before its brake released. My concern at that time was the VFD provides a PWM wave and would a relay function normally with this type of signal on its relay coil? I do not know? This is new territory for me. Is the Voltage Monitoring Relay different from a regular relay or are they same?

You are correct this VFD does not have Load Monitoring it is actually (in my opinion) the wrong VFD for this application. And I say this now because what this forum has taught me.

First let me apologize for having posted this topic twice. My mistake My bad. I wish the Forum Administrator could combine them or delete one of the Posts.


This is a copy of my original post of February 13, 2014, 07:45 AM:

ArmorStart Controller can release brake with no voltage on motor

"We have an ArmorStart Distributed Motor Controller (284G-FVD7P6D-25-RRG-CBG-DB1-EMI). It is used to run an Over Head Lift motor on a Hoist. One day someone disconnected the Motor Power Cord Connector at the Motor (which is on a platform about 15 feet off the ground). I did not know the power feed to motor was disconnected. The Hoist was in the UP position and holding a Car. When I went to the HMI screen and retracted the “Locking Pins” and then selected “Manual Lower Hoist” the ArmorStart Controller released the Brake and with no torque / voltage on the Motor (motor power feed disconnected @ motor) gravity brought the Car speedily down (about a five foot drop). The Lift’s wire cable unspooled from the Lifter’s Drum and did some other minor damage to Car and tooling.”

Jraef responded to above:

“If you have applied an Armorstart drive to a hoist and want it to control the brake release, you got bad advice. Hoist applications require a specialized drive application called "Torque Proving" in which the drive becomes capable of providing full torque at zero speed. That can ONLY be done with Flux Vector Control (or better) in which you use a shaft encoder on the motor. The Armorstart is not capable of that. PowerFlex 70, 700 and 750 drives are, but they are not of the "Distributed Control" architecture where you have that nice pre-packaged unit.”

jraef’s response was all enlightening to me. I took this information to my work place and after some time and meetings it was decided to keep the VFD drives we have. I think we all agreed the ArmorStart 284G VFDs Drives were not the ideal choice for our Hoist application. In the end it was decided to keep what we have and try to make our operation as safe as possible. With that idea in mind I thought I would return to this well of knowledge and see if anyone can help me again.

Luke_Mages said:

Do you have access to the program? Write some code to prevent Lower Hoist when there is no power? Not ideal but should cost next to nothing.

Click to expand...

That is the problem, there is no way to monitor whether there is voltage applied to the motor. The VFD supplies the voltage for the Motor but if the Lift Motor power supply cable is disconnected the Motor isn’t energized and the VFD does not monitor whether Motor is energized. When VFD thinks its energizes Lift Motor it also releases brake (brake electrical cable is a separate cable and is connected) and Hoist falls. That is my problem.

that is why I'm hoping there is some way to lock down the Motor electrical cable connections. So they can not be unscrewed by anyone.

If you mean on the VFD side, why not a guard over the plugs. There seems to be some structure there you could attach too.

So to get this straight, the real issue is that even though the motor was disconnected, the drive had no idea it was disconnected, so when someone hit the "Release" button, the drive went ahead and ENERGIZED the brake release coil. Then because the motor had no power at all, it dropped. Is that right? Just so you know, that's because the drive, without that "torque proving" capability, has no idea that the motor is not there, so it just does what you tell it to do. The torque prove would not ALLOW the Brake Release coil to energize unless the motor was ALREADY providing full torque. If you had the correct drive then, this could not happen... but let's move on, that horse is dead.

The challenge is to try to accomplish something WITHOUT major changes to the equipment you now have. If you could, changing the power cords so that each one has another wire, pin and sleeve connection could work. I believe those are all 4 pin (3 phases + ground) so make them all 5 wire, 3 phases, ground and monitor circuit. What I would do is to run the Brake Release coil circuit through the monitor circuit in the cords and pins as a loop, so that if anyone pulled a plug, it disconnected power to the Brake Release circuit. That way even if someone screwed up again and hit that button, the brake could not release. But I'm not sure if you could change those power cords and plugs anyway.

So kluge-rig #1 of that concept would be to run a separate wire circuit with quick disconnects on it at each of the plug/receptacle connection points to do the same thing, then strap it tightly to the power cords so that you can only disconnect the power cords if you ALSO disconnect those wires. I'd make it some sort of a interference system, where unless you unplug that small wire and get it out of the way, you can't even unscrew the quick-connects of the power cords.

Kluge-rig #2 might be to use a ground continuity relay in the same manner, monitoring the ground pins that are already in that cord and connectors. It might involve tapping into it inside of the Armostart behind the case, so that may prove difficult. Here's the kind of ground continuity monitor I'm talking about, they are used in mobile mining equipment all of the time for somewhat similar issues.
http://www.bender-us.com/products/continuity-relays.aspx
That one is not MSHA rated, but you don't care about that.

That's all I can think of, other than changing to the right kind of drive... (OK, I promise, last time...)

Good luck.

Yes jraef I have been trying to think of some fail safe way or interlocking everything so I would know when my Lift motor was not being energized and then preventing the brake from releasing. I can dream up scenarios but how do you make it a professional installation that is failsafe. I feel like I have to invent something but I do not want a hay wired piece of junk when I am done. I was hoping there might already be some kind of lockable sleeve available that I could put over the connectors to prevent any inadvertent disconnecting of cable.

I do not know anything about "ground continuity relays" but will investigate thanks to your tip.

jraef said,
"If you could, changing the power cords so that each one has another wire, pin and sleeve connection could work. I believe those are all 4 pin (3 phases + ground) so make them all 5 wire, 3 phases, ground and monitor circuit. What I would do is to run the Brake Release coil circuit through the monitor circuit in the cords and pins as a loop, so that if anyone pulled a plug, it disconnected power to the Brake Release circuit. That way even if someone screwed up again and hit that button, the brake could not release. But I'm not sure if you could change those power cords and plugs anyway."

I like this idea and will investigate when I return to work on Tuesday. There is a reason they call this drive Armor Start. It is not made to easily modify. thank you.

I am going to be away from computers the next couple days so I will be unable to respond to further post. But maybe I’ll get lucky and be able to use someone else’s to view PLC Talk.

jraef said:

So to get this straight, the real issue is that even though the motor was disconnected, the drive had no idea it was disconnected, so when someone hit the "Release" button, the drive went ahead and ENERGIZED the brake release coil. Then because the motor had no power at all, it dropped. Is that right? Just so you know, that's because the drive, without that "torque proving" capability, has no idea that the motor is not there, so it just does what you tell it to do. The torque prove would not ALLOW the Brake Release coil to energize unless the motor was ALREADY providing full torque. If you had the correct drive then, this could not happen... but let's move on, that horse is dead.

Click to expand...

Yes. You summed up my situation well.

jraef said:

That's all I can think of, other than changing to the right kind of drive... (OK, I promise, last time...).

Click to expand...

Yes, I agree with you and thank you for teaching me something I did not know and apparently some people at my Plant did not know either.

factoryrat said:

Also I was thinking /wondering if there might be some kind of a sleeve that could be secured or locked over the cable connection points to prevent cables from being easily unscrewed and disconnected? Has anyone ever see such Safety add-ons?

Click to expand...

Below is a picture of what I was looking for. I cannot believe I found it! It's a miracle. I was trying to see if Rockwell made any 3-phase power media trunk cables and connectors with more than 3 conductors and a ground wire and I found these Clips which I had visualize but wondered how I could fabricate?

Locking Clip for Three Phase Power Media Trunk Cable (M35) || 280MTR35LC
Article: 280-MTR35-LC
Name: Locking Clip for Three Phase Power Media Trunk Cable (M35)
Manufacturer: Allen-Bradley

Instead of copying old posts, just include a link back to your previous post. The preferred method is to add further information on the same topic to the end of your original February 2014 post here.

http://www.plctalk.net/qanda/showthread.php?t=85470

You can find your old posts by clicking on your avatar name and selecting "Find more posts by Factoryrat".

Lancie1 said:

Instead of copying old posts, just include a link back to your previous post. The preferred method is to add further information on the same topic to the end of your original February 2014 post here.

http://www.plctalk.net/qanda/showthread.php?t=85470

You can find your old posts by clicking on your avatar name and selecting "Find more posts by Factoryrat".

Click to expand...

Thank you Lancie1 for info. I will try to remember and use in future.