The GS20 Durapulse drives make it super easy to display the motor speed in RPM.
But, there are a couple things you need to be aware of.
The GS20 naturally starts up showing the drive's frequency setting – the frequency you want
the drive to be at.
If you hit menu, you see the drive's actual output frequency which is zero right now.
When you hit run, we see it ramp up to the frequency setting at whatever rate you set
the ramps to.
Hit menu again and we see the user display, which is currently showing the DC bus voltage.
We want to see motor RPMs here.
I’ve already set up the drive with these parameters.
Some of these are just defaults, I’m showing them here because we are going to use them
in this demo.
Parameter group zero, parameter 4, selects what is shown in the user display.
It defaults to a three which is DC bus voltage and is indicated by a V in the display.
That’s this funny looking character right here.
And look!
Option 7 is the “calculated” RPM and is indicated by a lower-case “r” in the user
display.
Let’s go to group zero, parameter 4 and enter a seven to show calculated RPM in the
user display.
I’ll hit that menu button until we get back to the user display.
We have an “r” here for RPM – that’s a good sign.
I’ll hit run … and rotate the frequency knob … and sure enough we see something
that looks like RPM.
But wait a minute, if I rotate the knob all the way the display says the motor is running
at 1800 RPMs.
But if I measure that with a hand-held tach, I get this.
And if I try to apply a load to the motor it would drop even more below 1800 rpm.
And the nameplate says the max RPM is 1725
but the display still says 1800.
What’s going on?
Well, the drive calculates it’s an 1800 RPM motor because the frequency is set to
60 Hz and the number of poles is set to 4.
Remember: a 2 pole motor has one north and one south pole, so it’s really one pole
pair.
When the drive’s output frequency is 60 hertz, that single pole pair is going to rotate
at 60 hertz, which is 60 rotations per second which is 3600 rotations per minute.
A four-pole motor - like we have – has two pole pairs so the same 60 hertz frequency
has to service twice as many poles per revolution, so you only get half the RPMs which is 1800
RPM.
But, that’s all the drive knows.
It doesn’t know how much load your motor is seeing right now so there is no way it
can tell you the exact motor RPMs without some kind of feedback, which we will do in
other videos.
So it simply calculates 1800 RPM from the number of poles and scales that as you change
the frequency when using parameter 4’s option 7.
But what if you wanted to get a little closer to the real motor RPM?.
Suppose you have a system and when operating under your normal load conditions you measure
the RPM at 60Hz to be ... I don’t know ... how about 1763 RPM?
Here’s a trick you can use to display that.
Instead of displaying the calculated RPM – which we know is blindly calculated from the number
of poles – we can display this guy which takes the input frequency and scales it by
parameter zero five.
It also displays a K in the user display.
Let’s change parameter 0.04 to a 31 to do that.
We want the display to be 1763 at 60 Hz, so our scale factor needs to be 29.38 so I’ll
enter that in parameter 5.
Now if I press the menu key to get to the user display and then hit run, sure enough
my top end is very close to the 1763 RPM we wanted!
We could go back and tweak that scaling factor if we want it to be exact, but this is close
enough for our demo.
But what is this?
Well, that’s the K we were expecting to see.
The 7-segment display can’t show us an actual K so it uses this as a representation of K.
You can see the full character set in this chart which is at the end of chapter three
in the user manual.
So now you have two ways to display the motor RPM.
First, just show the RPM calculated from the poles – and most of the time that will be
close enough.
But if you want something closer to your actual loaded motor speed, you can display a scaled
version of the input frequency value.
One more thing to be aware of: The GS20 drives power up showing the frequency setting by
default.
You can change that so it shows you the user display on power up by setting parameter group
zero, parameter 3 to a 2.
Now when I power up the drive, it defaults to the user display which we have set up to
display RPM.
Click here to see more GS20 variable frequency drive video tutorials.
Click here to subscribe so you will be notified when we publish new GS20 videos.
And click here to learn about AutomationDirect’s free award-winning support options.