Stator Winding Design Considerations (Electric Motors)
Increasing the value of resistance will move the speed of maximum torque down. The below diagram shows a squirrel cage induction rotor having aluminum bars short circuit by aluminum end rings. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles to ensure starting, regardless of rotor position and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets. It is used extensively in step motors and switched-reluctance motors. Social Meanings of a New Technology.
A major turning point came n , when Antonio Pacinotti first described the ring armature. This featured symmetrically-grouped coils closed upon themselves and connected to the bars of a commutator, the brushes of which delivered practically non-fluctuating current. In , Frank Julian Sprague invented the first practical DC motor, a non-sparking device that maintained relatively constant speed under variable loads.
Other Sprague electric inventions about this time greatly improved grid electric distribution prior work done while employed by Thomas Edison , allowed power from electric motors to be returned to the electric grid, provided for electric distribution to trolleys via overhead wires and the trolley pole, and provided control systems for electric operations. This allowed Sprague to use electric motors to invent the first electric trolley system in —88 in Richmond, Virginia , the electric elevator and control system in , and the electric subway with independently-powered centrally-controlled cars.
The latter were first installed in in Chicago by the South Side Elevated Railroad , where it became popularly known as the " L ". Sprague's motor and related inventions led to an explosion of interest and use in electric motors for industry. The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of an air gap between the rotor and stator.
Efficient designs have a comparatively small air gap. Louis motor, long used in classrooms to illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a modern motor. Electric motors revolutionized industry.
Industrial processes were no longer limited by power transmission using line shafts, belts, compressed air or hydraulic pressure. Instead, every machine could be equipped with its own power source, providing easy control at the point of use, and improving power transmission efficiency.
Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible.
Today, electric motors consume more than half of the electric energy produced in the US. The first alternating-current commutatorless induction motors were independently invented by Galileo Ferraris and Nikola Tesla , in and , respectively.
In , the Royal Academy of Science of Turin published Ferraris's research detailing the foundations of motor operation, while concluding that "the apparatus based on that principle could not be of any commercial importance as motor. One of the patents Tesla filed in , however, also described a shorted-winding-rotor induction motor. George Westinghouse promptly bought Tesla's patents, employed Tesla to develop them, and assigned C. Scott to help Tesla; however, Tesla left for other pursuits in Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the three-phase cage-rotor induction motor in and the three-limb transformer in This type of motor is now used for the vast majority of commercial applications.
Lamme later developed a rotating bar winding rotor. The General Electric Company began developing three-phase induction motors in In an electric motor, the moving part is the rotor, which turns the shaft to deliver the mechanical power. The rotor usually has conductors laid into it that carry currents, which interact with the magnetic field of the stator to generate the forces that turn the shaft. Alternatively, some rotors carry permanent magnets, and the stator holds the conductors. The rotor is supported by bearings , which allow the rotor to turn on its axis.
The bearings are in turn supported by the motor housing. The motor shaft extends through the bearings to the outside of the motor, where the load is applied. Because the forces of the load are exerted beyond the outermost bearing, the load is said to be overhung. The stator core is made up of many thin metal sheets, called laminations. Laminations are used to reduce energy losses that would result if a solid core were used.
The distance between the rotor and stator is called the air gap. The air gap has important effects, and is generally as small as possible, as a large gap has a strong negative effect on performance. It is the main source of the low power factor at which motors operate. The magnetizing current increases with the air gap. For this reason, the air gap should be minimal. Very small gaps may pose mechanical problems in addition to noise and losses. Windings are wires that are laid in coils , usually wrapped around a laminated soft iron magnetic core so as to form magnetic poles when energized with current.
Electric machines come in two basic magnet field pole configurations: In the salient-pole machine the pole's magnetic field is produced by a winding wound around the pole below the pole face. In the nonsalient-pole , or distributed field, or round-rotor, machine, the winding is distributed in pole face slots. Some motors have conductors that consist of thicker metal, such as bars or sheets of metal, usually copper , alternatively aluminum.
These are usually powered by electromagnetic induction. A commutator is a mechanism used to switch the input of most DC machines and certain AC machines. It consists of slip-ring segments insulated from each other and from the shaft.
The motor's armature current is supplied through stationary brushes in contact with the revolving commutator, which causes required current reversal, and applies power to the machine in an optimal manner as the rotor rotates from pole to pole. In light of improved technologies in the electronic-controller, sensorless-control, induction-motor, and permanent-magnet-motor fields, externally-commutated induction and permanent-magnet motors are displacing electromechanically-commutated motors.
A DC motor is usually supplied through slip ring commutator as described above. AC motors' commutation can be either slip ring commutator or externally commutated type, can be fixed-speed or variable-speed control type, and can be synchronous or asynchronous type. Universal motors can run on either AC or DC. Variable-speed controlled AC motors are provided with a range of different power inverter , variable-frequency drive or electronic commutator technologies. The term electronic commutator is usually associated with self-commutated brushless DC motor and switched reluctance motor applications.
Electric motors operate on three different physical principles: By far, the most common is magnetism. In magnetic motors, magnetic fields are formed in both the rotor and the stator. The product between these two fields gives rise to a force, and thus a torque on the motor shaft. One, or both, of these fields must be made to change with the rotation of the motor.
This is done by switching the poles on and off at the right time, or varying the strength of the pole. The main types are DC motors and AC motors,  the former increasingly being displaced by the latter. AC electric motors are either asynchronous or synchronous.
Once started, a synchronous motor requires synchronism with the moving magnetic field's synchronous speed for all normal torque conditions. In synchronous machines, the magnetic field must be provided by means other than induction such as from separately excited windings or permanent magnets. A fractional-horsepower FHP motor either has a rating below about 1 horsepower 0. Many household and industrial motors are in the fractional-horsepower class.
By definition, all self-commutated DC motors run on DC electric power. Most DC motors are small permanent magnet PM types. They contain a brushed internal mechanical commutation to reverse motor windings' current in synchronism with rotation.
A commutated DC motor has a set of rotating windings wound on an armature mounted on a rotating shaft. The shaft also carries the commutator, a long-lasting rotary electrical switch that periodically reverses the flow of current in the rotor windings as the shaft rotates.
Thus, every brushed DC motor has AC flowing through its rotating windings. Current flows through one or more pairs of brushes that bear on the commutator; the brushes connect an external source of electric power to the rotating armature.
The rotating armature consists of one or more coils of wire wound around a laminated, magnetically "soft" ferromagnetic core. Current from the brushes flows through the commutator and one winding of the armature, making it a temporary magnet an electromagnet. The magnetic field produced by the armature interacts with a stationary magnetic field produced by either PMs or another winding a field coil , as part of the motor frame. The force between the two magnetic fields tends to rotate the motor shaft.
The commutator switches power to the coils as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops like a compass needle does , but rather keeps rotating as long as power is applied. Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator.
Sparks are created by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections — and hence coil ends — momentarily while crossing the gaps.
Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity , limits the output of the motor.
The making and breaking of electric contact also generates electrical noise ; sparking generates RFI.
Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance on larger motors or replacement on small motors. The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor.
While most commutators are cylindrical, some are flat discs consisting of several segments typically, at least three mounted on an insulator. Large brushes are desired for a larger brush contact area to maximize motor output, but small brushes are desired for low mass to maximize the speed at which the motor can run without the brushes excessively bouncing and sparking.
Small brushes are also desirable for lower cost. Stiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses lower efficiency and accelerated brush and commutator wear. DC machines are defined as follows: A PM permanent magnet motor does not have a field winding on the stator frame, instead relying on PMs to provide the magnetic field against which the rotor field interacts to produce torque.
Compensating windings in series with the armature may be used on large motors to improve commutation under load. Because this field is fixed, it cannot be adjusted for speed control.
PM fields stators are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings. Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed amount of flux. However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines.
To minimize overall weight and size, miniature PM motors may use high energy magnets made with neodymium or other strategic elements; most such are neodymium-iron-boron alloy. With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed singly-fed synchronous and induction electric machines. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles to ensure starting, regardless of rotor position and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets.
In this motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position. Efficiency for a BLDC motor of up to The BLDC motor's characteristic trapezoidal counter-electromotive force CEMF waveform is derived partly from the stator windings being evenly distributed, and partly from the placement of the rotor's permanent magnets.
Also known as electronically commutated DC or inside out DC motors, the stator windings of trapezoidal BLDC motors can be with single-phase, two-phase or three-phase and use Hall effect sensors mounted on their windings for rotor position sensing and low cost closed-loop control of the electronic commutator.
They have several advantages over conventional motors:. Modern BLDC motors range in power from a fraction of a watt to many kilowatts. They also find significant use in high-performance electric model aircraft. The SRM has no brushes or permanent magnets, and the rotor has no electric currents. Instead, torque comes from a slight misalignment of poles on the rotor with poles on the stator.
The rotor aligns itself with the magnetic field of the stator, while the stator field windings are sequentially energized to rotate the stator field.
The magnetic flux created by the field windings follows the path of least magnetic reluctance, meaning the flux will flow through poles of the rotor that are closest to the energized poles of the stator, thereby magnetizing those poles of the rotor and creating torque.
As the rotor turns, different windings will be energized, keeping the rotor turning. SRMs are used in some appliances  and vehicles.
A commutated electrically excited series or parallel wound motor is referred to as a universal motor because it can be designed to operate on AC or DC power. A universal motor can operate well on AC because the current in both the field and the armature coils and hence the resultant magnetic fields will alternate reverse polarity in synchronism, and hence the resulting mechanical force will occur in a constant direction of rotation. Operating at normal power line frequencies , universal motors are often found in a range less than watts.
Universal motors also formed the basis of the traditional railway traction motor in electric railways. In this application, the use of AC to power a motor originally designed to run on DC would lead to efficiency losses due to eddy current heating of their magnetic components, particularly the motor field pole-pieces that, for DC, would have used solid un-laminated iron and they are now rarely used.
An advantage of the universal motor is that AC supplies may be used on motors that have some characteristics more common in DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the commutator.
Such motors are used in devices, such as food mixers and power tools, that are used only intermittently, and often have high starting-torque demands. Multiple taps on the field coil provide imprecise stepped speed control. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor causing the motor to run on half-wave rectified AC. Universal motors also lend themselves to electronic speed control and, as such, are an ideal choice for devices like domestic washing machines.
The motor can be used to agitate the drum both forwards and in reverse by switching the field winding with respect to the armature. Whereas SCIMs cannot turn a shaft faster than allowed by the power line frequency, universal motors can run at much higher speeds. This makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable.
They are also commonly used in portable power tools, such as drills, sanders, circular and jig saws, where the motor's characteristics work well. Many vacuum cleaner and weed trimmer motors exceed 10, rpm , while many similar miniature grinders exceed 30, rpm.
The design of AC induction and synchronous motors is optimized for operation on single-phase or polyphase sinusoidal or quasi-sinusoidal waveform power such as supplied for fixed-speed application from the AC power grid or for variable-speed application from VFD controllers. An AC motor has two parts: An induction motor is an asynchronous AC motor where power is transferred to the rotor by electromagnetic induction, much like transformer action.
An induction motor resembles a rotating transformer, because the stator stationary part is essentially the primary side of the transformer and the rotor rotating part is the secondary side. Polyphase induction motors are widely used in industry.
SCIMs have a heavy winding made up of solid bars, usually aluminum or copper, joined by rings at the ends of the rotor. When one considers only the bars and rings as a whole, they are much like an animal's rotating exercise cage, hence the name. Currents induced into this winding provide the rotor magnetic field. The shape of the rotor bars determines the speed-torque characteristics. At low speeds, the current induced in the squirrel cage is nearly at line frequency and tends to be in the outer parts of the rotor cage.
As the motor accelerates, the slip frequency becomes lower, and more current is in the interior of the winding.
By shaping the bars to change the resistance of the winding portions in the interior and outer parts of the cage, effectively a variable resistance is inserted in the rotor circuit.
However, the majority of such motors have uniform bars. In a WRIM, the rotor winding is made of many turns of insulated wire and is connected to slip rings on the motor shaft.
An external resistor or other control devices can be connected in the rotor circuit. Resistors allow control of the motor speed, although significant power is dissipated in the external resistance. A converter can be fed from the rotor circuit and return the slip-frequency power that would otherwise be wasted back into the power system through an inverter or separate motor-generator.
The WRIM is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low supply current from zero speed to full speed. This type of motor also offers controllable speed. Motor speed can be changed because the torque curve of the motor is effectively modified by the amount of resistance connected to the rotor circuit.
Increasing the value of resistance will move the speed of maximum torque down. If the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced.
When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal.
Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation and net efficiency is also very poor. A torque motor is a specialized form of electric motor that can operate indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage. In this mode of operation, the motor will apply a steady torque to the load hence the name. A common application of a torque motor would be the supply- and take-up reel motors in a tape drive.
In this application, driven from a low voltage, the characteristics of these motors allow a relatively constant light tension to be applied to the tape whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage, and so delivering a higher torque , the torque motors can also achieve fast-forward and rewind operation without requiring any additional mechanics such as gears or clutches.
In the computer gaming world, torque motors are used in force feedback steering wheels. Another common application is the control of the throttle of an internal combustion engine in conjunction with an electronic governor. In this usage, the motor works against a return spring to move the throttle in accordance with the output of the governor.
The latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup and, depending on the speed, makes small adjustments to the amount of current applied to the motor. If the engine starts to slow down relative to the desired speed, the current will be increased, the motor will develop more torque, pulling against the return spring and opening the throttle.
Bearings for supporting the rotating shaft. One of the problems with electrical motor is the production of heat during its rotation. To overcome this problem, we need a fan for cooling. For receiving external electrical connection Terminal box is needed. There is a small distance between rotor and stator which usually varies from 0. Such a distance is called air gap. Stator of Three Phase Induction Motor The stator of the three-phase induction motor consists of three main parts: Stator frame, Stator core, Stator winding or field winding.
Stator Frame It is the outer part of the three phase induction motor. Its main function is to support the stator core and the field winding. It acts as a covering, and it provides protection and mechanical strength to all the inner parts of the induction motor. The frame is either made up of die-cast or fabricated steel.
The frame of three phase induction motor should be strong and rigid as the air gap length of three phase induction motor is very small. Otherwise, the rotor will not remain concentric with the stator, which will give rise to an unbalanced magnetic pull. Stator Core The main function of the stator core is to carry the alternating flux. In order to reduce the eddy current loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.
All the stamping are stamped together to form stator core, which is then housed in stator frame. The stamping is made up of silicon steel, which helps to reduce the hysteresis loss occurring in the motor. Stator Winding or Field Winding The slots on the periphery of the stator core of the three-phase induction motor carry three phase windings. We apply three phase ac supply to this three-phase winding. The three phases of the winding are connected either in star or delta depending upon which type of starting method we use.
We start the squirrel cage motor mostly with star-delta stater and hence the stator of squirrel cage motor is delta connected.
We start the slip ring three-phase induction motor by inserting resistances so, the stator winding of slip ring induction motor can be connected either in star or delta. The starting points for the coils must meet the pattern of coil placements shown. This pattern places two adjacent coils strategically and equidistantly around the stator with each of the three phases having one set of two adjacent coils and three sets of single coils pattern from Veinott and Martin, As with any winding, there are advantages counterbalanced by disadvantages.
The following equation defines the tooth or slot pitch of the slot stator. Now, since this design is a four-pole BLDC design, there are two full electrical cycles for one full mechanical cycle. The next equation defines the relationship between electrical and mechanical degrees for any four-pole design. A full winding pitch would possess a value of 6 with a throw of 1 to 7. This winding pattern has a winding pitch of 56 fractional or a throw of 1 to 6.
This group of fractional-pitch windings has a pitch less than 1 when an even stator slot count is employed. This winding pattern is used in larger three-phase ac motors to decrease the harmonic content of both the voltage and mmf waveforms.
This technique is very similar to that of short-pitch lap windings used in brush dc motors. The final winding type is the half-pitch winding, which has the simplest winding pattern.
It is used extensively in step motors and switched-reluctance motors. A nine-slot eight-pole winding is very popular, as well as a nine-slot six-pole. This winding by definition is also a fractional-pitch winding. It is the most cost-effective and simplest winding with the shortest mean length of turn MLT and therefore the lowest resistance per coil. It does suffer from reduced torque, as all fractional pitch windings do.
The various winding factors that determine the reduced torque values are reviewed in the next subsection. Winding Factors for Different Winding Patterns. These factors can be identified as follows: Veinott and Martin developed a table with the distribution factor for each major winding pattern, summarized in Table 5.
Filling the Stator Slots. The first item in filling the stator slot is to compute the area of the slot. There are many types of stator slot shapes but the trapezoidal constant-tooth-width slot shown in Fig. One can use basic trigonometry to determine the slot area or obtain the actual slot area from the lamination vendor. There are three methods used to compute slot area and the total volume of copper magnet wire used. They use the following units: The square mils value is smaller and can be computed by modifying Eq.
It is strongly recommended that the insulated wire dimensions be used. The Phelps Dodge magnet wire chart Table 2. The more important parameter is turns conductors per square inch.
It yields the value of total number of turns—or, more appropriately, conductors—that can be placed in a slot, assuming percent fill. Now, the most copper fill in terms of a percentage of actual turns per square inch versus percent fill turns per square inch that this author has actually done by hand-insertion methods is 73 percent with 37 AWG and 63 percent with 21 AWG.
The practical limit is somewhere between 40 and 50 percent of this theoretical value depending on the type of winding machine, tooling used on the winding machine, length of stator stack, size of stator slot, etc. If one wanted to use 22 AWG, the turns conductors per linear inch would be Here the maximum value is conductors with 22 AWG magnet wire. Based on a practical slot fill of 45 percent, the maximum number of turns would probably be The actual number of turns to achieve the desired performance has not yet been determined but for the AWG size selected, 69 turns or conductors is the maximum practical limit.