FOLLOWUP -- Lathe and VFD -- Some results

Sounds good.

You probably can set a 1 second rampdown and it won't trip, or put the braking resistor on.

Be sure to leave good air circulation, it will generate some heat.

I still need to make a shroud so that there is no direct path for chips to fly into the unit. My VFD is up above the machine on the headstock end.

Good Luck, Bob

I will experiment. The issue is, obviously, higher spindle speeds, requiring longer rampdown.

The other thing I can do is this: the limit switch for tutrning the lathe on and off, has a normally closed and normally open switch. I obviously use the N.O switch for RUN. But I can use the N.C. switch for RESET.

The base is very big, there is plenty of space, and also the motor is in the providing some extra circulation. Should be fine.

The first of those VFDs is doing nicely on my Bridgeport. The second, along with the 3HP motor from my former RPC is looking like it will end up replacing the aged 1/2HP motor with failing starting cap on my lathe. I really need to find a bigger lathe too...

That's very nice to hear. I am delighted.

By the way, starting caps cost relatively little.

Why not? There is room to install it, being a 3ph motor it's pretty compact, and being on a VFD I can set limits wherever I want.

Yes, but I'm too lazy to chase after one for a motor I don't care for anyway.

Braking is GOOD, a real safety feature. Even if you have to cobble up a braking resistor, it is well worth it. I get less than one second stops on my 15" Sheldon, with a pair of 100 W vitreous enamel resistors wired up to the braking circuit.

Yes, that would be really nice. I have the original forward/reverse/stop buttons on my Sheldon hooked to the VFD, and it is quite nice. I added a jog button, that is nice for when the gears don't engage.

Jon

Sounds like fun. I hope that if something gets stuck, the bigger motor would not break gears or something.

Just have to set the current limits appropriately. Also with two stages of belt drive, a slipping belt is more likely.

Does your sheldon include a mechanical brake?

Are your 1 second stops, actually happening at high spindle speeds?

My drive already has a braking resistor, IIRC. Some of mine do and some don't. But even if not. I have some resistors.

I already did so, it is awesome.

What is your jog frequency?

Myself, when gears don't engage, I just grab the chuck and rock it a little.

What you may find, and here I am NOT speaking from position of knowledge, is that your big motor will draw big current even at low load. What will change with load, is more power factor than current.

But who knows.

You're absolutely correct. I tried it to detect jams on a conveyor. The current difference between unloaded and jammed was too small for the VFD to discriminate. My lathe runs unloaded at about 60% current, it's grunting at 70%.

you REALLY DO NOT want a rampdown if you are going to use the mechanical brake - set it to coast

Correct, else the VFD will be fighting the mechanical brake

I had some bad news last night.

As the drive was running the motor, the motor leaks a lot of current through its insulation. (like Joe Gwinn story) Not good. Phase converter does not create this effect, so it is due to voltage spikes from the drive that this old insulation cannot handle. I do not want to ignore this issue by grounding, as I feel that it can ruin the motor.

I may try to locate a line reactor to dampen this, and shorten some cables, but for now I will go back to the phase converter.

You don't quantify the amount of motor insulation leakage current, or describe how it is that you became aware of it.

This effect is probably present in many home shop machines that users have converted to VFD operation, of which many of them are probably unaware.

Proper earth grounding of a machine is mandatory, regardless of what the power source is. There are no perfect insulating materials, only good ones and better ones. Insulation in motors is rated for maximum voltage and temperatures.

Once the insulation materials are compromised, (arc-over or similar path is established), in any electrical device, the earth ground path is critical for safety.

The motor insulation current leakage (in power line operated, or VFD-driven motors) is documented for those interested in looking for the info.

Motors that are rated for VFD operation are typically labeled Inverter Rated Duty or something similar. The insulation in these motors is specifically chosen to minimize/reduce leakage currents for the fast rise times of the VFD output voltages (variable time square waves, in appearance).

A motor that isn't rated for inverter duty could be a dual voltage motor rated at 240 or 480VAC. If the motor is operated with a VFD output at 240V, the insulation is generally up to the task of minimizing the current leakage. If that motor were operated with a VFD output of 440V or greater, then the insulation's intended rating will be exceeded.

WB ......... metalworking projects

Dropping the drive's carrier frequency should also help, and is free. You may notice more audible noise. Some drives also have a setting that softens the corners of the chopped output and that will also help. I assume this lowers the efficiency of the drive, but that's presumably of little concern here unless the drive is not well ventilated.

I measured it yesterday with an clamp on ammeter, it came out as 9 amps (!). Really a shocking amount.

that's why I decided to discontinue.

This is a single voltage (two speed) 220v motor. :-(

Nine amps?...!! If that's true, the motor is toast, and should be replaced. Immediately. When we talk of leakage, we mean a few milliamps.

The VFD is not causing this.

Unless the clamp-on is confused by the high-frequency output of the inverter drive.

One way to tell is to hook all three windings of the motor together and connect them to 110 volts through a light bulb (not through the VFD), and measure the current to ground. In a perfect motor, the current will be zero. (The motor will not even try to run, or be in any danger, and single-phase is what to use.)

Do not use an ordinary ohmmeter for this. The test must be made with full power voltage, or more. Or a megger, if you have one: .

If the current is in the amps, at least one winding is shorted to the frame, and the motor must be replaced or rewound.

This is *exactly* why one firmly grounds the frame of a machine tool.

Joe Gwinn

Probably a measurement error due to spikes.

No leakage without VFD.

That's my feeling.

I think that I have a megger.

I think that the motor is not leaking 9 amps and it was a measurement issue. I will try using a analog clampon next time. But there is leakage of some, unspecified amount.