MM420 speed problem!!!

NoName · Jan 26, 2004

Hello all.I'm working with C7635 OP, and a micromaster 420 inverter.And when I try to send the speed setpoint something weird happens. If I send 2000, the frequency is set to 50% that is 25HZ.But if i Try to send 1900 to the inverter as the speed setpoint, it should be around 40% so some where around 22Hz, but it's not it's 19,8 HZ. How come that i can't set the speed setpoint no where around 20 to 25 HZ???Have any of you had this problem, so any advice would be helpfull.Do I just have to set somekind of parameter or what??? Please help me.

Thank you.
>Aleš<

Tom Jenkins · Jan 26, 2004

I don't know your PLC, but it looks to me like your analog output is 12-bit. That means that 0% output is 0 data, and 100% output is 4095 data.

You don't say, but I suspect that your output is 4-20 mA. Make sure the drive and the output are both set for 4-20 mA. One of them may be 0-20 mA.

Another thing to check is if the VFD has been set to skip certain frequencies. Sometimes, to avoid critical frequencies and vibrational resonance, the VFD is set up to not operate at certain speeds.

NoName · Jan 26, 2004

Sorry, but I forgot to tell that I'm comunicating with the drive via Profibus.No through any analog inputs/outputs or something like that.Sorry it's been a long day

and i'm starting to forget stuff.

>Aleš<

The Dark Knight · Jan 26, 2004

Hmmm

It depends on the format (word or decimal) you are using
When I do my calcs, I get 19.53 Hertz..
if 1900 is transmitted as a word.
Here's what I got

1900Hex -> 6400Decimal

100%= 16384Decimal = 4000Hex

6400Decimal / 16384Decimal * 50Hz=19.53Hz

You can not just say 1900Decimal / 4000Decimal*50Hz=X

You will get the wrong answer

dchartier · Jan 26, 2004

Hello, NoName;
Dark Knight has a valid point. You need to write the speed setpoint values in hexadecimal format to the Profibus address selected.
Say your speed setpoint register address has been selected to PQW258;
then MOVE the hex value you want to that address:

in STL, it would look like

L W#16#4000
T PQW258 // requesting 100% of max. speed parameter
...

L W#16#2000
T PQW258 // requesting 50% of max. speed parameter
...

L W#16#1900
T PQW258 // requesting 47.5% of max. speed parameter

Hope this helps
Daniel Chartier

NoName · Jan 27, 2004

I am sending it as a word(HEX) to the inverter, so I realy don't know why it doesn't work like it should. Is it possible that a parameter could be set wrong???

Here is how I send the speed setpoint.
NW1

A(
A M 124.4
A M 28.5
JNB _01d
L W#16#1900
T PQW 258
SET
SAVE
CLR
_01d: A BR
)
JNB _01e
L W#16#47F
T PQW 256
_01e: NOP 0

Hope this helps.
Thank you.
>Aleš<

The Dark Knight · Jan 27, 2004

what i'm saying is that you should get close 19.5Hz. You say that you are getting 19.8, so the drive is responding properly.

NoName · Jan 27, 2004

No it's probably 19.5, I just wrote that value from the top of my head.Sorry. So what you are saying is, I can't set the speed no where around 22HZ or is there a way??

Thank you.

Steve Bailey · Jan 27, 2004

Of course you can set the speed to 22 Hz.

22 Hz / 50 Hz = X / 16384

X = 7209 decimal = 1C29 hex

Even if you're limited to steps of 100 counts, there are still six steps between 1900 and 2000 hex;

1900 = 19.53 Hz
1A00 = 20.31 Hz
1B00 = 21.09 Hz
1C00 = 21.88 Hz
1D00 = 22.66 Hz
1E00 = 23.44 Hz
1F00 = 24.22 Hz
2000 = 25.00 Hz

NoName · Jan 27, 2004

AAAAAAAAAAAAA. I was thinking in a totaly diferent direction.I totaly forgot that i was sending a HEX value,to the inverter.I just thougt that 1900 is around 23Hz,I wasn't thinking right.Thank you all for all your help,as you can see sometimes I have tunnel vision. Thanks to ALL!!!! :site:

JRW · Jan 27, 2004

Another tidbit-
If you want to see your profibus value sent to the drive-
Look at
r2050.0- Control word from your master (in decimal)
r2050.1- speed value from your master (in decimal)
r2050.2- Spare
r2050.3- Spare

MM420 speed problem!!!

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