In the previous video, we learned about Field Oriented Control – also called Vector Control
where the drive uses an encoder to keep track of where the rotor is,
so it can optimize the orthogonal current components to get superior speed and torque
control of the motor all the way down to zero RPM. It turns out, that as long as there is enough of
a magnetic field in the motor’s rotor for the drive to track it, the drive can do a really good
job of controlling the motor without using a sensor. Which is why we call it sensorless vector control.
Of course, for the rotor to have enough of a magnetic field for the drive to work with,
it has to be rotating faster than some minimum speed.
Let’s show that graphically. Above some minimum speed or frequency, the drive can use sensorless
vector to dynamically control the current to optimize the speed and torque of the motor.
Parameter 182 contains that cutoff frequency and it defaults to 30 RPM, which for our 60Hz
setup would be 1Hz. The programming manual for the WEG CFW500 drive recommends staying above 18
RPM (pg12-5], which for our 60Hz motor would be around 0.6 hertz. That’s pretty good,
especially when you compare it to volts hertz, which struggles below a few hertz.
What happens if you try to run the drive below that cutoff speed?
The drive automatically drops out of sensorless vector and goes into something called I/f mode.
That is, it simply pumps a fixed amount of current into the motor. It’s kinda like volts hertz,
except it controls the current instead of the voltage. The scalar current value sent
is in Parameter 183. This is the percentage of motor magnetization current in Parameter 410,
which is determined during tuning – it’s not a percentage of the rated current.
By the way, if Parameter 182 is less than 3 RPM, then sensorless vector will always be
on – it won’t transition to the I/f mode. Other than that low frequency cutoff,
this is identical to the vector control with the encoder we did in the previous video, except there
was no cutoff – full vector with the encoder could control the motor all the way down to zero hertz.
Let’s do an example. I’m using the exact same
setup as the previous video, except I removed the encoder wires just to be sure we are really using
sensorless vector mode. Since the hardware is identical, the only difference between
the two is the mode we are operating in so we can do a true apples-to-apples comparison.
And once again, I need to stress that all “Inverter Duty" or "Inverter Rated” motors
are not created equal. It’s critical that you choose the right motor for your VFD application
and, in particular, choose the right motor for the mode the drive is using. To get the most out
of these vector control modes, you need to have a motor that is optimized for vector control .
This is what we need to do to – it looks a lot like the list we had in the previous
video – doesn’t it? The only difference is we don’t have an encoder module to install, we
need to tell the drive we want to use sensorless vector control and we’ll use a different tuning
method. I’ve also added the sensorless vector low-frequency cutoff parameters for reference.
There’s also one additional parameter we are going to need in just a minute.
I’ll gray our all the reference, read only, and default parameters, so you can see we really don’t
have much to do in this block of tuning steps. Ok, let’s power up the drive. Uh-oh – we have a
fault. What’s going on? Well, Fault 79 tells us the encoder isn’t wired correctly. Remember – I
have the encoder module installed, but I removed the wires to be sure we weren’t using the encoder
in this demo. So yeah, the encoder isn’t wired correctly. We could remove the encoder module,
but we might be using some of its I/Os or have plans to upgrade to an encoder in the future.
We could ignore the fault, but that’s annoying. Instead, look at this. Parameter 343 is a mask
where we can turn off things we don’t need. In particular, setting Parameter 343 to an
87 hex turns off the encoder module fault. Hit this to clear the fault, and yep – it’s gone.
Perfect. Don’t forget to turn that back on when you decide to add the encoder back in!
I reset this drive to factory default before I started the video. If we did it again right now,
we would have to go disable that encoder fault bit in Parameter 343 again,
so let’s not spend time on that. The drive defaults to volts hertz mode,
so if I hit run, it spins up to its default 3Hz min frequency. This is important:
verify that the motor is spinning in the same direction as this arrow on the display.
If not, remove power from the drive, swap any two motor leads and try again. The motor should now
be spinning in the correct direction. Ours looks good, so I’ll hit Stop.
Drop into the parameter groups and scroll down to the startup group
that walks us through configuring the drive just like we did in the previous video.
That brings up Parameter 317, where we tell the drive we want
to configure it for one of the vector or field-oriented modes by setting it to a 1.
That sends us to Parameter 202. We set that to a three
to tell the drive we want to use sensorless vector mode.
This is the line voltage we expect. The service factor is fine.
And the default rated voltage is fine. This motor's nameplate says it is rated
for 3 amps, so I’ll enter that here. 60 hertz is what we are using,
and the motor's faceplate says it is rated for 1725 RPM, so I’ll enter that here.
This is the motor we are using, and we’re not using an encoder, so we don’t care about this guy.
I’m surprised the sensorless mode would even ask for that.
The motor is self-ventilated, and we are done!
Go to Parameter 408. We want to use Mode 2, which is the best tuning we can do without a sensor.
This mode expects there to be no load on the shaft. Since this pulley isn’t really much
of a load, I’m going to leave it on to make it easier for you to see when the shaft is turning.
I’ll bring up a stop watch and fast forward the video.
When tuning is done, the drive restarts. That took around three minutes. Not bad at all.
I’ll hit run, and the drive spins the motor up to the default min frequency of 3Hz.
Let’s go to Parameter 133 and change the min frequency to zero hertz so
we can mess around with the lower speeds. Remember – sensorless vector can’t go all the way
down to zero RPM and we used the default cutoff of 30 RPM, which for our 60Hz motor is 1 hertz.
That means at 1Hz the drive should switch over to I/f mode where it just injects a fixed current.
So watch the current bar as I scroll down to the 1 hertz cutoff for sensorless vector
control – it’s running at about 3 bars or 30 to 40 percent of rated current.
If we go below that 1Hz cutoff frequency – yep – the current
jumped to 5 bars and it stays there. Why? Because we told it to turn off sensorless
vector below 1Hz and switch to the I/f mode. In that mode the drive is supposed to inject 120%
of the motor's magnetization current. That’s in Parameter 410 which we see is 1.7 amps. 120% of
that is around 2 amps. If we go to Parameter 0003 which shows us the current being output,
yep – the drive is forcing exactly 2 amps into the motor. Exactly what we expect.
Let’s go back up to 2Hz where we know the drive is using sensorless vector control.
And just like with the full vector control using the encoder,
if we watch the current on the bar graph and I grab the pulley, we see the drive try to do
everything it can to get that pulley moving again at the rpm we requested.
Volts hertz won’t react like that. So it looks like sensorless
vector is doing its thing. Cool. And if we stop the motor, we see it won’t
free spin, and the drive is still sending current out. Again, just like in full vector control,
the motor’s magnetization current defaults to always on when in any vector mode – even when
the motor is stopped. If you don’t want to burn that energy while the motor is stopped, then you
can tell it not to inject current while the motor is stopped by going to Parameter 181 and setting
it to a 1. Now we see the current output is zero and the motor’s rotor is free to spin.
That ought to be enough to get you started, but there are a number of things you need to be aware
of if you want to take full advantage of the vector control modes. Like how to choose the
right inverter duty motor for your application – all motors are not created equal. Why the motor
full load amperage has to be above some minimum. Should you tune with the motor warm or cold,
and why? How to minimize power consumption when in a vector control mode. How to deal with overloads.
How vector control affects braking. And any other loose ends I think of. Join us in Part 3,
where we will show you how to get the most out of using the vector control modes.
Meanwhile, click here to learn more about the WEG CFW500 drive.
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