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#EEwebinar

This webinar will be available afterwards at www.eeworldonline.com & email

Q&A at the end of the presentation

Hashtag for this webinar: #EEwebinar

Before We Start

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Meet your Speakers

MODERATOR FEATURED SPEAKERDal Y. OhmPresidentDrivetech, Inc.

Aimee KalnoskasModeratorEE World

#EEwebinar 4

FOC (Field oriented Control) for AC motors• Synchronously rotating reference frame

o Analogy: Geo-centric vs Helio-centric view• AC motor current can be divided into two components

o Torque producing current componento Field flux current component

• PM motor – Zero normally, Nonzero for IPM• Induction motor – Magnetization Current

• Independent control of two current components• Operation is very similar to separately excited DC motors

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Frame Transform (1) - Clarke• 3 Phase to 2 Phase Transform

S 1 cos120 cos120 Sa S =1.5 0 -sin120 sin120 Sb So 0.5 0.5 0.5 Sc

• All polyphase currents produceso Rotating mag flux!

• Equivalent 2-Phase Machine• Identical magnitude, Reversible• Not invariant power

Sa,S

S

Sb

Sc

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Frame Transform (2) - Park• Coordinate attached to the rotor

(rotating)o D-axis on rotor N poleo Q-axis on N-S mid point

Id cos sin Ix Iq = - sin cos Iy

Ix cos - sin Id Iy = sin cos Iq

x

+yd

q

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Dynamic Equations of Brushless Motors• Voltage Model in Synchronous Frame

Vq = (Rs + Ls p) Iq + Ls Id + mVd = (Rs + Ls p) Id - Ls Iq

• Correspondent to DC Motor equation• Can be extended to Induction Motors

m

+

Rs Ls

+-

VqIq +

Ls Id

Rs Ls

+-

VdId

Ls Iq

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Synchronous Current Regulator (SR)

o Zero steady-state phase error with PI control“Internal Model Principle”

o Id* is nonzero for phase angle advanceo Easy to add back emf & cross-coupling compensation

PWM (SVM)

VaVbVc

IaIbIc

eCross-

Coupling Compensation

Inverse Park

RegulatorPI

RegulatorPI

Iq* + -

Id* + -

Iq

Id

Vd

Vq

e

Vector Saturatio

nAlgorith

m

Inverse

Clarke

V

V

ClarkePark

I

Ie

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Characteristics of FOC Drives• Inherent “zero steady-state tracking error”• Simple to add “disturbance feed-forward”

o Back emf and cross-coupling• Results

o Accurate commutation at any speed!!o Maximum torque per ampere

• With nonzero Id* for IPMo Improved Dynamicso Induction motor flux weakening by changing Id*

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Disturbance Feedforward Techniques

• Compensates for Back emf and cross-coupling terms

• Quick integrator response – Improved performance

Transform

abc -> dq

PI

PI

Transform

dq -> abc

IaIbIc

Va*Vb*Vc*

-+

-+

Iq*

Iq

Id*

Vq*

Id

Vd*

e - Lq Iq

Ld Id + m

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Analytic Tuning of Velocity PI (1)• Used for initial gain before fine tuning• Based on simple 1st order plant model• (1) Cancellation Tuning for bw of v

Kp( s + i)s

KtJm s + Bm

+-

c

Gc(s) = 1/{Jm / (Kp Kt) s + 1} v = (Kp Kt / Jm)

Kp = v (Jm / Kt), i = Bm / Jm

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Analytic Tuning of Velocity PI (2)(2) Pole placement Tuning

o Based on critically damped poleso No control of closed-loop zeros

Kt ( Kp s + i) Jm s2 + (Kt Kp + Bm) s + Kt iGc(s) =

Kp = 2 v (Jm / Kt), i = (1/2) v

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Gains for Per Unit Systems• Conversion of S.I. Gains to Per Unit Systems• Kp, i, max, Imax are in SI units• Kp’, max’, Imax’ are in per unit units• i’ = i, Kp’ from two identical block equations• Note: id = i Ts (digital controller gain)

Kp( s + i)s

i*

Kp’( s + i’)s

i* max’max

ImaxImax’

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Tuning of Current Loop

• Cancellation tuningKp = c Lq, i = (Rs/Lq)

• Pole-placement tuningKp = 2 c Lq, i = c

2 Lq• Gain translation for Per-unit systems

– similar to Velocity-loop

Kp( s + i)s

(1/Rs)(Lq/Rs) s + 1

+-

IqIq* Vq

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Low-cost Sensors & Sensorless Control• Hall sensors (with linear interpolation)• Magnetic Rotary Hall Position Sensors (AMS, Avago) • Sensoless - Estimation of Flux Angle & Velocity

o Popular technologiesVemf detection (6-step)Model-based Observer (Estimator)Carrier injection (IPM, Audible noise)

o PerformanceBW limited, limited accuracyChallenges at startup and near-zero speed, high torque

applicationModel of nonlinearities (Extra voltage drops)Algorithm selection and tuning – key to success

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Carrier Injection Method (IPM)

Current Regulator

PWMVoltageInverter

LPF

BPF

HeterodyningCalculation

LuenbergerObserver

FrameTransform

InverseTransform

Carrier VoltageInjection (0.5 – 2

kHz)

+ -

θr^

θr^

Iqds

θr^

VqdsIqd*

Iqde

motor

(Sensored Block)

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Observers (Linear or Sliding mode)

• Applied to 2-ph stationary quantities (I, I, E, E, etc.)o 2-ph Synchronous variables can also be used.

• Luenberger: Select K so that observer bw is >>10X motor model

• Sliding mode: Sliding gain L > max(E, E), K = o Chattering, heavy LPF

• Atan() or Phase Locked-Loop

MotorModel

KIs~ +

Is-V

s Es s

(Optional outer loop)

(Saturate to +/-L)

LPFATAN(

)

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Sensorless Start-up Strategies• Open-loop forced start

o Kt Iqs = J d/dt + TL o Startup current Iqs and acceleration rate should beo Slightly higher than worst case J and TL

• Inductance-based angle detection for motors with high ΔLo V – IR = L di/dto Voltage injection with Current rise-time measurment

• Carrier Injection Startup for motors with high ΔL

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Power Factor Correction (PFC)

(Pavg)p.f. = ---------------------------

(Vrms) (Irms)• A figure of merit that measures how effectively energy is

transmitted• p.f = (Is1/Is) cos

o Distortion factor (Is1/Is) o Displacement factor cos

• IEEE 519 recommends <5%• Dedicated PFC Converter IC or Processor Controlled (1

Phase)

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Conclusion• High motor reduces size and total operating

cost• Sensorless or Low-cost feedback reduces cost • Proper control algorithm & optimally tuned

driveallows precise commutation to improve

efficiency, maximum torque, and dynamics• Selection of uC, Gate drives and Power

switches for low-cost and compact design

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Questions?

MODERATOR FEATURED SPEAKERDal Y. OhmPresidentDrivetech, Inc.ohm@drivetechinc.com

Aimee KalnoskasModeratorEE Worldakalnoskas@wtwhmedia.com

@DW_Aimee

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