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Chapter 5 special motor
 

Chapter 5 special motor

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    Chapter 5 special motor Chapter 5 special motor Document Transcript

    • CHAPTER 5 SPECIAL MOTOR AND SINGLE-PHASE INDUCTION MOTOR5.1 STEPPER MOTORA stepper motor (or step motor) is a brushless, synchronous electric motor that candivide a full rotation into a large number of steps. The motors position can be controlledprecisely, without any feedback mechanism. Stepper motors are similar to switchedreluctance motors (which are very large stepping motors with a reduced pole count, andgenerally are closed-loop commutated.)Fundamentals of OperationStepper motors operate differently from DC brush motors, which rotate when voltage isapplied to their terminals. Stepper motors, on the other hand, effectively have multiple"toothed" electromagnets arranged around a central gear-shaped piece of iron. Theelectromagnets are energized by an external control circuit, such as a microcontroller. Tomake the motor shaft turn, first one electromagnet is given power, which makes the gearsteeth magnetically attracted to the electromagnets teeth. When the gears teeth are thusaligned to the first electromagnet, they are slightly offset from the next electromagnet. Sowhen the next electromagnet is turned on and the first is turned off, the gear rotatesslightly to align with the next one, and from there the process is repeated. Each of thoseslight rotations is called a "step," with an integer number of steps making a full rotation.In that way, the motor can be turned by a precise angle.Stepper motor characteristics1. Stepper motors are constant power devices.2. As motor speed increases, torque decreases.3. The torque curve may be extended by using current limiting drivers and increasing thedriving voltage.4. Steppers exhibit more vibration than other motor types, as the discrete step tends tosnap the rotor from one position to another.5. This vibration can become very bad at some speeds and can cause the motor to losetorque.6. The effect can be mitigated by accelerating quickly through the problem speeds range,physically damping the system, or using a micro-stepping driver. 114
    • 7. Motors with a greater number of phases also exhibit smoother operation than thosewith fewer phases.Open-loop versus closed-loop commutationSteppers are generally commutated open loop, ie. the driver has no feedback on where therotor actually is. Stepper motor systems must thus generally be over engineered,especially if the load inertia is high, or there is widely varying load, so that there is nopossibility that the motor will lose steps. This has often caused the system designer toconsider the trade-offs between a closely sized but expensive servomechanism systemand an oversized but relatively cheap stepper.A new development in stepper control is to incorporate a rotor position feedback (eg. anencoder or resolver), so that the commutation can be made optimal for torque generationaccording to actual rotor position. This turns the stepper motor into a high pole countbrushless servo motor, with exceptional low speed torque and position resolution. Anadvance on this technique is to normally run the motor in open loop mode, and only enterclosed loop mode if the rotor position error becomes too large -- this will allow thesystem to avoid hunting or oscillating, a common servo problem.TypesThere are three main types of stepper motors. 1. Permanent Magnet Stepper 2. Hybrid Synchronous Stepper 3. Variable Reluctance StepperPermanent magnet motors use a permanent magnet (PM) in the rotor and operate on theattraction or repulsion between the rotor PM and the stator electromagnets. Variablereluctance (VR) motors have a plain iron rotor and operate based on the principle of thatminimum reluctance occurs with minimum gap, hence the rotor points are attractedtoward the stator magnet poles. Hybrid stepper motors are named because they use use acombination of PM and VR techniques to achieve maximum power in a small package5.2 UNIVERSAL MOTORA wound field DC motor with the field and armature windings connected in series iscalled either a "series-wound motor" or a "universal motor," because of its ability tooperate on AC or DC power. The ability to operate on AC or DC power is because thecurrent in both the field winding and the armature (and hence the resultant magneticfields) will alternate (reverse polarity) at the same time, and hence the mechanical forcegenerated is always in the same direction.The torque of a series-wound or universal motor declines slowly with speed. Althoughthis can be advantageous for some applications, it also means that, unloaded, the motor 115
    • may "run away" and speed up to the point of mechanical failure. However factors such asexternal load and internal mechanical resistance may adequately limit the speed.Operating at normal power line frequencies, universal motors are very rarely larger thanone kilowatt (about 1.3 horsepower). Universal motors also form the basis of thetraditional railway traction motor in electric railways. In this application, to keep theirelectrical efficiency high, they were operated from very low frequency AC supplies, with25 and 16.7 hertz (Hz) operation being common. Because they are universal motors,locomotives using this design were also commonly capable of operating from a third railpowered by DC.An advantage of the universal motor is that AC supplies may be used on motors whichhave some characteristics more common in DC motors, specifically high starting torqueand very compact design if high running speeds are used. The negative aspect is themaintenance and short life problems caused by the commutator. As a result such motorsare usually used in AC devices such as food mixers and power tools which are used onlyintermittently, and often have high starting-torque demands. Continuous speed control ofa universal motor running on AC is easily obtained by use of a thyristor circuit, while(imprecise) stepped speed control can be accomplished using multiple taps on the fieldcoil. Household blenders that advertise many speeds frequently combine a field coil withseveral taps and a diode that can be inserted in series with the motor (causing the motorto run on half-wave rectified AC).Universal motors generally run at high speeds, making them useful for appliances such asblenders, vacuum cleaners, and hair dryers where high RPM operation is desirable.Motor damage may occur due to overspeeding (running at an RPM in excess of designlimits) if the unit is operated with no significant load. On larger motors, sudden loss ofload is to be avoided, and the possibility of such an occurrence is incorporated into themotors protection and control schemes. In some smaller applications, a fan bladeattached to the shaft often acts as an artificial load to limit the motor speed to a safevalue, as well as a means to circulate cooling airflow over the armature and fieldwindings."Universal" or "Series-wound" motors generally operate better with DC current, but theyhave the ability to operate with AC current as well, making them very versatile for abroad range of applications. However, there is little to no means to control the motorsspeed accurately. Unlike induction motors, the "goal" of this motor is to run a load at thehighest speed possible, which has specific advantages for appliances such as vacuumcleaners and blenders and such. Many automotive starter motors are either series-woundor compound-wound motors because of the high starting torque. 116
    • 5.3 SERVO MOTORA servo motor is a dc, ac, or brushless dc motor combined with a position sensingdevice(e.g. a digital decoder). In this section, our discussion will be focused on the three-wire DC servo motors that are often used for controlling surfaces on model airplanes. Athree-wire DC servo motor incorporates a DC motor, a geartrain, limit stops beyondwhich the shaft cannot turn, a potentiometer for position feedback, and an integratedcircuit for position control.Servos are extremely useful in robotics. The motors are small and are extremely powerfulfor thier size. A standard servo such as the Futaba S-148 has 42 oz/inches of torque,which is pretty strong for its size. It also draws power proportional to the mechanicalload. A lightly loaded servo, therefore, doesnt consume much energy. The guts of a servomotor are shown in the picture below. You can see the control circuitry, the motor, a setof gears, and the case. You can also see the 3 wires that connect to the outside world. Oneis for power (+5volts), ground, and the white wire is the control wire.OperationThe servo motor has some control circuits and a potentiometer (a variable resistor, akapot) that is connected to the output shaft. The potentiometer allows the control circuitryto monitor the current angle of the servo motor. If the shaft is at the correct angle, thenthe motor shuts off. If the circuit finds that the angle is not correct, it will turn the motorthe correct direction until the angle is correct. The output shaft of the servo is capable oftravelling somewhere around 180 degrees. Usually, its somewhere in the 210 degreerange, but it varies by manufacturer. A normal servo is used to control an angular motionof between 0 and 180 degrees. A normal servo is mechanically not capable of turning anyfarther due to a mechanical stop built on to the main output gear. The amount of powerapplied to the motor is proportional to the distance it needs to travel. So, if the shaft needsto turn a large distance, the motor will run at full speed. If it needs to turn only a smallamount, the motor will run at a slower speed. This is called proportional control. How doyou communicate the angle at which the servo should turn? The control wire is used tocommunicate the angle. The angle is determined by the duration of a pulse that is appliedto the control wire. This is called Pulse Coded Modulation. The servo expects to see apulse every 20 milliseconds (.02 seconds). The length of the pulse will determine how farthe motor turns. A 1.5 millisecond pulse, for example, will make the motor turn to the 90degree position (often called the neutral position). If the pulse is shorter than 1.5 ms, thenthe motor will turn the shaft to closer to 0 degress. If the pulse is longer than 1.5ms, theshaft turns closer to 180 degrees.5.4 RELUCTANCE MOTORA reluctance motor is a type of synchronous electric motor that induces non-permanentmagnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenonof magnetic reluctance. 117
    • A reluctance motor, in its various incarnations, may be known as a: • Synchronous reluctance motor • Variable reluctance motor • Switched Reluctance Motor • Variable reluctance stepping motorReluctance motors can have very high power density at low-cost, making them ideal formany applications. Disadvantages are high torque ripple when operated at low speed, andnoise caused by torque ripple. Until recently, their use has been limited by the complexityinherent in both designing the motors and controlling them. These challenges are beingovercome by advances in the theory, by the use of sophisticated computer design tools,and by the use of low-cost embedded systems for motor control. These control systemsare typically based on microcontrollers using control algorithms and real-time computingto tailor drive waveforms according to rotor position and current or voltage feedback.Design and operating fundamentalsThe stator consists of multiple salient (ie. projecting) electromagnet poles, similar to awound field brushed DC motor. The rotor consists of soft magnetic material, such aslaminated silicon steel, which has multiple projections acting as salient magnetic polesthrough magnetic reluctance. The number of rotor poles is typically less than the numberof stator poles, which minimizes torque ripple and prevents the poles from all aligningsimultaneously—a position which can not generate torque.When a rotor pole is equidistant from the two adjacent stator poles, the rotor pole is saidto be in the "fully unaligned position". This is the position of maximum magneticreluctance for the rotor pole. In the "aligned position", two (or more) rotor poles are fullyaligned with two (or more) stator poles, (which means the rotor poles completely face thestator poles) and is a position of minimum reluctance.When a stator pole is energized, the rotor torque is in the direction that will reducereluctance. Thus the nearest rotor pole is pulled from the unaligned position intoalignment with the stator field (a position of less reluctance). (This is the same effect usedby a solenoid, or when picking up ferromagnetic metal with a magnet.) In order to sustainrotation, the stator field must rotate in advance of the rotor poles, thus constantly"pulling" the rotor along. Some motor variants will run on 3-phase AC power (see thesynchronous reluctance variant below). Most modern designs are of the switchedreluctance type, because electronic commutation gives significant control advantages formotor starting, speed control, and smooth operation (low torque ripple).6.5 CAPACITOR START-INDUCTION RUN MOTORA capacitor start induction run motor is similar in some ways to the split phase type. Themain difference is that a capacitor is placed in series with the start winding. The capacitorstart motor produces considerably more starting torque than the split phase motor. After 118
    • the start winding is disconnected from the circuit, the performance is nearly identical withthe split phase motor. Logically, capacitor start motors should be used where the loadacceleration requirements exceed the capacity of a split phase motor.A motor capacitor,such as a start capacitor or run capacitor, including a dual runcapacitor, is an electrical capacitor that boosts the current to an electric motor, such as inair conditioners, hot tub/jacuzzi spa pumps, or forced air heat furnaces. A round dual runcapacitor (described below) is used in some air conditioner compressor units, to boostboth the fan and compressor motors.Run capacitorsRun capacitors are designed for continuous duty, and they are energized the entire timethe motor is running. Run capacitors are rated in a range of 3–70 microfarads (µF), withvoltage classifications of 370 V or 440 V. Single phase electric motors need a capacitorto energize a second-phase winding. If the wrong run capacitor is installed, the motor willnot have an even magnetic field, and this will cause the rotor to hesitate at those spotsthat are uneven. This hesitation can cause the motor to become noisy, increase energyconsumption, cause performance to drop, and cause the motor to overheat. However, amotor will not be ruined just because a run capacitor is faulty.Start capacitorsStart capacitors briefly increase motor starting torque and allow a motor to be cycled onand off rapidly. Start capacitors have ratings above 70 microfarads (µF), with three majorvoltage classifications: 125 V, 250 V, and 330 V. A start capacitor stays energized longenough to rapidly bring the motor to 3/4 of full speed and is then taken out of the circuit,such as by a centrifugal switch that releases when rotating at or around that speed.Examples of motor capacitors are: a 35 µF, at 370 V, run capacitor, or an 88–108 µF at250 V start capacitor6.6 SHADED POLE MOTORA shaded-pole motor is a type of AC single-phase induction motor. As in otherinduction motors the rotating part is a squirrel-cage rotor. All single-phase motors requirea means of producing a rotating magnetic field for starting. In the shaded-pole type, a partof the face of each field pole carries a copper ring called a shading coil. Currents in thiscoil delay the phase of magnetic flux in that part of the pole enough to provide a rotatingfield. The effect produces only a low starting torque compared to other classes of single-phase motors. 119
    • These motors have only one winding, no capacitor nor starting switch, making themeconomical and reliable. Because their starting torque is low they are best suited todriving fans or other loads that are easily started. Moreover, they are compatible withTRIAC-based variable-speed controls, which often are used with fans. They are built inpower sizes up to about 1/6 hp or 125 watts output. For larger motors, other designs offerbetter characteristics.The first photo is of a common C-frame motor. With the shading coils positioned asshown, this motor will start in a clockwise direction as viewed from the long shaft end.The second photo shows detail of the shading coilsConstruction Of Shaded-Pole MotorsThe three types of shaded-pole motor are salient-pole, skeleton and distributed-winding.Salient-pole construction has many main-winding coils. The number of poles is the sameas the number of coils. In most cases there are four coils. Some of the older refrigeratorfans are silent-pole, shaded-pole motors. They used a felt soaked in oil for lubricating therotary part.The skeleton type is used for horsepower from 0.00025 to 0.03. This type of motor usestriple shading. Three shading coils of different throw for each pole are used. In this typebearings are self-aligning iolite. Wick-type oilers are used to spread the lubrication sothat it covers the whole rotating shaft. A squirrel-cage rotor to be slightly offset from thepole, and it easily compressed as the coil is energized. The sucking effect of the coilcauses the rotor to be pulled back inside the hole in laminated core. This type of motor isused on can openers, small knife sharpeners, clocks and timers. It has an advantage overthe synchronous type originally used in clocks, as it is self-starting and will start again ifthe power fails. The old clock-type synchronous motors had to be started by hand.The third type of construction of shaded-pole motors is the distributed-winding. Thestator laminations are similar to those used for single-phase or polyphase inductionmotors. The main winding is also similar to the start winding except that it is short-circuited upon itself. It is displaced from the main winding by less than the 90º usually ininduction motors.Performance Of Shaded-Pole MotorsIf this type of motor of motor is properly lubricated in most cases the skeleton type issealed it will last (continuously operated) for over 25 years. Most clock motors and fanmotors are limited only by the physical abuse they receive. If they are kept plugged in (inthe case of a clock) and operating continuously as timers or similar devices, withoutoverloading, there is no reason why they cannot last indefinitely. There is only one coil inthe skeleton-type motor. If the voltage is stable and the temperature is normal, there is noreason why it will not continue to operate without maintenance of any kind. Varioustypes of physical and electrical abuse can cause them to fail, however. 120
    • If the timer motor has been used to power the timing mechanism of an oven, then it willbe only a matter of eight to ten years of sporadic use that will cause it to lose itslubrication. The heat from the oven can cause it to lose its oil and the seals to breakdown. This shade pole motor has smooth running, high efficiency, low noise and longlife.Shaded pole motors have low starting torque and are available only in fractional andsubfractional horsepower sizes. Slip is about 10%, or more at rated load. A time varyingflux is induced in the poles by the main winding. When the pole flux varies, it induces avoltage and a current in the shading coil which opposes the original change in flux. Thisopposition retards the flux changes under the shaded portions of the coils and thereforeproduces a slight imbalance between the two opposite rotating stator magnetics fields.The net rotation is in the direction from the unshaded to the shaded portion of the poleface.These motors are often used to drive electric clocks and, occasionally, phonographturntables. In these applications, the speed of the motor is as accurate as the frequency ofthe mains power applied to the motor.Even by the standards of shaded pole motors, the power output of these motors is usuallyvery low. Because there is often no explicit starting mechanism, the rotor must be verylight so that it is capable of reaching running speed within one cycle of the mainsfrequency. Alternatively, the rotor may be provided with a squirrel cage, so that themotor starts like an induction motor, once the rotor is pulled into synchronism with itsmagnet, the squirrel cage has no current induced in it and so plays no further part in theoperation. A further development dispenses with the shading rings altogether. Theapplication of power giving the magnetised rotor enough of a flick to move it fastenough to establish synchronism. A mechanical means prevents the rotor from starting inthe wrong direction. This design will only work satisfactorily if the standstill load is nearto zero and has very little inertia. 121