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EE 442-642

(AC) Motor Drives

Fall 2012

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Control Structure of Drives

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Servo Drives

The basic structure is the same regardless of the

drive that is selected

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An Example of Adjustable Speed Drives

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Two Commonly-Used Coupling Mechanisms

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Simplified Circuit of a Drive

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Control of Servo Drives

The structure is application dependent

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Limiters in the Control Structure

By providing ramp limiters, for example, a drive can be

prevented from “tripping” under sudden changes.

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AC Motor Drives: Pump Application - Adjustable

Flow rate

Fixed versus adjustable speed drive

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AC Motor Per-Phase Representation

Assuming sinusoidal steady state

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Important Relationships in an Induction Machine

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Torque-Speed Characteristics

The linear part of the characteristic is utilized in

adjustable speed drives

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Plot of Normalized Rotor Current

It increases with slip and slip frequency

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Acceleration Torque at Startup

Intersection represents the equilibrium point

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Torque Speed Characteristics at various Frequencies of

Applied Voltage

The air gap flux is kept constant

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Adjusting Speed of a Centrifugal Load

The load torque is proportional to speed squared

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Phasor Diagram at Small Value of Slip Frequency

The rotor branch is assumed to be purely resistive

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Voltage Boost to Keep Air Gap Flux at its Rated Value

Depends on the torque loading of the machine

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Induction Motor Drive Capability Curves

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Generator Mode of Operation

Rotor speeds exceed the synchronous speed

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Regenerative Braking Mode to Slow Down

Machine is made to go into the generator mode

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Per-Phase Equivalent Circuit at Harmonic Frequencies

The magnetizing branch is ignored

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Torque Pulsations due to Harmonics

Rotations of fields due to the fifth and the seventh

harmonics are in opposite directions

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Classification of Converter Systems

PWM-VSI is now most commonly use

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PWM-VSI System

Diode rectifier for unidirectional power flow

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PWM-VSI System

Options for recovered energy during regenerative

braking

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General-Purpose Speed Controller

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Change in Switching Frequency based on the required

Fundamental Frequency

Can be significant in large power ratings

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Static Slip Recovery (Type 3 Wind Gen.)

Applications in very large power ratings where the speed

is to be adjusted over a very limited range