OT: Inverter speed control?

Gazzr

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Join Date
Nov 2006
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UK
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64
Hi all, not really a plc question. But a question I am sure somebody will be able to answer.
I have a machine that has a Siemens micromaster inverter used to vary the speed of a motor. Currently this is controlled via a potentiometer on the panel. I would like to replace this with some kind of digital input with lcd display. Hopefully the display would show 0 - 100%
One option is to configure the drive for digital inputs (+speed -speed)and then use the analogue output from the inverter to signal a Ma meter. Scaled to achieve the numbers I want (0-100). But I have tried this and its not very accurate, sometimes the meter would read above 100. Only slightly, but still it doesn't seem right and hope that there may be something on the market that will achieve what i require.

Many thanks in advance.
 
You can set up for Inc. Speed and Dec. Speed digital inputs on most drives. Some call this "floating point" control and others call it "MOP" control. There may be other names but they all do basically the same thing. There is no speed pot or analog speed input.

As for the ma output having a lot of error, I can only suggest that, if you are operating the drive in open loop scalar mode, sometimes called V/Hz mode, the accuracy will vary with motor load within the range of the motor nameplate slip. You will need to operate the drive as a sensorless or flux vector drive to tighten up the speed control and, consequently, the speed output accuracy. Siemens sensorless vector control will probably get you around one quarter of motor slip for error and flux vector (with an encoder) will give you basically no error.
 
The method your are now using has the great advantage of being able to run in the event that the speed control device should fail.

I have a lot of drives being controlled with speed pots in a washdown area that will stop when a pot goes nuts in hte middle of a production run, so I have been considering going with the MOP style control like yours.

What you may want to do to make the display "look better" is to drive the analog output with the command reference rather that the actual motor speed. Then the display should show what the drive is trying to do, instead of what it is doing. I am not familiar enough with siemens drives to know if this is configurable.

Of course, if there is a speed error and the drive is unable to maintain speed accurately enough for your process, that may be a bad thing if the display does not reflect it.

The operators and technicians will have to understand what the display means, and may need to use the drive display itself, at times, in troubleshooting.

Paul
 
OkiePC said:
What you may want to do to make the display "look better" is to drive the analog output with the command reference rather that the actual motor speed. Then the display should show what the drive is trying to do, instead of what it is doing. I am not familiar enough with siemens drives to know if this is configurable.
I will have to look into this, sounds like what I want.

OkiePC said:
Of course, if there is a speed error and the drive is unable to maintain speed accurately enough for your process, that may be a bad thing if the display does not reflect it.
This shouldn't be a problem as the shaft being driven has a completely seperate rpm readout. (Proxi switch and a flag) Allthough this too is a little erratic. (Another issue to be remedied)


Thankyou both for your comments.
 

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