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brianafischer

Encoder Connections to VFD

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In the process of designing a closed-loop motor control system, I noticed some conflicting information regarding encoder connections. The system utilizes an Emerson Control Techniques UniDrive with a Danaher encoder. After thoroughly reading the documentation, each manufacturer seems to recommend conflicting advice! Please see the attached excerpts from the user manuals. Emerson Connect the cable shields to the 0V common and clamp the encoder cable at BOTH ends. I also don't appreciate the following verbage: It seems that if my case of doubt so happened that the encoder case WAS grounded (through the motor and surrounding frame), and I connected the encoder case to the drive ground, I would be creating a nice ground loop between the motor ground and the encoder! Danaher The standard practice I have seen: Ground the encoder case ONCE using either the mounting bracket or the case lead (if no continuity to system ground on the case lead) Use twisted-shielded pairs Connect the shields to GND only at the instrument end I find the Emerson manual quite interesting. So, the question of the day is: when have you used the Emerson technique and how/when is it applicable to ignore the encoder/sensor manufacturers suggestions and follow the drive manufacturers suggestions? Thanks, Brian danaher_encoder_suggestions.pdf unidrive_encoder_suggestions.pdf

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There is no black or white rule here as it all depends on the particular encoder construction (isolated body, earthed common etc) and the frequency of the signals you are trying to protect. With RF (radio frequency, above 1 MHz) signals "plane grounding" techniques must be used as connection inductance (to ground) comes to play. In effect you ground the shield at as many points as possible to ensure a minimum inductance connection to reference (the "ground"). At lower frequencies, wiring inductance is manageable and "single point grounding" techniques must be used. By grounding at a single point you ensure that no current flows through the cable/body shield (and in effect turning it to a noise generating/receiving antenna) and that no noise-generating "ground loops" are formed. Grounding should ideally be done at the receiver or weakest signal end to ensure minimum noise at the input processing stages. In industrial electronics most situations do not involve RF signals and single point grounding is preferably used as it does not interfere/mix with any protective/safety earth (unless intented). So the way to go is to take into consideration the manufacturer instructions (the encoder construction), the signal frequencies involved and the overall wiring/cabling layout to judge how noise is generated and conducted/transmitted. I would start with the manufacturer recommendation if their example/model is the same as the installation at hand (in theory they have checked it and works OK) or with "single point grounding" and make the final decision in the field. Regardless, a tidy and clean installation with good ohmic connections is a must before looking at any noise problems.

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