AN905 Brushed DC Motor Fundamentals Author: Reston Condit Stator Microchip Technology Inc. The stator generates a stationary magnetic field that surrounds the rotor. This field is generated by either permanent magnets or electromagnetic windings. TheINTRODUCTION different types of BDC motors are distinguished by the construction of the stator or the way the electromag-Brushed DC motors are widely used in applications netic windings are connected to the power source.ranging from toys to push-button adjustable car seats. (See Types of Stepping Motors for the different BDCBrushed DC (BDC) motors are inexpensive, easy to motor types).drive, and are readily available in all sizes and shapes.This application note will discuss how a BDC motorworks, how to drive a BDC motor, and how a drive Rotorcircuit can be interfaced to a PIC® microcontroller. The rotor, also called the armature, is made up of one or more windings. When these windings are energizedPRINCIPLES OF OPERATION they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the opposite polesThe construction of a simple BDC motor is shown in generated by the stator, causing the rotor to turn. As theFigure 1. All BDC motors are made of the same basic motor turns, the windings are constantly beingcomponents: a stator, rotor, brushes and a commutator. energized in a different sequence so that the magneticThe following paragraphs will explain each component poles generated by the rotor do not overrun the polesin greater detail. generated in the stator. This switching of the field in the rotor windings is called commutation.FIGURE 1: SIMPLE TWO-POLE BRUSHED DC MOTOR N NORTH SOUTH Brushes Commutator Field Magnet Axle Armature or Coil 2004 Microchip Technology Inc. DS00905A-page 1
AN905Brushes and Commutator Shunt-WoundUnlike other electric motor types (i.e., brushless DC, Shunt-wound Brushed DC (SHWDC) motors have theAC induction), BDC motors do not require a controller field coil in parallel (shunt) with the armature. Theto switch current in the motor windings. Instead, the current in the field coil and the armature are indepen-commutation of the windings of a BDC motor is done dent of one another. As a result, these motors havemechanically. A segmented copper sleeve, called a excellent speed control. SHWDC motors are typicallycommutator, resides on the axle of a BDC motor. As the used applications that require five or more horsepower.motor turns, carbon brushes slide over the commutator, Loss of magnetism is not an issue in SHWDC motorscoming in contact with different segments of the so they are generally more robust than PMDC motors.commutator. The segments are attached to differentrotor windings, therefore, a dynamic magnetic field is FIGURE 3: SHUNT-WOUND DCgenerated inside the motor when a voltage is applied MOTORSacross the brushes of the motor. It is important to notethat the brushes and commutator are the parts of aBDC motor that are most prone to wear because they Brushare sliding past each other. DC Shunt Voltage Field SupplyTYPES OF STEPPING MOTORS ArmatureAs mentioned earlier, the way the stationary magneticfield is produced in the stator differentiates the varioustypes of BDC motors. This section will discuss thedifferent types of BDC motors and the advantages/ Series-Wounddisadvantages of each. Series-wound Brushed DC (SWDC) motors have the field coil in series with the armature. These motors arePermanent Magnet ideally suited for high-torque applications because the current in both the stator and armature increases underPermanent Magnet Brushed DC (PMDC) motors are load. A drawback to SWDC motors is that they do notthe most common BDC motors found in the world. have precise speed control like PMDC and SHWDCThese motors use permanent magnets to produce the motors have.stator field. PMDC motors are generally used in appli-cations involving fractional horsepower because it ismore cost effective to use permanent magnets than FIGURE 4: SERIES-WOUND DCwound stators. The drawback of PMDC motors is that MOTORSthe magnets lose their magnetic properties over time.Some PMDC motors have windings built into them to Seriesprevent this from happening. The performance curve Field(voltage vs. speed), is very linear for PMDC motors. DCCurrent draw also varies linearly with torque. These Voltage Armaturemotors respond to changes in voltage very quickly Supplybecause the stator field is always constant. BrushFIGURE 2: PERMANENT MAGNET DC MOTORS Armature Brush DC Voltage Supply Permanent Magnet PolesDS00905A-page 2 2004 Microchip Technology Inc.
AN905Compound-Wound Note that in each circuit there is a diode across the motor. This diode is there to prevent Back Electromag-Compound Wound (CWDC) motors are a combination netic Flux (BEMF) voltage from harming the MOSFET.of shunt-wound and series-wound motors. As shown in BEMF is generated when the motor is spinning. WhenFigure 5, CWDC motors employ both a series and a the MOSFET is turned off, the winding in the motor isshunt field. The performance of a CWDC motor is a still charged at this point and will produce reversecombination of SWDC and SHWDC motors. CWDC current flow. D1 must be rated appropriately so that itmotors have higher torque than a SHWDC motor while will dissipate this current.offering better speed control than SWDC motor. FIGURE 6: LOW-SIDE BDC MOTORFIGURE 5: COMPOUND-WOUND DC DRIVE CIRCUIT MOTORS VCC Series Field Brush DC D1 Voltage Shunt BDC Supply Field Motor Armature To Controller R1BASIC DRIVE CIRCUITSDrive circuits are used in applications where a control- R2ler of some kind is being used and speed control isrequired. The purpose of a drive circuit is to give thecontroller a way to vary the current in the windings ofthe BDC motor. The drive circuits discussed in thissection allow the controller to pulse width modulate thevoltage supplied to a BDC motor. In terms of power FIGURE 7: HIGH-SIDE BDC MOTORconsumption, this method of speed control is a far more DRIVE CIRCUITefficient way to vary the speed of a BDC motorcompared to traditional analog control methods. VCCTraditional analog control required the addition of aninefficient variable resistance in series with the motor. To Controller VCC R2BDC motors are driven in a variety of ways. In somecases the motor only needs to spin in one direction.Figure 6 and Figure 7 show circuits for driving a BDC R1motor in one direction. The first is a low-side drive andthe second is a high-side drive. The advantage to usingthe low-side drive is that a FET driver is not typicallyneeded. A FET driver is used to: D11. bring the TTL signal driving a MOSFET to the BDC Motor potential level of the supply voltage,2. provide enough current to drive the MOSFET(1),3. and provide level shifting in half-bridge applications. Note 1: The second point typically does not apply Resistors R1 and R2 in Figure 6 and Figure 7 are to most PICmicro® microcontroller important to the operation of each circuit. R1 protects applications because PIC microcontroller the microcontroller from current spikes while R2 I/O pins can source 20 mA. ensures that Q1 is turned off when the input pin is tristated. 2004 Microchip Technology Inc. DS00905A-page 3
AN905Bidirectional control of a BDC motor requires a circuit Note the diodes across each of the MOSFETs (D1-D4).called an H-bridge. The H-bridge, named for its These diodes protect the MOSFETs from current spikesschematic appearance, is able to move current in either generated by BEMF when the MOSFETs are switcheddirection through the motor winding. To understand off. These diodes are only needed if the internalthis, the H-bridge must be broken into its two sides, or MOSFET diodes are not sufficient for dissipating thehalf-bridges. Referring to Figure 8, Q1 and Q2 make up BEMF current.one half-bridge while Q3 and Q4 make up the other The capacitors (C1-C4) are optional. The value ofhalf-bridge. Each of these half-bridges is able to switch these capacitors is generally in the 10 pF range. Theone side of the BDC motor to the potential of the supply purpose of these capacitors is to reduce the RFvoltage or ground. When Q1 is turned on and Q2 is off, radiation that is produced by the arching of thefor instance, the left side of the motor will be at the commutators.potential of the supply voltage. Turning on Q4 andleaving Q3 off will ground the opposite side of themotor. The arrow labeled IFWD shows the resultingcurrent flow for this configuration.FIGURE 8: BIDIRECTION BDC MOTOR DRIVE (H-BRIDGE) CIRCUIT VSUPPLY CTRL1 Q1 D1 C1 C3 D3 Q3 CTRL3 Motor R1 IFWD R3 BDC IRVS IBRK CTRL2 Q2 D2 C2 C4 D4 Q4 CTRL4 R4 R2The different drive modes for and H-bridge circuit are There is one very important consideration that must beshown in Table 1. In Forward mode and Reverse mode taken into account when designing an H-bridge circuit.one side of the bridge is held at ground potential and All MOSFETs must be biased to off when the inputs tothe other side at VSUPPLY. In Figure 8 the IFWD and IRVS the circuit are unpredictable (like when the microcon-arrows illustrate the current paths during the Forward troller is starting up). This will ensure that theand Reverse modes of operation. In Coast mode, the MOSFETs on each half-bridge of the H-bridge willends of the motor winding are left floating and the never be turned on at the same time. Turningmotor coasts to a stop. Brake mode is used to rapidly MOSFETs on that are located on the same half-bridgestop the BDC motor. In Brake mode, the ends of the will cause a short across the power supply, ultimatelymotor are grounded. The motor behaves as a genera- damaging the MOSFETs and rendering the circuittor when it is rotating. Shorting the leads of the motor inoperable. Pull-down resistors at each of the MOSFETacts as a load of infinite magnitude bringing the motor driver inputs will accomplish this functionality (for theto a rapid halt. The IBRK arrow illustrates this. configuration shown in Figure 8).TABLE 1: H-BRIDGE MODES OF OPERATION Q1 Q2 Q3 Q4 (CTRL1) (CTRL2) (CTRL3) (CTRL4) Forward on off off on Reverse off on on off Coast off off off off Brake off on off onDS00905A-page 4 2004 Microchip Technology Inc.
AN905SPEED CONTROL The ECCP module (short for Enhanced Capture Compare and PWM) provides the same functionality asThe speed of a BDC motor is proportional to the voltage the CCP module with the added capability of driving aapplied to the motor. When using digital control, a full or half-bridge circuit. The ECCP module also haspulse-width modulated (PWM) signal is used to gener- auto-shutdown capability and programmable deadate an average voltage. The motor winding acts as a band delay.low pass filter so a PWM waveform of sufficientfrequency will generate a stable current in the motor Note: Microchip Application Note AN893 gives awinding. The relation between average voltage, the detailed explanation of configuring thesupply voltage, and duty cycle is given by: ECCP module for driving a BDC motor. The application note also includes firmware and drive circuit examples.EQUATION 1: VAVERAGE = D × VSUPPLY FEEDBACK MECHANISMS Though the speed of a BDC motor is generally propor-Speed and duty cycle are proportional to one another. tional to duty cycle, no motor is ideal. Heat, commutatorFor example, if a BDC motor is rated to turn at 15000 wear and load all affect the speed of a motor. InRPM at 12V, the motor will (ideally) turn at 7500 RPM systems where precise speed control is required, it is awhen a 50% duty cycle waveform is applied across the good idea to include some sort of feedback mechanismmotor. in the system.The frequency of the PWM waveform is an important Speed feedback is implemented in one of two ways.consideration. Too low a frequency will result in a noisy The first involves the use of a speed sensor of somemotor at low speeds and sluggish response to changes kind. The second uses the BEMF voltage generated byin duty cycle. Too high a frequency lessens the the motor.efficiency of the system due to switching losses in theswitching devices. A good rule of thumb is to modulate Sensored Feedbackthe input waveform at a frequency in the range of 4 kHzto 20 kHz. This range is high enough that audible motor There are a variety of sensors used for speed feed-noise is attenuated and the switching losses present in back. The most common are optical encoders and hallthe MOSFETs (or BJTs) are negligible. Generally, it is a effect sensors. Optical encoders are made up ofgood idea to experiment with the PWM frequency for a several components. A slotted wheel is mounted to thegiven motor to find a satisfactory frequency. shaft at the non-driving end of the motor. An infraredSo how can a PIC microcontroller be used to generate LED provides a light source on one side of the wheelthe PWM waveform required to control the speed of a and a photo transistor detects light on the other side ofBDC motor? One way would be to toggle an output pin the wheel (see Figure 9). Light passing through theby writing assembly or C code dedicated to driving that slots in the wheel will turn the photo transistor on. Aspin(1). Another way is to select a PIC microcontroller the shaft turns, the photo transistor turns on and off withwith a hardware PWM module. The modules available the passing of the slots in the wheel. The frequency atfrom Microchip for this purpose are the CCP an ECCP which the transistor toggles is an indication of motormodules. Many of the PIC microcontrollers have CCP speed. In the case of positioning applications, anand ECCP modules. Refer to the product selector optical encoder will also provide feedback as to theguide to find the devices having these features. position of the motor. Note 1: Microchip Application Note AN847 FIGURE 9: OPTICAL ENCODER provides an assembly code routine for pulse-width modulating an I/O pin in firmware. slottedThe CCP module (short for Capture Compare and wheelPWM) is capable of outputting a 10-bit resolution PWMwaveform on a single I/O pin. 10-bit resolution meansthat 210, or 1024, possible duty cycle values ranging Photo Transistorfrom 0% to 100% are achievable by the module. The IR LEDadvantage to using this module is that it automaticallygenerates a PWM signal on an I/O pin which frees up Front View Side Viewprocessor time for doing other things. The CCP moduleonly requires that the developer configure the parame-ters of the module. Configuring the module includessetting the frequency and duty cycle registers. 2004 Microchip Technology Inc. DS00905A-page 5
AN905Hall effect sensors are also used to provide speed Back Electro Magnetic Flux (BEMF)feedback. Like optical encoders, hall effect sensorsrequire a rotary element attached to the motor and a Another form of velocity feedback for a BDC motor isstationary component. The rotary element is a wheel BEMF voltage measurement. BEMF voltage and speedwith one or more magnets positioned on its outer rim. A are proportional to one another. Figure 11 shows thestationary sensor detects the magnet when in passes locations where BEMF voltage is measured on aand generates a TTL pulse. Figure 10 shows the basic bidirectional drive circuit. A voltage divider is used tocomponents of a hall effect sensor. drop the BEMF voltage into the 0-5V range so that it can be read by an analog-to-digital converter. The BEMF voltage is measured between PWM pulsesFIGURE 10: HALL EFFECT SENSOR when one side of the motor is floating and the other is grounded. At this instance in time the motor is acting magnet wheel like a generator and produces a BEMF voltage magnet proportional to speed. hall effect sensor Front View Side ViewFIGURE 11: BACK EMF VOLTAGE MEASUREMENT VSUPPLY CTRL1 Q1 C1 C3 Q3 CTRL3 R1 R3 Motor BEMF BDC BEMF CTRL2 Q2 C2 C4 Q4 CTRL4 R4 R2DS00905A-page 6 2004 Microchip Technology Inc.
AN905All BDC motors behave slightly differently because ofdifferences in efficiency and materials. Experimenta-tion is the best way to determine the BEMF voltage fora given motor speed. A piece of reflect tape on theshaft of the motor will allow a digital tachometer tomeasure the RPM of the motor. Measuring the BEMFvoltage while reading the digital tachometer will give acorrelation between motor speed and BEMF voltage. Note: Microchip Application Note AN893 provides firmware and circuit examples for reading the BEMF voltage using a PIC16F684.CONCLUSIONBrushed DC motors are very simple to use and control,which makes them a short design-in item. PICmicrocontrollers, especially those with CCP or ECCPmodules are ideally suited for driving BDC motors.REFERENCESAN893 Low-Cost Bidirectional Brushed DC MotorControl Using the PIC16F684.AN847 RC Model Aircraft Motor Control.www.howstuffworks.comwww.engin.umich.edu/labs/csdl/me350/motors/dc/index.html 2004 Microchip Technology Inc. DS00905A-page 7
AN905NOTES:DS00905A-page 8 2004 Microchip Technology Inc.
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AN887 AC Induction Motor Fundamentals created naturally in the stator because of the nature of Author: Rakesh Parekh the supply. DC motors depend either on mechanical or Microchip Technology Inc. electronic commutation to create rotating magnetic fields. A single-phase AC induction motor depends on extra electrical components to produce this rotatingINTRODUCTION magnetic field.AC induction motors are the most common motors Two sets of electromagnets are formed inside any motor.used in industrial motion control systems, as well as in In an AC induction motor, one set of electromagnets ismain powered home appliances. Simple and rugged formed in the stator because of the AC supply connecteddesign, low-cost, low maintenance and direct connec- to the stator windings. The alternating nature of the sup-tion to an AC power source are the main advantages of ply voltage induces an Electromagnetic Force (EMF) inAC induction motors. the rotor (just like the voltage is induced in the trans-Various types of AC induction motors are available in former secondary) as per Lenz’s law, thus generatingthe market. Different motors are suitable for different another set of electromagnets; hence the name – induc-applications. Although AC induction motors are easier tion motor. Interaction between the magnetic field ofto design than DC motors, the speed and the torque these electromagnets generates twisting force, orcontrol in various types of AC induction motors require torque. As a result, the motor rotates in the direction ofa greater understanding of the design and the the resultant torque.characteristics of these motors.This application note discusses the basics of an AC Statorinduction motor; the different types, their characteris- The stator is made up of several thin laminations oftics, the selection criteria for different applications and aluminum or cast iron. They are punched and clampedbasic control techniques. together to form a hollow cylinder (stator core) with slots as shown in Figure 1. Coils of insulated wires areBASIC CONSTRUCTION AND inserted into these slots. Each grouping of coils, together with the core it surrounds, forms an electro-OPERATING PRINCIPLE magnet (a pair of poles) on the application of ACLike most motors, an AC induction motor has a fixed supply. The number of poles of an AC induction motorouter portion, called the stator and a rotor that spins depends on the internal connection of the stator wind-inside with a carefully engineered air gap between the ings. The stator windings are connected directly to thetwo. power source. Internally they are connected in such a way, that on applying AC supply, a rotating magneticVirtually all electrical motors use magnetic field rotation field is created.to spin their rotors. A three-phase AC induction motoris the only type where the rotating magnetic field isFIGURE 1: A TYPICAL STATOR 2003 Microchip Technology Inc. DS00887A-page 1
AN887Rotor Speed of an Induction MotorThe rotor is made up of several thin steel laminations The magnetic field created in the stator rotates at awith evenly spaced bars, which are made up of synchronous speed (NS).aluminum or copper, along the periphery. In the mostpopular type of rotor (squirrel cage rotor), these bars EQUATION 1:are connected at ends mechanically and electrically by f-the use of rings. Almost 90% of induction motors have N s = 120 × -- Psquirrel cage rotors. This is because the squirrel cage where:rotor has a simple and rugged construction. The rotor NS = the synchronous speed of the statorconsists of a cylindrical laminated core with axially magnetic field in RPMplaced parallel slots for carrying the conductors. Each P = the number of poles on the statorslot carries a copper, aluminum, or alloy bar. These f = the supply frequency in Hertzrotor bars are permanently short-circuited at both endsby means of the end rings, as shown in Figure 2. Thistotal assembly resembles the look of a squirrel cage, The magnetic field produced in the rotor because of thewhich gives the rotor its name. The rotor slots are not induced voltage is alternating in nature.exactly parallel to the shaft. Instead, they are given a To reduce the relative speed, with respect to the stator,skew for two main reasons. the rotor starts running in the same direction as that ofThe first reason is to make the motor run quietly by the stator flux and tries to catch up with the rotating flux.reducing magnetic hum and to decrease slot However, in practice, the rotor never succeeds inharmonics. “catching up” to the stator field. The rotor runs slower than the speed of the stator field. This speed is calledThe second reason is to help reduce the locking ten- the Base Speed (Nb).dency of the rotor. The rotor teeth tend to remain lockedunder the stator teeth due to direct magnetic attraction The difference between NS and Nb is called the slip. Thebetween the two. This happens when the number of slip varies with the load. An increase in load will causestator teeth are equal to the number of rotor teeth. the rotor to slow down or increase slip. A decrease in load will cause the rotor to speed up or decrease slip.The rotor is mounted on the shaft using bearings on The slip is expressed as a percentage and can beeach end; one end of the shaft is normally kept longer determined with the following formula:than the other for driving the load. Some motors mayhave an accessory shaft on the non-driving end for EQUATION 2:mounting speed or position sensing devices. Betweenthe stator and the rotor, there exists an air gap, through Ns – Nbwhich due to induction, the energy is transferred from % slip = ------------------- x100 - Nsthe stator to the rotor. The generated torque forces the where:rotor and then the load to rotate. Regardless of the type NS = the synchronous speed in RPMof rotor used, the principle employed for rotation Nb = the base speed in RPMremains the same.FIGURE 2: A TYPICAL SQUIRREL CAGE ROTOR End Ring Conductors End Ring Shaft Bearing Bearing Skewed SlotsDS00887A-page 2 2003 Microchip Technology Inc.
AN887TYPES OF AC INDUCTION MOTORS phase induction motor is required to have a starting mechanism that can provide the starting kick for theGenerally, induction motors are categorized based on motor to rotate.the number of stator windings. They are: The starting mechanism of the single-phase induction• Single-phase induction motor motor is mainly an additional stator winding (start/• Three-phase induction motor auxiliary winding) as shown in Figure 3. The start wind- ing can have a series capacitor and/or a centrifugalSingle-Phase Induction Motor switch. When the supply voltage is applied, current in the main winding lags the supply voltage due to theThere are probably more single-phase AC induction main winding impedance. At the same time, current inmotors in use today than the total of all the other types the start winding leads/lags the supply voltage depend-put together. It is logical that the least expensive, low- ing on the starting mechanism impedance. Interactionest maintenance type motor should be used most between magnetic fields generated by the main wind-often. The single-phase AC induction motor best fits ing and the starting mechanism generates a resultantthis description. magnetic field rotating in one direction. The motorAs the name suggests, this type of motor has only one starts rotating in the direction of the resultant magneticstator winding (main winding) and operates with a field.single-phase power supply. In all single-phase Once the motor reaches about 75% of its rated speed,induction motors, the rotor is the squirrel cage type. a centrifugal switch disconnects the start winding. FromThe single-phase induction motor is not self-starting. this point on, the single-phase motor can maintainWhen the motor is connected to a single-phase power sufficient torque to operate on its own.supply, the main winding carries an alternating current. Except for special capacitor start/capacitor run types,This current produces a pulsating magnetic field. Due all single-phase motors are generally used forto induction, the rotor is energized. As the main applications up to 3/4 hp only.magnetic field is pulsating, the torque necessary for the Depending on the various start techniques, single-motor rotation is not generated. This will cause the phase AC induction motors are further classified asrotor to vibrate, but not to rotate. Hence, the single- described in the following sections.FIGURE 3: SINGLE-PHASE AC INDUCTION MOTOR WITH AND WITHOUT A START MECHANISM Capacitor Centrifugal Switch Rotor Rotor Input Main Input Power Power Main Winding Winding Start Winding Single-Phase AC Induction Motor Single-Phase AC Induction Motor without Start Mechanism with Start Mechanism 2003 Microchip Technology Inc. DS00887A-page 3
AN887Split-Phase AC Induction Motor FIGURE 5: TYPICAL CAPACITORThe split-phase motor is also known as an induction START INDUCTION MOTORstart/induction run motor. It has two windings: a start Capacitor Centrifugal Switchand a main winding. The start winding is made withsmaller gauge wire and fewer turns, relative to the main Rotorwinding to create more resistance, thus putting the startwinding’s field at a different angle than that of the mainwinding which causes the motor to start rotating. Themain winding, which is of a heavier wire, keeps the Inputmotor running the rest of the time. Power Main WindingFIGURE 4: TYPICAL SPLIT-PHASE AC INDUCTION MOTOR Start Winding Centrifugal Switch They are used in a wide range of belt-drive applications like small conveyors, large blowers and pumps, as well Rotor as many direct-drive or geared applications. Permanent Split Capacitor (Capacitor Run) AC Induction Motor Input Power Main A permanent split capacitor (PSC) motor has a run type Winding capacitor permanently connected in series with the start winding. This makes the start winding an auxiliary Start Winding winding once the motor reaches the running speed. Since the run capacitor must be designed for continu-The starting torque is low, typically 100% to 175% of the ous use, it cannot provide the starting boost of a start-rated torque. The motor draws high starting current, ing capacitor. The typical starting torque of the PSCapproximately 700% to 1,000% of the rated current. The motor is low, from 30% to 150% of the rated torque.maximum generated torque ranges from 250% to 350% PSC motors have low starting current, usually less thanof the rated torque (see Figure 9 for torque-speed 200% of the rated current, making them excellent forcurve). applications with high on/off cycle rates. Refer toGood applications for split-phase motors include small Figure 9 for torque-speed curve.grinders, small fans and blowers and other low starting The PSC motors have several advantages. The motortorque applications with power needs from 1/20 to design can easily be altered for use with speed control-1/3 hp. Avoid using this type of motor in any applications lers. They can also be designed for optimum efficiencyrequiring high on/off cycle rates or high torque. and High-Power Factor (PF) at the rated load. They’re considered to be the most reliable of the single-phaseCapacitor Start AC Induction Motor motors, mainly because no centrifugal starting switch is required.This is a modified split-phase motor with a capacitor inseries with the start winding to provide a start “boost.”Like the split-phase motor, the capacitor start motor FIGURE 6: TYPICAL PSC MOTORalso has a centrifugal switch which disconnects the Capacitorstart winding and the capacitor when the motor reachesabout 75% of the rated speed. RotorSince the capacitor is in series with the start circuit, itcreates more starting torque, typically 200% to 400% ofthe rated torque. And the starting current, usually 450%to 575% of the rated current, is much lower than the Input Power Mainsplit-phase due to the larger wire in the start circuit. WindingRefer to Figure 9 for torque-speed curve.A modified version of the capacitor start motor is the Start Windingresistance start motor. In this motor type, the startingcapacitor is replaced by a resistor. The resistance start Permanent split-capacitor motors have a wide varietymotor is used in applications where the starting torque of applications depending on the design. These includerequirement is less than that provided by the capacitor fans, blowers with low starting torque needs and inter-start motor. Apart from the cost, this motor does not offer mittent cycling uses, such as adjusting mechanisms,any major advantage over the capacitor start motor. gate operators and garage door openers.DS00887A-page 4 2003 Microchip Technology Inc.
AN887Capacitor Start/Capacitor Run AC Shaded-Pole AC Induction MotorInduction Motor Shaded-pole motors have only one main winding andThis motor has a start type capacitor in series with the no start winding. Starting is by means of a design thatauxiliary winding like the capacitor start motor for high rings a continuous copper loop around a small portionstarting torque. Like a PSC motor, it also has a run type of each of the motor poles. This “shades” that portion ofcapacitor that is in series with the auxiliary winding after the pole, causing the magnetic field in the shaded areathe start capacitor is switched out of the circuit. This to lag behind the field in the unshaded area. Theallows high overload torque. reaction of the two fields gets the shaft rotating. Because the shaded-pole motor lacks a start winding,FIGURE 7: TYPICAL CAPACITOR starting switch or capacitor, it is electrically simple and START/RUN INDUCTION inexpensive. Also, the speed can be controlled merely MOTOR by varying voltage, or through a multi-tap winding. Mechanically, the shaded-pole motor construction Start Cap Centrifugal Switch allows high-volume production. In fact, these are usu- ally considered as “disposable” motors, meaning they Run Cap are much cheaper to replace than to repair. Rotor FIGURE 8: TYPICAL SHADED-POLE INDUCTION MOTOR Shaded Portion of Pole Copper Ring Input Power Main Winding Start WindingThis type of motor can be designed for lower full-loadcurrents and higher efficiency (see Figure 9 for torque-speed curve). This motor is costly due to start and run Supply Linecapacitors and centrifugal switch. Unshaded Portion of PoleIt is able to handle applications too demanding for anyother kind of single-phase motor. These include wood-working machinery, air compressors, high-pressure The shaded-pole motor has many positive features butwater pumps, vacuum pumps and other high torque it also has several disadvantages. It’s low startingapplications requiring 1 to 10 hp. torque is typically 25% to 75% of the rated torque. It is a high slip motor with a running speed 7% to 10% below the synchronous speed. Generally, efficiency of this motor type is very low (below 20%). The low initial cost suits the shaded-pole motors to low horsepower or light duty applications. Perhaps their larg- est use is in multi-speed fans for household use. But the low torque, low efficiency and less sturdy mechanical features make shaded-pole motors impractical for most industrial or commercial use, where higher cycle rates or continuous duty are the norm. Figure 9 shows the torque-speed curves of various kinds of single-phase AC induction motors. 2003 Microchip Technology Inc. DS00887A-page 5
AN887FIGURE 9: TORQUE-SPEED CURVES OF DIFFERENT TYPES OF SINGLE-PHASE INDUCTION MOTORS Capacitor Start and Run 500 Changeover of Centrifugal Switch Torque (% of Full-Load Torque) Capacitor Start 400 Split-Phase 300 PSC 200 Shaded-Pole 100 20 40 60 80 100 Speed (%)THREE-PHASE AC INDUCTION Wound-Rotor MotorMOTOR The slip-ring motor or wound-rotor motor is a variationThree-phase AC induction motors are widely used in of the squirrel cage induction motor. While the stator isindustrial and commercial applications. They are the same as that of the squirrel cage motor, it has a setclassified either as squirrel cage or wound-rotor of windings on the rotor which are not short-circuited,motors. but are terminated to a set of slip rings. These are helpful in adding external resistors and contactors.These motors are self-starting and use no capacitor,start winding, centrifugal switch or other starting The slip necessary to generate the maximum torquedevice. (pull-out torque) is directly proportional to the rotor resistance. In the slip-ring motor, the effective rotorThey produce medium to high degrees of starting resistance is increased by adding external resistancetorque. The power capabilities and efficiency in these through the slip rings. Thus, it is possible to get highermotors range from medium to high compared to their slip and hence, the pull-out torque at a lower speed.single-phase counterparts. Popular applicationsinclude grinders, lathes, drill presses, pumps, A particularly high resistance can result in the pull-outcompressors, conveyors, also printing equipment, farm torque occurring at almost zero speed, providing a veryequipment, electronic cooling and other mechanical high pull-out torque at a low starting current. As theduty applications. motor accelerates, the value of the resistance can be reduced, altering the motor characteristic to suit the load requirement. Once the motor reaches the baseSquirrel Cage Motor speed, external resistors are removed from the rotor.Almost 90% of the three-phase AC Induction motors This means that now the motor is working as theare of this type. Here, the rotor is of the squirrel cage standard induction motor.type and it works as explained earlier. The power This motor type is ideal for very high inertia loads,ratings range from one-third to several hundred horse- where it is required to generate the pull-out torque atpower in the three-phase motors. Motors of this type, almost zero speed and accelerate to full speed in therated one horsepower or larger, cost less and can start minimum time with minimum current draw.heavier loads than their single-phase counterparts.DS00887A-page 6 2003 Microchip Technology Inc.
AN887FIGURE 10: TYPICAL WOUND-ROTOR TORQUE EQUATION GOVERNING INDUCTION MOTOR MOTOR OPERATION Wound Rotor The motor load system can be described by a fundamental torque equation. Brush EQUATION 3: dω m dJ T – T l = J ----------- + ω m ----- - - dt dt External Rotor where: Slip Ring Resistance T = the instantaneous value of the developed motor torque (N-m or lb-inch) Tl = the instantaneous value of the load torque (N-m or lb-inch)The downside of the slip ring motor is that slip rings and ωm = the instantaneous angularbrush assemblies need regular maintenance, which is velocity of the motor shaft (rad/sec)a cost not applicable to the standard cage motor. If the J = the moment of inertia of the motor –rotor windings are shorted and a start is attempted (i.e., load system (kg-m2 or lb-inch2)the motor is converted to a standard induction motor),it will exhibit an extremely high locked rotor current –typically as high as 1400% and a very low locked rotor For drives with constant inertia, (dJ/dt) = 0. Therefore,torque, perhaps as low as 60%. In most applications, the equation would be:this is not an option.Modifying the speed torque curve by altering the rotor EQUATION 4:resistors, the speed at which the motor will drive a dω m T = T l + J ----------- -particular load can be altered. At full load, you can dtreduce the speed effectively to about 50% of the motorsynchronous speed, particularly when driving variable This shows that the torque developed by the motor istorque/variable speed loads, such as printing presses counter balanced by a load torque, Tl and a dynamicor compressors. Reducing the speed below 50% torque, J(dωm/dt). The torque component, J(dω/dt), isresults in very low efficiency due to higher power called the dynamic torque because it is present onlydissipation in the rotor resistances. This type of motor during the transient operations. The drive acceleratesis used in applications for driving variable torque/ or decelerates depending on whether T is greater orvariable speed loads, such as in printing presses, less than Tl. During acceleration, the motor should sup-compressors, conveyer belts, hoists and elevators. ply not only the load torque, but an additional torque component, J(dωm/dt), in order to overcome the drive inertia. In drives with large inertia, such as electric trains, the motor torque must exceed the load torque by a large amount in order to get adequate acceleration. In drives requiring fast transient response, the motor torque should be maintained at the highest value and the motor load system should be designed with the low- est possible inertia. The energy associated with the dynamic torque, J(dωm/dt), is stored in the form of kinetic energy (KE) given by, J(ω2m/2). During deceler- ation, the dynamic torque, J(dωm/dt), has a negative sign. Therefore, it assists the motor developed torque T and maintains the drive motion by extracting energy from the stored kinetic energy. To summarize, in order to get steady state rotation of the motor, the torque developed by the motor (T) should always be equal to the torque requirement of the load (Tl). The torque-speed curve of the typical three-phase induction motor is shown in Figure 11. 2003 Microchip Technology Inc. DS00887A-page 7
AN887FIGURE 11: TYPICAL TORQUE-SPEED CURVE OF 3-PHASE AC INDUCTION MOTOR Pull-out Torque 7 x FLC Full Voltage Stator Current Current (% of Motor Full-Load Current) Torque (% of Motor Full-Load Torque) LRC 6 x FLC 2 x FLT 5 x FLC 4 x FLC Full Voltage Start Torque LRT 3 x FLC 1 x FLT 2 x FLC Pull-up Torque 1 x FLC Sample Load Torque Curve 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Rotor Speed (% of Full Speed)STARTING CHARACTERISTIC The LRT of an induction motor can vary from as low as 60% of FLT to as high as 350% of FLT. The pull-upInduction motors, at rest, appear just like a short cir- torque can be as low as 40% of FLT and the breakdowncuited transformer and if connected to the full supply torque can be as high as 350% of FLT. Typically, LRTsvoltage, draw a very high current known as the “Locked for medium to large motors are in the order of 120% ofRotor Current.” They also produce torque which is FLT to 280% of FLT. The PF of the motor at start isknown as the “Locked Rotor Torque”. The Locked typically 0.1-0.25, rising to a maximum as the motorRotor Torque (LRT) and the Locked Rotor Current accelerates and then falling again as the motor(LRC) are a function of the terminal voltage of the motor approaches full speed.and the motor design. As the motor accelerates, boththe torque and the current will tend to alter with rotorspeed if the voltage is maintained constant. RUNNING CHARACTERISTICThe starting current of a motor with a fixed voltage will Once the motor is up to speed, it operates at a low slip,drop very slowly as the motor accelerates and will only at a speed determined by the number of the statorbegin to fall significantly when the motor has reached poles. Typically, the full-load slip for the squirrel cageat least 80% of the full speed. The actual curves for the induction motor is less than 5%. The actual full-load slipinduction motors can vary considerably between of a particular motor is dependant on the motor design.designs but the general trend is for a high current until The typical base speed of the four pole induction motorthe motor has almost reached full speed. The LRC of a varies between 1420 and 1480 RPM at 50 Hz, while themotor can range from 500% of Full-Load Current (FLC) synchronous speed is 1500 RPM at 50 Hz.to as high as 1400% of FLC. Typically, good motors fall The current drawn by the induction motor has two com-in the range of 550% to 750% of FLC. ponents: reactive component (magnetizing current)The starting torque of an induction motor starting with a and active component (working current). The magne-fixed voltage will drop a little to the minimum torque, tizing current is independent of the load but is depen-known as the pull-up torque, as the motor accelerates dant on the design of the stator and the stator voltage.and then rises to a maximum torque, known as the The actual magnetizing current of the induction motorbreakdown or pull-out torque, at almost full speed and can vary, from as low as 20% of FLC for the large twothen drop to zero at the synchronous speed. The curve pole machine, to as high as 60% for the small eight poleof the start torque against the rotor speed is dependant machine. The working current of the motor is directlyon the terminal voltage and the rotor design. proportional to the load.DS00887A-page 8 2003 Microchip Technology Inc.
AN887The tendency for the large machines and high-speed In most drives, the electrical time constant of the motormachines is to exhibit a low magnetizing current, while is negligible as compared to its mechanical time con-for the low-speed machines and small machines the stant. Therefore, during transient operation, the motortendency is to exhibit a high magnetizing current. A can be assumed to be in an electrical equilibrium,typical medium sized four pole machine has a implying that the steady state torque-speed curve ismagnetizing current of about 33% of FLC. also applicable to the transient operation.A low magnetizing current indicates a low iron loss, As an example, Figure 12 shows torque-speed curveswhile a high magnetizing current indicates an increase of the motor with two different loads. The system canin iron loss and a resultant reduction in the operating be termed as stable, when the operation will beefficiency. restored after a small departure from it, due to aTypically, the operating efficiency of the induction motor disturbance in the motor or load.is highest at 3/4 load and varies from less than 60% for For example, disturbance causes a reduction of ∆ωm insmall low-speed motors to greater than 92% for large speed. In the first case, at a new speed, the motorhigh-speed motors. The operating PF and efficiencies torque (T) is greater than the load torque (Tl). Conse-are generally quoted on the motor data sheets. quently, the motor will accelerate and the operation will be restored to X. Similarly, an increase of ∆ωm in the speed, caused by a disturbance, will make the loadLOAD CHARACTERISTIC torque (Tl) greater than the motor torque (T), resultingIn real applications, various kinds of loads exist with in a deceleration and restoration of the point ofdifferent torque-speed curves. For example, Constant operation to X. Hence, at point X, the system is stable.Torque, Variable Speed Load (screw compressors, In the second case, a decrease in the speed causesconveyors, feeders), Variable Torque, Variable Speed the load torque (Tl) to become greater than the motorLoad (fan, pump), Constant Power Load (traction torque (T), the drive decelerates and the operatingdrives), Constant Power, Constant Torque Load (coiler point moves away from Y. Similarly, an increase in thedrive) and High Starting/Breakaway Torque followed by speed will make the motor torque (T) greater than theConstant Torque Load (extruders, screw pumps). load torque (Tl), which will move the operating pointThe motor load system is said to be stable when the further away from Y. Thus, at point Y, the system isdeveloped motor torque is equal to the load torque unstable.requirement. The motor will operate in a steady state at This shows that, while in the first case, the motora fixed speed. The response of the motor to any selection for driving the given load is the right one; indisturbance gives us an idea about the stability of the the second case, the selected motor is not the rightmotor load system. This concept helps us in quickly choice and requires changing for driving the given load.evaluating the selection of a motor for driving aparticular load. The typical existing loads with their torque-speed curves are described in the following sections.FIGURE 12: TORQUE-SPEED CURVE – SAME MOTOR WITH TWO DIFFERENT LOADS ωm T Tl ωm T X Y Tl 0 0 Torque Torque 2003 Microchip Technology Inc. DS00887A-page 9
AN887Constant Torque, Variable Speed Loads FIGURE 15: CONSTANT POWER LOADSThe torque required by this type of load is constantregardless of the speed. In contrast, the power islinearly proportional to the speed. Equipment, such as Torquescrew compressors, conveyors and feeders, have thistype of characteristic. PowerFIGURE 13: CONSTANT TORQUE, VARIABLE SPEED LOADS Speed Torque Constant Power, Constant Torque Loads This is common in the paper industry. In this type of Power load, as speed increases, the torque is constant with the power linearly increasing. When the torque starts to Speed decrease, the power then remains constant. FIGURE 16: CONSTANT POWER,Variable Torque, Variable Speed Loads CONSTANT TORQUEThis is most commonly found in the industry and LOADSsometimes is known as a quadratic torque load. Thetorque is the square of the speed, while the power is thecube of the speed. This is the typical torque-speed Torquecharacteristic of a fan or a pump. PowerFIGURE 14: VARIABLE TORQUE, VARIABLE SPEED LOADS Speed High Starting/Breakaway Torque Torque Followed by Constant Torque Power This type of load is characterized by very high torque at relatively low frequencies. Typical applications include Speed extruders and screw pumps. FIGURE 17: HIGH STARTING/Constant Power Loads BREAKAWAY TORQUEThis type of load is rare but is sometimes found in the FOLLOWED BYindustry. The power remains constant while the torque CONSTANT TORQUEvaries. The torque is inversely proportional to thespeed, which theoretically means infinite torque at zerospeed and zero torque at infinite speed. In practice,there is always a finite value to the breakaway torquerequired. This type of load is characteristic of the trac-tion drives, which require high torque at low speeds for Torquethe initial acceleration and then a much reduced torquewhen at running speed. SpeedDS00887A-page 10 2003 Microchip Technology Inc.
AN887MOTOR STANDARDS • Design A has normal starting torque (typically 150-170% of rated) and relatively high startingWorldwide, various standards exist which specify vari- current. The breakdown torque is the highest of allous operating and constructional parameters of a the NEMA types. It can handle heavy overloadsmotor. The two most widely used parameters are the for a short duration. The slip is = 5%. A typicalNational Electrical Manufacturers Association (NEMA) application is the powering of injection moldingand the International Electrotechnical Commission machines.(IEC). • Design B is the most common type of AC induction motor sold. It has a normal startingNEMA torque, similar to Design A, but offers low starting current. The locked rotor torque is good enough toNEMA sets standards for a wide range of electrical start many loads encountered in the industrialproducts, including motors. NEMA is primarily associ- applications. The slip is = 5%. The motor effi-ated with motors used in North America. The standards ciency and full-load PF are comparatively high,developed represent the general industry practices and contributing to the popularity of the design. Theare supported by manufacturers of electrical equip- typical applications include pumps, fans andment. These standards can be found in the NEMA machine tools.Standard Publication No. MG 1. Some large AC motorsmay not fall under NEMA standards. They are built to • Design C has high starting torque (greater thanmeet the requirements of a specific application. They the previous two designs, say 200%), useful forare referred to as above NEMA motors. driving heavy breakaway loads like conveyors, crushers, stirring machines, agitators, reciprocat- ing pumps, compressors, etc. These motors areIEC intended for operation near full speed withoutIEC is a European-based organization that publishes great overloads. The starting current is low. Theand promotes worldwide, the mechanical and electrical slip is = 5%.standards for motors, among other things. In simple • Design D has high starting torque (higher than allterms, it can be said that the IEC is the international the NEMA motor types). The starting current andcounterpart of the NEMA. The IEC standards are full-load speed are low. The high slip valuesassociated with motors used in many countries. These (5-13%) make this motor suitable for applicationsstandards can be found in the IEC 34-1-16. The motors with changing loads and subsequent sharpwhich meet or exceed these standards are referred to changes in the motor speed, such as inas IEC motors. machinery with energy storage flywheels, punchThe NEMA standards mainly specify four design types presses, shears, elevators, extractors, winches,for AC induction motors – Design A, B, C and D. Their hoists, oil-well pumping, wire-drawing machines,typical torque-speed curves are shown in Figure 18. etc. The speed regulation is poor, making the design suitable only for punch presses, cranes, elevators and oil well pumps. This motor type is usually considered a “special order” item.FIGURE 18: TORQUE-SPEED CURVES OF DIFFERENT NEMA STANDARD MOTORS Design A Torque (% of Full-Load Torque) 300 Design D Design C 200 Design B 100 20 40 60 80 100 Speed (%) 2003 Microchip Technology Inc. DS00887A-page 11