Motor Basics

[Music] you the Technical Training Department of escala America incorporated presents induction motor basics understanding how an induction motor works hi I'm Steve Kaler 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 they're simple construction relative 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 will 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 will 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 rotor 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 and valves contain bearings which allow the rotor shaft to turn freely on its axis now the type of enclosure can vary depending on the motors 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'll start with a 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 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 stators 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 16.6 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 stators 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 electro-motive force always produces current that opposes the force of the source magnetic field so the rotors 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 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 motors 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 s 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 over P where F is the frequency of the AC power supplied and P is the number of motor poles will talk about motor poles in a second so with some simple math you can see that a for Paul 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 a motors rated load you can usually find the value for the rated speed on the motors nameplate indicating the general speed of the rotor at a 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 stators rotating magnetic field and the mechanical speed of the rotor is called slip the amount of slip depends on the amount of the motors load the greater the load on the motor the slower the rotor turns in relationship to the stators 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 this 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 rotors speed the same as the stators 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 motors 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 equals 7200 7200 divided by the number of poles in this case 2 shows us that the stators magnetic field will rotate at 3,600 revolutions per minute but what happens to the motors 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 2 or 4 pole configurations if more torque is needed most people will opt for a physically larger motor instead of using a 6 or 8 pole machine well it's time for yet another quiz question if we have a motor with a rated speed of 1700 and 74 RPM how many Poles does it have and the answer is for polls we know that our 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 seventeen hundred and seventy four 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 150 percent of rated torque pull up torque is the amount of motor torque available as the motor accelerates toward its rated speed if the motors 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 motors 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 okay 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 motors 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 becomes 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 though typically there's an overload relay that will protect the motor from damage while here comes another quiz question if we look at the speed torque curve at what point is the torque at 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 AE motors are allowed a maximum slip of 5% they are similar to design B 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 lowers stator 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 five to 13% low starting current to withstand full voltage starting and very high two 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 loads applications like stamping presses alright 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 rated speed you can calculate horsepower by multiplying the motors speed times the amount of torque in foot-pounds and then dividing that sum by 50 250 to torque is a measure of the turning or twisting force supplied by the motor to the load to calculate torque in foot-pounds multiplied horsepower times 50 252 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% tolerance above or below the rated voltage shown on the nameplate motor rated current which is listed on the nameplate as f la 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 VF d 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 polls 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 ABC ordy 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 en ciel 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 te env enclosure uses cooling fins to dissipate heat instead of a fan or vent opening they're designed for installation indoor or outdoor in dirty and or slightly damp conditions a totally enclosed fan cooled or te FC enclosure is cooled by a motor shaft connected to an exterior fan though they're not waterproof te FC enclosures are used outdoors in dirty locations and the final type is a totally enclosed blower cooled or te BC enclosure which is cooled through forced convection by a rear mounted blower you'll find te BC enclosures in both indoor and outdoor applications other types of induction motor enclosures include totally enclosed air / totally enclosed washdown explosion-proof enclosures and hazardous location enclosures and now here comes our final question what does te n V stand for t en V stands for totally enclosed non-ventilated very good T env 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 comm we've come to the end of this training program but it definitely isn't the end of our commitment to make the Escala 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 your skyla 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 you

Channel: Yaskawa America

Views: 309,573

Rating: 4.931531 out of 5

Keywords: drive, inverter, vfd, motion, control, automation, automate, factory, mechatronics, Yaskawa, machine, industrial, Electric Motor (Engine Category), three phase, Induction Motor (Engine Category), induction, Motor, speed torque curves, nameplate, theory, component identification, Basics, induction motor basics

Id: qpKybaLURy0

Length: 28min 41sec (1721 seconds)

Published: Tue May 14 2019