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I have been trying to choose incremental rotary encoder for speed control of three phase induction motor. My problem is that I am not sure what type of output should I choose. There is following quadrature encoder interface (please see the enclosure - signals from encoder are connected to terminals 1,2,3 in J9) on my control board. I don't know whether this interface is intended only for incremental rotary encoder with open collector output or it is possible to use different output types. Can somebody give me an advice. Thanks in advance.enter image description here

I have found an encoder which is sufficient in terms of shaft diameter, supply voltage and output type. There are two parameters that are confusing for me in datasheet. The first one is maximum current requirements 180 mA. I suppose that this current is the maximum current that can be sink by the encoder's output transistor. In my case the transistor must sink \$\frac{5\,V}{4.7\,k\Omega}=1.06\,mA\$. Could this be a problem? The second confusing parameter is output load \$50\,mA\$ (including \$3.3\,k\Omega\$). What does this parameter mean? (\$\frac{5\,V}{3.3\,k\Omega}\neq50\,mA\$). Here is the datasheet of chosen encoder: enter link description here

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  • \$\begingroup\$ Please include links to any parts referenced in your post and repost that image - properly sized and cropped - so that it's legible to any future readers. That looks like an incremental encoder with open collector outputs. For speed monitoring you just need one output. \$\endgroup\$ – Transistor Jun 23 '16 at 8:07
  • \$\begingroup\$ Thank you for your reaction. So you mean that I should choose only the encoders with open collector output? \$\endgroup\$ – Steve Jun 23 '16 at 8:31
  • \$\begingroup\$ I think the board is suited for open collector 5V encoder. \$\endgroup\$ – Marko Buršič Jun 23 '16 at 8:52
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Reading your schematic:

  • I see the A and B encoder phases which are standard for incremental encoder. - The "index" signal usually pulses once per revolution and may or may not be useful in your application.
  • The inputs have 4k7 pull-up resistors and this suggests it is designed for open-collector encoder. Since the pull-ups are connected to +5 V they will also support a 5 V TTL output encoder.
  • There are RC circuits to ground on your inputs. These seem to be performing a filter function and since the resistance is relatively high they won't affect the pull-ups significantly.

Usually encoders are used for fine position control and VSD/VFD induction drive doesn't do this very will. A more typical application is to fit a tachometer or encoder feedback to monitor motor speed. In this case either the A or B encoder output would suffice. In your case, however, the drive seems to be designed to accept A-B-Z type of encoder so you should be fine.

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  • \$\begingroup\$ Transistor, thank you for nice explanation of my circuit. It is very helpful for me. Only for assurance, the terminal number 4 is intended for encoder power? \$\endgroup\$ – Steve Jun 23 '16 at 10:55
  • \$\begingroup\$ Have a look at my answer to Open collector circuit needs pull-up resistor to measure frequency where I explain the likely inner workings of the encoder. There you will get an idea of why and what voltage the encoder needs. \$\endgroup\$ – Transistor Jun 23 '16 at 13:01
  • \$\begingroup\$ I have found an encoder which is sufficient in terms of shaft diameter, supply voltage and output type. There are two parameters that are confusing for me in datasheet. The first one is maximum current requirements 180 mA. I suppose that this current is the maximum current that can be sink by the encoder's output transistor. In my case the transistor must sink \$\frac{5\,V}{4.7\,k\Omega}\$=1.06mA. Could this be a problem? The second confusing parameter is output load 50 mA (including 3.3 k\$\Omega\$). What does this parameter mean? (\$\frac{5\,V}{3.3\,k\Omega}\$≠50mA) \$\endgroup\$ – Steve Jun 24 '16 at 7:26
  • \$\begingroup\$ Pop all that info into the original question (as an edit / update) and link the datasheet. \$\endgroup\$ – Transistor Jun 24 '16 at 9:20
  • \$\begingroup\$ Are you using the "KI, HL, KS: Push pull output with short-circuit protection" model? \$\endgroup\$ – Transistor Jun 24 '16 at 15:02

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