PWM Impedence

[rosssyscon]

We are using a variable frequency drive that has a frequency output relative to output current of the drive. The input value is very flacky and unstable. We contacted the manufacutrer and they realized they had a problem. They suggest an RC circuit. Here is what they suggested.

For a 0-5VDC output, R1 = 698 ohms
R2 = 0.23 x Z, where Z is the input impedance of the digital instrument or PLC
C1 = 0.02 ? R2, select the next larger standard value of capacitance.

I have an entire page from them, but didn't want to bore you. Please advise.

Thanks again,

[support]

I am not sure what is your objective. Are you using the PLC's PWM output to control the VFD? Normally VFD are either controlled by a analog input or a RS232/RS485 input. If your VFD can be controlled by RS485 that is the best way as you can get perfect values into the VFD. Although the PWM output may be low pass filtered to produce a pseudo analog output it may still be too noisy for device. We don't have a ready formula for such computation. You can try out the RC value they suggested to see if it will work better.

[rosssyscon]

Sorry for the confusion. WE are controlling the VFD speed via 4-20mA output. That works great.

It's the feedback signal from the VFD that is a
--> "PWM open collector analog output at 2 kHz (30V max, imped 1kOhm, 10mA max)" - or -

--> "Can be configured as a Open collector logic output (30V max, imped 100kOhm, 50mA max) "

It's a cheap drive! They have the schematic on the RC circuit. Basically, I need the impedence (Z) on input #3 to calculate the resistor size (R2) required.

Thanks.

[support]

So the feedback signal is a 2KHz PWM? The duty cycle gives you the data you need? In that case, just connect it to input #3 and run PMON. Then use PULSEWIDTH(1)  function measure the pulsewidth of the PWM signal. You can get very accurate measurement of the pulse width (resolution of 1 microsecond)  and that can give you good feedback signal.

I don't see why you need the RC circuit unless the purpose is to convert the PWM signal into an analog signal and feed into ADC?

[Jay]

I'm still looking for the impedance of input #3. I am trying to change the pwm to an analog signal. The pwm is jumping around and I'm up for trying anything at this point.

[support]

Let try to see If I understand you correctly:

1)  you are trying to measure the frequency of the output signal from the VFD which gives you a measurement of the current?

2) This VFD output is to be connected to input #3 of the PLC?

3) YOu are currently using the PMON function to measure the frequency using PULSEFREQUENCY function, but the reading obtained is unstable?

4) You are trying to convert the variable frequency output signal from the VFD into an analog value and feed it to the ADC #n of the PLC?

Let me know if any of the above assumption is not true. I am really not sure what impedance value are you looking for. If it is the input #3 then I am not sure whether we can say exactly what is the "input impedance" since the inputs are NPN type. The resistance in the path of the NPN inputs is 10K ohm.

If you are refering to the analog input (0-5V) then the input impedance is 1 M Ohm or higher.