VFD enclosure with no external controls. After looking at the link I think I'm missing something.

[rons]

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All I have is a door with interlock switch. The is a 240 VAC to 120 VAC transformer and the VFD.
The 120 VAC is used outside the VFD enclosure box. It powers a digital pressure switch and some other circuits.
From the small 120 VAC box a command signal is returned to the big box with the VFD.

I think at the very least I want an emergency off button.
The way it works now is apply power to everything when the door knob is rotated right. A rotate left disconnects all power and the door is ajar.

One thing I like about the Invertek VFDs is the external rotary switch. So I was thinking about a rotary switch with
three positions.
Center: off
Right: Normal On
Left: Service Mode

 

[BajaFire]

What's your question?

 

[JST]

You are wanting an E-Stop. One way to do it is to wire the E-stop in the power to the box. You would need a high power E-stop switch. Hit the switch, and all power is cut, no questions, no nonsense.

To just cut power to the motor, then it should be able to go in the demand switch line from the pressure switch box back to the main box. It would cut the control signal that closes the motor contactor.

That would not be hard, but it would also not cut off power if the contactor got stuck, etc, so it is a lower grade of E-stop than one that chops all power.

 

[SomeoneSomewhere]

E-stops are usually used where rapid response is required, such as risk of being trapped or caught on a rotating part (guarded belts?). For general locking out for maintenance, a lockable rotary isolator is best (like you have). Also good for remote locations.

Critical E-stops are usually monitored by a safety relay because they can fail invisibly - contactors weld or stick, buttons get gooed up, wires get shorted together when the cable gets squished. An isolator breaking the main power is much simpler and more reliable.

A separate manual-off-auto control switch (i.e. not safety) is fine, though I'm not really sure why you'd need one on a compressor.

 

[rons]

What's your question?

How a professional design would sequence the power and provide for a standby state where a guy could test wiring, change parameters, etc.

 

[rons]

E-stops are usually used where rapid response is required, such as risk of being trapped or caught on a rotating part (guarded belts?). For general locking out for maintenance, a lockable rotary isolator is best (like you have). Also good for remote locations.

Critical E-stops are usually monitored by a safety relay because they can fail invisibly - contactors weld or stick, buttons get gooed up, wires get shorted together when the cable gets squished. An isolator breaking the main power is much simpler and more reliable.

A separate manual-off-auto control switch (i.e. not safety) is fine, though I'm not really sure why you'd need one on a compressor.

Right now my E-stop is to rotate the door handle from locked to unlocked.
The thought of a runaway compressor makes me ask what more I could do.

 

[rons]

To just cut power to the motor, then it should be able to go in the demand switch line from the pressure switch box back to the main box. It would cut the control signal that closes the motor contactor.

That would not be hard, but it would also not cut off power if the contactor got stuck, etc, so it is a lower grade of E-stop than one that chops all power.

The motor connects directly to a VFD. I could cut the signal to the digital input on the VFD. Hopefully the VFD will always respond.
Maybe one of those e-stop buttons that double as a power switch. I mean the one that twists and pops out then push in again.

 

[SomeoneSomewhere]

Many drives can be powered from a 24VDC source, which enables the control electronics but nothing more. That's an option, as is fitting a remote HMI to the outside of the box. Or just defeating the interlock, which is what happens 95% of the time.

Normally, an E-stop wouldn't merely switch a digital input - wrong programming or a software error is too risky. Standard is either contactors to disconnect incoming power to the drive (duplicated and with safety relay monitoring if the hazard justifies it), or a drive with a duplicated safety-certified 'safe torque off' input.

Having a second, backup overpressure cutout (that doesn't rely on software) is a good idea, but doing it via human e-stop and intervention IMHO isn't really sensible.