Vfd and motor isolation

Densam · Mar 9, 2016

I have seen two systems of isolation 1.isolator between inverter and motor with this all three phases go through the isolator.
2. The phases go straight to the motor and we isolate the control which I turn stops the inverter
Which one is most recommend. Thank you.

keithkyll · Mar 9, 2016

3. STO. See this thread.

Densam · Mar 10, 2016

That's great thanks keithkyll

James Mcquade · Mar 10, 2016

The UK has rules for this and you MUST abide by them.
Can't remember the section, sorry.
Its in the European Union guidelines.

cheers,
james

nehpets · Mar 10, 2016

I have always installed a local isolator to the motor, I think somewhere in the old IEEE regs it used to stipulate that there should be local isolation within three metres of the motor, that said, it is easier to fault find a system fault if you can isolate the motor but have the VFD running, besides I know personally, I would rather trust an isolator than a bunch of semi-conductors that should be off.

Steve

Pete.S. · Mar 10, 2016

What exactly do you guys mean when you say "Isolator"?

And for what purpose exactly?

nehpets · Mar 10, 2016

A local disconnect switch

Steve

Pete.S. · Mar 10, 2016

Thanks Steve.

Usually a local disconnect switch needs to be local so it's connected after the VFD.

And the switch is also routed back to the PLC so the program knows that the switch is disconnected. That way the PLC doesn't send a start signal to the VFD.

STO (Safe Torque Off) is not the same thing. You could use STO with or without a local disconnect switch. STO is to make sure the motor doesn't move (and hurts someone). There could still be a voltage on the motor windings.

From ABB drives manual:
The STO may be used where power removal is required to prevent an unexpected
start. The function disables the control voltage of the power semiconductors of the
drive output stage, thus preventing the inverter from generating the voltage required
to rotate the motor (see diagram below). By using this function, short-time operations
(like cleaning) and/or maintenance work on non-electrical parts of the machinery can
be performed without switching off the power supply to the drive.

If it's a machine it's the machinery directive that is the law in the EU. Basically it's the risk assessment that controls what you can and cannot do. Safety functions is in the ISO 13849-1 standard.

damiens · Mar 10, 2016

ABB probably makes the best low voltage disconnects but their product support is horrible.

jraef · Mar 10, 2016

STO is for use in part of a safety procedure when working on a machine doing normal tasks without having to lock-out and tag-out a disconnect 20 times per day.

The disconnect (isolator) is still required if someone is going to want to work on the MOTOR. Where that switch is located is subject to local codes and circumstances. Here in North America we use the term "within direct line of sight" to the motor. But even then there are exceptions.

YMMV.

DickDV · Mar 11, 2016

A power line disconnect AHEAD of the VFD is acceptable for PowerDown isolation but it is not acceptable for E-Stop purposes. Due to the stored energy in the VFD, the output can remain energized for seconds and even minutes after the input power is removed. jraef's description of Safe Torque Off and motor isolation applies if protection is to be done on the output side of the VFD.

shooter · Mar 14, 2016

option 1 is safer, however before opening the switch between inverter and motor be sure the inverter is at stopped position. thus an interlock is needed.

Vfd and motor isolation

Densam

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keithkyll

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Densam

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James Mcquade

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nehpets

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Pete.S.

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nehpets

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Pete.S.

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damiens

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jraef

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DickDV

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shooter

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