Schaublin 125-CNC retrofit.

Aciera wrote:

I know there are people out there who have been running motors with isolation class B on VFD's for years. I guess what I'm trying to say is that using a VFD on motors with isolation class lower than F can indeed destroy the motor. So really it comes down to how badly it's going to hurt to replace that motor.
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I wonder if it is relevant that I have the motor cables running through a common-mode chole at the VFD outlet?

If you have 3-phase and you are keeping the variator then you have no reason to run the spindle motor from a VFD.

Does insulation failure and shorting cause a fire risk outside the motor, or just a stinky dead motor?

I my case it just ended with an insulation break down to the metal body (ie. phase-earth fault) so the circuit braker got activated. If you have your motor properly grounded and your electrical installation is in order I don't see a terribly increased risk. Although having said that, I once witnessed the windings of a rather small 3kW motor go up in smoke in a sawmill. The amount of smoke coming out of that thing was unreal. Even after cutting all power to it it continued to smoke for what felt like a very long time. There was no VFD involved though.

So what did I learn from this?
If I do work for somebody else I refuse to hook up a VFD to a motor that is not isol class F but if it's my shop I might try.
Not sure if this helps you in any way though.

[edit]
Really considering that you have a way to automate the variator I don't see why you would need a VFD. I'm not sure a VFD will actually save all that much space if you modernize the contactors but I might be wrong.

[edit2]

I wonder if it is relevant that I have the motor cables running through a common-mode chole at the VFD outlet?

maybe, but really I have no idea. To be honest, I didn't bother much with insulation classification before that motor on my mill failed. Just like I used to think electromagnetic shielding was mostly a nice to have but not relevant in my life. That one really hit me on the head one day and I shield everything now.

RotarySMP wrote:

Space.
I removed the enormous electrical cabinet, and need to shoe horn everything into the pneumatic bay. 5 contactors and their wiring take up more space than one contactor and a VFD.
Also those existing contactors need 48VAC for their coils. I am pulling the huge Transformer, so replacing all of them is dearer than a VFD.

How about using SSRs controlled by HAL logic?
www.amazon.de/-/en/X-DREE-3-32VDC-semico...16efccb9d559b528ebea
Easily triggered by GPIO, so no 48VAC needed. You would need one for each direction of the Dahlander, I assume. I would probably keep one big clunky contactor as the master power breaker.