© 2002 by CRC Press LLC

10

AC MachinesControlled asDC Machines(Brushless DC

Machines/Electronics)

10.1 Introduction10.2 Machine Construction

Permanent Magnets • Stator Windings

10.3 Motor Characteristics

Mathematical Model

10.4 Power Electronic Converter

Unipolar Excitation • Fault-Tolerant Configuration • Current Source Inverter

10.5 Position Sensing

Position Sensorless Control

10.6 Pulsating Torque Components10.7 Torque-Speed Characteristics10.8 Applications

10.1 Introduction

Brushless DC (BLDC) motors are synchronous motors with permanent magnets on the rotor andarmature windings on the stator. Hence, from a construction point of view, they are the inside-out versionof DC motors, which have permanent magnets or field windings on the stator and armature windingson the rotor. A typical BLDC motor with 12 stator slots and four poles on the rotor is shown in Fig. 10.1.

The most obvious advantage of the brushless configuration is the removal of the brushes, whicheliminates brush maintenance and the sparking associated with them. Having the armature windings onthe stator helps the conduction of heat from the windings. Because there are no windings on the rotor,electrical losses in the rotor are minimal. The BLDC motor compares favorably with induction motorsin the fractional horsepower range. The former will have better efficiency and better power factor and,therefore, a greater output power for the same frame, because the field excitation is contributed by thepermanent magnets and does not have to be supplied by the armature current.

These advantages of the BLDC motor come at the expense of increased complexity in the electroniccontroller and the need for shaft position sensing. Permanent magnet (PM) excitation is more viable insmaller motors, usually below 20 kW. In larger motors, the cost and weight of the magnets become

Hamid A. Toliyat

Texas A&M University

Tilak Gopalarathnam

Texas A&M University

© 2002 by CRC Press LLC

excessive, and it would make more sense to opt for excitation by electromagnetic or induction means.However, with the development of high-field PM materials, PM motors with ratings of a few megawattshave been built.

10.2 Machine Construction

BLDC motors are predominantly surface-magnet machines with wide magnet pole-arcs and concentratedstator windings. The design is based on a square waveform distribution of the air-gap flux densitywaveform as well as the winding density of the stator phases in order to match the operational charac-teristics of the self-controlled inverter [1].

Permanent Magnets

BLDC motors obtain life-long field excitation from permanent magnets mounted on the rotor surface.Advances in permanent magnet manufacturing and technology are primarily responsible for loweringthe cost and increasing the applications of BLDC motors. Ferrite or ceramic magnets are the most popularchoices for low-cost motors. These magnets are now available with a remanence of 0.38 T and an almoststraight demagnetization characteristic throughout the second quadrant. For special applications, mag-netic materials with high-energy products such as neodymium-iron-boron (Nd-Fe-B) are used. The highremanence and coercivity permit marked reductions in motor frame size for the same output comparedwith motors using ferrite magnets. However, the size reduction is at the expense of increased cost of themagnets.

The primary considerations while choosing the magnetic material for a motor are the torque per unitvolume of the motor, the operating temperature range, and the severity of the operational duty of themagnet [2]. For maximum power density, the product of the electric and magnetic loadings of the motor

FIGURE 10.1

Three-phase BLDC motor with four poles on the rotor and 12 stator slots.

© 2002 by CRC Press LLC

must be as high as possible. A high electric loading necessitates a long magnet length in the direction ofmagnetization and a high coercivity. A high power density also requires the largest possible magnetvolume. Exposure to high temperatures tends to deteriorate the remanent flux density and coercive forceof permanent magnets. Hence, the highest operating temperature must be considered while choosingthe magnets. Magnets can also be demagnetized by fault currents such as short-circuit currents producedby inverter faults. Hence, protective measures are usually taken in the inverter and control electronics tolimit the magnitude of the armature currents to a safe value.

The magnets are constructed in the form of arcs, radially magnetized, and glued onto the surface ofthe rotor with adjacent rotor poles of opposite magnetic polarity as shown in Fig. 10.1. The number ofrotor poles is inversely proportional to the maximum speed of rotation, and is frequently chosen to meetmanufacturing constraints. Most BLDC motors have four, six, or eight poles, with four the most popularchoice.

Stator Windings

BLDC motors are often assumed to have three phases, but this is not always the case. Small motors forapplications such as light-duty cooling fans have minimal performance requirements, and it is cost-effective to build them with just one or two phases. On the other hand, it is preferable to use a highphase number for large drives with megawatt ratings. This reduces the power-handling capacity of asingle phase, and also incorporates some degree of fault tolerance. Machines with as many as 15 phaseshave been built for ship propulsion. Although these are special-purpose designs, motors with four andfive phases are quite common.

The number of stator slots is chosen depending on the rotor poles, phase number, and the windingconfiguration. In general, a fractional slots/pole design is preferred to minimize cogging torque [3]. Themotor of Fig. 10.2 has six slots, which is not a multiple of the number of poles, and is hence a fractionalslots/pole design. The windings could be lap-wound or concentric-wound, and the coil span could befull-pitch or short-pitch, depending on the crest width of the back-emf desired. There are virtually infinite

FIGURE 10.2

Three-phase BLDC motor with six slots and four poles.

A+

A−

A+

A−

C−

C−

C+

C+

B−

B−

B+

B+

© 2002 by CRC Press LLC

combinations of the above design factors, and it is up to the ingenuity of the designer to select one thatis best suited to the inverter characteristics and meets design specifications.

10.3 Motor Characteristics

The air-gap flux-density waveform is essentially a square wave, but fringing causes the corners to besomewhat rounded. As the rotor rotates, the waveform of the voltage induced in each phase with respectto time is an exact replica of the air-gap flux-density waveform with respect to rotor position. Becauseof fringing, the back-emf waveform takes on a trapezoidal shape. The shape of the back-emf waveformdistinguishes the BLDC motor from the permanent magnet synchronous motor (PMSM), which has asinusoidal back-emf waveform. This has given rise to the terminology “trapezoidal motor” and “sinusoidalmotor” for describing these two permanent magnet AC (PMAC) machines.

The back-emf voltages induced in each phase are similar in shape and are displaced by 120

°

electricalwith respect to each other in a three-phase machine. By injecting rectangular current pulses in each phasethat coincides with the crest of the back-emf waveform in that phase, it is possible to obtain an almostconstant torque from the BLDC motor. The crest of each back-emf half-cycle waveform should be asbroad as possible (

≥

120

°

electrical) to obtain smooth output torque. This condition is satisfied by the12-slot motor of Fig. 10.1 because it has full-pitched coils, but not by the six-slot motor of Fig. 10.2because the coil spans are shorter than the pole arcs. The two back-emf waveforms calculated using thefinite-element method are plotted in Fig. 10.3 and it can be seen that the six-slot motor has a smallercrest width, and is hence not suitable for 120

°

bipolar excitation. However, it can be used with otherexcitation waveforms as discussed in the section on unipolar excitation.

FIGURE 10.3

Back-emf waveforms of the 12-slot and the 6-slot motors.

0 20 40 60 80 100 120 140 160 180-50

-40

-30

-20

-10

0

10

20

30

40

50

Bac

k-em

f (V

)

Rotor position (deg)

12 slots

6 slots

© 2002 by CRC Press LLC

The ideal back-emf voltage and 120

°

phase current waveforms for a three-phase BLDC motor areshown in Fig. 10.4. The inverter switches that are active during each 60

°

interval are also shown corre-sponding to the inverter circuit of Fig. 10.5. The simplicity of this scheme arises from the fact that duringany conduction interval, there is only one current flowing through two phases of the machine, whichcan be sensed using a single current sensor in the DC link. Because there are only two inverter switchesactive at any time, this is also called the two-switch conduction scheme, as opposed to the three-switchconduction scheme used in PMSM motor drives.

The amplitude of the phase back-emf is proportional to the rotor speed, and is given by

(10.1)

FIGURE 10.4

Back-emf and phase current waveforms for three-phase BLDC motor with 120

°

bipolar currents.

FIGURE 10.5

Schematic of IGBT-based inverter for three-phase BLDC motor.

EI Phase A

Phase B

Phase C

360180

S5S6

S1S6

S1S2

S3S2

S3S4

S5S4

S5S6

Activeswitches

T eTorque

B L D CMotor

S1 S3 S5

S4 S6 S2

D 2D6D4

D1 D3 D5

ph A

ph B

ph C

+

-

Vdc

E kfwm=

© 2002 by CRC Press LLC

where

k

is a constant that depends on the number of turns in each phase,

φ

is the permanent magnetflux, and

ω

m

is the mechanical speed.During any 120

°

interval, the instantaneous power being converted from electrical to mechanical isthe sum of the contributions from two phases in series, and is given by

(10.2)

where

T

e

is the output torque and

I

is the amplitude of the phase current. From Eqs. (10.1) and (10.2),the expression for output torque can be written as

(10.3)

where

k

t

is the torque constant. The similarity between the BLDC motor and the commutator DC motor can be seen from Eqs. (10.1)

and (10.3). It is because of this similarity in control characteristics that the trapezoidal PMAC motor iswidely known as the BLDC motor, although this term is a misnomer as it is actually a synchronous ACmotor. But it is also not a rotating field machine in the AC sense, because the armature mmf rotates indiscrete steps of 60

°

electrical as opposed to a smooth rotation in other AC machines.

Mathematical Model

Because of the nonsinusoidal nature of the back-emf and current waveforms, transformation of themachine equations to the

d

-

q

model is cumbersome, and it is easier to use the phase-variable approachfor modeling and simulation. The back-emf can be represented as a Fourier series or by using piecewiselinear curves [4]. The circuit equations of the three windings in phase variables can be written as [4]

(10.4)

where

v

a

,

v

b

,

v

c

are the phase voltages,

i

a

,

i

b

,

i

c

are the phase currents,

e

a

,

e

b

,

e

c

are the phase back-emfvoltages,

R

is the phase resistance,

L

is the self-inductance of each phase, and

M

is the mutual inductancebetween any two phases.

The electromagnetic torque is given by

(10.5)

where

ω

m

is the mechanical speed of the rotor.The equation of motion is

(10.6)

where

T

L

is the load torque,

B

is the damping constant, and

J

is the moment of inertia of the drive.The electrical frequency is related to the mechanical speed by

(10.7)

where

P

is the number of rotor poles.

Po wmTe 2EI= =

Te 2kfI ktI= =

va

vb

vc

R 0 0

0 R 0

0 0 R

ia

ib

ic

L M– 0 0

0 L M– 0

0 0 L M–

ddt-----

ia

ib

ic

ea

eb

ec

++⋅=

Te eaia ebib ecic+ +( )/wm=

ddt----- wm Te TL Bwm––( )/J=

we P/2( )wm=

© 2002 by CRC Press LLC

10.4 Power Electronic Converter

BLDC motor drives require variable-frequency, variable-amplitude excitation that is usually provided bya three-phase, full-bridge inverter as shown in Fig. 10.5. The switches could be BJTs, MOSFETSs, IGBTs,or MCTs. The decreasing cost and drastic improvement in performance of these semiconductor deviceshave accelerated the applications of BLDC motor drives. The inverter is usually responsible for both theelectronic commutation and current regulation [5]. The position information obtained from the positionsensors is used to open and close the six inverter switches. For the given phase current waveforms, thereare only two inverter switches—one upper and one lower that conduct at any instant, each for 120

°

electrical. If the motor windings are star-connected and the star point is isolated, the inverter inputcurrent flows through two of the three phases in series at all times. Hysteresis or pulse-width-modulated(PWM) current controllers are typically used to regulate the actual machine currents to the rectangularcurrent reference waveforms shown in Fig. 10.4. Either soft chopping or hard chopping could be employedfor this purpose. The flow of currents during one 60

°

interval when switches

S

1

and

S

6

are active is shownin Fig. 10.6a for soft chopping and Fig. 10.6b for hard chopping. When

S

1

and

S

6

are in their on state,the current builds up in the path shown by the solid lines. In soft chopping, the current regulatorcommands the turn-off of switch

S

1

once the current crosses the threshold. The current then decaysthrough diode

D

4

and switch

S

6

as shown by the dashed lines. Alternatively,

S

6

could be turned off, andthe current would then decay in the loop formed by

S

1

and

D

3

. The fall time of the current can be madesmaller by hard chopping, in which both the active switches are turned off. The current then freewheelsthrough

D

4

,

D

3

, and the DC link capacitor, feeding energy back to the source. The freewheeling diodes thusprovide important paths for the currents to circulate when the switches are turned off and during thecommutation intervals.

The discussion thus far has concentrated on the operation of the BLDC machine as a motor. It can,however, operate equally well as a generator. The polarity of the torque can be reversed by simply reversingthe polarity of the phase current waveforms with respect to the back-emfs. This can be used to advantagefor regenerative braking operation, in vehicle propulsion, for example. Special arrangements may needto be made in the power converter to accept the energy returned by the machine, as conventional diodebridge rectifiers are incapable of feeding energy back to the AC supply. The situation is considerablysimplified if the source is a battery, as in automotive applications.

Unipolar Excitation

Unipolar current conduction limits the phases to only one direction of current, and the commutationfrequency is half that of a bipolar or full-wave drive. The unipolar motor needs fewer electronic partsand uses a simpler circuit than the bipolar motor. For these reasons, unipolar-driven motors are widelyused in low-cost instruments. A typical application of BLDC motors of this class can be found in diskmemory apparatus [6]. Unipolar excitation results in an inefficient winding utilization compared withbipolar excitation, but they have the following advantages over bipolar circuits [7]:

FIGURE 10.6

Illustration of soft chopping (a) and hard chopping (b) for current regulation.

-+

a. b.