Schaublin 125-CNC retrofit.

The motor has an over long shaft to mount the variator. If I replace the motor with a generic induction motor, then I will also have to replace the variator with a simple belt drive, probably poly-Vee.

Peter (Trilobite) replaced his motor, and used a VFD. I am tempted to risk it as is. If I kill this motor, it is no big deal, then I have to replace it and replace the variator with a poly vee belt drive. It would seem the risks are a slightly increased fire hazard and the financial risk of the insulation shorted motor taking the VFD with it over the Jordan.

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

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.

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.

Short question to the VFD. Would it be OK to use the VFD as a soft statt or should it start at 40hz right away?

andypugh wrote:

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.
.

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

It probably helps a bit by reducing the phase--> ground high frequency currents
that cause insulation heating. A real load inductor on the VFD outputs would help a
lot more (by greatly reducing the high frequency components in the VFD output)

This kind of device:

www.automationdirect.com/adc/shopping/ca...ine_reactors/lr-4015

Darn, I just found another strike against doing the speed control solely though the variator and Dahlander windings...
The cons :
- Needs the 5 big contactors
- Needs 48VAC transformer for control of the existing contactors (48VAC was just one tap of an enormous Transformer which is not going back in).
- Replacing the 5x 48VAC coil contactors with 24VDC coiled ones will be nearly as expensive as a VFD.
- Needs the 126-19 Relay board.
- Can't really do CSS any way.

Pro
- Stump pulling torque at low speed.

Realistically this is a small lathe, with a max swing of only 270mm / 11". A 4hp motor running through the 1:6.5 back gear is probably going to have enough torque at the spindle to make decent depth of cut even at resonably low motor supply frequency.

I am tending more and more to simplifying it and just using a VFD. I could still connect the variator control, and position it manually to bias to torque or speed, but not use it in the primary speed control loop.

I appreciate your feedback and discussion. You have brought up a number of aspects I didn't consider, so the chat really helps to drill down on a solution.


Mark

The motor contactor was probably electrically sized for motors locked rotor amp rating being higher amp than the running amp requirement. So they are larger and have more robust contacts than a lower amp rated contactor .
SSRs can have some nice features such as zero crossing turn on / off -- phase on / off , but not aware if they are rated for motor control applications .