It’s tempting to select a variable frequency drive based on horsepower alone.
After all, if you select a drive that has the same or more horsepower than your motor
you are using, then you should be good to go – right?
Well … not necessarily.
There are six other things you need to consider when specifying a drive: To make sure the
drive can handle the motors current demands, just read the motors full load current requirement
right off the name plate and find a drive that is rated for at least that full load
current.
That is a great starting point, but you also need to make sure the drive can handle any
overload conditions you may expect during startup or during periodic or intermittent
extra loading.
Looks like this one is rated for 150% for constant torque applications and 120% overload
for variable torque applications.
So, make sure you know what your system overload conditions are.
If you expect to exceed these overload specs, then up size the drive until you find one
that can handle.
You will also need to know if your application is for variable Torque like fans and pumps
or constant torque like conveyors and machine control.
If you are not sure, assume constant torque.
Knowing the application type is important because all the spec tables are segregated
by application type.
As the altitude increases, the air becomes less dense, which reduces its ability to cool.
Since there isn’t as much cooling available you may need to oversize the drive to compensate
for the decrease in cooling.
GS4 AC drives are designed to operate at 100% capacity at altitudes up to 1000 meters.
By the time you get to 3000m, the drive can only be expected to generate about 80% of
its rated current.
Note that this curve assumes these temperatures for these temperatures for the different types
of standards.
At 1000m GS4 drives can be operated up to 50- or 40-degrees C. But at 3000 meters the
max air temperature can only be 40- or 30-degrees C.
For example, Denver is at 1600 meters.
So GS4 drives can only be expected to output about 95% of their rated current.
Which brings us to temperature.
AC Drives generate a lot of heat so if the drive is in an enclosure there is the possibility
that you will exceed the max temperature ratings of the drive if the enclosure doesn’t have
additional cooling or ventilation.
This table in the user manual tells us Drives up to 30 or 40 hp can operate up to 40 or
50 deg C depending on if the ventilation cover is removed or not.
Of course, removing that cover does change your IP Rating.
We have similar max temperatures with the larger drives.
They can operate up to 40 or 50 deg C depending on whether they have a conduit box installed
or not.
They don’t have ventilation covers and that conduit box restricts the airflow.
Can you go above those limits?
Sure, but you have to derate the drive like this which is 2% for every 1 degree C above
the max. with an absolute maximum operating temperature of 60 degrees C. So make sure
you have measured or calculated what the temperature in your cabinet or room will be when the drive
is running to be sure you don’t exceed these limits or if you do exceed the normal limits,
that you derate the drive accordingly.
A variable frequency drive takes the three-phase sinusoidal input, rectifies it and then chops
it so the voltage sent to the motor looks like this on each phase.
The VFD very accurately controls the width of these pulses – we call that pulse width
modulation - so the motor feels like it is getting this, when it is really just getting
a bunch of square pulses.
The rate that the pulses are sent is what we call the carrier frequency.
The higher the carrier frequency the more accurately we can reproduce the effective
sine wave into the motor.
If we have a low carrier frequency which gives us only a few pulses to work with per cycle,
then we’ll get a really crude approximation of a sine wave.
So, you would think we always want a really high carrier frequency – right?
Well, no.
In fact, you usually want the lowest carrier frequency you can handle.
Parameter 2.10 is for setting the carrier frequency.
And down here at the bottom there is a great little chart.
It tells us if you want to minimize motor noise and vibration, then you want a higher
carrier frequency because you get a smoother sine wave.
But most of the time you want to minimize EMI, you want to minimize heat and minimize
power consumption, which means you want a lower carrier frequency.
What carrier frequency should you use?
Most of the time the default is fine.
But if you find need to reduce the noise or reduce the power consumption or reduce the
heat dissipation … being able to modify the carrier frequency is a great option to
have.
So, you need to select a drive that is rated for the motor you want to use, can handle
any temporary overload conditions you expect, is designed for the type of application you
have, that will still support your motors needs at the altitude, and can output the
current your motor needs while operating in the temperature of your environment.
And you can tune the carrier frequency if you need to lower the power consumption and
operating temperature of the drive.
Click here to learn more about GS4 variable frequency drives, click here to learn more
about AutomationDirect’s free award-winning support options, and here to subscribe to
our video channel so you are notified when we post new videos.