There’s more to selecting a drive than just grabbing one with a current rating that matches
your motors current rating. Let’s walk through this check list of things to think about when
selecting a Variable Frequency Drive. Before we get started though, let’s make
sure everyone understands that Variable Frequency Drives are for controlling three phase motors.
You can’t use a VFD with single phase motors. If your motor has a starting capacitor and/or
a run capacitor, then it’s a single phase motor and you can’t use it with a VFD.
Of course, you do need to start with making sure the drive can handle your three phase
motor’s the full load amperage shown on the motors face plate.
For example, here’s a 1 horse power general purpose inverter duty motor. Its faceplate
says for a 60 Hz supply it needs three amps at 230 volts or 1.5 amps when run at 460 volts.
Here’s a 20-horse power farm duty motor. Its faceplate says it needs 46.6 amps when
run at 230 volts. And this 2 horsepower washdown motor needs
5.4 amps at 230 volts, 5 amps at 190 volts. So while each motor’s faceplate might display
it a little differently, they all contain the same amperage information.
Ok, given the full load amperage of the motor, we just select a drive to match that – right?
No. There are two other things you need to check before selecting a drive.
It’s important to know what your overload conditions might be. Most drives are designed
to handle temporary overload conditions so when you get that large rush of current when
you rapidly accelerate the motor or get a brief impact load the drive doesn’t fault
out. Most AC drives are designed to handle around 150% of overloaded current for around
60 seconds and the ACN drives are no exception. The ACN drives can also handle up to 200%
overload for no more than 4 seconds. If your application is going to need to handle
more than the drives rated overload capacity, then you may want to consider selecting a
larger drive that CAN handle that overload condition to avoid faults and to get the performance
you need. But be careful. Don’t just arbitrarily oversize
the drive. Why? Because if the drive gets too big, then it won’t be able to adjust
the motor protection parameters low enough to keep from burning up the motor. A good
rule of thumb to prevent that would be to limit your drive oversizing to around 200%
of the motors full load. If you need more than that, then its’s time to re-think your
system design. You can also use things like Auto-torque boost
or more advanced controlling methods, such as Sensorless Vector or Field Oriented Control,
etc. to get the higher torques needed to accelerate an excessive load w/o overloading the drive
or motor beyond their capacities. One side note: If you are replacing a simple
Motor Starter with a Variable Frequency Drive, then beware: some motor starters can handle
up to 600% overload conditions! If you are replacing a Motor starter then you are probably
doing it to soften that instant load anyway. Just don’t expect to get the same startup
performance with a VFD as you get with a motor starter.
Finally, know your application type. Do you have a variable torque requirement like a
fan or a pump? Those are usually easier to start and are less demanding. Most other applications
are considered Constant Torque. Like machine control, tensioning applications or conveyors
that need to keep things moving regardless of load fall into the constant torque category.
If you don’t know what your application will be, then assume the more demanding constant
torque and make sure your drive can handle that. The good news is, the ratings for all
the ACN drives do assume the more demanding constant torque.
Some drives, like the popular AutomationDirect GS20 and GS4 families show both the variable
and constant torque specs so you know exactly what they can do.
So, know what your application is and that the ratings of the drive you select are appropriate
for that application. Bottom line, taking these things into account
when selecting a drive will save you a lot of headaches later on and can even extend
the life of the drive. Speaking of extending the life of the drive
… please be sure to condition – some folks call it re-form – the drives capacitors
before using the drive with a motor. Not doing that shortens the life of your drive and can
even damage it outright. Every drive manufacturer has a different procedure.
The ACN drives are easy – if the drive has been sitting for more than a year, just apply
power for two hours, with no motor connected. That properly recharges the capacitors, so
they are ready when the motor demands that first big surge of current. If the capacitors
aren’t fully charged and the conductive oxide layers inside the capacitors aren’t
carefully burned off, you run the risk of damaging the capacitors and killing the drive.
So if you don’t know how long the drive has been sitting on the shelf, go ahead and
recharge the capacitors according to the manufacturers recommended procedure.
That should be enough to get you started with selecting and prepping a drive.
Click here to learn more about the ACN family of variable frequency drives. Click here to
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