Mitsubishi VFD control by PLC

D

Thread Starter

Daniel

This is my first time designing a control panel for a motor and my first time using a commercial VFD so sorry if this seems like a dumb question to people who've done it before.

I want to be able to digitally set the frequency of the motor up to 60 Hz (144rpm, 31.8m/min) of a Mitsubishi FR-E720-S30 VFD (they call them inverters) with a Mitsubishi Q00UCPU. I know it can be done with Modbus (or CC-Link etc.) but I think that requires a comm module.

I thought section 6.6.3 of the Mitsubishi E700 user manual had the answer ("use contact signals to perform continuous variable-speed operation, without using analogue signals") but then it turns out that the frequency is still controlled by the inverter in this case and that the digital input it refers to is simply accelerate (RH) and decelerate (RM) based on time values set in the inverter unit. What I was really hoping for (wishful programmer-thinking actually) was a simple way to send an integer between the two devices but I don't think that's possible without an I/O card like the QJ71MT91, so instead I was thinking of outputting a PWM signal to the VFD. Section 5.2.4 says which pins to use for an analogue voltage input using a potentiometer, I thought maybe using a HF capacitor would smooth the PWM output to an average voltage. Maybe that'd work (probably don't even need the smoothing cap) but that's a lot of conversions (frequency -> PWM signal -> average voltage -> frequency) so there must be a better way of doing it.

In fact, looking back over this I probably should just go with the QJ71MT91 module. Any one agree/disagree? Any other advice you can give me (remember I'm new to this)?
 
S
One option that you don't mention that might work for you is that many drives allow you to create a list of speeds and select from the list with binary-coded digital I/O. So if you had three PLC outputs wired to three drive inputs, for example, you could select between 8 speed presets. That approach might give you enough resolution if Mitsubishi drives have it and depending how many inputs they'll use like that.

Since you're just starting to do this kind of thing, I'll add a general observation from several decades in the business. There ARE times to try to accomplish a lot with very little hardware, but if you're not careful, you risk costing the project more in labor than you save.
 
> many drives allow you to create a list of speeds >and select from the list with binary-coded digital I/O.

Mitsubishi does have this option and that would be an easy solution but I'd rather be able to have better control over the motor speed than that.

> There ARE times to try to accomplish a lot with very little hardware, but if you're not careful, you
> risk costing the project more in labor than you save.

Thanks for the advice, I think that's a good lesson for me to take away from this project so far :)

Any comments on the PWM option though? Even though that's not the way I'm going do you think connecting the PWM output directly to the voltage input ADC or through a capacitor would have worked?
 
S
Yes, I think the PWM would probably have worked, so long as the "ON" voltage will be tolerated by the drive analog input. In fact some signals interpreted by the recipient as analog are in fact PWM.

Even if the drive didn't like that, you can buy devices that convert analog signals to frequency to read analog values into PLC's with no analog inputs. They probably make the reverse item as well (frequency to analog), so you could use one of those.
 
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