Bypassing control circuit with VFD?

I'm moving from a location with 3 phase power to one with only single phase and in anticipation of the move I'm wiring in a VFD to run my lathe. So I got it all hooked up yesterday and everytime I try and kick the lathe in it trips the drive. It's a 220v 7.5hp motor and a TECO Fluxmaster 500 10hp drive. So I had an electrician in this afternoon who spent 3 hours trying to make it work (at $100+ an hour!!) and he finally shrugged his shoulders. He said that the drive appears to be set up and working properly, and we know the electrics in the lathe work, but for some reason it can't get past the control circuit of the lathe. I put a similar VFD on my milling machine and it worked well, but no control circuit, just a switch and a motor. So I decided that rather than continuing to try and troubleshoot this I'd just run the drive direct to the motor, bypassing the control circuit entirely. My lathe only has a single drive motor with a separately controlled lubricator motor, so I think that the functions within the VFD can take over all the functions of the control circuit. A few questions have appeared in my tiny little mind however:

1. Does this make sense!!???

2. My motor has six leads labeled UVW and ZXY. Just looking at the wiring diagram here, I'm assuming these are forward and reverse? So then if I wire my drive to only three of the leads, can I switch between forward and reverse electronically? I don't understand three phase...

3. Why aren't things ever easy, or cheap???

VFD is too small

whiskeykid said:

It's a 220v 7.5hp motor and a TECO Fluxmaster 500 10hp drive.

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I haven't pulled the documentation on your specific drive, so this is based on common application rules for VFDs.

When using a VFD designed for three-phase input power from a single-phase source, you must derate the drive by 50%. So to power a 7.5 HP motor you would need a drive rated 15 HP.

Connections to the motor should be the same as they were when running from true three-phase power.

There are only three wires. To reverse the motor you exchange any two of these; doesn't matter which ones.

- Leigh

Is your lathe a clutch type or a DOL type?

Can you post the error the VFD trips with?

The VFD should be connected directly to the motor and any existing overload (heaters) removed all control of the motor should be though the VFD control lines.

Did your electrician megger the motor? as some VFD's are sensitive to ground leakage.

HelicalCut said:

Is your lathe a clutch type or a DOL type?

Can you post the error the VFD trips with?

The VFD should be connected directly to the motor and any existing overload (heaters) removed all control of the motor should be though the VFD control lines.

Did your electrician megger the motor? as some VFD's are sensitive to ground leakage.

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I'm not certain what a DOL type is. My lathe is a Voest DA250. The motor is bolted directly to a two lever gearbox, and the spindle is belt driven from there, no clutch.

I'm also not familiar with the term 'megger'.

Oh, and the error code is just OC, over current.

Stop powering the control crircuitry from the VFD and wire the VFD direct to the motor. Power the control circuits from the line. That should solve the problem.

whiskeykid said:

I'm also not familiar with the term 'megger'.

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A megger is an ohmmeter designed to measure very high resistances, measured in Megohms, hence the name.

These devices typically apply 500 to 1000 volts to the test circuit. They're used to measure insulation resistance, as between a motor winding and its case for example.

- Leigh

Megger: There are two versions, the other measure very low resistance value, as in ground resistance testing.
In some cases both models are combined.
M.

WhiskeyKid

Your lathe is a DOL (Direct On Line) type, Being an OC trip I supect as Pierce Butler mentioned that your control circuitry (contactors and heaters) is still being used to control the motor rather than the correct way of using the VFD to control the motor.

Thye UVW and ZXY motor leads are NOT for reversing. It might be a two speed motor, but most likely it is an older 6 Lead Y-Delta wound Dual Voltage motor. In IEC countries, they do Dual Voltage motors differently than we do. For them, the two industrial voltages are 380V and 220V, so the ratio between them is 1.732:1, or the square root of 3. So connecting the motor leads in Y allows you to apply the higher voltage, connecting them in Delta allows for the lower voltage (the ratio difference between Y and Delta is 1.732:1).

You want the lower voltage connection (assuming you are using 220 or 230V power). There is then a specific pattern to which those wires must be connected. While it is true that you would only connect the VFD output to 3 of them (U,V,W by the way), the ZX&Y leads must be properly connected for your line voltage. For 230V operation (motor connected in Delta), connect U and Z together, V and X together, then W and Y together. If you left ZX&Y all tied together, you had a Y connection and the motor would overload relatively quickly when you gave it 230V because it was expecting to see 380V 50Hz or 440V 60Hz. Giving it 1/2 the expected voltage results in 1/4 the normal output torque, so the motor slip gets too high, the motor pulls twice the nrmal current, and the VFD trips on OC.

Also, even if you had that connection right, you CANNOT leave the control circuit connected to any of the output leads of the VFD. If the lathe was built in Europe, they typically used 220V control, but that is phase to ground, not phase to phase, so one side is likely grounded and you are shorting the VFD output.

Ok, got it all hooked up today and it's working great. I failed to mention that the motor was dual voltage (hence the six wires) so it took some buggering to get the drive wired in, but I finally got it running. I wired a potentiometer into my drive and mounted it to the panel on my lathe, it's really nice to have that kind of fine tuning at hand. I would like to fart around and get the safety switch on the apron working, but other than that it's golden.

Thank you everyone for your assistance.

Wire the safety switch to the VFD!

The safety switch is easy- wire it's contacts into the start/stop control wiring of the VFD. In doing so, you'll allow the VFD to perform dynamic braking for a very fast stop. No fancy engineering here, just use it as a dry contact, and loop the Stop/Run signal through it.