EEE 471Brushless DC Motor (BLDC)

Dr. A Hasib ChowdhuryDept. of EEE, BUET

August 2014

Brushless DC Motor (BLDC)

• Conventional dc motors are highly efficient • Their characteristics make them suitable for use as

servomotors: current and torque, voltage and rpm are linearly related

• Functions of commutator and brushes implemented by solid-state switches brushless dc motor maintenance-free motor

• Formal definition: A BLDC is a synchronous electric motor powered by dc electricity – has an electronically controlled commutation system, instead of

a mechanical commutation system based on brushes

Brushless DC Motor (BLDC) contd.

• Combines advantages of PMDC motor and synchronous motor– No excitation losses, and no wear out– Low reactive current, similar to DC motor, current

proportional to torque– Measuring shaft torque is easy by detecting DC current

in DC link– High supply voltage allowed

• only limited by power transistors in frequency inverter and winding insulation of motor

– Like DC motor, BLDC needs speed control and speed sensor– Like synchronous motor, BLDC needs its frequency inverter

Basic Structures• It is an inside out dc motor

– Armature on stator instead of rotor commutator do not need rotating parts, electronic commutator used

– Rotor is a permanent magnet no power supply needed

Structure of a typical three-phase BLDC

Basic Structures contd.

• Stator windings similar to those in a polyphase ac motor

• Rotor composed of one or more permanent magnets

• BLDC different from ac synchronous motors incorporates means to detect rotor position to produce signals to control electronic switches

• Most common position/pole sensor is Hall element, some motors use optical sensors

• Sensorless control also possible done by measuring back EMF in undriven coils to infer rotor position

Variation in Construction

Several different physical configurations

Conventional (also known as 'inrunner') configuration

• Permanent magnets mounted on spinning armature (rotor)

• Three stator windings surround rotor

Outrunner configuration

• Radial-relationship between coils and magnets reversed; stator coils form center (core) of motor

• Permanent magnets spin on an overhanging rotor which surrounds core

Flat type

• Stator and rotor plates, mounted face to face used where there are space or shape limitations

Outrunners typically have more poles, set up in triplets to maintain three groups of windings, and have a higher torque at low RPMs

Variation in Construction contd.

• Two electrical configurations having to do with how windings are connected to each other (not their physical shape or location) – Delta configuration – Wye configuration

• Delta configuration gives low torque at low rpm, but can give higher top rpm

• Wye configuration gives high torque at low rpm, but not as high top rpm

• Wye winding normally more efficient– At any given time, two-thirds of windings in a delta configuration are

running at half the voltage of the other, which has an impact on efficiency– Wye winding always powers only two windings in series, so higher

voltages (or lower-resistance windings) can be used

Variation in Construction contd.

• Two-phase BLDCs also very commonly used for simple construction and drive circuits

Cross section of a two-phase motor having auxiliary salient poles

Advantages Over Brushed DC Motors

BLDC motors and conventional dc motors are similar in static characteristics, but remarkable differences in some aspects

Conventional dc motor commutation by brushes and commutatorBLDC motor commutation by using semiconductor devices (eg. transistors)• Higher efficiency and reliability• Reduced noise• Longer lifetime (no brush erosion)• Elimination of ionizing sparks from commutator• Overall reduction of electromagnetic interference (EMI) • Electromagnets can be cooled by conduction to motor casing, requiring no

airflow inside motor for cooling so motor's internals can be entirely enclosed

• Maximum power that can be applied to a BLDC motor is exceptionally high, limited almost exclusively by heat

Main Disadvantage

• Higher cost BLDC motors require complex electronic speed controllers to run

• Many practical uses have not been well developed in commercial sector

• Under high mechanical loads, BLDC motors and high-quality brushed motors are comparable in efficiency

Comparison of Conventional and Brushless DC Motors

Drive Circuits

Unipolar drive

A simple 3-phase unipolar-operated motor(optical sensors as position detectors)

Drive Circuits contd.• Phototransistors PT1, PT2, PT3 placed at 1200 intervals

– Exposed to light in sequence through a revolving shutter• As rotor S-pole faces P2, PT1 detects light and turns transistor Tr1 on • S-pole, created at pole P1 by current in winding W1, attracts rotor N-pole

to move it in the direction of arrow• When N-pole comes to position to face pole P1, shutter shades PT1, and

PT2 will be exposed to light transistor Tr2 turned on

Drive Circuits contd.• A current flows through winding W2, creates a S-pole on P2• Rotor N-pole revolves in the direction of arrow and face P2• At this moment, shutter shades PT2, and PT3 exposed to light• These actions steer current from winding W2 to W3• Thus P2 de-energized, while P3 energized creates S-pole• Hence N-pole on rotor further travels from P2 to P3 without

stopping• By repeating such a switching action in sequence, permanent

magnet rotor revolves continuously

Drive Circuits contd.

Switching sequence and rotation of stator's

magnetic field

Drive Circuits contd.

Bipolar drive• Efficiency highest

when a 3-phase BLDC motor is driven by a 3-phase bridge, since current flows through each winding

• This drive is called 'bipolar drive' (since a winding is alternately energized in south and north poles)

Three phase bipolar-driven BLDC motor

Drive Circuits contd.• 6 phototransistors placed at equal intervals to detect rotor position• These photo elements exposed in sequence to light through a revolving

shutter• The relation between the ON/OFF state of the transistors and the light

detecting phototransistors are quite complex. • The simplest relation is set when the logic sequencer is arranged in such a

way that when a phototransistor marked with a certain number is exposed to light, the transistor of the same number turns ON.

• Fig.6 shows that when currents flow through Tr1, Tr4, and Tr5, terminals U and W have the battery voltage, while terminal V has zero potential.

• In this state, a current will flow from terminal U to V, and another current from W to V as illustrated in Fig.7.

Motor from a 3.5" floppy disk drive. The coils, arranged radially, are made from copper wire coated with blue insulation. The rotor (upper right) has been removed and

turned upside-down. The grey ring inside its cup is a permanent magnet.

The four poles on the stator of a two-phase brushless motor. This is part of a computer cooling fan; the rotor has been removed.