Motor Basics

Motor Basics


Yaskawa eLearning The Technical Training Department of
Yaskawa America Incorporated Presents… Induction Motor Basics Understanding How an Induction Motor Works Hi, I’m Steve Koehler. Three phase induction motors are the most commonly used AC motors around, you’ll find them in literally thousands
of applications with the ability to produce a lot of torque, three phase induction motors typically
power large industrial machines used for product manufacturing, processing and
other applications. Their simple construction, relatively
low cost and low maintenance are the main reasons for their popularity. In this program we’re going to look at
how an induction motor works and provide you with some basic information on the
induction process. We’re going to start by listing the
components that make up a three phase induction motor then to help you understand how they all work together and the role that each component plays we’ll review the theories of induction and electromagnetism. Next, we’ll explain the difference between synchronous speed and rated speed. How the number of poles affect the
stator and rotor and discuss speed torque curves We’ll define and compare NEMA motor designs then take a look at the information that is typically found on an induction motor nameplate and explain what each specification means. And finally, we’ll take a look at the
various types of motor enclosures. Now, at the end of each section we are
going to ask you some questions to review the points that we have covered. Oh, one note before we start… in many applications the speed of a
three phase induction motor is often controlled by a variable frequency drive the information we present today will
give you a good understanding of three phase induction motors and prepare you
for the next video in our series titled Variable Frequency Drive Basics Let’s get started. Let’s start by identifying the components of a typical three-phase
squirrel cage type induction motor. Inside the stator frame of the motor
are the stator and the stator windings. The stator is made up of a series of
stacked, insulated and compressed iron slices with cutouts or slots through
which we run the stator windings. We use stacked metal slices to reduce
electrical losses in the system. Centrally located within the stator is the rotor. The rotor is basically a cylinder with
an iron core which is also made up of laminated slices just like the stator. The rotor has conducting end caps on
each end and conducting bars running through the slots in the laminated
slices that connect to the end caps. The result is a rotating cage that looks
similar to a squirrel cage. That’s why induction motors are commonly
referred to as a squirrel cage motor. The rotor is attached to the motor shaft. Bearings support the motor shaft
allowing the shaft and roller to rotate as it remains centrally positioned
within the stator enclosure. The shaft transports the mechanical
energy created from the rotor to the load and an air gap between the stator and
rotor eliminates any physical contact between the two components. Protecting the components is the
enclosure which consists of the motor frame and the end bells. The end bells contain bearings which
allow the rotor shaft to turn freely on its axis. Now the type of enclosure can vary
depending on the motor’s application. We’ll talk more about the different types of
enclosures later. All right, before moving on here’s the
first question… What are the two major components of an
induction motor? The answer is the stator and the rotor –
these components work together to convert electrical energy into
mechanical energy. In this next section we’ll talk about how these two components interact with each other to create electromagnetic induction. We’lll start with the definition. So, what is electromagnetic induction? Electromagnetic induction is a process
in which current is created in a conductor by moving it through a
magnetic field or by having the magnetic field move or change around a stationary conductor. In a three phase induction motor the
stator produces the varying, or to be more exact, rotating magnetic field
needed to initiate and maintain the induction process. And let’s look at how that’s
accomplished. As we described earlier, the stator has a
core of individual laminated plates that are laminated together these plates have
slots in them which allow insulated electrical windings to pass through the
core. The stator windings are distributed
around the stator in such a way that when current flows through them the stator produces a magnetic field. To better understand how the magnetic
field is produced, think about the electromagnetic demonstration you may
have seen back in science class. An insulated copper wire is wrapped
around a nail. When voltage is applied across the wire
current flows through it and the nail becomes an electromagnet. In this case you can think of the nail
as the stators core and the copper wire as the windings. Certain metals such as iron, like this
nail, have magnetic properties where the atoms are randomly oriented. This random scattering of atoms cancels
any natural magnetic field, but when current is passed through the coil of
wire it produces a magnetic field which forces the atoms to align in the nail.
These aligned atoms combine to produce a strong magnetic field from the nail and
the same is true of the stator’s core. Three-phase induction motors use
balanced three-phase power. The phases are electrically separated
from each other by 120 degrees. When you apply alternating current (or AC)
the flow of electrons reverses direction halfway through the cycle. To understand how quickly this happens,
remember that the standard AC power frequency in North America is 60 Hertz
and that means that current flowing through the stator repeats one
electrical cycle every sixteen point six milliseconds. And that’s faster than the
blink of an eye! Here’s an example of a simplified model. You can see that the stator is
mechanically stationary but when we combine three-phase alternating current
with the correct placement of the stator slots, we create a rotating magnetic field. Now let’s look at the rotor and see what
role it plays in the induction process. The rotor with its iron core, conducting
bars and conducting end caps is constructed to conduct electromagnetic
current. As the stator’s magnetic field rotates, it
induces voltage in the bars of the rotor. As the rotor bars are shorted at both
ends using end rings there is now a closed path for the flow
of current in the rotor bars. This in turn aligns the atoms in the
iron cores of the rotor to produce its own magnetic field, which opposes the
magnetic field produced by the stator. This happens because any induced
electromotive force always produces current that opposes the force of the
source magnetic field. So, the rotor’s magnetic field opposes the stator field which causes it to repel from like poles and attract to opposite poles this causes a constant chase as the
rotor is attracted and repelled by the rotating magnetic field of the stator. We’ve covered the stator and the rotor
and we have seen how each has their individual function in the induction
process, but for induction to take place there’s a third element needed and
that’s relative motion. Generally speaking, relative motion is a
calculation between the speed of one moving object relative to the speed of
another moving object. In this case, the relative motion between the rotating magnetic field of the stator and the rotating rotor. As you can see the rotor rotates slower
than the stator how much slower depends on the motor’s
external load and how much energy is lost internally from friction, induction
leakage and other causes. Now, here’s another question. Is the rotor connected to the
three-phase incoming power? The answer is no. The stator creates a
rotating magnetic field which induces a voltage in the rotor bars which are
shorted on the ends. That allows current to flow in the rotor which then creates its own opposing magnetic field to the stator field. The difference between the speed of the
magnetic field and the speed of the rotor is called slip. To help you understand slip, let’s take a
moment to define some terms that you’ll need to know as we move forward. The speed of the rotating magnetic field
in the stator is called synchronous speed. You can calculate synchronous or sync
speed by using the formula 120 times f /p Where f is the frequency of the AC power supplied and p is the number of motor poles. We’ll talk about motor poles in a second. So with some simple math, you can see
that a four pole motor connected to a 60 Hertz power source would have a sync speed of 1800 RPM. The mechanical speed of the rotor is
called the Rated Speed. A rated speed is based off of a motor’s
rated load. You can usually find the value for the rated speed on the motors
nameplate indicating the general speed of the rotor at rated load. Using our previous example of a four
pole motor running on a 60 Hertz supply, the rated speed would normally be
between 1725 and 1750 rpm. So, as you can see with AC induction
motors the rotor always rotates slower than the magnetic field of the stator. The difference between the sync speed of
the stator’s rotating magnetic field and the mechanical speed of the rotor is
called slip. The amount of slip depends on the
amount of the motor’s load. The greater the load on the motor, the slower the rotor turns in
relationship to the stator’s rotating magnetic field. The more this difference increases, the
more slip increases. To illustrate slip, pay attention to what
happens to the speed of the motor as the load is increased and decreased. You can see that as the motor load
increases, the mechanical speed of the motor decreases. And as the load is
decreased the speed increases. This increase and decrease in motor
speed due to load is called slip. Well, let’s take a moment to review. Now here’s another question. Is the
rotor’s speed the same as the stator’s rotating magnetic field speed? No. The rotor speed is always slower than
the rotating magnetic field of the stator in a positive torque application. If both were traveling at the same speed, there would be no induction and the
rotor would not be able to create a magnetic field. Induction motors can be constructed to handle various loads and various speeds. One way is to change the number of poles in the stator. You can increase or decrease
torque by adding or subtracting the number of poles in the stator. The more poles there are, the slower the
magnetic field rotates. Using the sync speed formula we can
explain why that is true. Revolutions per minute equals f which is
the motor supply frequency in Hertz, times 120, divided by the number of poles. So, say you have a two pole motor powered
at 60 Hertz. 60 times 120=7200. 7200 divided by the number of poles, in this case, two, shows us that the
stators magnetic field will rotate at 3,600 revolutions per minute. But what happens to the motor
synchronous speed if we add more poles? If, like the example just given the motor
is powered at 60 Hertz and we have six poles instead of two, the synchronous speed
produced by the stator decreases to 1200 RPM. You can see by this table that sync
speed decreases as you increase the number of poles. So high torque induction
motors have slower sync speeds and lower torque. Motors of the same size have less
torque and higher synchronous speeds. One thing to note as pole count
increases, so does the cost of manufacturing. So most induction motors are two or four
pole configurations. If more torque is needed, most people
will opt for a physically larger motor instead of using a six or eight pole
machine. Well, it’s time for yet another quiz
question. If we have a motor with a rated speed of
1774 rpm how many poles does it have? And the answer is four poles. We know that are rated speed is going to be a little bit less than our sync speed. Looking at this table from earlier we
see that a four pole motor has a sync speed of 1800 RPM. This tells us that if we include slip, we
are around 1774 RPM. In this next section, we’ll discuss speed
torque curves and what they mean. Induction motors are used to produce
work or to complete a physical task. As they work, induction motors use electrical energy to produce the torque needed to accomplish that task. A Speed Torque Curve shows you how the torque produced by an induction motor varies throughout the different phases of its operation. Starting Torque is the amount of torque
an induction motor produces as it ramps up from a standstill. Looking at this example of a Speed
Torque Curve we can expect the starting torque to be about one hundred and fifty
percent of rated torque. Pull Up Torque is the amount of motor
torque available as the motor accelerates toward its rated speed. If the motor’s Pull Up Torque is less than the amount required to accelerate the
load, the motor will never reach its rated speed. As the motor continues to accelerate
toward its rated speed, it encounters its Breakdown Torque. Breakdown Torque is the greatest amount
of torque a motor can generate. When the motor has accelerated itself to
its rated speeds, the motor should be producing between 80 to 100 percent of
its rated torque. That is, of course, if the machine has
been designed properly. Now, let’s look at our example again but
this time let’s pay close attention to the motor torque. As the load is increased and the motor
speed decreases, the amount of torque produced by the motor increases. Watch it follow this line you can see
that torque and current are proportional. That means an induction motor draws more current as you increase the load. So, as the load increases, it increases
the amount of current that the motor draws and consequently the amount of
motor torque produced. Ok, let’s look at another example. Let’s
say the machine load gets very large, so large that it causes the motor to
produce torque near the motor’s Breakdown Torque rating and then beyond it. You can see as the load increases the
amount of torque increases and follows this curve. Because of this, the speed of the motor
begins to decrease and the amount of current flowing to the motor increases. Current flow in the rotor begins to
increase as the rotor become saturated. The current in the system as a whole is going to increase. When the motor slips beyond its
Breakdown Torque it begins to produce less torque which
then causes the motor speed to decrease even more, and in many cases, stall. This situation usually results in damage
to the motor if left in this state, due to overheating of the stator. So typically there’s an overload relay
that will protect the motor from damage. Well, here comes another quiz question. If we look at the Speed Torque Curve, at
what point is the torque and its maximum? Did you come up with Breakdown Torque? I hope you did, as Breakdown Torque is
the greatest amount of torque a motor can generate. Now let’s discuss the NEMA design types
and why they are important. An organization called the National
Electrical Manufacturers Association or NEMA establishes technical standards for
the manufacturing of electronic products NEMA has established standards for four
different designs of electrical induction motors which are A, B, C and D
respectively each standardized design has unique
speed torque and slip capabilities depending on the work they perform. NEMA Design A motors are allowed a
maximum slip of five percent they are similar to design be motors in
respect to torque output however these motors are not limited on
their starting current this allows for lower winding impedance
which in turn our state or resistance making Design A one of the most
efficient motors from an energy standpoint. Design A motors often offer greater
breakdown torque than Design B centrifugal fans and pumps are typical
Design A applications NEMA Design B motors are the most
commonly used induction motors in the industry they have a maximum of five percent slip
and speed torque characteristics that are similar to Design A motors but with
a NEMA mandated limit to their starting current because they can provide good pull up
torque Design B motors are used in a wide variety of applications Design B motors can also take impact or
burst loads at full speed without stalling. applications using NEMA Design A and B
motors are best suited for drives, a topic that will be covered more in our
next installment with Drive Basics. A NEMA Design C motor also has a
maximum of five percent slip. Design C motors are built to power equipment
requiring high breakaway torque like positive-displacement pumps and
conveyors. Similar to Design C motors a NEMA
Design D motor is a squirrel cage motor designed with a maximum slip of 5 to
13% Low starting current to withstand full voltage starting and very
high locked rotor torque Like Design C motors, you’ll find
Design D motors powering equipment with high starting torque requirements like
cranes or hoists Design D motors are also well suited to
high impact load applications like stamping presses All right, before moving on here is
another review question Which NEMA design motor will be the best
for a fan or pump application? The answer…Design A and Design B are
both suitable for this application due to their low amount of slip and high
breakdown torque. In this next section we will give you an
understanding of the information on an induction motor nameplate. In North America NEMA also establishes
the standards for the information provided on the nameplate this information is vital to determining
the motors characteristics. Let’s look at a typical nameplate Horsepower is a measure of the motors
mechanical output rating and its ability to deliver the needed torque for the
required load and at the rate of speed you can calculate horsepower by
multiplying the motor speed times the amount of torque in foot pounds and then
dividing that sum by 5252 Torque is a measure of the turning or
twisting force applied by the motor to the load to calculate torque in foot-pounds
multiply horsepower times 5252 which is a constant obtained by dividing 33,000
by 2 pi and then dividing that number by revolutions per minute or RPM motor rated voltage is the optimal
performing voltage of the motor because line voltage fluctuates motors
are rated with a 10-percent tolerance above or below the rated voltage shown
on the nameplate Motor Rated Current which is listed on
the nameplate as FLA for Full Load Amps is the amount of amperage the motor
needs when it is operating at full load torque and horsepower the motor rated frequency is the
frequency at which the motor is designed to operate. in North America, the rated frequency is 60 Hertz some motors are designed to work with a variable
frequency drive or VFD they are rated to run at different frequencies Motor Rated Speed or full load RPM is
the approximate RPM at which the rotor is rotating when the motor is operating
under full load Motor Rated Speed is expressed in revolutions per minute Motor Poles indicates the number of
poles inside the stator of a three-phase motor Motor Phase is the number of AC power
lines supplying the motor of course with a three-phase motor there are three
power lines the NEMA design letter indicates the
motors NEMA design type either A, B, C or D the letter designation describes the
motors torque and current characteristics insulation is crucial in an induction
motor, the insulation class describes the thermal tolerance of the motor windings the letter indicates the motor windings
ability to withstand operating temperatures for specific lengths of time Motors controlled with a variable
frequency drive and our motors that run at lower speeds usually have a higher
insulation class. Service Factor represents the percentage
of overloading a motor can handle over short periods when operating at rated
voltage and frequency. The frame size describes the mounting
dimensions including the foot hole mounting pattern and shaft dimensions. Now it’s time for a question on Nameplates. What is usually given on the motor nameplate synchronous speed or the rated speed? Rated speed is usually given for a motor with this we know how fast the shaft is spinning at the rated load and can easily figure out the synchronous speed if needed. Standards have been established by NEMA for the types of Induction Motor Enclosures the standards are based on
the motors use and are designated on the nameplate as ENCL. An Open Drip-Proof or ODP enclosure is
typically used for indoor applications the open drip proof enclosure allows
outside air to circulate over the windings while preventing any liquid
from entering the enclosure within 15 degrees from vertical. A Totally Enclosed Non-Ventilated or TENV enclosure uses cooling fins to dissipate heat instead of a fan or vent opening. They’re designed for installation indoor
or outdoor injury and/or slightly damp conditions. A totally enclosed fan-cooled or TEFC
enclosure is cooled by a motor shaft connected to an exterior fan though they’re not waterproof TEFC enclosures are used outdoors in
dirty locations. And the final type is a Totally Enclosed
Blower Cooled or TEBC enclosure which is cooled through forced convection by a rear-mounted blower you’ll find TEBC enclosures in both
indoor and outdoor applications. Other types of induction motor
enclosures include Totally Enclosed Air Over Totally Enclosed Wash Down Explosion Proof enclosures and Hazardous Location enclosures And now here comes our final question What does TENV stand for? TENV stands for Totally Enclosed, Non-Ventilated very good TENV enclosures are designed for
damp and dirty environments. As I mentioned at the beginning this
program is just a basic introduction to how AC induction motors work There’s much more to learn and a good
place to do that is at yaskawa.com Well we’ve come to the end of this
training program but it definitely isn’t the end of our commitment to make
Yaskawa drives and motion products the best in the industry The commitment to quality continues in
the way we work with our customers and with our vendors it’s in the way we train our associates it means we deliver product on time we answer questions quickly and we never say we can’t Yaskawa quality is reflected in the effort
our associates bring to work every day to us quality means doing everything we
can to make our customers, partners and employees experience a great one. We commit to that we make it happen we can because to us it’s personal.

11 Replies to “Motor Basics”

  1. The speaker has a really good voice and the graphics are excellent. Thanks Yaskawa for producing this video!

  2. Dear sir your Explaining method is easy and efficient.
    Please make demo video on multiple drive A1000 communication via profibus.

  3. I am certain in that yaskawa do a great job for us I mean by us that's people that want learn automation technology but I feel sorry because that videos not have enough likes… But thank yaskawa for the hundreds of videos and demos that you make it to us.. Thanks to that engineer's spent him time to us that's videos never go for nothing…. Thank yaskawa

Leave a Reply

Your email address will not be published. Required fields are marked *