VFD Series Collection

VFD SERIES COLLECTION

THE W HAT, HOW, AND WHY OF VARIABLE FREQUENCY DRIV ES

VFD SERIES COLLECTION

The Complete Series

Thank You for Reading!

“I’d like to start this book by thanking everyone who has clicked, read, shared, and commented on the VFD
blogs that have been posted to date. When you’re spending the time to write, rewrite, edit, rewrite these over
and over so many times, it truly helps make the effort worth it knowing those of you out there are getting
something out of these. So, my heartfelt thanks for making it all worthwhile.” – Kevin Beach, Automation
Specialist - Power Control

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What the Heck is a Variable Frequency Drive? Page 4
Page 8
The Environment Can Kill Page 12
Page 16
Motor Cable Length Matters Page 22
Good Panel Design Page 28
How Do I Choose an Electric Motor? Page 32
Is Safe Torque Off for Me? Page 36
How to Connect the Shield in VFD Cable
EMC Filtering

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What the Heck is a Variable Frequency Drive?

Let’s Dive into Variable Frequency Drives

We all hear the terms AC drive, variable frequency drive, and variable speed drive, but what the heck does this
all mean to you? How can you use them? How do you benefit from their use? This book will look at the uses and
cautions when implementing variable frequency drives. From this very basic primer to installation concerns to
designing a panel incorporating a variable frequency drive or VFD will be covered. So, let’s get started.

CHANGING THE SPEED

Since the days of water-driven line shafts there has always been
a need to change the speed of our equipment; be it fans, cutting
tools, pumps, or whatever. To change the speed, the operator had
to move the belt to different diameter sheaves to change ratios.
And he did it while the belt was moving!! Many fingers, hands,
and lives paid the price for productivity and feeding the family in
the early days of the industrial revolution. Obviously, there was a
need to improve this system. With the introduction of electricity
in manufacturing plants, innovation took off. Simple rheostats for
the small motors to motor-generator sets for larger motors to
the advent of true variable speed DC drives. All of these worked
adequately for the times, but a better solution was needed.

MOVING TO AC DRIVES

Variable speed DC drives were still in wide use by original equipment manufacturers into the 1980s but were
slowly being replaced by AC six-step drives. These drives converted AC to DC then sent the DC power in

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six “steps” to simulate AC sine wave to the motor. There were a number of problems with six-step drives, one
being poor torque control. A different issue I personally came across was at the old Cone-Blanchard Machine
Company in Windsor, Vermont. In the early 1980s, they looked to AC as a way to combat the lower-cost
competition from offshore on their flagship machine, the Blanchard Grinder. Everything worked great except
you could clearly see the steps in the steel after it was surface ground. The steps couldn’t be measured but they
were clearly visible to the eye. They stuck with DC.

INSULATED-GATE BIPOLAR TRANSISTORS

In 1983 the insulated-gate bipolar transistor (IGBT) was invented and started to come into widespread use in
the ’90s. The IGBTs along with the exponential improvement in processor speeds changed the AC drive world
forever. Using the high-speed processors, IGBTs could switch on and off at rates unheard of just a few years
before. These high switching frequencies, today up to 8 kHz and higher, smooth out the steps dramatically.
This high-speed technology is called pulse width modulation (PWM). PWM is most simply defined as getting
analog results with digital means. Digital control is used to create a square wave, a signal switched between on
and off. The changeover to PWM took a few years but it vastly improved VFDs. All PWM drives work in the
same fashion and has three general sections: the input rectifier, intermediate DC circuit, and output inverter.
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VARIABLE SPEED AC MOTORS

The diode bridge rectifier changes the AC input to DC and
charges the DC circuit/bus. The stored power in the bus allows
for a stiff power supply to the inverter section where the IGBTs
switch on and off at very high frequency creating a “sine wave”
output to the AC motor. Output frequency and voltage are
controlled electronically by controlling the width of the pulses of
voltage to the motor. VOILA!! Variable speed AC motors!! This
is very much an oversimplified description, but it does describe
how it works. Today’s AC VFDs are quite sophisticated, making
them capable of doing some pretty remarkable things and are
available in line voltages to over 6K. A PowerFlex® 755 AC
VFD has over around 2000 parameters that you can use to fine-tune the drive to meet precisely your needs,
and it can even be used in basic positioning applications!! Kind of like a servo drive. A VFD’s basic operating
modes are volts per Hz (VHz), sensorless vector (SV), and flux vector (FV) and can be used on induction
motors and permanent magnet motors. This allows for using this drive in many places in a modern plant or mill,
from a simple fan and pump drives (VHz) to coordinated drive systems on a paper line or steel rolling mill (FV).

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OPEN AND CLOSED LOOP

When in open loop, VHz mode a PowerFlex 755 will provide speed regulation of .5%
of base speed and in a closed-loop, FV mode the same drive will give .001% speed
regulation. You just add the feedback and change the Motor Control Mode parameter.
Remember, this speed regulation is of the base speed of the motor so an 1800 RPM
motor being controlled in VHz mode will vary as much as 9 RPM from the set speed.
Now, don’t get all concerned about all the parameters and things you have to do to
make your VFD work. Rockwell Automation® drives come with a start-up wizard in the
HIM as well as in the Connected Components Workbench software.

HUMAN INTERFACE MODULE

First, let’s talk about HIM. HIM stands for Human Interface Module and it can be mounted on the front of
the drive or on the panel door. HIM allows you to make changes, troubleshoot, and do manual operation
easily. Navigation is straightforward and the internal start-up wizard prompts you on each step. Connected
Components Workbench™ software (CCW) is a free download from Rockwell Automation that gives you
the ability to configure, troubleshoot, and start your drive. In addition to configuring every drive Rockwell
Automation currently provides, it also can be used to program the Micro800™ series of PLCs, PanelView™ 800
operator terminals, soft starters, and the MSR57 and Guardmaster® 440C safety relays.

We’ll take a little deeper dive into these tools in this series. I know this has been a pretty basic overview of VFDs,
but it is important to get a foundation on this subject. As this series progresses, we will be looking into a number
of topics regarding the application and commissioning of VFDs.

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The Environment Can Kill You

Making the Right VFD Selection

The selection of a VFD typically is a relatively straightforward process. You provide things like HP, voltage,
motor nameplate FLA, etc. and we can shoot out a basic proposal in no time. But there are some “gotchas” that
can come back to haunt you if they are not addressed early on in the selection process.
This section takes a somewhat deeper dive into the environmental issues that can prematurely turn your hair
grey. While it is essential to make certain the motor is properly selected for its environment, this post addresses
only the VFDs. In the past, it was very common to see drives hanging on plant walls in their own enclosure but
this is becoming increasingly less popular. It is now quite common to place drives in an MCC (motor control
center) or drive room. This will not only allow for a climate-controlled environment it also keeps tighter controls
on the electrical safety aspects, most notably arc flash (a topic for an upcoming section of this book).

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CONSIDER AN INDUSTRIAL ENVIRONMENT

When a drive is located out on the plant or mill floor, it is susceptible to all the crud floating around in the room.
Every industry has its list of environmental “gotchas” that need to be considered. For example, in the wastewater
industry, the drives that are mounted outdoors actually are subjected to fewer environmentally related problems
than those installed in the plant. When outdoors they predominantly deal with heat/cold and moisture such as
rain and snow but indoors, they are subjected to a variety of airborne contaminants such as hydrogen sulfide,
active organic nitrogen, or chlorine dioxide, in addition to the heat and moisture issues. Rockwell Automation
has addressed this as best they can by conformal coating the circuit boards but only on the places that do not
need to conduct power or signals to another part of the drive. Anything used for an electrical connection cannot
be conformal coated since they become nonconductive so a well-chosen enclosure can make the difference
between success and premature failure.

As you review locations for your drive installation be certain to watch for various factors
such as temperature and humidity. We suggest using a data logger to get a longer-term
look at the environment so trends can be addressed. A trend could be the sun shining
through windows creating temperature variations over the course of the day.
Once you are certain of the drive environment, then we can make an educated suggestion
as to enclosure ratings such as IP20 (NEMA 1), IP54 (NEMA 12), IP66 (NEMA 4X)
or whatever is best suited to combat the contaminants. Let’s look at these in order of
increasing costs.

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ENCLOSURE RATINGS – NEMA AND IP

IP20 rating provides an enclosure that will keep moderately sized objects such as material
scrap and, of course, fingers from getting to the electronics and causing a failure or injury.
It won’t keep out dust or fluids, both of which are detrimental to electronics. This enclosure
could be used in any relatively dust-free and dry environment and are commonly used in
MCC or drives rooms.
IP54 enclosures provide a higher rating for keeping out contaminants, especially when they
are gasketed with fans and filters. They will provide increased protection from airborne
contaminants like sawdust, paper particles, etc. but are not fluid-tight so definitely don’t
consider this wash down because they are not. IP54 is what you would commonly see in
floor-mounted drive cabinets.

IP66 does provide a sealed environment for your electronics but at a large price adder.
Rockwell Automation offers several packaged drives in a factory-made IP66 enclosure.
When available, this is a great solution as the enclosure was specifically designed to not only
work in a moist environment but to dissipate the heat adequately. The main environmental
consideration for IP66 packaged drives is to meet Rockwell Automation’s published
environmental specifications. These enclosures are rated for both indoor and outdoor use but
when being used outdoors remember to consider the huge extremes in temperature that can
be encountered. Consider not only cooling the cabinet but heating it in colder climates.

Of course, you can always install an IP00 (open) drive into your own enclosure but now you are responsible
for handling the thermal considerations including the heat loss for all devices and the manufacturer’s minimum
spacing around their devices. And this spacing includes wire-ways and other items in the box; literally, anything
that could restrict airflow over the heatsinks.

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ENCLOSURE UPGRADES AND ENHANCEMENTS

Further enclosure enhancements that can help are fans and filters,
air conditioning, providing outside air, etc. All of this can help
your drive become a long-term asset in your facility. Sometimes
it looks like an IP66 is your solution but, while this rating is one of
the most expensive available, it only gets even more expensive
because generally it is gasketed and sealed with no incoming air
to aid in cooling the devices inside, so it needs to be larger for
airflow. The cost continues to rise as outside air cooling or air
conditioning is added. A/C can oftentimes cost more than the
enclosure!

Which brings up another “gotcha”. Just because you added air
conditioning or some other type of cooling to your enclosure
doesn’t mean you can “cheat” on the manufacturer’s specifications
on air gap around your heat-producing devices inside the
cabinet to make the cabinet smaller and therefore less expensive.
Airflow over the heatsinks is critical as most of the heat-
producing devices are directly attached to the heatsinks. If there
is insufficient airflow over the fins on the heatsink, then you will
absolutely have a premature failure, even though the cabinet is
air-conditioned. The picture here shows an IP66 enclosure with
air conditioning but the improper spacing of the drives. The heat
built up so badly in this enclosure the plastic wire-ways melted.

Rockwell Automation has produced a document that addresses many industry-specific requirements called
Industry Installation Guidelines for Drives. This informative publication addresses issues and concerns
which are found in a typical plant in the automotive, chemical, food and beverage, forest products, paper and
converting, marine, mining, steel, tire, and water/wastewater industries. Feel free to download it using the link
here. Having this at your fingertips can be the difference between success and frustration.

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Motor Cable Length Matters

Grounding and Bonding

We are often asked by customers “How long can my motor cables be when using a VFD?” Well, the simple
answer is, there really isn’t a simple answer. This post looks at motor cable lengths and how to get the longest
length possible. First and foremost!! If your system is not grounded and bonded correctly, then none of
what follows applies. That being said, let’s talk about the motor cable itself as using the wrong cable can get us
off to a really bad start.

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CABLE OPTIONS

Today we have VFD rated cables that provide not only the conductors
and jacketing, but also shielding to contain the noise within the cable
and keep the radiated noise from getting to the drive. When the shield
is properly bonded at both ends, it keeps the inverter generated noise
contained within the drive/motor circuit. Another option is the armored
cables. These provide the full complement of conductors and employs
continuous aluminum armor that provides most of the advantages
of shielding and adds considerable mechanical strength and higher
resistance to moisture. While the VFD rated cable doesn’t really make
a difference in motor lead length, it will make a big difference with noise
issues. This is Belden’s VFD Cable Selection Tool which will assist in
VFD cable sizing.
Now, how far away from the motor can we mount the drive? Wiring and Grounding Guidelines for Pulse
Width Modulated (PWM) AC Drives from Rockwell Automation is a complete guide to everything about
wiring and grounding, both of which need to be done correctly to maximize motor cable lengths.

THE POWERFLEX 750 CLASS CHART

Let’s review this chart taken from the manual mentioned above and try to see what it’s telling us. First, be certain
you’re using the correct drive and voltage as the distances change with voltage (blue box above). Then look for
the proper frame size and HP (red box above). Keeping in mind, of course, that there can be 2 frame sizes for a
given HP (or kW). Now we look at the green box. This reflects the motor you’re using, specifically the Corona
Inception Voltage (CIV) of the motor itself. One of the phenomena associated with all energized electrical
devices, including electric motors, is corona. The localized electric field near a conductor can be sufficiently
concentrated to ionize air close to the conductors. This can result in a partial discharge of electrical energy
called corona discharge, or corona. If you remember hearing the buzzing noise from high-voltage lines, this is
the audible noise from the corona.

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CIV IMPACT

So how does the motor’s CIV impact us? There are typically
three sources of impedance in any VFD system; the drive; the
cable, and the motor. When the drive sends output sine waves
down the cable, they reach the motor and, due to the motor’s
impedance, reflect back towards the drive, while the drive is
continually sending sine waves to the motor. These sinewaves
are at 650 volts and when they combine or stack (out and back)
they total 1300 volts. Now, if your motor is rated for 1000 volts,
well, let’s just say this is a very bad thing for the motor, with a lot
of black smoky stuff resulting.

For this reason, today’s vector duty motors are 1600 CIV, and, as a result, are far better suited for use with
VFDs. So, let’s look at those columns (blue boxes where they intersect with orange. With just a drive and vector
duty motor, the longest lead length is 500 feet. And this is absolutely total cable length including up to the
ceiling and down to the submersible pump; not just how far apart they are from each other in a straight line.

Not that there aren’t options to attain longer lengths because there certainly
are but these are all at additional cost. The takeaway here is it is always
preferable to locate the drives as close as possible to the motor.

If this is not possible, the first option would be a terminator box. This mounts
at the motor and gives runs up to 600 feet with no voltage drop. Using this
device we can get the drive and motor combination to about 600 feet total
cable length.

REACTORS

Looking at the columns in the chart for Frame 6 750 class drives (below,
1600V under the red boxes), the next option would be to add an output
line reactor between the motor and VFD. This device mounts at the drive
and slows the rise time and allows longer cable lengths but there is a voltage
drop.

The third option is a Reflected Wave Filter or RWR drives (below, 1600 V
under the green boxes). This device mounts at the drive as well and limits
the peak voltage on the line. In doing so there can be cable lengths of 1200
feet but with voltage drop.

The line reactor and reflected wave filter options are in the 1600V columns
below the orange and green boxes in the following chart.

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VOLTAGE DROP???

So, what’s this thing about voltage drop? I mentioned it three times in the last three paragraphs so it must be
important. Well it is!! Most simply it is the charging current of the cable itself that can be fairly substantial in very
long lengths. With a VFD rated cable, this could be as much as an amp for every 100 feet of cable length. This
is of particular concern when using smaller VFDs as shown by the limited distance of 600 feet for the frame 1
PowerFlex 755, the smallest frame offered. The frame 6, 7 and 8 drives can go to 1200 feet as you can see in the
chart above.

Now, what do I do if I’ve got a problem in the field and don’t have any of the options in place? There is an
adjustable factor that may come in play. In PWM-based VFDs, there is something called carrier frequency that
is the rate at which output transistors are gated or turned on, usually from 2 to 15 kHz. Higher values yield better
current waveform, but more VFD losses. Try incrementally reducing the carrier frequency to as low as 2 kHz
and see if that helps. If not, contact your Drive Specialist for help in selecting the correct solution. We can bring
many different technical resources to aid in determining the best solutions for your needs.

As always, this is really intended to be a quick snapshot of the topic but there could be a thing or two in this you
might not have known previously. For more complete information on all this, contact your Drive Specialist, we
are here to help. Really!!

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Good Panel Design

Good Panel Design is Everything –
Or, how to not be your own worst enemy.

If I had a dollar for every time I was shown a complete mess of an electrical panel, I’d have…well, a whole lot of
dollars for sure. One time while at an end-user site with a Rockwell Automation Field Service Engineer I heard a
comment about panels that I think is well worth repeating here.
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This particular end-user was asking why he was getting communication failures on his PowerFlex VFDs. At one
point, he took us out and showed us, what he called, a gorgeous panel. And it was, there were around 60 low
HP drives in it as well as a lot of process type controls and an Allen-Bradley® ControlLogix® system. When the
end-user technician finished talking about the gorgeous panel, the Field Service Engineer said, “I’ve seen lots
of good-looking panels that simply weren’t designed right,” which has certainly stuck with me ever since. In this
particular case, there was not sufficient space above and below the drives to allow for adequate airflow over the
heatsink on the back of the drives which resulted in a communication failure as the communication processor
was sitting right on the heatsink. Oops!! They kept getting heat-related failures and, even worse, they were
usually at 2:00 in the morning. We want to help you avoid that. So let’s talk about designing a drive panel for
success.

5 Questions and Considerations

1. Consider the environment – Where is the panel going?
2. Are you following documented clearance guidelines?
3. Consider using flange mounted drives.
4. Are you following proper grounding & bonding guidelines?
5. How are you routing wires and wireway?

#1. CONSIDER THE ENVIRONMENT – WHERE IS THE PANEL
GOING?

First, consider the environment the panel is going to be mounted within. For good panel design, ideally, it
should be in a dedicated motor control room that would allow for a controlled environment and eliminating the
issues of potentially corrosive or other harmful gasses. It also could prevent unauthorized entry into what is an
electrically dangerous environment; we’ve all seen those arc flash videos.

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Environmental challenges can increase the risk of arc flash incidents in electrical panels.
But if that’s not possible and the drive is going to be out on the plant floor, be aware of any potentially corrosive
gases or vapors that could be present and also the temperature and humidity at that location. For good panel
design, select an enclosure rating that will protect all the devices inside it from corrosive gases or vapors and
cool it accordingly. You will need to know all the heat-producing devices before you can talk about cooling. Your
Horizon Solutions, a Rexel Banner, Account Manager can help you make the proper selection of a Hoffman
enclosure.
We have discussed much of this in previous sections of this book.

#2. ARE YOU FOLLOWING DOCUMENTED CLEARANCE
GUIDELINES?

Now from a drives perspective, as the heatsink fins align along the long axis of the drive, it is essential to
have sufficient clearance above and below each drive to allow the fan to push adequate air across those fins.
Most of today’s drives are “bookshelf” design so that they can be mounted against each other like books on
a shelf allowing for potentially using a smaller enclosure but the spacing above and below MUST meet the
manufacturer’s specifications. And when they say clearance, they do mean clearance. That Panduit wireway you

put between the drives does NOT count as clearance.
The above is taken from the Installation Manual for Rockwell Automation
PowerFlex® 750 Class VFDs.
Note, the minimum clearance above AND below the drive is 3 inches while the
drives are touching each other on the sides, the aforementioned bookshelf design.
The wire way would need to be more than 3 inches away from the bottom or top
of the drive. Oh and any other drives mounted near these would ALSO need to
be 3 inches above and below the drive. So, the spacing above these drives to the
next drives above them would need to be 6 inches PLUS the width of the wireway.
Anything less than that will absolutely present a problem for you or your customer
down the road as the airflow will be restricted.
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#3. CONSIDER USING FLANGE MOUNTED DRIVES

Another way to help reduce heat-related issues within a drive panel is to use “flange mounted” drives. These

are designed to have the heat sinks fins outside the enclosure putting the major heat source away from your
electronics.
Looking up at the bottom of a flange mount PowerFlex® 70, the orange line shows the back of the cabinet.
Proper grounding and bonding are essential in today’s industrial environment. Gone are the days where we
could hire someone off the farm and count on them to keep your plant running what with all the computers,
precision instrumentation, data collection, and internet requirements in today’s manufacturing environment.

#4. ARE YOU FOLLOWING PROPER GROUNDING AND
BONDING GUIDELINES?

So what’s the difference between grounding and bonding?
Grounding is simply connecting all the “current carrying”
elements of your system to earth such as the neutral of a power
transformer. There are several different ways to ground your
system but the “best practice” is Delta / Wye with grounded wye
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neutral.
The benefits of doing this are increased safety, providing a direct path for common mode current, controlled
common mode noise current, and others.
Bonding is connecting all the conductive elements that normally aren’t supposed to be carrying current to put

them all at the same potential, creating a low impedance path back to the source. But the system still needs to
be grounded.
A properly grounded and bonded system is safe and helps control common mode noise.
A final piece of advice on grounding, skip those gorgeous painted back panels and go for galvanized to improve
the grounding. If you have to use painted, then be certain to scrape off the paint at connection
points. Function over form.

#5. HOW ARE YOU ROUTING WIRES AND WIREWAY?

Another consideration is the wireway you use in the panel and how you route the different
voltages through your cabinet. The picture to the right shows power, signal wire, unshielded
Ethernet and 24VDC all mixed together. This is a sure-fire way to get that 2:00 AM call.
Always try to keep the most space possible between your various cables and wires within your
enclosure. Even something as simple as a shield between your wireways can help you sleep
better at night.
For more information, you should check out Rockwell Automation’s Industry Installation
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Guidelines for Drives for more complete and industry-specific installation information (PDF file download).
Following the best practices we have outlined and asking yourself the right questions will help improve your
drive panel design. Consider the environment. Double check; are you following documented clearance
guidelines? Are you following proper grounding and bonding guidelines?

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How Do I Choose an Electric Motor?

A Walk Down Technical Lane

AC electric motors are the engine of manufacturing and keep the economy running. They have been driving
industry since shortly after Nikola Tesla patented the AC induction motor in 1888. If you’ve ever looked at an
industrial motor catalog, you know finding the right motor in an industrial motor catalog can be a daunting task.
With a huge selection of motors in the same voltage, HP, and mounting, which one do you choose? We can
make it a bit simpler but first, let’s take a quick walk down technical lane and go through how an electric motor
works.
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In the cutaway above we can see the internal components of a three-phase electric motor. The stator is
mounted in the housing and is therefore stationary and the fan and rotor are mounted to the shaft and will
rotate. The stator consists of a bunch of overlapping windings that are offset by 120° electrically. When power is
applied to it a magnetic field is created that rotates at what is called synchronous speed.
At the same time, windings in the rotor are closed and electromagnetic force or emf is generated in the
opposite direction from the stator creating torque and causing rotation of the shaft. The rotor will never quite
reach the speed of the stator and that difference in speed is called motor slip. Therefore, under load, an 1800
RPM base speed motor will actually operate around 1740-1770 RPM at 60 HZ.
This is a totally simplified explanation but should serve our purpose for this book.

BACK TO THE MOTOR CATALOG

Now back to that mind-numbing motor catalog. As with anything
else in an industrial environment, we always start with the application.
And with any application, we must look at the work being done, the
environment, the power needed, and the voltage being used. Let’s start
with an easy one. The motor will be located in a climate-controlled,
clean, industrial environment like an electronic parts manufacturing
plant, belted to the drive pulley of a conveyor and 480 VAC is plant
power.
If it just uses a contactor to start, it then we can use any motor with the correct HP, mounting, and voltage. This
section is not intended to help with motor sizing but rather the correct type of motor for an application. So, if
we were given a need for 2 HP and 1800 RPM base speed, then we would pick a foot mounted motor since it is
belted to the load. The specs would be 2 HP, 1800 RPM base speed, 145T frame, and three full load amps. As
we talked about previously the 1800 RPM base speed will not be seen due to the slip inherently in the motor,
so the actual speed will be more like 1750 RPM. Click on the picture for the complete data sheet on this motor.
Yeah, yeah; I know there are some additional considerations when using a belt drive on a motor, like a roller
bearing on the drive end, but that is beyond the scope of this section.

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The next day the application has been changed and the motor speed needs to be varied depending on
the product being run at the time. So now we are not using a contactor but a VFD instead. The additional
information we need to gather is the operating speed range, and we should really take a deeper dive into the
application. The speed range can vary greatly across electric motors but the most important thing in the motor
is the insulation class. When the motor will run at speeds less than base speed it is important to know how the
motor cools itself. Totally enclosed fan cooled (TEFC) is the most common industrial motor housing and is fine
for most applications. But when you’re running it at reduced speed it is critical to remember the fan is attached
to the non-drive end of the motor shaft turning at motor speed and the fan’s airflow drops by the cube law. In
other words, a 1750 RPM motor running at half speed, 875 RPM, is only moving 1/8 the volume of air over the
cooling fins than at the base speed. This why insulation is like way important.

INSULATION CLASS

Most new industrial motors are insulation class F or H which is what we are
looking for in a VFD controlled motor. The insulation classes are based
upon the maximum temperature in the windings of the motor.

These are all based upon NEMA MG1 Guidelines established for
adjustable speed motor applications. These guidelines apply to motor
torque, speed, noise, and other factors but this section is only considering
motor insulation. As we already discussed, the airflow drops drastically with
speed so generally, the higher the insulation class or winding temperature
the wider a speed range the motor will be able to accommodate. There are,
of course, always exceptions to this.

In our sample application, the conveyor needs to be able to run from full speed down to 10% of max speed. For
our purpose, we’ll assume max speed is motor base speed.

So, going back to selecting the motor, most industrial motors today are what
is generally referred to as Inverter Duty which means they can be used with
a VFD, but it may not give the speed range needed. The speed range can
range anywhere from 2000:1 down to 2:1. Our conveyor application requires
1800 to 180 RPM or 10:1. The motor we had already selected has a constant
torque speed range of 1000:1 which is more than ample for our needs, so the
motor does not to be changed. If it had been 2:1 a different motor would be
needed. To be safe if this motor had been rated for 10:1 we may still want to
change motors.

Our application is a moving target and now due to space constraints, it’s been
decided they need to use a right-angle worm gearbox between the motor
and drive belt. Because they said they want to mount the motor directly to
the gearbox then we need to add some way to mount the motor. We need a
mounting flange on the drive end of the motor. The most common standard
flanges are referred to as C-Face and are a NEMA standard across all motors
and gearboxes. Looking at our motor catalog, we see the standard flange for

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2 HP is 145TC. The smaller NEMA frame sizes usually come with both the mounting base and the C-Flange.
This flange will bolt directly to a worm gearbox with the correct 145TC adaptor on the input.

This is a pretty basic primer on selecting a “normal” industrial motor in a
“normal” environment, but in my day-to-day world I run into mostly more
specialized motor needs. So, let’s take a quick look at some of those.

The food and beverage and pharma industries have cleanliness issues that are
dictated by FDA regulations. Their equipment cleaning requirements range
from cleaning solvent wipe down to high-pressure washdown. And the motor
manufacturers do have both epoxy-painted motors and complete stainless-
steel housings to accommodate them. When cost is an issue, epoxy paint is
generally a less expensive option.

Paper and steel mills are rather hot and nasty places. Red hot temperatures
in the case of steel, steam, and water in paper create a need for some type
of extra cooling for the motors. The most common way to accomplish this
is with a blower cooled motor (TEBC), and these blowers come in a couple
of mounting styles: axial or piggyback. Axial means the blower is mounted
on the non-drive end of the motor and will add additional length to the
motor which will need to be considered during selection.

The piggyback blower adds very little to the overall length, but it is
substantially taller than a standard motor which, again, needs to be
considered.

For both, piggyback and axial blower cooled motors a starter is needed for the blower motors.

MOTOR INDUSTRY AND APPLICATION INSIGHT

The aggregate industry has a set of really different issues that need to be addressed. Since they are used on
some pretty shock-filled applications like crushers and pulverizers they use 100% cast iron construction, high
strength steel for the shafts, extra high starting torque, roller bearings on the drive end, special seals, epoxy paint,
even rodent screens. They are designed for use outdoors in the harshest environments including heat, rain,
cold, snow, you name it. And here in the north country where I live and work, they have to sit idle outside buried
in snow all winter and start right up in the spring. They are technically referred to as aggregate duty but also
“crusher” duty.

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Pump motors are another pretty specific motor that can bite you on the butt
if you don’t know them. When mounted on the pump they appear to be a
standard NEMA flange motor but, like an iceberg, what’s below the surface is
what sinks you.
These pumps often have special mounting flanges and pump specific shafts.
These shafts are generally longer so they can reach down into the pump
and often have features machined in so the pump impeller can be mounted
directly on the shaft. Most motor manufacturers have a large selection of
motors they have developed in conjunction with the pump manufacturers and
meet pump specific standards. They are often designated as J Frame and
would show as 56J rather than 56C or whatever the frame number is followed by a J.
An increasingly common motor today is the permanent magnet AC motor.
The PM DC motor has been around for decades, but the AC version is a
relative newcomer. As its name implies, there are permanent magnets located
in the housing of the motor but unlike AC induction motors, no energy is
induced to the rotor. As a result, they are more efficient (2-4%), have no-slip
and are considered synchronous motors, run cooler, and have higher power
factors. Since they use rare-earth magnets, they are up to 2X as powerful as
standard permanent magnets allowing for smaller and lighter motors versus
AC induction. It is important to note line starting is not an option and a VFD
must be used with PMAC motors. PMAC motors are being used extensively
in energy savings projects today.
Most motor companies have an extensive amount of technical information available to help you in the final
selection process. Just click on the motor images and then click on the catalog number to see the technical data
for each of them. Once there you can scroll down and get the performance curves and the drawings.

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Pictured: Screenshot of a Marathon Motors web page; note the purple buttons for the datasheet, outline, 2D
CAD, and 3D CAD.
Beyond what we have discussed here, there are lots of other specialty motors such as farm duty/agricultural,
HVAC, fan motors, car wash motors and many more but this section is dedicated to industrial motor selection.

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Is Safe Torque Off for Me?

To STO or Not to STO

In today’s safety-conscious manufacturing environment, this is a very reasonable question to ask. With all the
new safety-rated switches, PLCs, and other devices, is safe torque off (STO) something you could really use,
and is it worth the investment?
Right off, I’ll say, given the minimal cost involved with drive safe
torque off, the investment is truly a no-brainer even if you don’t
use it at first. STO is designed to remove power from the gate
firing circuits of the drive output power devices (IGBTs). With the
power removed, the drive output power devices cannot turn on to
generate AC power to the motor. When used in combination with
other safety devices, it can satisfy the requirements of IEC 61508,
IEC 61800-5-2 SIL 3, ISO 13849-1 PLe, and Category 3 for safe
torque off (STO). The safety rating also depends upon the drive
selected, so be certain to work with your Specialist. One really nice
side benefit is the drive control remains powered, so you don’t lose
communications resulting in further loss of valuable production time.

ASSESS THE RISK

So, let’s get started. If this is a new, new-to-you, or extensively rebuilt piece of equipment, then you should
absolutely start with a risk assessment. This would be done by a team of people who all have a need to touch
the machine at various times and should include the operators, engineers, maintenance people, and anyone
responsible for cleaning the machine. There are many ways to evaluate the risk but a common one is called the
hazard rating number (HRN) system where numerical values are assigned to likelihood of occurrence (LO),
frequency of exposure (FE), degree of possible harm (DPH), and number of persons affected by a situation
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(NP). NP is why it is so important to represent everybody. These values are used in the following calculation:

LO X FE X DPH X NP = HRN

The values for each of these varies tremendously. For example, LO goes from .033 for almost impossible to 15
for certain, no doubt of it happening, so obviously designing the risks out has a huge influence on the HRN.
And given the final score needs to be less than 10, minimizing contact with the machine and designing the risk
out is a pretty good way to get started.

For a deeper dive into risk assessment, visit Tom Hopkins’s two-part blog starting with “5 Machine Safety
FAQs Answered.”

And a little something to keep in mind, when it comes to safety standards in the US, there is no such thing as
being grandfathered. When a standard is deactivated and replaced, the new standard is the law.

WHEN TO USE SAFE TORQUE OFF

So, when does STO come into play? First, let’s be certain when STO
is not an option. If there is a need for electrical or mechanical repair or
machine component replacement, you must go through the normal
LOTO (lockout/tagout) process to ensure all energy is dissipated in
the system, including electrical, pneumatic, hydraulic, and inertial—any
stored energy that could be released causing unexpected movement and
injuries.

STO is designed for use during routine machine operation for things
like clearing jams and the like. So, how does it work? Quite simply, STO
inhibits the output of a VFD and does not allow the drive to generate
sufficient current to create torque. When the motor has stopped, it
cannot be started again until the STO function is removed. Depending
on the drive, the device could meet PLe SIL3 safety.

The picture to the right is a good example of where STO should be
considered; one hand in the machine and another on the start button, not
to mention the lack of safety glasses, cut gloves, etc.

From the perspective of safety, two stop categories must be considered.
Stop Category 0 is achieved with immediate removal of power resulting in coast to stop. In some situations, this
is acceptable, but, when it is not, you should use Stop Category 1, which leaves power on the drive for enough
time to achieve a controlled stop. In either stop category, no further motion can happen so long as the STO
function is in place. There is a Stop Category 2, but that is considered a production stop.

When used in conjunction with approved safety rated switches or PLCs and Interlocking and/or Guard Locking
guard switches, an acceptably safe environment is created for the machine operator to enter the machine for
routine work.

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HOW ABOUT SAFE SPEED

There is, of course, a little twist in this, and it is called safe
speed monitor (SSM). This option is used when the process
requires some motion in the machine, such as threading a
paper machine. One application I was personally involved
with was on a textile machine. Periodically, the operator
needed to go in the machine with gloves and a Teflon pad
to clean the chrome rolls. Obviously, this couldn’t be done
at operating speeds, so the SSM would allow the Guard
Locking Switches to be opened by the operator when the safe
speed was reached. The SSM should always have a thorough
understanding of the machine function and be dictated by the
results of the risk assessment performed on the machine.

Traditionally, all safety-related components were hard-wired
to the controller creating a number of pain points, such as lots
of wire and “where the hell is the short?” Today, safety can be
accomplished over Ethernet and STO or SSM is no exception,
just be certain the device is designed for CIP Safety. In the
case of Rockwell Automation PowerFlex 750 class drives, STO
would be 20-750-S for hardwired, 20-750-S1 for hardwired
SSM, and 20-750-S3 for STO using Ethernet. While the
20-750-S3 can also be hardwired, it is important to remember
the 20-750-S is hardwired only with no Ethernet. Rockwell
Automation also offers an “all-in-one” 20-750-S4 card for the
PowerFlex 755 and PowerFlex 755T drives, which incorporates
safe torque and safe speed either hardwired or over Ethernet.

So, to answer the question this section started with, “Is safe torque off for me?”, look closely at your process.
Does it require the operator to enter the working area for routine and normal operations? If the answer is
yes, then safe torque off is something you should be seriously reviewing. And if you need help making that
determination, contact us.

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How to Connect the Shield in VFD Cable

It Depends

So far, we’ve talked about VFD cables, panel design best practices, and noise mitigation, and we hope we
helped reduce some of the gotchas when dealing with VFD and the harmonic issues they create. A question we
still field routinely is, “Do I need to bond both ends of my motor cable, or do I leave one loose?” Well, the simple
answer is “DEPENDS”! Back in the day, one end would be left loose, but as VFDs have started using very
high switching frequencies (See the “What the Heck is a Variable Frequency Drive?” section of this book), the
electrical noise created by the drive has become an ever-increasing issue in modern process systems. There are
two separate but equally important components of this:
1. Motor cable
2. Feedback (signal) cable betweek the motor and VFD

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These both have decidedly similar but really opposite functions. Motor cables need to keep radiated electrical
noise from leaking into other components, and feedback cables need to keep that same noise from entering the
cable and corrupting the signals. We’ll look at the motor (load) side first. Load or motor cables need to be able
to contain the electrical noises created by the drive. These cables should meet the NFPA-79 Standard (See
“Choosing VFD Cables Just Got Easier”), which calls for a braided shield to be under the jacket of the cable.
When both ends of the shield are bonded to both the motor and the drive, that will keep the noise from leaking
out. The field in the windings switching on and off at high frequencies (typically 2-8 kHz) it induces voltages that
can affect any low voltage signals, such as encoder signals causing them to become erratic and inaccurate. By
grounding the motor housing by way of the shield helps to isolate the induced noise back to the source.

Possible fully grounded system using shielded cables on both the line and load side.
The picture to the right shows an optional EMC grounding plate. This gives
a clean method to pass the cables through the EMC cores and land them
on the drive. This plate is grounded to the drive and gives a continuous path
for the electrical noise.
But why are there two, you ask? Well, the line (input) cable also should be
shielded as it too is a conductor of noise. Don’t panic though, as that only
has to go between the drive and line reactor. The standard power cable can
go from there to the power source.
The cores are not necessary all the time and are used primarily in CE
certified applications to reduce RF emissions. Not all drives will use ferrite
cores for this, so consult your specialist for what devices are required to
meet CE.

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Not every application requires feedback cables or signal cables, but when they do, it is always best to route low
voltage and high voltage separately. Of course, this is not always practical. So, in the case where they will be near
each other, the bonding of the feedback cable is also of major importance. The reason, in layman’s terms, is with
a shielded signal cable, you want to capture noise that might be induced elsewhere in the system (i.e., motor
cable) and keep the potential and current flow grounded in one direction to protect the clean signal inside the
shield.
So, in a nutshell, power cable shields are bonded to both the motor and drive and the feedback to the drive only.
If you need help connecting the shield to VFD cable, our Automation Specialists are here to help. Contact us
today!

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EMC Filtering

To Filter or Not to Filter

THAT IS THE VFD/EMC QUESTION

In today’s high-tech manufacturing world, the issues of electrical noise and methods to mitigate it are of
supreme importance. For example, a small bit of harmonics on the electrical system in a semiconductor plant
can take out some critical instrumentation costing hours of production and millions of dollars. With so many
things available to help, what should you do?
Today we are looking at the EMC filtering options available internally or externally in VFDs.

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IEEE-519

One of the more common questions we get is, “What percentage of voltage and current distortion does EMC
filtering provide?” It’s similar to what you would get with expensive line-side harmonic solutions, such as passive
or active filters. This question is typically asked relative to distortion limits mandated in IEEE-519. IEEE-519 is
a system guideline setting limits for harmonic distortion sent out of electrical devices onto the electrical power
system.
If you get nothing else from this section, while using EMC filtering in the drive is certainly a best practice, they
have little effect on IEEE-519 compliance. These filters are designed to provide compliance with CE directive
2014/30/EU. While requirements for compliance with this directive are rare in North America, if you are
shipping a machine with drives on it to the EU, then EMC filtering must be used to be in compliance.

ELECTRICAL NOISE

While we have gone over this in previous sections and webinars, let’s have a quick review of what is meant by
the term electrical noise. We’ll start with the fact it has nothing to do with audible noise. Among other causes
like the drive cooling fans, audible noise is created by the switching frequency of the IGBTs and changes with
the rate of switching. Rather, the term electrical noise is defined as voltage spikes generated by electrical devices
when in routine operation that can interfere with the routine operation of other selected devices.

The left picture shows a typical PWM sine wave. Note the waves aren’t smooth and that is caused by the
switching of the IGBTs in the drive. The right image shows those same waves but with a longer motor cable and
the electrical noise is obvious. The addition of EMC filtering will not help with distortion of this magnitude and
line distortion of this magnitude is way out of the limits allowed in IEEE-519.
Among other causes, common sources of electrical noise are:
⏹ Mechanically switched inductive loads create intense intermittent noise.
⏹ PWM drive power outputs create intense continuous noise.
⏹ Switch-mode DC power supplies can create continuous noise

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Obviously, the second bullet applies to pulse width modulated AC drives. In the case of VFDs, these spikes
are generated by the high-speed switching on the output IGBTs in the drive and, as we have already seen, are
further impacted by motor cable lengths. These can’t be mitigated by EMC filtering enough to meet IEEE-519.
For more information on this subject see the “Motor Cable Length Matters” section and the webinar “Proper
Cabling from VFD to Motor.”

Before we continue, as we have stated MANY times in the past, nothing in this section or any of our other
drives sections and blogs and webinars applies if you do not have a properly grounded and bonded system. For
a little deeper dive into that see our webinar “Proper Drive Installation and Dealing with those Pesky Power
Jumpers” and the “Motor Cable Length Matters” section of this book.

EMC FILTERING AND POWERFLEX DRIVES

So now that we understand their limitations, let’s circle back to EMC
filtering. First, the Rockwell Automation® PowerFlex® 750 class drive
comes with these filters embedded as standard and the PowerFlex 520
class is available with filtering as an option. The PowerFlex 525 part
number changes from 25B-D4P0N104 to 25B-D4P0N114. This applies
to both the PowerFlex 525 and 523. However, if you find out after the fact
you need filtering, the filters can be ordered as separate component. In
frame sizes A-D, the filter is mounted to the enclosure back panel and the
drive is mounted on the filter. For Frame E, the drive is mounted on the
panel as normal and the filter is mounted on the side of the drive.

Whether you are using a PowerFlex 750 drive or PowerFlex 520 drive
class with EMC filters, we strongly recommend using the optional EMC
mounting plate. This plate gives you a convenient and inexpensive
place to land all your motor cables, the shield in the cable (you are, of
course, using VFD rated cables), and the ferrite cores, which are a part
of the EMC filtering system. Once you’ve used these EMC plates and
experienced how easy it is to mount motor cables and shields, you will
want to consider them for all your drive requirements even without the
EMC filtering mandate.

In the picture to the right, you will note, the PowerFlex 520 mounting
plate does not come with the ferrite cores as they are included with the
drive when you order the EMC filter option with the drive.

The PowerFlex 750 EMC mounting plate comes with the cores as they
are not included with the base drive.

The cores are needed when CE directive 2014/30/EU is required. Please
note, the PowerFlex NEMA 1 Conduit Box does not work with the EMC
plates, it is either one or the other.

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BEST PRACTICES

So, in conclusion, the EMC filters are not intended to get your drive into compliance with IEEE-519 but are
absolutely essential if the machine is heading to the EU. Yes, if you miss that, it can be added later, but the
additional cost for the changes will far outweigh the relatively low cost of having them in the original design.
They definitely are a “best practice” whenever the drive is used in a water or wastewater environment or really
any industrial application but not essential.
And the EMC plates are super convenient.
But if you need to deal with serious harmonic issues, use the appropriate devices and don’t try to get away on
the cheap with the EMC filters. The correction will cost far more after the fact. As the adage goes “it is far less
expensive to design correctly than to fix it in the field”, especially when that in the field location could be an
ocean away.

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