Mitsubish FR-A8AP resistance across terminals

[pedrovg 1]

We are working with two VFDs FR-A820 Series, each of them driving a motor that obey a master-slave relationship. One of them is in speed control, while the other is in torque control.

Each of the VFDs has one of the Mitsubishi FR-A8AP encoder reading cards that collect the data from an Avtron A56 encoder. While testing our system it came our realization that, if one measures the resistance across the terminals of the encoder cards mounted on the VFD these are not the same. Concretely, this is the reading we get: (see attached)

 

We noticed that this difference in the resistance causes a voltage drop in the encoder signal when looked over a oscilloscope. We are surprised that this resistance is different: a) between the cards, b) between the terminals A and B in the VFD.

We wanted to ask:

• Is this behavior expected? Should the resistances in both cases be different?
• Is this something we should be concerned about?

So far our application works and our motor turns as they should, but we are quite surprised that this is the case.

Edited 31 Oct 2023 by pedrovg

[Dave W. 15]

Hello
I'm not sure if I understand your question correctly.
If you measure the resistances while the encoder is connected and supplied with voltage, then no reasonable value can be measured.
If the encoder is connected but not supplied with voltage, it can also be the encoder that leads to these measurements. Depending on whether track A or track B or both or neither is active, this could lead to different resistance values.

BR, Dave