Hi all

Working with newer motor drives, they feature "Safe Torque Off" circuitry which permits a motor dead stop without removing power to the drive. This would have several advantages, not the least of which is eliminating re-charging the drives. But, is it recognized as safe enough for an E-Stop here in the US with prevailing regulations?

Regards

cww

 

It seems to me to be a pretty reliable solution for removing torque from a motor. the gate signals to the semiconductors feeding the motors are interrupted. there is no longer a way for them to act in concert with each other to generate any torque.

--
Bob
http://ilbob.blogspot.com/

 

The safe torque off features are specifically designed for machine safety applications and are considered to be Category 2 stops under NFPA 79 as well as the international standards. They are entirely appropriate for stops generated by machine safeguarding devices, however they are not appropriate for an e-stop application, which requires either a category 1 or category 0 type stop. The primary purpose for developing this safe stop technology was to avoid the need to recharge, re-home etc after a "safety stop."

Many designs still do not separate the e-stop function from the safety stop function and as such can not take advantage of this new technology. It is paramount to remember that e-stop is not a safety stop environment and must be treated differently per the standards.

 

We went around and around on this getting safety gurus in, redoing safety analysis on 2 5 axis robots with their 6000 series servo drives. got documentation from Rockwell on how they were compliant. Our conclusion was they were and they have been great......removes power from the drives and have been running for 2 summers now flawlessly. We installed removable key switches with safety relays to light curtains with the safeoff in the loop and have been happy.

Dave Ferguson
Control Systems Engineer, CAP

 

I've seen it used at times, primarily along with a safety relay.

 

Yes, that is the reason for having it in the drive, it complies to the "SIL" specifications.

IIRC it makes the drive a SIL-3 device when properly used.

 

Curt,

As a well respected contributor on this site, your question has already been answered by yourself.

without "re-charging the drives" indicates power may be available.

A E-Stop should remove all power.

Some folks consider a E-Stop as a additional HOLD point.

Best regards

 

Thanks all

That's kinda what I thought, that this was a lesser stop, and not meeting the requirements for an E-Stop. For roller conveyors, there aren't many reasons for a safety stop as there are few hazards to guard against. And since this is a line of many short conveyors, neither a premium for the drives, or the spendy components to complete the system would be appropriate without some major advantage. Some day, electronics will be considered as reliable as relays, but I doubt I'll be concerned by that time. Ought to be interesting charging all those drives when recovering. Had to check it out and information is quite sketchy.

Regards
cww

 

the STO is for machine safety only, it is not suitable where injury or loss of life is a major cost issue. It does not cover maintence.

STO does not apply where multiple or simultaneous failures are considered a possibility and requires a full certification testing of the installed equipment. The latter is rarely performed in field erected equipment where multiple contractors are involved and start up schedules are pressing.

a. To avoid an electric shock hazard, verify that the voltage on the bus capacitors has discharged before performing any work on the drive.
Measure the DC bus voltage at the +DC and -DC terminals or test points. The voltage must be zero.

b. In safe-off mode, hazardous voltages may still be present at the motor. To avoid an electric shock hazard, disconnect power to the motor and verify that the voltage is zero before performing any work on the motor.

 

Curt,

If you do go with Relays and need to charge the input caps consider using a time delay soft start circuit (Power resistor and time delay relay) to keep the peak current down. Inrush limiting thermistors may also be appropriate and are simpler (although they run hot).

KEJR

 

Yeah I thought of that, but I hate to add any complexity to the estop circuit. It kinda defeats the purpose. I think I'll look at smalle groups.

Regards
cww