3 Phase Motor Danfoss Drive control

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Thread Starter

Mickbeeee

I am installing a Motor Speed control "Danfoss VLT 2800". I am going to use Two digital signals from a PLC to the Drive, to give me Four fixed Motor speeds

For me to control the New Drive, I am using two 24v output siganls from the SLC 500 Alen Bradley PLC within the same Control panel as the new VLT 2800,

Can I input these signals straight into the Drive, or do I have to use Relays switched by the PLC and volt free contacts to utilise the Drive's own 24v Supply????
 
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James Ingraham

In theory, you should be able to use a 24VDC PLC output directly to a VLT 2800. However, the 24VDC supply to the PLC must have the same reference as the drive. The easiest way to achieve this is to attach the common side (0V) of your 24VDC power supply to the common of the drive (pin 55, I believe). We also generally tie all of
our commons to ground.

In practice, we find this doesn't work very well. The transistor outputs of the PLC don't work well without a load. Try it; make sure the output is off and put a meter on it. You'll read 24 volts every time. Then hook up a load such as a lamp. Now the meter will read 0V. The load of the drive input is so small that the PLC output
tends to stay floating high, causing your drive to get spurious signals. You can get around this by adding a load parallel to the drives input, or by using a mechanical relay. I'd say 90% of the time people use the relay, because it's easier. You can use a relay card in the PLC, a 24VDC out card with a relay breakout
module (IFM), or put in little-bitty single pole relays.

Good luck.

-James Ingraham
Sage Automation, Inc.
 
You should be OK. Most of the AB 24 VDC Output modules are ground referenced (sourcing). Connect -24 V from your PLC DO supply to terminal 20 Connect the -24 of analog supply to terminal 55
They may be the same supply in which case you may leave the wire off terminal 20.

You need to verify that your modules use a common -24VDC.

Roy
 
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