1K pot/5K pot

Hello everyone.

Would someone be able to tell me if there is a difference using a 1K potentiometer or a 5K potentiometer? I am working on 2 Danfoss VLT5000s. One is a VLT5027 (master) and the other is a VLT5008 Flux (slave). I am using a reference signal from the master and it is going into a pot. The manual says to use a 1K pot, but all I have are 5K, 10K, 100K pots. Will there be a difference in using the 5K instead of the 1K shown in the diagram?

There will be a slight increase in electrical noise sensitivity with the higher resistance pot. If your leads are short and shielded properly, I'd not worry about it.

Depends on the circuit

IF a simple voltage divider then no effect on the drive as it still gets the same voltage reference and the current drain on the supply is less

IF a 0 to 20mA input then yes you will have to up the supply voltage to get the required mA

yet again it depends on your circuit If you want more info the post your cct

Typical is 5 volts on the pot. With a 1K, the signal will be at 5mA.
WIth a 5K pot, the signal will be 1mA.
The higher the current, the more noise immunity you have. 5K will probably be okay. Make sure you don't route the cable near solenoid or motor wires. If that's a must, use high quality shielded cable, grounded at the controller end only (pot side floating).

keithkyll said:

... shielded cable, grounded at the controller end only (pot side floating).

Click to expand...

QFT.

And regardless of the system, at the very least, you should always run a shielded cable for analog signals. 95 times out of 100, if you don't, you probably won't see any ill effects. It's troubleshooting those remaining 5 times that lead to frustration, wasted time, (and, in my case, often egg on the face).

Thanks guys, I got the application working. One more question, I am having trouble with this. My boss would like for the slave to run 1/2 the speed of the master. I turn down the pot to 1/2 way and its at 1/2 the master. Now, if I change the speed of the master, the slave doesnt change to 1/2 the master. It is about 3/4 of the master. I found the voltage did not change that is being sent to the analog input of the slave. With the master running at 30.6Hz and with the pot all the way "on", the slave is at 30.6 Hz. I bring the pot 1/2 way down and the slave is at 15.3Hz. With the pot 1/2 way down, I change the speed of the master to 17Hz. The slave is now running at 15.3Hz. The voltage is the same at the 1/2 way mark on the pot, about 2.3V with the master being 30.6 and 17. Any ideas?

I am confused without a schematic. There is 2 types of calibration for pots. Linear and logarithmic. In simple terms, linear is used for industrial, logarithmic for audio controls. Make sure your pots are linear. Are you using 1 or 10 turn pots? Do you have 2 pots mechanically ganged together?

If you have flexability, take the analog retransmit output from the master (0-5 volts), and set the slave for 0-10 volt input.

Im sorry for being confusing. It is a linear pot. I am using an analog output from the master VFD to the voltage side of the pot. The wiper is then connected to an analog input on the slave and the 0v side of the pot is in the slave. The master is sending 0-20mA, but I put a 510Ohm resistor in the 0v and analog input on the slave (Danfoss said it will make mA to volts).

This is difficult without a wiring diagram but let's try this.

I'm assuming the Master drive has a 0-20ma current loop output which tracks with Hz or rpm up to 20ma = max speed.

I'm assuming the Follower drive has a 0-10VDC speed reference input with 10V being Follower max speed.

I'm not real familiar with DanFoss drives but, almost surely the Master drive has a scaling or gain parameter for the speed output. Likewise, the Follower drive has a pair of parameters one defining the analog speed minimum speed and the other defining the analog speed max speed.

In order to convert the 0-20ma output to match the 0-10V input, the Master + output is connected to the Follower + input and the Master - output is connected to the Follower - input. In addition, a 500 ohm resistor is added between the Follower + and - inputs to convert ma to V.

To get the proportional speed scaling between the two drives, you can simply adjust the Master output scaling or gain parameter so, at full Master speed the output is 10ma instead of 20ma.

Or, if that is somehow how available, change the Follower max speed parameter to be half of the Master full speed so, as the input voltage sweeps from 0 to 10V, the Follower runs from 0 to half of the Master speed.

The above is assuming that 0ma on the Master and 0V on the Follower both represent 0 speed.

There should be no need to do this with any potentiometer unless the ratio of speeds between the two drives is to be constantly adjusted by an operator.

Are you using the actual Master 'Current Speed' analog output as a reference to your slave pot (high side) ?

Oh, and if you are actually getting a 0-20mA signal, you might find it much easier to use a signal isolator/converter to change that over to 0-10VDC.

Hi DickDV,

Thanks for the reply. You are correct in your assumptions. The only thing that is different from what you said to what I have available is the adjustment on the Master output. The only thing I can choose from is 0-20ma or 4-20ma. The application requires the ratios between the 2 to be trimmed. I dont see a problem with how it is now other than when the speeds change, the pot needs to be turned. I was just seeing if there was something I could do to make it "easier" on the operator if he/she decides to decrease or increase the speed on the fly. The manual said a 510ohm resistor and another guy said 500ohm and you said 500 as well. Is the 510 too much?

The object with the resistor is to convert 20ma to 10V so ohms law gives you 500 ohms. 510 ohms is plenty close enough.

When you say trimmed, do I understand you to mean that the operator is going to be adjusting the ratio frequently?

If so, and the adjustment is so the follower never has to go faster than the master, then, instead of a fixed 500 ohm resistor, use a 500ohm pot. The wiring then is the same as for the fixed resistor I laid out above as far as the - terminals are concerned but on the + side, the master + lead goes to the high speed end of the pot. The + terminal on the follower drive goes to the wiper on the pot. The low speed end of the pot goes to the - connection between the two drive - terminals.

The max speed parameter on the follower drive input will be the fastest speed you want the follower to go when the master is at full speed and the pot is set to max speed.

The slave is feeding product to the master. Its on a folder/gluer machine. So, the slave needs to be slower than the master, but a change in speed of the master needs to to have a direct proportional change in the slave. I know I could change the max speed of the slave so it will always be the same ratio at different speeds, but the ratio needs to be able to change on the fly. So my question is what would be causing the slave to not still go 1/2 of the new master speed with the pot still at 50%? Could it be I am using a 5k pot? Would your suggestion of just a 500Ohm pot work better?

Rich, something is wrong because the speed control should be proportional as you desire.

Get your voltmeter out and measure the voltage across the potentiometer end to end, not to the wiper. Do this at full speed, 3/4 speed, and half speed. You should see about 10V, 7.5V, and 5.0V on a 500 ohm pot.

OOPS!!! There's the problem. If you are trying to put the 20ma thru a 5000 ohm pot, the voltage would be 100V, 75V and 50V which is too high for the current loop power supply to produce. Find a 560 ohm fixed resistor and connect it from end to end across the 5K pot. Now the loop sees 560 in parallel with 5000 which is close to 500 ohms. Go back and check the voltages for 10,7.5, and 5 again.

You know, this would be a lot simpler with a 500 ohm pot. Is that so hard to come by where you are?

Anyway, now with the 560 ohm resistor added, let's check our circuit out again. You have a jumper from the Master - to the Follower -. One side of the 560 ohm resistor and the slow end of the pot also tie to this jumper.

On the positive side, you have a wire from the Master + to the high speed end of the pot. The remaining lead from the 560 ohm resistor also connects to this wire.

Finally, you have a wire from the pot wiper to the Follower + input.

Check those voltages across the pot as the Master changes speeds and let me know. Then we'll take it to the next test point.

If your master drive has an analog output that you can configure to be proportional to it's command speed, that may give you a better result. You just wire it's output to the slave drive analog input, and scale the slave drive max speed accordingly.

Then, the slave drive will be commanded to run at whatever speed the master drive is actually running, regardless of where the master gets its command from.

JMHO
Paul