MMB 5th MM: Control of AC Machines

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5th mini module

Principles for field-oriented control

1. Block diagram of current-fed IM2. Principles for field-oriented control: decoupled flux and torque control3. The current-controlled voltage-source converter4. Direct and indirect rotor flux oriented control5. Other variants of flux-oriented control

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1. Block diagram of current-fed IM

The general model of the IM

Model in a synchronous rotating coordinate system (x, y)

C Setting ? K = ? s implies

C The special case with ? K = ? s? (the angular speed of the rotor flux ? r) is now studiedLet and

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C For a stator current fed machine (i.e. is controlled externally), we get

C Splitting into x- and y-components:

where

is the slip frequency

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Current and rotor flux space vectors in different reference frames

C d is called the load angle

C It follows that:

which implies

MMB 5th MM: Control of AC Machines

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Block diagram of current-fed IM

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2. Principles for field-oriented control: decoupled flux and torque control

C We now know that in rotor-flux coordinates

where is the rotor time constant

C Typically Tr is in the range of 0.11.0 sec

C Therefore, the instantaneous torque should be controlled by controlling isy

C The rotor-flux ? r is controlled by isx only (through a first-order lag element)

C The idea of FOC is to control torque and flux separately by decoupled control ofthe stator currents components isx and isy

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C Notes:

1. Here, the idea of FOC was explained in rotor-flux coordinates

2. Decoupled control of torque and flux may also be realized in a reference frame attached to

either the stator flux or the magnetizing flux space vector

3. Under ideal conditions, the same performance may be obtained

4. However, practical concerns, such as detection/estimation of ?s, parameter sensitivity, etc.,

may render performance differences in practical applications

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3. The current-controlled voltage-source converter

C Most FOC strategies require a current-fed AC machineY controllable current source is needed (both phase and amplitude)

C For a VSI with fsw > 1 kHz this leads to inner current loops with

1. high bandwidth

2. no steady-state error

C Input: stator current references (3 or 2 phase)Output: stator voltage vector

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C Using stator current control, we get:

1. control of the instantaneous stator current

2. no over-current trips

3. a possibility to get very good dynamic performance

4. sinusoidal currents in steady state

5. a free compensation for VSI non-linearities

C Many different principles for current control exist, including:

1. hysteresis and bang-bang

2. linear series controller (typically PI)

3. predictive schemes

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Three free-running hysteresis controller

+ Simple, robust, very fast

The inverter switching frequency depends on (a) the dc-link voltage, (b) the load, and (c) H-band

Mutually un-synchronized switching instants on phase A, B, and C

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Stationary reference frame hysteresis controller

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Typical waveforms

Number of switchings

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Synchronous reference frame linear controller

+ Complies with the theory of linear system making a theoretical analysis simple

+ Excellent steady-state performance. Good dynamics. Uses standard PWM

Not quite as fast as hysteresis controllers

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4. Direct and indirect rotor flux oriented controlDirect rotor-flux oriented controller

C Main difficulty: flux estimation

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Indirect rotor-flux oriented controller

C Position feedback requiredC Sensitive to rotor time constant variations

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5. Other variants of flux-oriented control

C Two main strategies have been reviewed: (a) direct (feed back) and

(b) in-direct (feed forward)

rotor-flux oriented control

C Both give (under ideal conditions) the same performance, i.e.(a) flux and

(b) torque

can be regulated independently of each other

C A fast current control loop is required to convert a voltage-source into a current source

C Many other strategies exist

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Example: Direct torque and flux control

Principle

C The VSI impresses the stator-flux, because

implies

for rs = 0 O

C Hence, by controlling ? s, we control the torque also

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C The torque may be controlled by(a) active forward vector (stator field acceleration)

(b) active backward vector (stator field backward acceleration)

(c) zero vector: the stator field stops

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Basic control structure

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C Note the rapid phase reversal of the stator flux

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Summary of DTC

C Excellent torque dynamicsC No current controllers (and no build-in current limiters)

C No pulse-width modulatorC No transformations to/from a synchronous reference frame

C Flux and torque estimation required (like in direct FOC)C Nearly sinusoidal stator flux (depends on the hysteresis band)

C The average switching frequency depends on the hysteresis settingsC High sampling frequency required (40 kHz, typically)