Specifications

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Motor Control

Overview

96 Maxim Industrial Solutions

AC induction motors: simplicity

and ruggedness

An AC induction motor is popular in

industry because of its simplicity and

ruggedness. In its simplest form, this

motor is a transformer with the

primary-side voltage connected to

the AC-power-voltage source and the

secondary side shorted to carry the

induced secondary current. The name

“induction” motor derives from this

induced secondary current. The stator

carries a three-phase winding and the

rotor is a simple design, commonly

called a “squirrel cage,” in which the

copper or aluminum bars are short-

circuited at both the ends by cast-

aluminum end rings. The absence of

rotor windings and brushes makes

this motor design especially reliable.

Rotor and stator of an induction motor.

When operated from the 60Hz voltage,

the induction motor operates at a

constant speed. However, when power

electronics and a microprocessor-

based system are used, the motor’s

speed can be varied. The variable-

speed drive consists of an inverter,

signal conditioner, and microprocessor-

based control. The inverter uses three

half bridges in which the top and the

bottom switch are controlled in a

complementary fashion. Maxim

offers multiple half-bridge drivers

like the MAX15024/MAX15025

which control the top and bottom

FETs independently.

Precise measurement of three-

phase motor current, rotor position,

and rotor speed are necessary for

efficient closed-loop control of

an induction motor. Maxim offers

many high-side and low-side current

amplifiers, Hall-effect sensors, and

simultaneous-sampling analog-

to-digital converters (ADCs) to

accurately measure these parameters

in the harshest environments.

A microprocessor uses data on the

current and position to generate

logic signals for the three-phase

bridge. A popular closed-loop

control technique called vector

control decouples the vectors of field

current from the stator flux so that it

can be controlled independently to

provide a fast transient response.

Brushless DC motors: high

reliability and high-output power

A brushless DC (BLDC) motor has

neither commutator nor brushes, so

it requires less maintenance than a

DC motor. It also offers more output

power per frame size compared to

induction and DC motors.

The stator of the BLDC motor is quite

similar to that of the induction motor.

The BLDC motor’s rotor, however,

can take different forms, but all are

permanent magnets. Air-gap flux

is fixed by the magnet and is unaf-

fected by the stator current. The

BLDC motor also requires some

form of rotor position sensing. A

Hall-effect device embedded in the

stator is commonly used to sense

the rotor’s position. When the rotor’s

magnetic pole passes near the Hall-

effect sensors, a signal indicates

whether the north or the south pole

passed. Maxim offers several Hall-

effect sensors like the MAX9641*,

which simplifies designs and reduces

system costs by integrating two

Hall-effect sensors and digital logic

to provide both positional and

directional outputs of the magnet.

The importance of sensors,

signal conversion, and data

interfaces

Several types of sensors provide

feedback information in the motor-

control loop. These sensors also

improve reliability by detecting

fault conditions that can damage

the motor. The following sections

examine the role of sensors in motor

control in greater detail. Specific

attention will be given to current-

sense amplifiers, Hall-effect sensors,

and variable-reluctance (VR) sensors.