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Old June 22nd, 2018, 07:56 AM   #1
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dylantzips is offline
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Can someone explain the basics of STO Modules in Powerflex VFDs?

Title says it all. I am new to PLCs in general and could not find a good explanation.
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Old June 22nd, 2018, 08:59 AM   #2

widelto is offline
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Send me a PM with your email, I got a good whitepaper from emerson.
My two cents.
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Old June 22nd, 2018, 09:18 AM   #3
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dmargineau is offline
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For PF 750 Series:


For PF 523/5s, at Page 235 of :


For PF 527s, at Page 87 of :

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Old June 22nd, 2018, 02:05 PM   #4
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STO is a verified safe method of disabling the output of the drive so that, even though there might be detectable leakage current through the power devices that can be read with a meter, it is incapable of generating torque in the motor. So you can use STO for when the MACHINE needs to be stopped for normal process functions, such as loading or removing material, clearing jams, setting up templates etc., but you do NOT want to have to do a full power down with LO/TO procedures every time you want to access whatever that drive is controlling.

STO however does NOT make it safe to perform ELECTRICAL work on the machine, for that you still need to power down and use LO/TO procedures. The reason you want STO however is because for the routine every day stuff mentioned above, powering down a VFD and powering it up again every time puts stress on some of the VFD components (specifically the "pre-charge circuit"), leading to premature failure of the drive. So you use STO for those routing functions and hard power downs for electrical maintenance work. But STO can only achieve certain levels of Safety Integration and Performance, so a Safety Evaluation needs to be performed to see what's needed first.

How it works is that there is a verified safety circuit that, when the STO input is opened (or commanded via the network), directly shunts the transistor base emitters to ground, making it so that there is no way to make the transistors fire even if you wanted to. This circuit is hardware, not software and is AHEAD of any internal intelligence in the VFD. As such, the STO circuit then has to be tied to a Safety Controller of some sort. That's where the different versions separate;

  • The STO in the PF525 requires an external Safety Relay or Safety Controller that is hard wired into the STO terminals of the drive, even though you may be controlling the drive via an Ethernet I/P network.
  • The STO function in a PF527 is capable of being controlled via the Ethernet I/P network, because the 527 can ONLY be used in a network control application with a Logix processor with Motion Control capability anyway, so by using a Safety version, you can attain STO over that network.
  • In the PF750 series, STO is done with an add-on card, one of which would be hard wired (the 753 can only use that one, but so can a 755), the other can be done over the Ethernet I/P network (755 and 755T only, with caveats).
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Old June 23rd, 2018, 12:22 AM   #5
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lostcontrol is offline
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Excellently explanation jraef!!
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