I have a customer w/ a 480V 3/4 HP motor with a break and a 24vdc powered encoder.  They are looking at installing a filter on the motor to prevent noise on the encoder?  Not sure this will fix it?

Is the encoder cable run with the motor leads? That's the only time I can think of that I ever had problems with noise on an encoder.

In my previous life we had similar hook up but motor@ 600 volt ;we had noise problem we were able to have corrected by seperate adding a ground at encoder to motor frame.

The Encoder Cable must be shielded. The shield has to be only connected at the electronic chasis. The Encoder's Electronics DC Supply must be isolated from the Motor Power Supply. The Electronic's
0V must also be isolated from electonics chasis.

If the encoder is a single signal (incremental), you should run a signal and its inverse to the receiving end, That way the common mode noise can be rejected with a differential amp. You may need some 'glue logic' at the encoder.

I agree with Farzad and Jim, from aspects of installation you need to choose a well shielded calbe segregating from HV cables addtionally if it's an incremental encoder the configuration of bipolar is a good noise-immunized,which means you need 4 wires A/A-,B/B- at least from the encoder. If it's extreme circumstance you can choose a fibre as media.
of course installing a reactor /dv/dt filter to the output of aVFD will reduce common noise and a shielded power cable is highly recommended to use.

You can buy VFD's that have the chokes/reactors in them and may or may not need external reactors

Depending on the speed range required you can get an AC motor rated 20:1 constant torque that may mean you dont need the encoder

i agree with michel grossi, the vfd will correct the noise on the line. in todays world the vfd at 460vac is very inexpensive, i prefer A-B

You also need to look at how far away the encoder is from where you are taking the reading. Along with shielding, you also want to try and route the cable to avoid interference with anything else, some might call that isolating.