Need A.C current (high) measuring electronic circuit for conductive filament

[shriragini]

Hi,
Actually, I have a mechanism to protect motor bearings from variable frequencies of the shafts for a.c voltages.In the phenomenon,iam using an external conductor filament(fixed to motor) to attract the shaft currents and ground it,thus preventing bearings from getting damage.My requirement is I want to measure the current from the external conductor ring to make sure that it is working.So please suggest a method to measure the current drop.


ThanQ....in advance.........

 

[duffy]

For AC current readings you can use a clamp-on or feedthrough transformer, like this one:
https://www.electro-tech-online.com/custompdfs/2012/06/csla2cd.pdf
The magnetic field is proportional to the current.

The problem is a little confusing, I know bearings can be ruined by galvanic effects arising from a voltage differential, but I don't understand why the motor shaft has any current running through it at all...?

 

[cachehiker]

We electrically isolate the motors and then ground the frames through an ammeter to assess bearing currents in PMDC designs.

The problem with doing that so easily with our induction motor systems is the capacitive coupling between the windings and the stator dominate and it is difficult to assess how much current is coupled through the stator and how much is coupled through the rotor.

I think I would pursue a mounting means that included separate grounds, one going to the motor frame and one going to the shaft grounding. The results might be useful but I don't thing they would be particularly accurate. When we measure across a 1.0Ω shunt, we get all sorts of different measurements depending on what meter or scope we're using.

The inductance of the motor resonates a bit with the scope probe or meter capacitance. With such a small signal and only 1.0Ω of damping, the resulting ringing can throw off the measurement by a thousand percent. We have to calculate and insert an appropriate amount of damping (30KΩ last time) between the motor case and the scope probe and correct for it later if necessary.

Let's say it brings consistency to all the meter and scope measurements but we don't consider the exact results entirely trustworthy.

but I don't understand why the motor shaft has any current running through it at all...?

AC current flows in the rotor at a frequency proportional to the motor slip. Unavoidable imperfections in the motor construction create enough eddies and asymmetries that it just happens at high frequencies.

 

[duffy]

Is it not practical to insulate the bearing in the bearing-block, or is the problem that the insulator would have to be so thin that the capacitive reactance would be low enough to permit conduction at high frequencies?

 

[cachehiker]

It depends. The bearing currents are often low enough that a bigger bearing is often cheaper and easier. Plastics used to support live parts or for insulation in higher voltage systems have to be flame rated. Flame rated plastics are neither cheap nor easy to work with. The bearing current issue is generally associated with high reliability, long life applications or Harbor Freight / Walmart motors for fixed speed appliances that have been cheapened up so much that the now tiny bearings will no longer tolerate inverter drives.

Ungrounded moving parts also generate static charges. When the static charge accumulates to the point where it jumps across the nearest gap, it can create ground bounces or other spurious signals and general interference. One of our famous experiments had an isolated 120VDC motor being run as a generator, picking up charge from the belt like a Van de Graaff, and creating loud, 2-3mm arcs every few seconds.

If you were watching the big game and the blender being used to make margaritas was arcing to ground and pixelating everything every few seconds would you be happy?

About half of the cheaper, smaller motors I work with have one metal end cap and one plastic end cap. The first for cost (a simple stamping) and the second because of a complicated shape that integrates some electrical isolation. The grounded (drive end) bearing is big and handles up to about 8-10mA of bearing current. The isolated (fan end) bearing is only half the size and sees no bearing current.

 

[shriragini]

Variable frequency drives which is in contact with the motor is continuously rotating which inturn produces currents which passes through bearings of the motor.So these damaging currents entering through bearings may cause bearing damage and motor premature failure.To prevent this,damaging currents are attracted by a conductive filament (circular ring body fixed via motor shaft to motor) and are grounded,thus protecting the motor bearings.

In my research I found a method to measure the actual bearing current produced while variable frequency drives are acting upon which is helpful to know the amount of current produced totally because of VFD (variable frequency drives).

What I want next is,a method/circuit to measure the bipassed currents through conductive ring i.e conductive filament and to know whether the conductive filament is properly working or not? means all the current is getting bipassed through the conductive filament or not?Can U please help me.....???

 

[johansen]

wrap a rogowski coil around the motor shaft, inside the motor.

then, disconnect the winding shield.
next, put a shunt inline with the shields and read the current.

 

[cachehiker]

Rogowski coil. Thanks.

It might be a little overkill for most of what we do but the next time middle management gets funneled down through all the "expert" design engineers to me, a lowly quantitative analyst and testing engineer, and resents hearing something they don't want to believe, I've got another tool to prove they're up in the night.

That's why I like posting (or is it mostly lurking?) in this forum.

 

[johansen]

you will need to wind a custom coil, or contact (the only company i know of is pemuk dot com) and they may have coils intended for this purpose.