Safe Torque Off Tutorial in a WEG CFW500 Variable Frequency Drive (VFD) - Part 2

In part 1 we saw how to connect a simple ESTOP  switch with dual contacts directly to the drive’s   safety module to quickly disable the output  of the drive in both the standard STO mode   and the time-delayed mode which gave us more  control over how the motor’s torque is released.   That seemed to work well, so why  would you want to add a safety relay?   Safety is all about risk reduction.  And the level of safety you get   requires you do a thorough risk  assessment to help ensure you get   the system integrity and performance  levels you need for your application.   Safety relays are designed specifically to  help you reduce risk to an acceptable level.   In this video, we’ll show you how to use  the safety module, but it’s important to   understand that it falls on you to figure out  if it is appropriate for your application.   And if so, how best to use it in your system  to reduce risk in an industry-compliant way.   I’m using the same single phase one horsepower  drive and safety module we used in Part 1. See   that video if you need a refresher on how  to install and use the safety modules modes.   We’ll also use the I/O module’s 24-volt  supply to power everything again.   You can use the drive’s safety module with any  safety device. Maybe you want to shut a machine   down when someone breaks light curtain beams. Or  a door gate opens and trips a safety switch,   or a cable pull interlock switch gets tripped  or a trapped key system gets activated.   You just pick the safety relay that is designed  to work with that kind of safety device   and wire it to the safety module. For this video, we’ll stick with the   ESTOP button we used in the previous video,  so we’ll use this two-channel safety relay   that’s specifically designed for things  like ESTOP buttons and safety gates.   Let’s take a minute to understand  how this safety relay works.   It will give you a better appreciation  for why safety relays are so important.   The ESTOP button is wired like this. The  safety relay sends known voltage pulses out S11   and looks for those pulses on S12 to continuously  verify that contact is wired correctly. It also   sends different voltage pulses out S21 and expects  to see that on S22 to test the other contact.   And if the relay’s cross-fault  detection switch is in the down   position the relay will even detect  if the contacts are cross-wired.   So if a wire gets cut or even if some other  wire shorts to it, the safety relay is going   to know it’s not correct because it won’t  see the expected coded voltage pulses.   And all of that ensures that the ESTOP switch is  actually going to work when you need it.   As long as the ESTOP is not active – these  contacts are closed – and there is power to the   safety relay, then these contacts will be closed,  which allows power to flow into the safety module,   which allows power to go out to the motor. If the safety relay loses power or either   of these ESTOP contacts opens, then these guys  open and the drive cuts off power to the motor.   And remember, the drive totally cuts power  to the motor leaving it to free spin down   by itself either via the STO mode or the time  controlled mode that we covered in Part 1.   All of that assumes you have these  two pins shorted and the safety relay   has automatic start mode enabled via  this switch inside the safety module.   If instead, you switch the module to MANUAL  mode and then we connect those terminals to   a momentary normally open switch, then the  safety relay has to see the ESTOP reset   and then this contact close before it will allow  power to flow to the motor. That’s for safety   systems that require you to release the ESTOP and  press a second button to force you to acknowledge   that you have verified that everything is  OK before it will allow power to flow.   For example, maybe you have a machine in  a gated cage to protect the operators.   When someone enters the cage a safety switch  trips a safety relay which triggers the drive’s   STO and shuts the machine down. When the operator  closes the gate, the STO condition goes away.   But, the machine doesn’t start yet because  all it knows is the cage door is closed.  It doesn’t know if the operator actually  left the cage. With the manual reset,   the operator has to manually press a button  outside the cage to reset the safety relay.   That ensures the operator is clear of the machine  and it’s OK to restart. So that manual restart   is really important for applications where  there is immediate danger to personnel.   Hopefully, you can see why these safety relays  are so important. They continuously verify that   all the safety circuitry is functional using  coded signals to ensure the wiring is correct   and not corrupted by other wring which gives you  confidence it will work when you need it. And they   can require you to manually verify everything is  OK before allowing power to flow to the motor.   Well, that was a lot of talking for  something pretty simple. The good news is,   all we need to do to get this to work is wire  the system as shown here and push these two   white DIP switches down to tell the module we are  using a safety relay and not direct dry contacts.   There are no parameters to set or any drive  configuration at all for the basic STO mode.   Here’s our ESTOP button, the safety relay, the  CFW500 drive and the manual reset pushbutton.   I’ll push those two switches on the safety module  down to tell it we are using an OSSD compliant   safety relay. OSSD is Output Signal Switching  Device – that just means the device provides those   coded voltage waveforms to test the wiring instead  of the dry contacts that just provide continuity.   Power up the drive. Uh oh – we have a 160 alarm code.   That says the STO is engaged. And, if we look at the safety relay   we see these LEDs are off which says  the internal relay contacts are open.   Why? Because we have the relay in manual mode.   We have to manually tell the relay it’s ok to let  the motor run either after a fault or on power-up.   So I’ll hit the manual reset button telling the  relay I have verified that everything is ok,   and yep, the alarm went away and we see the  safety relay's LEDs say it’s good to go.   Let’s spin up the motor … and hit the ESTOP.  The motor free spins down because the drive’s   safety module is currently configured for the  basic STO mode. That’s gonna take a while.   OK, the motor finally stopped spinning,  so I’ll release the ESTOP. Hmm ..   we still have an STO alarm and the relay's  LEDs aren’t lit. Why? Because we haven’t   manually told the safety relay everything is  ok. I’ll do that and yep, the alarm went away.   If I remove this cover and switch the  relay to automatic mode – I’ll put that   cover back - and spin up the motor … and hit  the ESTOP .. and let the motor spin down. Now if I release the ESTOP the alarm gets cleared  automatically. The relay doesn’t require the extra   verification step. Exactly what we expected. We could repeat all of that using the time   delay version of STO, but it will work the  exact same way as in Part 1 so we’re not   going to repeat that here. But, it is worth  noting that the CFW500 drive's time delay mode   does still get you the SIL3/PLe/Cat4 rating. That ought to be enough to get you started   with using safety relays and the  CFW500 Variable Frequency Drives.   Click here to learn more about the CFW500 family  of variable frequency drives. 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