[chjade84]

I have a water cooled 2.2kW ebay special spindle driven by a Hitachi VFD. Two 120 lines going to the input terminals of the VFD (for 240 volts total) and the earth connected to the grounding terminal. U, V and W output are connected to the spindle's three input pins, the fourth is not connected on the VFD end or inside the spindle (I checked). The spindle is in an aluminum clamp which is bolted to the machine (obviously) and the only other wiring going back to the control box are the stepper wires.

I was kneeling on the stainless control box in shorts trying to troubleshoot why when I turn on the spindle the steppers refused to run and several lights on the TinyG control board would flicker variably as the spindle changed RPM. I thought it was a noise issue but couldn't figure it out. Then, while the spindle was turning only several RPM I reached up to grab it and make it stall - which is super easy at very low speeds cause it has no torque. As I touched the spindle I felt the familiar vibration of AC current flow through me and was, pardon the pun, shocked!

Somehow the case was live. I assumed one of the solder joints on the spindle connector was touching the metal enclosure I just finished installing on the power connector so I tore it apart and checked with it off - this time with a multimeter. 120V from the spindle case to ground. I looked at a lot of things and finally removed both the spindle and VFD and put them on the bench so they were isolated. Still, 120V from the case to ground. This only happens when the motor is turning and it seems to ramp up to 120V through the low RPMs then stays the same.

I checked continuity between the input terminals and the case - nothing. Checked the resistance between each of the pins and it was 1.8 ohms, coil to coil. Infinite resistance between any of the pins and the case. It appears nothing is touching anything that shouldn't be but I'm somehow getting a steady voltage through the case of the spindle.

Finally I just ran a ground wire from the spindle to the ground terminal of the VFD and it is now not hot anymore, at least. It doesn't freak out the control board anymore so I can use the machine but that doesn't explain why this would happen in the first place.

The electrical engineers at work were stumped. Any ideas? Is it just mine or a thing with the cheapo Chinese spindles? I love it though, super quiet and about 0.0001 runout on the spindle. I can live with that for $180!

[Al_The_Man]

It has been discovered by those that dismantled some of these spindles that in spite of having a ground pin, the wire itself inside the spindle was actully insulated from earth ground.
Al.

[chjade84]

I did partially dismantle mine in an attempt to troubleshoot this and did see that the "earth" pin #4 was not connected to anything. No wire, nothing. Although I had heard this was the case so I didn't bother having an earth wire connected to it. (I assumed being held to an all-metal machine with something like 12 square inches of aluminum to stainless contact that I wouldn't need to.) I can add the wire no problem but is the voltage through the case a symptom of a three phase motor that isn't properly grounded (i.e. will all motors behave this way) or is it a defect or design flaw of this particular motor that is alleviated by just grounding the case?

Thanks!

[Al_The_Man]

It is possible to get inductive leakage that will register on a sensitive meter, but being Chinese origin, I would not stake a lot on quality of insulation.
Better safe than sorry.
Most 3ph motors have a definate ground terminal that is obviously connected to the frame, the Chinese not so obvious.
Al.

[hub]

I did partially dismantle mine in an attempt to troubleshoot this and did see that the "earth" pin #4 was not connected to anything. No wire, nothing. Although I had heard this was the case so I didn't bother having an earth wire connected to it. (I assumed being held to an all-metal machine with something like 12 square inches of aluminum to stainless contact that I wouldn't need to.) I can add the wire no problem but is the voltage through the case a symptom of a three phase motor that isn't properly grounded (i.e. will all motors behave this way) or is it a defect or design flaw of this particular motor that is alleviated by just grounding the case?

Thanks!

All the 3 phase AC motors I have used have ground (PE) connected to the casing.

[chjade84]

Oh, this was definitely not "sensitive meter" voltage, it was straight wall outlet, 120V 10A voltage - I've been shocked enough to know that feeling, lol.

[chjade84]

All the 3 phase AC motors I have used have ground (PE) connected to the casing.

Same here. I don't know why they didn't bother to with this one... just a tapped hole in the end cap and a 3" length of wire is all it will take.

[mactec54]

Oh, this was definitely not "sensitive meter" voltage, it was straight wall outlet, 120V 10A voltage - I've been shocked enough to know that feeling, lol.

You need to have a Ground wire from your spindle to the VFD, Then the VFD to the main input power supply, for safety these need to be perfect & you need to connect the Ground wire of the 4th pin to the body of the spindle

[hub]

Same here. I don't know why they didn't bother to with this one... just a tapped hole in the end cap and a 3" length of wire is all it will take.

Their quality standards/regulations etc aren't that good. One of the reasons the prices are lower

[chjade84]

Thanks for the response, but I do realize it really needs to be grounded - my question is more of a theoretical one as in why there is normally voltage present through the body of the motor at all times during use and if this is a common occurrence in all ungrounded 3 phase motors, all motors in general, all of these Chinese spindles or just a defect in mine.

I don't mean to sound ungrateful for the responses or anything. It's just that I'm aware of the safety problem of a lack of ground and that's been resolved. Now I'm more on the "why" of the problem - especially since either of the electrical engineers who work with motors daily could explain it or had even encountered it from a perfectly functional motor.

Like most of us DIY-ers I just have to know WHY!

[hub]

If I disconnect ground from the motor, I can measure a little over 100V AC between motor casing & ground. But I can't feel it when touching, too low amps I guess.
There might be an insulation problem. Windings touching the frame?

[chjade84]

If I disconnect ground from the motor, I can measure a little over 100V AC between motor casing & ground. But I can't feel it when touching, too low amps I guess.
There might be an insulation problem. Windings touching the frame?

Interesting...

I was thinking insulation at first but thought if that was the case I could read some resistance between one of the windings and the case but the meter just reads open.

[hub]

Interesting...

I was thinking insulation at first but thought if that was the case I could read some resistance between one of the windings and the case but the meter just reads open.

It might be different when the motor is running.. Just guessing.

[chjade84]

I'll try and measure that tonight but I'm not sure if you can measure resistance while there is current flowing through it?

[hub]

I'll try and measure that tonight but I'm not sure if you can measure resistance while there is current flowing through it?

Try measuring voltage & amps while running.

[dharmic]

Guessing it's inductive coupling - the motor windings set up a magnetic field per design, but that field induces eddy currents in the case and spindle which in turn create a voltage drop against earth.