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There is a VFD which controls an AC motor through a 0-10V control. I know that this control voltage is DC voltage.

I'm wondering would a PWM work for this application? I mean instead of 2VDC, what happens if one applies 0-10V 500Hz pulse train with 20% duty cycle? In theory this corresponds to 2VDC but since it is PWM, I'm hesitating to even try.

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    \$\begingroup\$ You could low pass filter it, like in a DAC. \$\endgroup\$ – PlasmaHH Oct 18 '18 at 9:06
  • \$\begingroup\$ Yes I could do even with an opmap filter but that is not what Im asking. Im wondering if you use plain PWM. If a filter is really needed or not I mean. \$\endgroup\$ – atmnt Oct 18 '18 at 9:08
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    \$\begingroup\$ @– atomant: That obviously depends on how that receiving thing is handling the input, nobody can tell. \$\endgroup\$ – PlasmaHH Oct 18 '18 at 9:08
  • \$\begingroup\$ This higly depends on the exact type of VFD used. Some offer programmable filtering, some offer PWM inputs. Please specify. \$\endgroup\$ – Jeroen3 Oct 18 '18 at 14:20
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They may or may not have sufficient filtering in there for whatever PWM signal timebase you can generate.

As a guess, they probably would put some filtering that would handle mains ripple on the input line so if you had a PWM timebase in the kHz it might have low enough ripple to not cause issues.

Worst case will be 50% duty cycle and I would expect the effect of too much ripple would be a beating of frequencies (sampling vs. PWM) resulting in a modulation of RPM with time.

Since you have to generate the PWM anyway and likely have to boost the voltage from logic level to 10V, you may as well filter it and buffer it properly, in my opinion. The few parts (say an LM324 and a few resistors and capacitors) cost virtually nothing compared to playing around with this kind of thing.

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It would work, by adding a RC lowpass filter. But you have to tell how would you wire 10Vcc to an output and more, what is the output type. If it is a push-pull then no problem, else you might need to make an interface between yours PLC and inverter.

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You can expect that the speed of a motor controlled by a VFD will closely follow any change in the level of the analog speed reference input. The ability of the VFD to make the motor speed respond to a step input will be limited to the mechanical capability of the motor and load combination to cause a speed change, not the ability of the VFD to change frequency. Meeting that expectation would tend to defeat any expectation that speed could be controlled by a PWM input.

Would a VFD manufacturer offer a speed reference input that would accept a PWM input? That is not very likely. You could study specifications for VFD products on the market.

What other alternatives to analog an analog speed reference input might be available? Some VFD manufacturers once offered a pulse train input that controlled speed in proportion to the frequency of the pulse train. Today, the most common alternative is likely a serial communication input.

Note also that most VFD manufacturers offer a 4-20 mA current signal input.

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