Eliminating Motor Shaft Currents through Tachometer & Encoder

You can also use a lawn mower or a toaster with not much different effect on reducing your bearing current S.M.

I can't even imagine why you would even consider an encoder for earthing?

Why are bearing currents a problem in the first place?

The mind just boggles!

But I do like the idea of a lawn mower running against the shaft

JRaef, I have worked on insulated bearings and seen them fail for silly reasons. So I do know why they are there.

One silly reason was someone innocently leaving a crowbar leaning against an insulated pedestal.

Thanks for your reply.i had already gone through the link you have passed on regarding baldore -inverter driven induction motors shaft and bearing current solutons.but recently i came across a paper ------------eliminating damage from motor shaft currents through tachometers and encoders. by brain w.winter of avtron manufacturing inc.i am reproducing his introduction para below.

since the introduction of variable speed electronic controls for dc and ac motors users have noted a troubling problem, voltage potential is sometimes induced on the shafts of the motor.the resulting current can discharge through gearbox bearings,through motor bearings or through the encoder or tachometer attached to the motor.this discharge causes the premature failure of motor bearings and /or encoder bearings.

kindly guide me thanks

This just means that tachometers and encoders CAN be damaged too. Nothing else. S.M.

The author only has cautioned that shaft currents might find a path thro the tachometer / encoder. That definitely does not mean that they are used for routing the shaft currents.

I think you need be tuited on English more than on electricity.

The most effective way of reducing these effects is to filter out the transients before they get to the motor. A sine wave filter, or isolating transformer are the preferred methods. These also allow the motor to be positioned further from the VFD and also generate less interference along the wiring.

Baldor / Reliance / ABB / Avtron have it right. They are not in collusion with each other.

I have seen instances with VFD's that used insulated bearings and then a tachometer was added with a conductive coupling. The result was damage to the tachometer bearings due to the electrical discharge now occurring through the tachometer bearing. The solution was to use an insulated coupling, so the shaft potential no longer discharged through the tachometer bearing.

thanks for the reply.Still request all the forum members to let me know is encoder / tachometer used for eliminating shaft current.or these are now obselete methods.your reply will be highly appreciated

NEVER USE an encoder or tachometer as a means to discharge shaft currents. It takes the bearing right out of the device.

Protect your instruments, don't abuse them!

Plain enough?