Disconnecting Multiple Motors with One Switch? 3

[JimCook]

I have an application where I need to disconnect and lock out four VFD motors (all are 3 HP) with one switch. But I have been unable to find a suitable switch. Does anyone know of a switch that might work?

This switch is to allow the operator to lock out four sections of belt drivs so that he/she can step out onto the belts to adjust or repair sheets of wood veneer. The operator's switch is only for "minor servicing activities, performed during normal production operations, and are routine, repetitive, and integral to the use of equipment for production."

All motors are independently controlled by their drives. We thought about disconnecting a common power feeder to the drives but the VFDs (A-B 700) would then require too much time to restart. We will have 30A NEMA-style disconnect switches for each motor for servicing the belts or motors.

 

[itsmoked]

Yeah what a pain..

Perhaps four contactors with a single switch. Use a switch with early contacts to disable all the drives before the contactors dropout. Otherwise, someone is sure to hit the switch with the drives running which invites a bunch of failed drives. This scheme also provides some redundancy in keeping the motors from running by disabling the drives AND disconnecting the power to the motors.

The conveyor doesn't need the drives to prevent roll back does it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If you are looking for one switch that will allow you to lockout four independent motor circuits I doubt that you will find an approved switch.
Disconnecting power to a group of motor starters is common but the motor starters must be approved for group fusing.
The VFDs may be unhappy if a disconnect between the drive and the motor is opened at an inappropriate time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JimCook]

We thought about using a safety relay to disable the PowerFlex output transistors but explaining how all this works to OSHA is assumed to be problematic. The operator would have a keyed switch with the key captured in the closed position. To use, the operator stops the line, opens the switch and pulls the key. He/she steps out on the line to repair the sheet. Operator returns to contol panel, inserts key and closes switch. Operator hits Start to resume production.

We'd take two contacts from the keyed switch to a safety relay wired to the safe-off input of the four drives. With the relay de-energized, power would be removed the gate circut on the drives. We dread the thought of going to OSHA with this for their approval...

 

[PaulBr]

I assume that drives you have are Allen Bradley PowerFlex 700S. Currently these drive can have Safe-Off option board. This board is serving to stop and disable drives without physically dicconnetcting power wiring. Check with Rockwell if you can add these boards to you drives.

PaulBr.

 

[JimCook]

Bill,

Our standard is to incorporate an Aux switch on our disconnects so that the drive "knows" that we are disconnecting the motor. Saves us from a lot of drive faults.

Lots of switches could be used for this with the conductor size and load, but as you point out, I haven't found any that are approved motor disconecting means.

 

[rbulsara]

You may want to consider grouping all four switches next to each other and reroute the wiring as needed.

To meet motor's local disconnect requirement of NEC and since OSHA is applicable, your options are limited to:

1. Disconnect the main common feeder to the VFDs. If it is not within sight, you need to lock it OFF.

2. Provide separate disconnects at each motor and if any one of them can start the belt, you need to lock OFF each one of them too if all are not within sight. (Hopefully VFDs are turned off before the maintenance need).

Contactor/relay rig would not meet the intent of the Code for having a local disconnect, imho. In fact it would be no different than a starter/controller.




Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Further to the lockout relay scheme;
That is too similar to the old stop/locks that were once used for safety lockoffs.
This was a cheap addition to a standard industrial class stop button. With the stop button depressed, the device could be flipped over to hold the stop button down. There was provision to then install a padlock to hold the button down and prevent unauthorized operation.
In Western Canada when I was just getting into industrial work, the legislation was changing and these devices were no longer being accapted as a safe lockoff device. Wye? Not safe, to many failure modes. to many people injured and killed.
Any OSHA officer who approves a lock-off device other than a complete disconnect of all energized conductors may not understand his job very well.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

PaulBr,

Semiconductors aren't normally accepted as isolation devices because they have off-state leakage and don't introduce a physical air gap. I'm amazed if OSHA would allow a semiconductor as a safety isolation, but I don't think they do. Maybe I'm wrong?


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If we learn from our mistakes I'm getting a great education!

 

[PaulBr]

According to NFPA 79 paragraph 9.2.5.4.1.4
"Drives or solid state output devices designed for safety related function shall be allowed to be final switching element, when designed according to relevant safety standard"
This was added in NFPA 79 of 2007. I was working for printing press manufacturer and this was relief for us. We have been using this function on Rockwell PowerFlex drives, Control Technicues drives, and Indramat servo drives without disconnecting power.

 

[waross]

So you are relying on the control circuit to hold the power off. Read my lips, read my post. They used to do that but stopped about 40 or 50 years ago because people were getting hurt and dying. If you don't learn from history you are doomed to repeat it.
Do let us know when you eventually have an incident due to a failure in a control circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

It's a secure way of disabling the power output stage to prevent the motor being driven. It certainly doesn't meet the requirements for an isolating device to allow personnel to work safely, at least not by the European and IEC definitions. The secure disable functions are dealt with by the machine safety standards like EN 954. Two different functions.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[HRSADI]

I have a sugguestion,
Use 4 pole Isolator after the VFD output and take one phase from each motor and pass it through the Isolator.So by this way you disconnect one phase of each motor by one action as well as the motors will not getting start with two phase.

Best regards

 

[HRSADI]

I have a suggestion,
Use 4 pole Isolator after the VFD output and take one phase from each motor and pass it through the Isolator.So by this way you disconnect one phase of each motor by one action as well as the motors will not getting start with two phase.

Best regards

 

[rbulsara]

NFPA 70, NEC, is not only a "standard" but also a law with minor addenda in most states of the USA. Other NFPA standards may not have been adopted as laws, so NFPA 70 (and OSHA) prevails in most cases.

Local motor "disconnect" is a requirement, just de-energizing is not sufficient.

HRSADI: That would be single phasing,a no-no.

Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

According to NFPA 79 paragraph 9.2.5.4.1.4
"Drives or solid state output devices designed for safety related function shall be allowed to be final switching element, when designed according to relevant safety standard"

And the "relevant standard" is NFPA 70E - Standard for Electrical Safety in the Workplace. You are likely required to produce a complex lockout procedure/plan.If you can incorporate said procedures in your design, maybe OSHA will give you a nod. You'll have to make sure all your provisions are verifiable so that these things are safe.

 

[JimCook]

Thank you all for a very lively thread! To answer some of the questions raised:

We are still going to have traditional 30A NEMA disconnects for each motor for servicing. We will be fully compliant with NEC requirements.

All our disconnects have an aux contact back to the drive or PLC to indicate to the control system that the motor switch is open.

Please read the quote again in my first post. That language is directly from OSHA that permits "minor servicing" without full lockout. IEC has similar language in their standards.

If the gate circuit is disabled, the motor can't turn if the output transistors short. Induction motor can't turn with DC.

Because our state has it's own OSHA, we have to request a variance to use a safety relay system with the A-B drive safe off feature. While this type of system is widely used in Europe, it is rare in the US. "Selling" it to OSHA will be time consuming and difficult.

Those of you who haven't heard of safety relays or safety PLCs, should take some time to do some internet research. Just Google "safety relays" and you'll find lots of links that describe how these very clever systems work. I wish our OSHA people would do some research, too...

To avoid having to go to OSHA, I was hoping someone could direct me to a motor switch with four sets of three phase contacts (or some way to stack them) that could be opened with one motion. However, I am prepared to make our case to OSHA that a safety relay and the safe off feature meets the "minor servicing" requirements.

Again, I'm very pleased at the responses in this thread and I am very grateful for all your ideas.

Thanks!

 

[jraef]

OK, so I am assuming you are OK with routing all 12 conductors into one box then. Since the motor power is relatively small at 3HP, you can in fact get "Cam Switches" from various manufacturers which can have as many as 12 (maybe more?) contacts probably up to about 32A, with motor switching ratings as well. Example:

Companies like this will build you whatever you want. I would get a base mounted switch and a padlocakable operator, I would also get another contact that operates prior to any of the others opening that I would wire to the Safe Off circuits of the VFDs. That will also ensure that nobody opens that switch under load.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[waross]

Good suggestion Jeff. I'll second that and throw in a star.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JimCook]

Very nice jraef! You'll get a star from me, too! That was just what I was looking for. Because these are 3 HP motors, 12 t-leads in a box is really no big deal.

Thanks!