What Is a drive anyway.......?
Dedicated to Jennifer.
My home brewed 4 Gig, 2.19GHZ Intel Duo Core 2 Box.
have always loved girls with big memories.)
See what it costs to hold an Onsite AC Drives seminar (at your facility).
On with the AC drives article:
AC Drive Technicians
good AC drive technician understands the operation of the variable
drive and the functions of its components.
An outstanding AC drive technician also understands the effects of
the load on
the drive and the effects of the drive on the load."
AC Motor-Drive Systems
(Sometimes called Variable Frequency Drives or VFDs)
your objective is to learn how to troubleshoot variable frequency
drives or maybe you want to know how to properly set-up
a drive through programming. Maybe you just want the question
answered: how does an AC variable frequency drive work?
this article I will definitely cover the theory of operation
of the variable frequency drive (VFD) and several typical AC
drive applications including multi-motor AC drive applications.
any case, to gain a useful working knowledge of the drive it
is mandatory that you understand how AC induction motors produce
mechanical force, called torque, and the limitations of
AC motors. Therefore, what follows includes quite a lot of
AC motor theory followed by heavy AC drive theory.
very first thing you need to know is that the AC drive
does not produce any torque - in fact, it
does not produce
even one ounce of load moving torque. All the mechanical force
that moves your machine and al of the mechanical force that
is produced by the motor & drive
produced by the motor. The drive itself just has to provide
with the proper voltage and current necessary in a form that
is usable by the motor. The drive itself, is only an electrical
motor is the all-important prime mover. If a particular
motor does not have the capability to produce the required
and speed there is absolutely nothing you can do to the
drive, programmatically or otherwise, to enable the motor to
when designing a motor-drive system proper motor selection
all never, never, never
match a "High Efficient" design motor to an AC
am emphatic about this because I see it happen all the time
and it is the worst thing you
can do. (Well, not really the worst thing you can do, but
I do want to get my point across.)
not buy a high-effecient motor to operate on an AC drive!
you have an interest in this topic you should consider registering
in my AC
drive seminar and bring a co-worker (because no one
else back on the job is going to believe what I told you during
the seminar.) Also, keep reading, I will tell you things about
AC drives you never before heard and never will hear anywhere
Exactly what is an AC drive?
word "drive" is
used loosely in the industry. It seems that people
involved primarily in the world of gear boxes and
pulleys refer to any collection of mechanical and
electro-mechanical components, which when connected
together will move a load, as a "drive".
When speaking to these people, an AC drive may be
considered by them as the variable frequency inverter
and motor combination. It may even include the motor's
pulley - I am not sure.
in the electrical field and electrical suppliers usually
refer to a variable frequency
inverter unit alone, or an SCR power module alone (when discussing
DC drives) as the "drive" and the motor as the "motor".
of variable frequency drives (VFD) used to refer to the
drive as just that, a "variable
frequency drive". More manufacturers are referring to
their drive as an "adjustable speed AC drive".
To make matters worse when a motor is included in the package
it may be referred to as an "adjustable speed AC drive
variable frequency drive is an adjustable speed drive.
Adjustable speed drives include
all types; mechanical and electrical. Now is it clear? Don't
worry about it. It's not clear to anyone. As you read on,
when I refer to the "drive" I am referring
to the variable frequency inverter alone.
Little About AC Drives
main power components of an AC drive, have to be
able to supply the required level of current and
voltage in a form the motor can use. The controls
have to be able to provide the user with necessary
adjustments such as minimum and maximum speed settings,
so that the drive can be adapted to the user's process.
Spare parts have to be available and the repair manual
has to be readable. It's nice if the drive can shut
itself down when detecting either an internal or
an external problem. It's also nice if the
drive components are all packaged in a single enclosure
to aid in installation but that's about it.
paradox facing drive manufacturers today is that as they
drives easier to use, the amount of training with which
they must provide their users increases. This is because
as drives become easier to use they are purchased more and
more by people of less and less technical capability. As
less technical people get involved in drive purchases the
number of misapplications goes way up. I call this phenomenon
the "dumb trap". (When manufactures discover this
phenomenon they simultaneously discover how dumb they've
been. Some have not yet discovered it.)
action in an AC variable frequency drive system is
in the motor. This
is really where it all happens.
To be an AC drive application Wizard (which is several levels higher then Guru)
one must understand how motors use electric power. It is essential. I cannot
emphasize the importance of this.
All loads moved by electric motors
are really moved by magnetism. The purpose of every component
in a motor is to help harness, control, and use magnetic force.
When applying an AC drive system it helps to remember you are
actually applying magnets to move a load. To move a load fast
does not require more magnets, you just move the magnets fast.
To move a heavier load or to decrease acceleration time (accelerate
faster) more magnets (more torque) are needed. This is the
basis for all motor applications.
Where does the real action happen in a AC drive system?
a cross-sectional view a motor rotor and field magnetic core.
Looking from the side would look something like a looking at
can add magnets (and torque) to our drive system by using a motor
with a core that is either longer, larger in cross-sectional
diameter, or some combination of both.
A Side Note About
Fishing, Electro-magnets, Current, and Magnetic Conductivity
When we go fishing we put bait on a hook and throw it in water knowing that according
to generally accepted theory, a hungry fish will sooner or later, bite. Well
the truth is we don't know why the fish bite. No one to date, has talked to a
fish (well maybe a few people talk to fish). The fact the we get hungry and therefore
fish must too, seems like a safe assumption. But it doesn't really matter because
we do know that putting bait on a hook will get fish into the boat.
Magnetism and electricity are the
same way. We have some well accepted theories that we can use
to explain how magnets can move our load but no one really
knows what magnetism and electricity are (regardless of what
they say). When it comes to using magnetic force to move our
load, how it works just doesn't matter. We do know that it
works. We have even noticed a few peculiar things.
We have noticed that when you wrap
a coil of wire around a piece of iron and apply electric current
the piece of iron becomes magnetic. We call this an electro-magnet.
Schematic of Electro-Magnet
(The Torque Producers Inside Every Motor)
have noticed a lot of things about electro-magnets
that are very important
to the drive application wizard (you'll see why later):
we apply the electric current, the magnet field grows at
a finite rate to a finite
- After voltage is applied and
full current is reached, which always takes a little time,
the field quits growing and becomes a constant size. If we
increase the applied voltage the field grows and becomes
stronger, decrease the voltage and the field weakens and
- When we remove electric power
to the coil the field does not just disappear. It just decreases
in size until it does disappear. It collapses over time so
- The more current our coil draws
(which we can force by increasing the applied voltage level
) the stronger and larger our magnetic field becomes. I know
I said it twice. It's that important.
- When we increase voltage to
our electro-magnet, current will increase directly proportional
up to a point. After that point current increases exponentially.
THIS IS IMPORTANT! Generally accepted theory says that the
iron core or any material, can only conduct a limited amount
of magnetic flux. Once that point is reached current can
become very high with a very small increase in voltage. This
is called magnetic saturation and is sometimes seen in motor
applications. Motor life becomes very short when the core
reaches saturation - about 15 seconds in some cases. We will
look at this and some of the causes later.
- Some energy is consumed by simply
magnetizing the iron core. Different materials consume different
amounts of energy. This is usually considered an energy loss.
- Some energy is converted into
heat within the iron core. Different materials convert different
amounts of energy. This is also usually considered an energy
a core is magnetized, demagnetization and reverse polarity
re-magnetization consumes more energy
and takes quite a long time, relatively speaking. (Remember,
an existing field has to collapse over time.) The amount
of this loss is proportional to the frequency of polarization
reversals. This happens 120 times per second when operating
an AC motor at 60 hertz. We will touch on the importance
of this later. (Are you beginning to see where all this is
Magnets Within the Motor and Torque
motor stator shown below is a two-pole motor meaning it is
wound with with two field coils for each phase.
In the industry this would be called a "2-pole
simplicity, only one phase is shown. In reality, a 3-phase, two
pole motor requires six coils, evenly spaced around the core
- a minimum of two coils is required,
to generate two electro-magnetic poles, for each of the three phases.
4-pole motor will have four coils per phase or 12 total coils for
a 3-phase motor.)
are designed so that the electro-magnets are made as
strong as possible with acceptable risk of core saturation.
maximize the torque capability of the motor but also means
that during normal operation every motor may at some point,
close to saturation. How close a motor runs to saturation
depends upon the amount and type of core material used. So
this point varies from manufacturer to manufacturer. There
really is a difference in motors and you get what you pay for.
the voltage applied to a motor is increased current to the electro-magnets
increases resulting in higher field strength and increased
torque output. This is a commonly used technique, especially
in AC drive applications. It is a very good way to gain torque
capability when needed.
technique can increase motor torque it will also cause higher
than normal motor heating resulting in reduced motor life. Close
Avoid saturating the core!
Three Phases of a 2-Pole Motor
image to the right shows all three phases wound into a 2-pole
how the end connections of each phase are connected together
at the "Y" point. This allows for three lead wires to be brought
out of the terminal box to be connected to a 3-phase power
you could remove the coils from the above motor without
connection, and lay them side-by-side, this is what
you would have. What is shown are three phases: A,
B, and C phase connected together (see the arrow)
at a "star" or "Y" point. There
are other motor connection schemes but this is the
If a magnet is passed
along the coils, an electric current is generated
in each of the three phases. In fact, there is little
difference between AC generator and motor field windings.
|The faster you move
the magnet the higher the AC output frequency. Variable frequency
drives control the frequency electronically. We'll get to more
on that later.
an iron core is placed so a moving magnetic field
it, a magnet field is generated within the iron core.
It takes time to generate a field therefore, the
new field reaches peak strength after the peak of
the generating field has passed. The bar (rotor)
is "pulled" by
the magnetic field thus producing torque.
magnetic field has to pass through the rotor to generate a rotor
field and pull.
If the rotor travels at the same speed as the magnetic field,
induction into the rotor will cease, the magnetic field will
disappear and the rotor will loose its pull and slow down. Pull
(torque) is obtained when fields are passed through the rotor
in quick succession. Remember though, it takes a long time to
generate a field. If the frequency of fields passing through
the rotor is too low, effectiveness is lost. If the
frequency of the generating field is held constant, and the torque
enough to move the rotor, the rotor will reach an equilibrium
speed, where at any higher speed induction and torque are reduced
and the rotor slows back down to equilibrium.
About AC Induction Motors
Rotor (Rotating Part)
Stator (Stationary Part)
Motor Formula - Calculating the Synchronous Speed:
is a constant in the formula - don't worry about it.)
the RPM the motor would run if the rotor did not slip. All
AC induction motors slip. ("Synchronous motors",
a special kind of induction motor, do not slip- at least least
not supposed to. More about synchronous motors will have to
be covered in another article.)
note about nominal RPM ratings:
AC motor referred to, in the industry, as an 1800 RPM motor
will be name-plated with a
speed of something less, usually around 1735 RPM. 1735 RPM
is a typical
RPM rating but can be higher or lower but is always less
than the synchronous speed (1800 RPM). The difference between
the synchronous and the actual RPM is
Adjusting slip is an important technique in AC drive applications.
A lot more about slip will come later.
following is surprisingly simple but important! Don't let it
You do not have to memorize it - just understand it.
of Synchronous Speed
(The "Poles" are the number of electro-magnetic
poles wound into the motor. Motors can have any even number of poles
wound into them but a minimum of 2 poles for each of the 3
phases are required. The most common AC motors are wound with either
2, 4, 6, or 8 poles.)
at the calculation above you can see that a motor name-plated
approximately 3450 RPM and 60 HZ is obviously a 2-pole motor
with a synchronous
of 3600 RPM.
of % Slip
(Typical induction motors slip anywhere from 3% to 5% when
they are fully mechanically loaded.)
of % Slip Calculation
are some motors called "induction motors" and what is induction?
an electric current is applied to a conductor a magnetic
field builds around that conductor. If another conductor
is in close proximity so that the building magnet field "cuts" through
that conductor, a current of equal potential is produced
with flow in the opposite direction of the original current.
This conductor is called the secondary
circuit and the principal is called induction.
an electric current is applied to a conductor a magnetic field
builds around that conductor. If another conductor is in close
proximity so that the building magnet field "cuts" through
that conductor, a current of equal potential is produced with
flow in the opposite direction of the original current. This
conductor is called the secondary
circuit and the principal is called induction.
number conductors in the secondary is increased the output potential
is increased in direct proportion. The inverse is also true.
is called transformer action. It is because of
transformer action that a current is created in the rotor
of an AC
induction motor and a resulting magnetic force, within and
around the rotor, is also created.
the magnetic field reaches maximum strength and quits growing,
the current flow in the secondary returns to zero
regardless of the level of current flow
in the primary. In other words, there is a secondary current induced
only when the magnetic field around the primary is either increasing
or decreasing in size.
article will be continued on our sister web site: www.drivesys.com
is not much sense in having two long articles on
two web sites and causing you to wait so long for
this web page regarding AC drives, to take a long
time to load into your browser. (And you may not
even be interested in it.)
continuation of this article will therefore be found, sooner
or later, based on when I have time to write, at
you would like to drop us a note just click
here. I read
everyone of our comments from readers.
for reading so far. .... James Shumberg
Copyright © 2003-2010 James M. Shumberg & Drive Systems,