Definitely change the plug into one that has earth as well, and make sure the motor body is grounded to the frame, and that the frame is grounded to the earth connector in the new plug

The limits we look for in these cases (in germany) are:

1. less than 50V AC and
2. less than 0.5mA

(This is for touchable metal parts, that are not connected to earth)

So yeah, I'd try to avoid touching the motor spindle, but with that low of a current it shouldn't actually be a problem as any current flow at all will make the voltage drop significantly.

You can try grounding as many of the metal parts as possible, but the motor shaft will likely be difficult

Your last image shows that your sewing machine plug has no earth ground. I’m not well versed in sewing machines enough to know if this is typical for sewing machines, but typically any appliance with no earth ground will insulate the user from either conductor.

You cannot count on the neutral wire to safely ground anything that the user touches.

Your readings indicate that there’s a small amount of voltage coupling onto the housing. Voltage alone doesn’t indicate much if there’s no current behind it, so it could be safe, but as another user indicated, you probably want external metal bits to be earth grounded. That’s the whole point of earth ground.

I would say you're right about it being just parasitic/capacitive current. And 4.1 uA and 43uA is thousands of times smaller than what would be dangerous, so that's a no for the kill you part. If you're very woried about the reading, you could add a 10k-100k Ohm resistor between the machine body and the ground while measuring, if voltage drops to near zero it's fine.

If you decide to add a ground, please don't do it with a second plug. Replace the cord with a 3-core one, attach PE to both the machine body and motor.

30V won't kill you. Even with 170V it might not cause much harm, since the current is in micro Amperes. I hope you have a good MCB, and even even good curcuit breakers. If they are present and they don't get tripped while operating or testing, it should not be lethal.

That said, you don't know the reason for this leak. Better to take it a repair shop. If you like doing it yourself open up and see if there is any visible tear on power line insulation. If nothing is visible if can be some issue with winding of motor, some other component.

You can use it with gloves, a good foot wear, and a good set of circuit breakers, if it has to be used, but I won't recommend taking such risks. Since it is an old machine I would say you get it repaired if possible.

Smells like phantom voltage/current to me. Get a high value resistor at least 100kOhm, preferably 1MOhm to start with. Connect the chassis to ground via that resistor and take the same measurement again. The voltage you are seeing should collapse.

For reference, RCD (current leakage safety switches) pop at abour 30mA depending on the local rules. So you are orders of magnitude under that.

Cheap multimeters can show weird numbers when measuring high-impedance floating metalwork, especially around mains wiring and electrical noise. That does not prove it is safe, but the raw voltage reading may not mean what it first appears to mean.

The tingling I wouldn’t trust just because, tingling… BUT it can well be a thing with non grounded or poorly grounded gear. I get to tingling from my laptop even though it is connected through a power pack.

The big issue with metal chassis and single insulated housings is that without a ground if it does develop a fault (say something hot melts abd grounds out and r a motor winding cracks), then you are genuinely hot with nowhere for current to go. It really should be grounded somehow if it’s an old chassis that isn’t designed with multiple insulation.

At the very least. Get a safety protected plug (residual current device). They are cheap and effective. But they are you last line of protection not your first.

• not an engineer. YMMV. I don’t know anything and my advice probably faulty .. etc. but definitely if you know what you’re doing and can do it safely: test voltage with a big resistor (should be an easy test), get safety switch.

Thats a weird sympton. There usually isn't alot of electrical wiring in that kind of machine. Have you checked the wiring and socket for the light bulb? Maybe something with the power switch?

Seems unlikely that the new motor is bad in the same way as the old.

Okay this is fun. So first of your amp reading is no Bueno. As I can tell you have no real load on the circuit so your amp draw will be low. Until something acts as a load like a light bulb contacting the frame and the neutral line or your hands. So the amp will rise.

Now the next thing there is power going to the motor, with out the foot pedal engaged?

A quick hunch makes me say is the new foot pedal wired backwards? On ac circuits it's you cut power at the switch and the rest of the unit is connected to neutral line all the time.

Those sowing machine motors are typically "universal motors" they are brushed motors that can run on both ac and DC. So power flows from footpedals to brushes to armature, to windings back out the other brush. It's a bit on the weird side that the shaft has voltage. It could be from a brush sitting funny maybe. But I would most definitely just check the wiring orientation for starters. Then go from there. Adding a ground connection isn’t a solution. A ground wire is to prevent you from getting hurt when this happens.

@einfach_orangensaft Measure current. Might be capacitive through capacitor in motor. And check your GCD.