MC3PHAC: Ready-to-Use AC Induction Motor Controller IC For .

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Freescale SemiconductorWhite PaperMC3PHACWPRev. 0, 10/2005Ready-to-Use AC InductionMotor Controller IC for Low-CostVariable Speed Applicationsby: Dave Wilson and Bill LucasFreescale Semiconductor, Inc.1AbstractRecent advancements in PWM generation and controltechniques have been combined with state-of-the-artcontrol algorithms in a pre-programmed solutionreferred to as the MC3PHAC, which is designed todramatically minimize up front development costs andtime to market in variable speed AC motor controlapplications. Even though the device requires noprogramming, flexibility has been maintained, allowingthe user to customize the device to a particularapplication. Particular attention has been given to thesafety and fault processing features, including “deadcrystal” shutdown, hardware fault shutdown, and DC busmonitoring/protection. Freescale Semiconductor, Inc., 2005. All rights reserved.Table of Contents12345678Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Advanced Motor Control PWMs . . . . . . . . . . . . . . 3Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . 64.1 Standalone Mode. . . . . . . . . . . . . . . . . . . . . . 64.2 Host Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Bus Ripple Cancellation . . . . . . . . . . . . . . . . . . . . 9Velocity Pipelining and Interpolation . . . . . . . . . . 11System Monitoring and Protection . . . . . . . . . . . 13Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Introduction2IntroductionUse of variable speed control of AC induction motors has increased sharply over the last decade as thepromise of energy savings and more elegant control techniques are being realized. This is particularly truein applications that require full speed operation for only a small percentage of the time, such as certain fanand pump loads. Since line-connected AC motors have intractable speed characteristics, such load demandvariations have historically been handled by throttling between the motor and load, a technique which hasbeen compared to driving a car with the accelerator pedal to the floor, and controlling speed with thebrakes. However, by controlling the operating speed of the motor directly, energy savings of up to 75%have been cited for certain applications as compared to direct line-connected motor operation.Several techniques ranging in sophistication have been utilized to accomplish variable speed AC motorcontrol. If high bandwidth torque control is required across a wide operating speed, field orientedtechniques utilizing a rotor speed sensor, or using the motor itself as the feedback sensor may be employed.Since AC induction motors are asynchronous in nature, the calculations needed to accomplish this oftenrequire a high performance controller, such as a DSP. However, many (if not most, by volume) variablespeed AC motor control applications only require moderate torque control performance down to afrequency of about 5 Hz. In these cases, simply controlling the waveform voltage and frequency to themotor (volts-per-hertz control) is the most economical approach.Regardless of the control topology selected, it is undeniable that larger portions of the task of developinga variable speed drive are being devoted to the software effort, with its associated tools investment. Sincethe MC3PHAC requires no programming, it eliminates this investment requirement, which consistentlyreduces overall project development and debug time. Being a “fixed” solution, it is unavoidable that thetrade off for these advantages will come at the cost of some reduced flexibility. However, great care hasbeen taken to insure that most of the critical system parameters common to high performance AC drivesare dynamically configurable, allowing the MC3PHAC to work in a multitude of variable speedconfigurations. Also, the MC3PHAC utilizes a serial interface which implements a special communicationprotocol, that allows a PC or microcontroller to configure the operating characteristics and control themotor in real time as a host. For example, through the host software a computer can exercise completecontrol over the volts-per-hertz relationship, allowing the MC3PHAC to work in variable torque as wellas constant torque variable speed applications.Ready-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 02Freescale Semiconductor

Advanced Motor Control PWMs3Advanced Motor Control PWMsAt the core of the MC3PHAC’s capabilities is an advanced PWM module designed specifically to meetthe demanding requirements of high performance AC drives. The module is clocked at 8 MHz (125 nsbetween timer tics), and generates six center-aligned PWMs in three groups of complimentary PWMs.This allows the MC3PHAC to connect directly to inverters that are indigenous to almost all three-phaseAC motor drives, as shown in Figure 1. The polarity of the high-side PWM signals can be specifiedindependently from the low-side PWM polarities. Dead-time is inserted between the on-times of eachcomplimentary signal pair and may be adjusted to any value between 0 and 32 µs, in 125 ns increments.3 phaseAC motorMotorolaDave’sControlCenterAC InBus ward/ReverseSpeedAccelerationPWM FrequencyFault FeedbackSerial ptional)Figure 1. Typical 3-Phase AC Motor Drive Using the MC3PHACReady-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 0Freescale Semiconductor3

Advanced Motor Control PWMsThe PWM frequency can be specified as one of four values, as illustrated in Table 1, along with theeffective PWM resolution for each frequency. Each PWM output is synthesized from a 512-entry table,consisting of 8-bit values, as shown in Figure 2. While this limits the peak-to-peak resolution of the outputwaveform to eight bits, it does not necessarily mean that the PWM resolution itself is limited to eight bits.This is particularly true for smaller modulation indices. The PWM resolution defines how many distinctvalues can exist over the full modulation range (0% to 100%), which is different from the peak-to-peakresolution of the output waveform.Table 1. PWM Frequencies and Corresponding ResolutionsPWM FrequencyPWM Resolution5.291 kHz9.6 bits10.582 kHz8.6 bits15.873 kHz8 bits21.164 kHz7.6 bits2561921286400100200300400500Figure 2. Table Used for Waveform Synthesis in the MC3PHACAnother factor that can have a much greater impact on waveform distortion than the resolution of the wavetable entries, is the sampling frequency by which the motor waveforms are updated. Since the PWMmodule acts as a sample and hold function, the waveform will be distorted in two ways.1. Sample and hold functions generate phase lag, which increases as the sampling frequencydecreases. This is generally not a problem for open-loop waveform generation. However, it mustbe considered when performing any closed-loop functions such as bus-ripple compensation, whichwill be discussed later.2. Since the PWM value is held constant until the next update, it results in a “stair-stepped”waveform, which introduces amplitude distortion when compared to a reference sine wave. Thisdistortion is proportional to the first derivative of the waveform, which means that an outputwaveform synthesized from Figure 2 will experience more distortion when the waveform ischanging rapidly near the zero crossings.Ready-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 04Freescale Semiconductor

Advanced Motor Control PWMsSince the distortion is related to the phase uncertainty for all non-zero derivative functions, then thesampling frequency and the output motor waveform frequency also affects it. For all carrier frequenciesexcept 15.9 kHz, the MC3PHAC PWMs are updated at a sampling frequency of 5.3 kHz, which results ina timing jitter of 95 µs. For a 15.9-kHz carrier, the PWMs are updated at a 4-kHz rate, with a timing jitterof 126 µs. This results in a phase uncertainty as a function of the motor waveform frequency, which isillustrated in Figure 3. As the motor waveform frequency decreases below about 10 Hz, improvements tothe phase jitter are not observed since the phase resolution of the 512-point waveform table is reached. Forboth update rates, the result is motor waveforms with greater accuracy than can be achieved with designsthat utilize higher waveform resolution but a lower waveform update frequency.Worst Case Phase Jitter ( 2atete RadUp10020406080100120140Motor Waveform Frequency (Hz)Figure 3. MC3PHAC Phase Uncertainty as a Functionof Motor Waveform FrequencyFrom Figure 2, it can be seen that the waveform contains a third harmonic component added to the sinewave, which results in 15% greater phase-to-phase amplitudes compared to traditional sine modulation.However, this restricts the MC3PHAC usage to three-phase loads that have a floating neutral, since acommon-mode third harmonic frequency component results from this modulation technique. It also placeslimitations on the synthesis technique of the three-phase outputs, since the sum of the output voltagewaveforms no longer equals zero.Ready-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 0Freescale Semiconductor5

Modes of Operation4Modes of OperationThe MC3PHAC will operate in either of two modes: Standalone or Host. Mode selection occurs atpower-up as specified by the state of pin 20. Both modes are described in further detail below.4.1Standalone ModeIn this mode, the MC3PHAC operating parameters are configured during power-up via passivecomponents connected to the device. Once the MC3PHAC determines that there is no external host (pin20 is high), it begins interrogating the externally connected resistor network to obtain operating parameterssuch as Speed Range, Dead-Time, and Voltage Boost. Other parameters continue to be input in real timeas the system operates, such as Start/Stop, Forward/Reverse, Motor Speed, PWM Frequency, Bus Voltage,and Acceleration. Standalone mode is the most economical mode from an overall system cost point ofview, as no host controller is needed for MC3PHAC operation. Figure 4 shows an example circuit usingthe MC3PHAC in standalone mode.R2 can be made of severalresistors in series to preventvoltage arking. Limit voltageacross a resistor to 150vCONNECT TO HIGH VOLTAGE BUS 5V (A)R2R22R1Install JP1 for 0 - 128 HzRemove JP1 for 0 66 Hz6.8k3.3kC3JP6Base speed 50 Hz, - PWM polarityR34700 pFBase speed 50 Hz, PWM polarityAJP9Base speed 60 Hz, PWM polarityA 5V 5VF13R2 C11uFR14Acceleration Input4.7kR29Speed Input3OpenR15R2 and R3 divider computed for3.5 volts nominal @ DC BUSpinJP8Base speed 60 Hz, - PWM polarity1 5V (A)6.8kJP7R85.0k2OpenR16A1ACCELERATA IO 5V 34R410M5674.00MHzC50.1uF8Connect to theinverter'sgate drivers91011121314DC BUSACCELSPEEDMUX OST MODEDT FAULTOUTRBRAKEPWMPOL BASEFREQPWM U TopPWM U BotPWM V TopPWM V BotPWM W TopPWM W BotRETRY/TxdPWMFREQ/ RxdFAULTINU2Using a 4.00 MHzresonator withbuilt-in capacitorsR32MC3PHAC1 5V282726252423C9.022 uFC8.022 uFC7.022 18Voltage Boost / PCMaster SelectDead-Time / Fault Out1716Spd Range / Retry / TxDPWM Frequency / RxDConnect to Resistive BRAKE Driver 5VR20SW2Fwd/RevVoltage BoostR2115Connect to optional fault circuitIf no external fault isrequired, connect theFAULTIN pin to digitalgroundDead TimeR12Speed RangeR13PWM FrequencyFigure 4. Schematic of the MC3PHAC in Standalone Mode OperationReady-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 06Freescale Semiconductor

Modes of Operation4.2Host ModeThe second mode of operation is called Host Mode, which utilizes a PC running host software (availablefrom Freescale), or a microcontroller emulating the host software commands. Instead of using discretecomponents to specify operating parameters, they are controlled in software, directly from the host.Immediately following power-up, the MC3PHAC determines that an external host is present by readingpin 20 as a logic low level. After continuing to initialize to an inert, safe condition, it remains dormant,waiting for commands over its serial interface to specify operating parameters. The MC3PHAC will notallow the motor to be activated until certain critical parameters have been communicated, such as PWMPolarity and Dead-Time information. Host Mode allows an external controller to monitor and control allaspects of the MC3PHAC’s operation, and permits much wider control over the system’s operatingenvironment as compared to Standalone Mode, which is illustrated by Table 2.Table 2. Comparison Between Standalone and Hosted Operating ModesNameStandalone ModeHost ModeDescriptionCommandedPWM polarityTop and BottomAll Positive or All Negativeat 50 Hz or 60 HzBottom Positive, Top PositiveBottom Positive, Top NegativeBottom Negative, Top PositiveBottom Negative, Top NegativeAt 50 Hz or 60 HzSpecifies the polarity of theMC3PHAC PWM outputs.Dead-time5 to 6 µs0 to 32 µsSpecifies the dead-time usedby the PWM generator.Fault timeout1 second to –53 seconds25 seconds to 4.55 hoursSpecifies the delay timeafter a fault conditionbefore re-enabling the motor.Voltage boost0% to 35%0% to 100%Zero hertz voltageMaximum voltageFixed at 100%0% to 100%Maximum allowablemodulation index valueVbus decel valueFixed at 110%of nominal Vbus0% to 143% of nominal VbusVbus readings above this valueresult in reduced deceleration.Vbus Rbrake valueFixed at 110%of nominal Vbus0% to 143% of nominal VbusVbus readings above this valueresult in the R brake pinbeing asserted.Vbus brownout valueFixed at 50%of nominal Vbus0% to 143% of nominal VbusVbus readings below this valueresult in an undervoltage fault.Vbus overvoltage valueFixed at 125%of nominal Vbus0% to 143% of nominal VbusVbus readings above this valueresult in an overvoltage fault.In Host Mode, remote control over the Internet is even possible. By running a separate server application(also available from Freescale) connected to the MC3PHAC, a remote computer running theaforementioned host software can control a motor at one location in the world from another location in theworld.An example circuit utilizing the MC3PHAC in Host Mode is illustrated in Figure 5.Figure 6 shows a GUI utilizing Freescale’s interface host software to control the MC3PHAC.Ready-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 0Freescale Semiconductor7

Modes of OperationR2 can be made of severalresistors in series to preventvoltage arking. Limit voltageacross a resistor to 150vCONNECT TO HIGH VOLTAGE BUS 5V (A)R2R22R1Install JP1 for 0 - 128 HzRemove JP1 for 0 66 Hz6.8k3.3kC3JP6Base speed 50 Hz, - PWM polarityR34700 pFBase speed 50 Hz, PWM polarityAJP9Base speed 60 Hz, PWM polarityA 5V 5VF13R2 C11uFR14Acceleration Input4.7kR29Speed Input3OpenR15R2 and R3 divider computed for3.5 volts nominal @ DC BUSpinJP8Base speed 60 Hz, - PWM polarity1 5V (A)6.8kJP7R85.0k2OpenR16A1ACCELERATA IO 5V 410M4.00MHzC50.1uFConnect to theinverter'sgate drivers234567891011121314DC BUSACCELSPEEDMUX INVrefRESETVddaVssaSTARTFW DOSC2OSC1PLLCAPVssVddPWMPOL BASEFREQPWM U TopPWM U BotPWM V TopPWM V BotPWM W TopPWM W BotVBOOST MODEDT FAULTOUTRBRAKERETRY/TxdPWMFREQ/ RxdFAULTINU2Using a 4.00 MHzresonator withbuilt-in capacitorsR32MC3PHAC1 5V282726252423C9.022 uFC8.022 uFC7.022 18Voltage Boost / PCMaster SelectDead-Time / Fault Out1716Spd Range / Retry / TxDPWM Frequency / RxDConnect to Resistive BRAKE Driver 5VR20SW2Fwd/RevVoltage BoostR2115Connect to optional fault circuitIf no external fault isrequired, connect theFAULTIN pin to digitalgroundDead TimeR12Speed RangeR13PWM FrequencyFigure 5. Schematic of the MC3PHAC in Host ModeFigure 6. Host Software GUI Interface used with MC3PHACReady-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 08Freescale Semiconductor

Bus Ripple Cancellation5Bus Ripple CancellationIn many AC drives, the inverter is powered from a DC bus with a large capacitor connected in parallelacting as an energy reservoir. To prevent fluctuations on the bus from disturbing the motor waveforms, thiscapacitor is often oversized, especially if a standard rectifier converter powers the bus. These fluctuationsmay be the result of voltage surges on the AC mains, regeneration resulting from fast deceleration of themotor, or even higher frequency ripple resulting from rectification of the AC line. Due to the high busfeedback sampling frequency with the MC3PHAC, all of these distortions can be compensated for. Every189 or 252 µs, depending on the PWM frequency, the DC BUS input pin is sampled, and the reading isused to compensate the modulation index in real time to regulate the motor current. While many AC drivesimplement a similar function, they can only compensate for lower frequency distortions since they samplethe bus voltage too infrequently to permit real-time ripple rejection.Referring to Figure 7, assuming that the transistors are driven in a complimentary fashion with zerodead-time, the equation defining the average voltage of the output waveform is given by:vo (t ) t h (t )Vbus (t )TEquation 1where : vo ( t ) is the average output voltageth ( t ) is the high time of the PWM waveformT is the PWM periodVbus ( t ) is the voltage of the DC busVVbusbusthVVbusbusGNDTFigure 7. PWM Waveforms Generated from a Half-BridgeReady-to-Use AC Induction Motor Controller IC for Low-Cost Variable Speed Applications, Rev. 0Freescale Semiconductor9

Bus Ripple CancellationNotice that equation 1 does not assume that Vbus is a constant, but rather a function of time t. However,let’s assume that there exists an optimum value of Vbus that we will call Vnorm such that when Vbus(t)equals Vnorm, then vo(t) will equal the desired value vo(t)norm based on the specified PWM high time andperiod. However, when Vbus differs from Vnorm, it is still possible to drive vo(t) to equal vo(t)norm byapplying a correction factor to the modulation term th(t)/T in equation 1, as indicated in equation 2.vo (t )norm V t h (t ) norm Vbus (t ) V (t ) busTEquation 2where : Vnorm is the optimum or reference value for Vbus (t )[ ] term is the correction factorSince the Vbus(t) terms cancel out, we see that any perturbations in the bus voltage do not affect the outputvoltage. Also, since the ratio th(t)/T will always be a positive fractional value, we must make sure thatwhatever waveform is desired on the output will be properly scaled and biased to reflect this. For example,if sinusoidal modulation is desired, then the sine wave amplitude should be scaled so as to not exceed apeak-to-peak value of 1, and the waveform should be biased at ½ in order to achieve full utilization of thedynamic range. If we modify equation 2 to reflect this, and accounting for all three phases of the output,we obtain: 1 M2π ( x 1) Vnorm vo (t , x )norm sin ω o t Vbus (t )3 Vbus (t ) 2 2Equation 3where: x is the output phase number ( 1, 2, 3 )ωo is the frequency of the output waveformsM is the modulation index (0 through 1)Equation 3 results in total bus ripple cancellation of the output waveforms. However, this is not the optimalsituation because the output waveforms are biased around a fixed voltage of ½ Vnorm, NOT ½ Vbus(t), asthey should be. Note that the modulation waveform consists of two terms; a DC term of ½, and an AC sineterm. In equation 3, the correction is being applied to BOTH terms, when in fact, it s

between timer tics), and generates six center-aligned PWMs in three groups of complimentary PWMs. This allows the MC3PHAC to connect directly to inverters that are indigenous to almost all three-phase AC motor drives, as shown in Figure 1. The polarity of the high-side PWM signals can be specified independently from the low-side PWM polarities.

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