Using an SCP instruction with a 1769-IF8 Analog Module

[lsam]

Okay, this where things stand - if 'input_Min' = 4000 and the 'input_Max' = 20000, the output value starts at a value of '0' and goes to '-2' while the motor is running and if 'input_Min' = 3200 and 'input_Max' = 21000, the output starts at a value of '2' and goes to '0' while the motor is running.

When I check current outputof the transducer without the motor running and while online with the PLC using a FLUKE 123 scope meter, I got a reading of 1.441 amps. Isn't that a little steep? I have the meter in auto mode and set to 'amp, 10mV/mA(sensitivity), amp DC'.

 

[Greg Fowlds]

Without the motor running what is the SCP input value?

Without the motor running you should have approx 4ma.

 

[lsam]

Motor not running
Input = 4035 to 4065

 

[Mickey]

while online with the PLC using a FLUKE 123 scope meter, I got a reading of 1.441 amps. Isn't that a little steep?

If so you may have damaged the card. See picture. Do you have a milliamp generator to check the card?

 

[lsam]

No. I do not have a milliamp generator.
The voltage output of the transducer is 14.5 dc

 

[lsam]

Without anything connected to the output of the transducer, I get the same reading of 1.441amps also.

 

[Greg Fowlds]

On the transducer, the wire going through the center of the coil.
Is it going in the correct direction.
There should be an arrow somewhere. + in --> to the hole.

 

[lsam]

The attached photo shows the physical location and the two wires that go through the transducers. There is a red (12vdc) wire and a blue (24vdc) wire. the positive source of the power supply is from the left side of the photo.

Before someone jumps to conclusions - yes I have tried removing one of the wires and still get the same results. Only one circuit (12vdc or 24vdc) can be used at a time. Therefore only one wire will have current flowing through it at a time.

Each of the two wires go to their own isolated auto-reset circuit breaker which in itself will limit the amount of current flowing through the motor circuit. I just need to monitor the amount that is actually flowing while the motor is running hence the reason for getting the SCP instruction correct.

 

[lsam]

forgot to post the photo

 

[Ken Roach]

I think you are misunderstanding how 4-20 mA current loop instruments work. They will generate voltage sufficient to drive a specific current through the loop resistance.

The 1769-IF8 module presents a 250 ohm impedance to the loop, so to drive 4 milliamperes through the loop the Current Transducer will generate about 1 volt. That's ordinary Ohm's Law: V = I * R.

Are you seeing 14.5 volts across the transducer output with the 1769-IF8 connected, or disconnected ?


The second problem is that I think you are connecting the Fluke 123 Scopemeter incorrectly.

Your description of setting it up for a particular 10 mV / mA ratio suggests that you've set it up for a DC current probe like the 80i-110s, but that you are not actually using the DC current probe.

If you measured 14.5 volts with the Scopemeter set up like this, it would show about "1.45" Amperes on the screen. This is a misunderstanding of your milliampere loop measurement, not an actual measurement of the current flowing to the motor.

The ScopeMeter doesn't have a direct milliampere measurement mode the way many ordinary DMMs do. All measurements done with a Fluke 120-series ScopeMeter are voltage measurements. Current measurements are done only via special current probes.

I think you mentioned that all the DMMs in your shop have blown fuses on their ammeter circuits, so this is the time to go invest in some fuses.

 

[Greg Fowlds]

Can you confirm that the current flow is in the direction of the arrow on the side of the transducer?

I think it may be backwards.
You have a reading of about 4.x ma without a motor running.
With the motor running I think the count on the SCP input is going down instead of up.
These are bi directional units.
If set to +/- 10 volts will indicate if current is going or coming.

Please check the current flow against the arrow on the label on the side.

 

[lsam]

Absolutely. That's why 2 days ago when I found all the blown fuses, I immediately placed an order 50+ of each seing that they are nothing like we use elsewhere in our plant.

And yes, I am reading the same 14.5 vdc with all of the meters available to me at this location which total 6 in all.

 

[lsam]

At this point in time, until a supply of fuses arrive I'm dead in the water in regards as to measuring any dc current values.

The one thing that I am positive about is that the positive (+) output of my power supply comes from the left side of the photo to the transducer and exits out the right side of the transducer in the direction of the arrow on the transducer as shown in the previous photo and then proceedes to the circuit breaker. From there it leaves the circuit breaker and goes directly to the positive lead of the motor. So, I can only assume the manufacturer labeled its product correctly. If they had not done so, the motor either would not work at all or at least not very long without leaving an odor to remember.

 

[Greg Fowlds]

Ok,
What happens to the SCP input reading value when you disconnect one of the 4-20ma wires?
Does it go to "0" or stay at 4035.

Reconnect the 4-20ma.
With the motor running what is the SCP Input value, more or less than 4035?

 

[Ken Roach]

I don't think we have determined that the 1769-IF8 module is correctly connected yet.

Can you post a drawing or describe in detail how your two wires from the Current Transducer connect to the 1769-IF8 module ?

My recommendation is to connect each of these in Differential Current mode as illustrated on Page 9 of the 1769-IF8 Installation Instructions.