Pulse train from a VFD
Pulse train from a VFD
(OP)
We need to obtain a pulse per Hz signal from an A-B Powerflex 70 VFD in order to use it as a master synch pulse signal for a certain production line. The rest of the line is servomotor driven and is follower to this master pulse, no problem there. Right now we are getting this master synch pulse signal "mechanically" from an external synchronous motor/gear/encoder combination connected to the master drive along with the other motors (this single master VFD runs three small synch motors simultaneously). This arangement works OK but it is pretty cumbersome because of the additional motor required just for synch purposes, since we cannot couple encoders to the other synchronous motors.
If we could get this pulse/Hz signal directly from the drive, directly or through an option card or an external card/converter/whatever, we´d simply feed the pulses into a counter and generate the master pulse every N pulses.
Thanks a lot for your help.
If we could get this pulse/Hz signal directly from the drive, directly or through an option card or an external card/converter/whatever, we´d simply feed the pulses into a counter and generate the master pulse every N pulses.
Thanks a lot for your help.





RE: Pulse train from a VFD
Like this:
Phase A --- resistor ------------- optocoupler anode
| |
capacitor diode cathode
| diode anode
| |
Phase B --- resistor -------------- optocoupler cathode
Make the cut-off frequency low. I used 1 Hz. The voltage across the capacitor will then stay constant as speed (frequency) increases. For a 400 V drive, I used two 5 kohm resistors and a 10 + 4.7 uF capacitor. That makes your cut-off frequency a little higher than 1 Hz, but it works.
Remember that the full PWM voltage is driving current through the resistors. I used 25 W resistors. Yes, 25 watts - not 2.5 watts.
The capacitors never see more than the V/Hz voltage. So you can use almost any non-polarized capacitor. I used old Siemens MKL 63 V capacitors that happened to be available. But you can use much lower voltage.
And, of course, there should also be a current limiting resistor in series with the optocoupler. Make it something like 330 - 1000 ohms. Depending on what coupler you use.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Pulse train from a VFD
It looks to me like your network is stripping out the PWM pulses and passing the fundamental sine wave on to the optocoupler.
Am I seeing this right?
RE: Pulse train from a VFD
The optocoupler outputs a reasonably good square with same frequency as fundamental. And I think that is what the OP wanted. At least, that is what he gets from the electromechanical contraption he is using now.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Pulse train from a VFD
Clever!
RE: Pulse train from a VFD
The truth is that, for reasons more political than technical, I´d prefer to install something already made and proven rather than having it build, any other suggestions?
RE: Pulse train from a VFD
Hope that helps
Ed
RE: Pulse train from a VFD
The pulse train output that you want used to be a fairly common option, but I don't think it was used much and manufacturers don't often have it now. I would check first with your local drive supplier if you have not already done that. There may be an accessory supplier that makes something like Skogsgurra described. I am thinking of one or two companies that might do that and will try to remember the names.
RE: Pulse train from a VFD
RE: Pulse train from a VFD
RE: Pulse train from a VFD
If your Powerflex70 is to be configured simply to receive the pulse train from the encoder you already have, it might be worth looking at their encoder option to see if this can be configured as a SP follower rather than simply a closed-loop encoder feedback for speed set-point.
RE: Pulse train from a VFD
RE: Pulse train from a VFD
I do know that yaskawa has the pulse train where one drive sends out the pulse train of say speed and the rest follow. This would be a digital signal not an analog signal on most drives. Using the analog signal to send out this signal would give you slightly different results in speed on each drive since it is an analog signal. This would be the reason for the digital pulse train above which would be better.
In the MHE world this would be the difference in a smooth ramp up with no jams where multiple drives have to come up together at same speed.
RE: Pulse train from a VFD