-
A BRUSHLESS DC MOTOR FOR VEHICULAR AC/HEATER APPLICATIONS L. E.
Unnewehr, P. P i a t k w s k i , and D. Giardini
Sc ien t i f ic Research S ta f f , Ford Motor Company,
Dearborn, Michigan 48121
SUMMARY
A brushless DC motor is proposed as a p o t e n t i a l
replacement for DC commutator type motors presently used in a lmost
a l l automotive applications. A proto- type brushless motor has
been designed and assembled i n t o t h e AC blower assembly used
on many Ford vehic les i n o r d e r t o e v a l u a t e t h i s
motor in terms of perfor- mance, cost, maintenance, operating
problems, size, and weight as compared t o t h e e x i s t i n g
commutator mo- t o r used i n t h i s blower. The new motor
operates from the 12-volt vehicle battery through an electronic
con- t r o l l e r which r ep laces t he l o s sy r e s i s t i ve
con t ro l s used on the ex i s t ing blower system. The new
brushless motor is expected t o r e s u l t in the following
improve- ments over the present blower motors:
1. Elimination of the brush-connnutator system, which is present
ly a source of high maintenance and warranty cos ts on the
ACfheaters in vehicles . The motor is maintenance free, except for
bear ing w e a r . Data col- lec ted by t h e U.S. Department of
Defense f o r r e l i a b i l - i t y p r e d i c t i o n s shows t
h a t conmutator motors have a f a i l u r e rate of from two t o
six times that of brushless motors, depending upon operat ing
speed(l) .
2. Simpler construction of the b rushless machine in which,
besides eliminating the brush-coaamtator of t h e DC colllmrtator
type, a very simple, stationary, sole- no ida l electrical co i l r
ep laces t he complex, r o t a t i n g , armature winding of t h e
conmutator machine. No perma- nent magnets a r e used i n t h e f i
e l d .
3. Improved energy eff ic iency; the e lectronic control scheme
e l imina tes the energy loss in res i s tors p resent - l y used
to cont ro l speed .
4 . The e l e c t r i c a l winding in the b rushless motor is
no t r e s t r i c t ed as to shape or material, but can be adapted
t o s u i t t h e economic and t e c h i c a l r e s t r i c - t i
o n s of the t ime. Material can be copper, aluminum, or high
conductivity alloys, and constructed from round, square, str ip, or
braided conductors.
5. The motor is insensitive to t empera ture var ia t ions
compared t o pH motors.
6. Due to t he s imp le s t ruc tu re of t h e motor, t h e o v
e r a l l volume occupied by t h e blower assembly is re- duced
about 25%.
7. A damaged or burned out e lectr ical coi l can be quickly and
simply replaced without the need for re- placing any other motor
parts.
BACKGROUND
The proposed brushless motor is an adaptation of the d i sc re
luc tance (3) 9 (4) 9 (12) machines developed f o r e l e c t r i c
v e h i c l e d r i v e s a t Ford Motor Company. This development
r e su l t ed i n t he assembly and t e s t i n g of a number of
prototype motors and e l e c t r o n i c c o n t r o l l e r s and
the i s su ing of n ine U.S. patents on motor configu- r a t ions ,
e l ec t ron ic con t ro l l e r s , and posit ion sensors. During
t h i s development, i t was r ea l i zed t ha t in order to
achieve the high s tar t ing torques necessary for t ract ion appl
icat ions, considerable added cost and com- p lex i ty had to be bu
i l t i n to t he e l ec t ron ic con t ro l l e r . However, f o r
low star t ing torque appl icat ions, such as fan and cent r i fuga
l pump d r ives , t he e l ec t ron ic c i r -
cui t ry required is re la t ive ly s imple , in some cases ,
simpler than that required for the conmutator motor. A second f e a
t u r e of t h e e a r l y development which added t o t h e c o s
t and complexity of t h e d i s c motor system was the use of an
external posit ion sensing device and associated electronics.
Reluctance motors are basical- l y synchronous machines requiring
that the frequency of t he e l ec t r i ca l exc i t a t ion be kep
t in exact synchronism with the rotor mechanical speed. Subsequent
develop- ment work has eliminated the need for an external pos- i t
i on s enso r by using a low s i g n a l l o g i c c i r c u i t t
h a t senses t h e v a r i a t i o n of inductance in the machine
it- self. A thi rd complicat ing factor in t h e t r a c t i o n
mo- tor , a l so re la ted to the h igh s ta r t ing to rque requi
re - ment, i s t h a t r e l a t i v e l y lw star t ing to rque is
devel- oped i n a s ingle phase machine and t h e r e are a f ixed
number of d i scre te angular pos i t ions on t h e machine a t
which zero torque exists. Therefore, it is necessary to use two or
three phases or machine sections to in- sure s tar t -up. This
multiple-section technique multi- p l i e s t h e number of sets of
power e lec t ronic devices required in the e l ec t ron ic con t
ro l l e r by t h e same num- ber. These three f e a t u r e s a l
l tended to o f fse t the in- herently simple and lw-cos t cons t
ruc t ion of t he bas i c , single-phase disc motor. None of t hese
f ea tu re s i s necessary for low-power, low s ta r t ing to rque
appl i - cations such as blower or fan d r ives , and the bes t at-
t r i b u t e s of the disc motor , namely its claim as t h e most
simple configuration of r o t a t i n g machine known, can be used
to the fu l les t advantage .
Configurations very similar t o t h e Ford d i s c motor have
been used for severa l years as control type step- per motors , and
represent one of the cheapest type of s tepper present ly avai
lable . The f i r s t small motor appl ica t ion a t Ford,
developed af ter the t ract ion motor effort described above, was a
cen t r i fuga l fue l pump. A p i c t u r e of the p ro to type
fue l pump is shown in Fig- ure 1. This appl icat ion br ings out
another valuable f e a t u r e of t he d i sc con f igu ra t ion as
compared t o most other types of electrical motors in which t h e e
l e c t r i - cal windings must be located in certain d iscre te
pos i - t i o n s and wound according to certain f ixed rules; t h
e d i s c motor r o t o r and stator can be assembled with an a
lmos t i n f in i t e va r i a t ion in geometry. Thus, in t h e f
u e l pump, t h e motor r o t o r is a l s o t h e a c t i v e
element of t h e pump and t h e r e s u l t i n g package is
smaller, l i g h t e r , h a s f a r fewer component p a r t s ,
and has g r e a t e r f u e l flow than motor-driven centrifugal
fuel pumps.
BRUSHLESS MOTOR DESCRIPTION
The Ford d i s c motor belongs i n t h e c l a s s of motors
known as variable reluctance motors, or, simply, reluc- tance
motors. These are synchronous type motors nor- mally operated from
a fixed frequency electrical source, and are one of t h e more p r
o l i f i c motor types since most e lec t r ic c lock , t iming ,
and turntable motors are of th i s type . In recent years , a
polyphase version of t h e clock motor has been developed i n l a r
g e r power s i z e s and, with the advent of high-power s o l i d
state switch- ing devices, has supplanted many squirrel cage
induction motor d r ives , ma in ly i n t he t ex t i l e i ndus t
ry . Reluc- tance motors are a l s o sometimes c l a s s i f i e d
as "singly excited" motors, since they require only one electri-
cal winding and have no f i e l d winding o r permanent mag- ne t
exc i ta t ion . The reluctance machine r o t o r i s merely a
piece of soft iron, usually laminated, having
8
-
s a l i e n c i e s o r p o l e p f o j e c t l w r a t h e r t
h a n a smooth c y l i n d r i c a l shape. There are no brushes,
comwrta- t o r s , s l i p r i n g s , o r r o t a t i n g
electrical windings, the primary sources of most electrical =chine
mainten- an
Figure 1. Prototype Fuel Pump
The d i s c motor is similar to the convent iona l re- luctance
machine except for i ts e lec t r ica l winding which is designed
to permit operation from a pulsed DC source rather than from a
fixed frequency AC source. This great ly s implif ies the winding
construct ion which, in the disc motor , is merely a so leno ida l
co i l r a the r than the complex windings wound in s lo ts accord
ing to f ixed rules as required by the conventional reluc- tance
motors. This also considerably f rees the shape of the s ta t
ionary magnet ic member of t h e d i s c machine ( s t a t o r ) ,
and permi ts the func t iona l ro les o f ro ta t ing and s t a t i
o n a r y members to be interchanged almost a t w i l l . The
motors used i n t h e t r a c t i o n and f u e l pump appl ica t
ions are of t h e axial a i r gap configuration in which both the r
o t a t i n g and stationary members take on disc shapes. The
blower motor is a r a d i a l a i r gap configurat ion and has a
cyl indr ica l shape more like a conventional motor. A schematic
representation of the blower motor is sham in Figure 2. Photographs
of the prototype blower motor are shwn in Figures 3 and 4.
II
Figure 2. Cross- s e c t i o n a l Views of Brushless Motor
Blower
Motor ac t ion is due t o t h e a t t r a c t i o n between t h
e po le s o r s a l i enc ie s on the ro tor wi th those on t h e
sta- t o r when magnetically excited. The exc i ta t ion is ap- p l
ied when the two sets of poles are unaligned and must be turned off
s l ight ly before they are f u l l y a l igned o r t he ro to r mu
ld become locked i n t o t h i s a l igned pos i t ion , p revent
ing fur ther ro ta t ion as i n a relay or s tepper Botor . The
funct ion of t he con t ro l l e r Is t o t u rn t he exc i t i ng
cu r ren t , whlch magnetizes the motor, on and off a t prescribed
intervals depending upon t h e relative positions of s t a t o r
and ro tor po les . To achieve good power and torque
characteristics from a d i s c motor, these intervals must be r e l
a t ive ly sho r t and a t a r ap id r epe t i t i on rate, one
reason vhp such a motor would not be feas ib le wi thout the use of
modern s o l i d state switching devices , thyr is tors and
transis- tors . Al though not s ignif icant in t h e blower appli-
ca t ion , t he d i sc motor can be made t o o p e r a t e as an e
l - ec t r i ca l gene ra to r by a small change in t he t iming of
the exci t ing current pulses . Further descr ipt ions of t he d i
sc motor and cont ro l le rs a re g iven i n References ( 2 ) - ( 4
) .
In the blower motor configuration (Figures 2-4), the inner
member is the s t a t iona ry member and contains the exc i ta t
ion winding which is wrapped around its middle section between the
pole project ions on each end. The s t a t o r is formed in two t i
g h t l y f i t t i n g s e c t i o n s which can be separa ted ax
ia l ly f o r inser t ion of t h e bobbin containing the so lenoida
l co i l . The s t a t o r is f ixed a t one end t o a n aluminum p
l a t e which is mounted on the vehic le o r o ther suppor t ing s
t ruc ture . The motor r o t o r is merely six sets of laminated
bars of rectangu- lar cross section evenly spaced and bonded to t
he squ i r - rel cage member of t h e blower. These bars a r e of
ap- proximately the same cross sec t ion as each of t h e s i x
pole projections on each end of t h e s t a t o r , as is the
annular cross section of the laminated member ins ide of t h e c o
i l which is the magnetic - as well as physical - connection
between the two s t a t o r ends. These three so f t i ron s ec t
ions - ro to r ba r s , s t a to r and pole pieces, and inner
annular connecting member - along with the a i r gaps a t each end
of t h e s t a t o r between the pole projec- t i o n s and ro to r
ba r s form a complete magnetic circuit through and around the
solenoid when i n t h e a l i g n e d pos i t i on ( t ha t is,
when the ro to r ba r s are exact ly oppo- site the pole project
ions) .
The number of po le p ro jec t ions is one of t h e
ROTOR SECTORS ' A FAN SQUIRREL CAGE I- (POLES) I
COIL
/ STATOR
-
Figure 3. Dissassembled Blower Motor; Stator Circu i t and
Solenoidal Coil are on Right Figure 4 . Assembled Blower and
Motor
design parameters of th i s type of motor and plays a ro le qu i
te ana logous to tha t of the poles on conven- t i o n a l
machines, even though th is conf igura t ion is of ten ca l led
"homopolar" due t o t h e a x i a l f l o w of mag- ne t i c f l ux
. Up t o a c e r t a i n number, t h e motor output general ly
increases with increasing number of poles , but so does the cost of
magnetic parts, the magnetic leakage, and the required dynamic
performance of t h e e lec t ronic cont ro l . A t some number of
poles , the leakage becomes s u f f i c i e n t t o start
decreasing the motor power output. This number is generally smaller
i n machines of smal le r phys ica l s ize and is a l s o smaller f
o r t h e r a d i a l a i r gap designs than for axial designs. The
prototype blower motor has s ix poles.
The magnetic members can be constructed of lamin- a t ed s i l i
con o r n i cke l a l l oy steel as used in t r ans - formers and
small motors, powdered i ron , o r , poss ib ly , solid carbon
steel such as AIS1 type C1020. The la t - t e r should resul t in
the lowest mater ia l and assembly cos ts bu t will increase the
eddy cur ren t losses in the magnet ic c i rcui t and de te r io ra
t e t he r e luc t ance r a t io of t h e machine. Laminations of
very simple shape are possible (which is not t rue of t h e a x i a
l a i r gap con- f i g u r a t i o n s ) , and th i s t echnique
should resu l t in very good magnet ic charac te r i s t ics , low
eddy current loss- es , and reasonable costs. Powdered i ron
members u l t i - mately may prove lower in cost , but some
magnetic c h a r a c t e r i s t i c s w i l l be sacr i f
iced.
The exciting solenoid can be constructed of al- most any
standard or unconventional type of conductor. L i t z wire
(conductor formed from braided and trans- posed wires of small
cross sec t ion) and f l a t aluminum s t r i p were both applied
successfully on the l a rge r t r a c t i o n d i s c motors. Ohmic
losses are general ly higher in a d i s c motor than i n a DC motor
of similar rat ing due to the high current pulses required to ex- c
i t e t h e former and in machines of l a r g e r power rat- ing
some form of co i l cool ing i s often required. This is not
expected to be necessary in the blower motor.
COMPARISON OF THE BRUSHLESS MOTOR WITH EXISTING MOTOR-
The prototype brushless blower motor and the DC cormnutator
motor used on current automotive AC blowers are compared i n terms
of several important parameters:
P a r t s Count:
Table I is a summary of t h e parts required for
each type of motor obtained by count ing the par ts on two
disassembled units. The brushless machine is con- s t ruc ted of
less than ha l f the par t s of t h e commutator machine.
Weight and Size:
Table I a l s o shows tha t the b rushless machine is l i g h t
e r and occupies a smaller volume than the commu- t a t o r
machine.
Manufacturing Cost:
An accurate evaluat ion of t he cos t of manufacture fo r t he b
rush le s s machine has not been made a t t h i s time. On t h e b
a s i s of weight alone, one could expect t ha t t he b rush le s s
machine should be approximately 4 6 / 5 3 = 87% of t h e assembly
cost of t h e commutator machine, since the materials in both
machines, with the exception of the permanent magnets, a r e
similar. However, t h e method of motor assembly is e n t i r e l y
d i f f e r e n t f o r t h e two machines, and th i s d i f f e
rence shou ld r e su l t i n a considerable fur ther cost advantage
fo r t he b rush le s s machine. The coil winding machinery
required to wind the s lo t ted a rmature of t h e commu- t a t o r
motor is h i g h i n i n i t i a l and maintenance costs, has a
high percentage of down time, and t h e wound armatures which i t
turns out have a r e l a t ive ly h igh r e j e c t i o n rate. By
c o n t r a s t , t h e winding on t h e brush- less machine can be
assembled by much less complex, more r e l i a b l e machinery of
the type used to assemble the lowes t cos t e lec t r ica l
components, such a s elec- t ronic t ransformers , inductors , re
lays , solenoids , etc. Secondly, the assembly of t h e commutator
and e lec t r ica l ly connec t ing each bar to an a rmature co i l
are addi t iona l complex assembly processes which are completely
eliminated in the brushless machine assem- bly. A t h i r d major
difference in the assembly of t h e two types of machines is that
the brushless design re- qu i res no f i e l d permanent magnets.
The e f f e c t of t h i s change upon t h e assembly c o s t s is
d i f f i c u l t t o e v a l - ua te , bu t the e l imina t ion of
t h e h a r d , b r i t t l e f e r r i t e material should resu l
t in some sav ings ; a l so , t he so f t i ron s ec to r s shou ld
r e su l t i n a materials cost savings a t least over the
arc-shaped ferr i te magnets. There- fore , when t h e e f f e c t
s of t hese t h ree major simplifi- c a t i o n s i n t h e motor
assembly process are evaluated, i t is conceivable that a brushless
machine could be manufactured f o r as l i t t l e as 50-60% of t h
e commuta- t o r machine manufacturing costs. Controller costs are
d iscussed in a later sec t ion of th i s paper .
10
-
DC COtWUTA'IVR
TABLE I
PARTS COUNT COMPARISON BETWEEN EXISTING AND PROPOSED BLOWW
MOTORS
BRUSHLESS RELUCTANCE
A. Rotor P a r t s
1. Laminated armature steel s tack 2 . 10 copper wire (#l8 AWG)
c o i l s , 3 . 10 - i n s u l a t i n g s l o t l i n e r s 4 . 10
- bar copper commutator 5 . 2 - f iber spacing cyl inders 6 . 2 - s
t ack i n su la t ing sp ide r s 7 . s h a f t
B. Sta tor Par t s
10 turns each
1. 2 . 3 . 4 . 5 . 6 . 7. a. 9 .
10. 11.
2 - f e r r i t e , arc-shaped magnets c y l i n d r i c a l s o
f t iron housing
2 - bearings rear bearing housing lock washer f ront bear ing s
l inger guard
2 - carbon brushes 2 - brush holders and spr ings f i b e r
mounting p la te for b rush ho lders
C. Motor Weight
53 02.
D. Dimensions
6 .2" dia . x 3" + 3" dia . x 2.5''
Elec t r i ca l Cha rac t e r i s t i c s
1. Current Wave Form: The wave form of the b rushless machine c
o n s i s t s of a series of discrete current puls- es ra ther than
the s teady DC of t y p i c a l commutator motors. The cur ren t pu
lses may be of s inusoida l , tra- pezoidal, triangular, or other
shape, depending upon the type of control used. RMS cu r ren t l
eve l s are gen- e ra l ly h ighe r i n the brushless motor than in
t h e com- mutator motor for a given power o u t p u t , r e s u l
t i n g i n higher armature copper losses in the former.
2 . Electromagnetic Torque: In the brushless motor,
electromagnetic torque, (or motor developed torque) is a function
of the RMS current squared, which some- what l e s sens t he e f f
ec t on efficiency of the h igher copper losses discussed above. In
a connrmtator motor, th i s to rque is a funct ion of the product
of t h e aver- age armature current and the field magnetic
flux.
3 . Losses and Efficiency: It is hard to make gener- a l i z e d
comparisons of losses and e f f i c i ency fo r t he two types of
machines since many s ize , weight , and cos t t r adeof f s become
involved. Two of .the signifi- cant loss components i n t h e
cormnutator machine do not ex i s t i n t he b rush le s s machine:
brush IR drop loss and b rush f r i c t ion l o s s . Bowever,
copper losses and iron (magnetic) losses are higher in the b
rushless machine due t o t h e chopped current wave form in t h e
brushless machine and t h e e f f e c t of the nonconducting f e r
r i t e p o l e p i e c e s i n t h e commutator machine. Pres- en
t exper ience ind ica tes tha t the fu l l load , ra ted speed
efficiency i s s l igh t ly h ighe r i n t he commutator
6 - steel bars of rectangular crossect ion
shaf t
2 - sof t i ron po le p ieces aluminum end p l a t e 3 0 - turn
copper , solenoidal coi l 2 - bushings (or bearings) 2 - lock
washers 3 - machine screws
46 oz.
6 .2" dia . x 3"
mator but that partial load andfor par t ia l speed eff i - c
ienc ies may be higher in the brushless motor. Also, the b rushless
e f f ic iency tends to be more constant as load and speed are var
ied.
4 . Electromagnetic Noise: Again, the two types of machines
should be roughly comparable. The noise re- s u l t s from t h e
chopped cur ren t wave in t he b rush le s s design and is
character ized by r e l a t i v e l y few harmon- ics of s i g n i
f i c a n t magnitudes. The commutator motor no ise is more
gaussian in na ture and is due t o spark- ing and improper
conanutation at the mechanica l corn- t a t o r . Small,
permanent-magnet e x c i t e d c o r n t a t o r mo- to r s t end t
o be no i s i e r t han e l ec t r i ca l ly exc i t ed ma- chines
since the conventional compensation techniques f o r improving
commutation ( interpoles , e tc . ) cannot be appl ied to permanent
magnet machines.
5 . Magnetic Flux: The brushless machine o p e r a t e s a t s
ignif icant ly higher levels of magnet ic f lux densi ty than the
commutator machine due t o t h e u s e of f e r r i t e permanent
magnets in t h e latter. This is one of t h e reasons for the
reduced brushless machine weight.
ELECTRONIC CONTRQLLW FOR BRUSHLESS MOTOR
Ser ies SCR Control ler
The power c i r c u i t f o r a very simple type of elec- t ron
ic con t ro l l e r capab le of dr iv ing the b rushless blower
motor is sham schematical ly in Figure 5 .
11
-
RHS current would be reduced by a f a c t o r of {240 /360
square of RMS current , would be reduced to 2401360 o r two t h
i r d s of the ra ted va lue .
t or 81.5%; the developed torque, proportional to the
S1 This type of motor control has been used success- BATTERY f u
l l y on both the l a rger t rac t ion motors and t h e small
It can operate with any type of posi t ioning informa- t i o n
and requires a minimum of l o g i c c i r c u i t r y f o r proper
operation of t h e power SCRE. The sa l i en t f ea - t u r e s of
t h i s c o n t r o l may be summarized as follows:
1. Power c i r c u i t r y consists of t h e minimum number of t
he lowest cost power switching devices (SCRs). SCRs do not requi re
the h igh qua l i ty , h igh cos t dynamic cha rac t e r i s t i c
s a s soc ia t ed w i th i nve r t e r SCRs.
-- c e n t r i f u g a l f u e l pump during many hours of
operation. -- /Y C
COIL
J 2. Uses simplest logic system for control of power
devices.
Figure 5 . Ser i e s SCR Controller 3 . The capacitor is the
dominant cost item in t h i s con t ro l l e r .
pena l ty in e f f ic iency , s ince the magnetic current of t
he motor is taken through the complete swing between plus and minus
maximum f lux va lues , increas ing the mo- tor hys te res i s
losses over the un i la te ra l exc i ta t ion schemes.
5 . A t rated speed and torque, the current wave form of t h i s
motor approaches a sine wave and t h e motor is exci ted in a
manner very similar to conventional syn- chronous motors. Voltage
and current harmonics are probably minimized i n t h i s
scheme.
Transistor-SCR Controller
I A more sophis t ica ted cont ro l scheme I s shown in Figure 7
, with associated motor vol tage and current wave forms i l l u s t
r a t e d in Figure 8. This scheme achieves a current pulse
approaching a square shape which increases the RMS curren t for a
given value of
Figure 6 . Series Controller Current Wave Form maximum current
and greatly enhances the control over motor torque and speed. The
capac i to r s i ze is smaller than that of the series SCR cont ro
l le r bu t more power switching devices are required. This scheme
is used to d r ive the p ro to type brushless blower motor. The
t h i s wave form is an example of a "chopped" sine vave, i.e.,
a s i n e wave "chopped off" or turned off af ter each posit ive
and negative portion of t h e s i n e wave and followed by a short
off time. The on pulse is applied during periods of increasing
inductance (decreasing reluctance) . The width of the sine pulses
is deter- mined by the product of t he motor inductance and series
capacitance, and is designed to be about 10% longer than the per
iod of increasing inductance a t the des i red maximum speed. This
means t h a t a t very low speeds, the current pulse covers only a
shor t por t ion of the t ime period of increasing inductance resul
t ing in low RMS currents and low electranagnetic torque. The
torque a t s t a r t i n g and low speeds can be increased by
increasing the number of currmt pulses during each per iod of in-
creasing inductance, although this scheme is not con- s ide red
necessa ry fo r t he l i gh t s t a r t i ng l oads of f a n s and
blowers. Operation a t par t ia l speeds is achieved by skipping
the application of current pulses during a c e r t a i n number of
periods of increasing inductance dur- ing a c e r t a i n time
period. For example, i n t h e motor of Figure 2, a t r a t e d
speed and load, s ix current pul- ses per revolution would normally
be applied to the ex- c i t i n g c o i l . A t a speed of 3600
RPM, t h i s would re- s u l t i n a pulse rate of 360 pulses per
second. If the r a t e were reduced t o 240 pps, it can be shown
that the
sa l ien t fea tu ies o f - ;h i s cont ro l are summarized
below:
1. Ful l to rque cont ro l at a l l speeds.
2. optimum motor current wave form; u n i l a t e r a l current
.
3 . Smaller capacitance but more power switching com- ponents
required than in series con t ro l l e r .
Other Control Schemes
A t least 20 addi t iona l cont ro l c i rcu i t s have been s
tudied in cons iderable de ta i l and about 10 of these have been b
u i l t and tes ted with var ious disc motors . Further discussions
of t he c i r cu i t ope ra t ion and the- ory of some of these
schemes may be found in Refer- ences 4 and 6-11. The simplest type
of control is a s i n g l e power t r a n s i s t o r which is
switched on and off so as t o permit current flow during periods of
in- creasing motor inductance. This scheme has been used during
much of t h e c e n t r i f u g a l pump t e s t i n g , and may be
app l i cab le fo r r e l a t ive ly l ov power l eve l s i n app l
i - cat ions where e f f i c i ency is not an important factor. The
t h r e e main l imi t a t ions of t h e s i n g l e t r a n s i s
t o r switch are:
12
-
a. The current bui lds up a t a rate determined by t h e motor
winding inductance and battery voltage (rather than by means of
capaci tor forcing as in t h e above two cont ro l le rs ) .
b. The energy in t h e motor c o i l vhen the cur ren t pulse is
to be turned off is diss ipa ted in the trans- is tor , great ly
reducing system eff ic iency and increas- ing t he r equ i r ed t r
ans i s to r s i ze in o r d e r t o d i s s i - pa te this added
heat load. The use of freewheeling diode around the motor coil w i
l l remove the hea t d i s - s ipa t ion i n t he t r ans i s to r
bu t g rea t ly r educe t he maximum motor speed due t o t h e l o
n g t r a i l i n g o f f of the current pulse . Also, system
efficiency is l i t t l e improved since p a r t of the co i l
energy ends up devel- oping negative motor torque, slowing dawn t h
e motor.
Some of the o ther cont ro l schemes are being eval- uated
towards the goal of achieving the minimum cost cont ro l le r
capable of sa t i s fy ing the t echnica l per - formance
requirements of t h e motor. The cont ro l cir- cu i t conf igura t
ion w i l l doub t l e s s ly be d i f f e ren t i n each
application.
CONTROLLER LOGIC AND POSITION SENSING
Logic and position sensing c i r c u i t r y is required in o
rder to synchronize the cur ren t pu lses wi th the mechanical time
periods of increasing motor induc- tance, as noted above. Although
t h i s c i r c u i t r y i n - volves some of t h e most i n t e
re s t ing t echn ica l prob- lems and design challenges, i t s
cost w i l l general ly be a very small percentage of the to ta l
sys tem cos t . Hopefu l ly , fo r e i t he r con t ro l l e r A o
r B above, t h e t o t a l l o g i c and p o s i t i o n c i r c u
i t r y can be fabr icated on a single semiconductor chip.
Experience has shown tha t t he cos t of such c i rcui t ry becomes
very low i n the large quant i t ies associated with automotive
appl i - ca t ions and generally decreases with time even in in- f
l a t i o n a r y economic conditions. The only addi t ional item
required in the b rushless motor system is a t i n y ceramic
permanent magnet, t he cos t of which w i l l be miniscule.
Four major types of position sensing have been applied to the
disc motor:
1. l ight sensing with an encoder-type disc, using l i gh t ac t
iva t ed SCRs and diodes.
2. variable re luctance of an external magnetic cir- c u i t
.
3. permanent magnet ac tua t ion of reed switches.
4. permanent magnet enhanced sensing of t h e internal motor re
luc tance var ia t ions .
The la t ter system has proven t o b e t h e least cos t ly
method yet conceived and is adequa te t o s a t i s fy t he
technical performance requirements of small d i s c motors. Since
this concept is r e l a t i v e l y new and has not been ful ly
descr ibed, a shor t descr ip t ion is con- tained here:
Internal Posit ion-Sensing System:
In a var iab le re luc tance motor , i f a res idua l magnet ic
f ie ld ex is t s in the magnet ic c i rcui t , an output voltage w
i l l appear a t the motor windings dur- ing ro ta t ion which
relates t o t h e minimum and maxi- mum inductance points of t h e
motor. This magnetism =Y a lso be ex te rna l ly produced e i ther
th rough b ias cu r ren t s i n t he motor winding or a permanent
magnet p l aced i n t he v i c in i ty of the motor's magnetic cir-
c u i t . An operational demonstration of th i s sys tem
used t h e b i a s Current in t h e motor windings approach as p
a r t of t he l og ic con t ro l fo r t he power c i r c u i t of
Figure 7. The zero crossover points of t h e A.C. c- went of the
output vol tage wave form indica ted the minimum and maxiam
inductance points of t h e motor.
I
Figure 7. Transistor-SCR Contrsller
The s lope po la r i ty a t the c rossover po in t ident i f ied
which inductance point the motor was at . Therefore, de tec t ion
of t h e minislum inductance point was simply achieved through the
use of a s tandard polar i ty sensi- tive voltage comparator. Upon
detec t ion of t h e mini- mum inductance point, a one shot
multivibrator was f i r e d f o r a predetermined on-time
energizing a power t r a n s i s t o r which powered the windings
of t h e motor. For the dura t ion of t h i s on-time plus a small
amount of addi t iona l time to a l low the motor c u r r e n t s t
o se t t le , t h e comparator was gated off. This prevented
erroneous outputs from the comparator during this power cycle. When
t h e comparator was gated on again it was only necessary for it to
de t ec t t he ze ro vo l t - age crossover point of the proper s
lope polar i ty to initiate t h e new cycle. This w a s the basic
operat ion of the pos i t ion sense scheme and i ts associated
logic . In p rac t i ce , o the r c i r cu i t ry may be necessary.
For example, i t may be des i rab le to vary the on-time of t h e
power t r ans i s to r mu l t iv ib ra to r as a funct ion of motor
speed t o s a t i s f y motor torque requirements. This adjustment
can be made over several cyc les ra ther than on a cycle per cycle
basis . This technique can r e s u l t i n maximum motor torque
over its entire speed range. Addit ional c i rcui t ry is also
necessary to in- sure p roper s ta r t -up of t h e non-moving
machine, since no pos i t ion sense s igna ls a re ava i lab le a t
t h i s time.
The logic system developed for Figure 7 is i l l u s - t r a t e
d in the block diagram of Figure 8. The pos i t ion sensor monitors
the motor induced EKF during the un- exci ted port ions of t h e
motor cycle. Its input gate is switched off for the durat ion of t
h e power transis- t o r on time (power pu l se t o t he motor
windings) plus a small amount of addi t iona l t ime to allow cur
ren t s t o settle wi th in the motor winding. The variable t ime
one shot multivibrator is t r iggered by the pos i t i on sensor
and drives the power t r a n s i s t o r . An on-time detector
monitors the output of the variable t ime one shot and continually
adjusts i t s on-time t o 50% of the mechanical period. This
adjustment is accomplish- ed slowly over several cycles . The pulse
rate tacho- meter monitors the cyclic frequency and reduces the v a
r i a b l e time one shot on-time t o less than 50% when t h e
maximum desired speed has been exceeded. This
13
-
TACHCUETLR
I I
1 POSITION SESSiNG
CIRCUITRY
ONE ShOT vmLELE Ti?&
1 x)% OH.TIUE
DETECTOR
I . i
Figure 8. Logic Block Diagram for Cont ro l of Figure 7.
overr ides the 50% on-time detector function. The s ta r t -up c
i rcu i t ry , cons is t ing of a ba t te ry vo l tage detector and
a motor not-running de tec to r , bas i ca l ly holds the normal
log ic in a n o f f s t a t e u n t i l a mini- mum bat te ry vo l
tage has been achieved f o r a given length of time. It then gates
on t he normal log ic and t r i g g e r s t h e v a r i a b l e
on-time one shot once only in an e f f o r t t o start the machine
ro t a t ing . Should the motor no t ach ieve ro t a t ion w i th t
h i s f i r s t pu l se , t he re w i l l be no posit ion sense
signals ava i l ab le a t t h e motor winding. The motor not
running detector senses this absence of pos i t ion s igna ls and i
n i t i a t e s a new start-up sequence. This start-up cycling w i
l l cont inue un t i l pos i t ion s igna ls are achieved.
Logic fo r Se r i e s SCR Chopper Operation:
The normal running modes and s tar t -up mode is t h e same as
that descr ibed for the power t r a n s i s t o r logic with the
following exceptions: the posit ion sensor feeds the SCR chopper d
i r e c t l y , which elimin- ates the va r i ab le on-time one
shot mult ivibrator and its assoc ia ted c i rcu i t ry . The speed
regulator simply ga tes the SCR chopper off whenever the desired
speed has been exceeded. This logic descr ipt ion does not include
the logic necessary to properly sequence the f i r i n g of t he
chopper SCRs since this requirement i s determined by the chopping
scheme chosen.
EXPERIMENTAL RESULTS
The six-pole conf igu ra t ion i l l u s t r a t ed in Fig- ures
2-4 has been assembled and operated with several d i f f e ren t
con t ro l c i r cu i t s . The principal dimensions of the act ive
e lements of t he motor are as follows: The inner s ta t ionary
member has a t o t a l a x i a l l e n g t h of 1.95 in.; the sa l
ienc ies o r po les a t each end of t h i s member have an act ive
axial length of 0.35 inch- es; outer diameter of s a l i e n c i e
s ( a i r gap diameter) is 2.78 inches; the s ix rotor bars (a t
tached to the fan squirrel cage) have 1.70 inch axial length and
.35 inch rad ia l th ickness ; the ac t ive c i rcumferent ia l
length of t he s a l i enc ie s and the bars is 0.72 inches, giving
a r a t i o of a c t i v e pole arc l ength to po le p i tch length
of 0 .5; the exci t ing coi l has an inner diameter of
approximately 1 . 7 inch. The number of turns on t he c o i l have
been var ied in an e f for t to ach ieve the bes t motor
performance with a given set of con t ro l l e r power device
current and vol tage ra t ings . It is f e l t t h a t t h e
optimum number of co i l tu rns has no t been r ea l i zed a t t h
i s s t a g e of development, and fu r the r improvement i n motor
performance may be possible. All magnetic
members of t h e motor are constructed of .014 inch 3% s i l i c
o n steel laminations.
With 26 turns of #30 AWG magnet wire in t h e c o i l ,
inductance measurements were made of a f u l l y assembled motor
using a Marconi inductance bridge, Type TF1313A. a t 1000 &.
With the motor in an a l igned posi t ion ( ro tor bars d i rec t
ly oppos i te s ta tor sa l ienc ies ) , which r e s u l t s in t h
e maximum inductance, the measured inductance was 790 vh. This
gives a r a t i o of maximum t o m i n i m u m inductance of
3.29.
The principal purpose of the experimental program was t o
compare two ident ical b lowers , the one powered by a conventional
automotive DC conrmutator motor and the second powered by a
brushless motor/controller system. The conventional blower used was
a Motorcraft model DSAF, the squi r re l cage one which i s i d e n
t i c a l t o t h a t on the brushless configurat ion shown in Fig-
ures 2-4. The two blowers were operated in an ident i - cal
environment without the shrouds surrounding the squirrel cage fan a
t the same speeds. It was assumed t h i s r e s u l t e d in ident
ical torque loadings on t h e two machines. A t 3050 rpm, the
following measurements were made a t t he ba t t e ry t e rmina l s
in both cases:
DC Commutator Brushless
Volts, ave. 11.4 12.0 Current (amp.), ave. 16.05 12.5 Power
(watt) 183 150
Since measurements were made a t the ba t te ry t e rmina ls ,
con t ro l l e r l o s ses are included in the above measure-
ments. The improved power e f f i c i ency of the b rushless
configuration is apparent from the above measurements. This d i f
fe rence in eff ic iency is widened a t p a r t i a l speed
conditions, where an armature resistance is used in the convent
ional system for speed control .
PROBLEMS AND LIMITATIONS
It has not been possible to evaluate the large- volume
electronic control ler cost dur ing the develop- ment of the
brushless motor system. This evaluation w i l l r equ i r e a f a i
r l y major e f f o r t and considerable in t e rac t ion between
automotive manufacturing engineers and e l ec t ron ic component
suppl iers . However, t h i s evaluation must be made before ser
ious considerat ion can be given to adopt ion of t h i s blower
motor f o r automotive applications. The e l ec t ron ic con t ro l
l e r w i l l cost more than the present ly used simple resist-
ance control ler . However, t h e motor developers feel tha t the
to ta l b rush less motor /cont ro l le r package cost can be less
cos t ly than the ex is t ing DC commutator motor system. Achieving
t h i s g o a l w i l l depend l a rge ly upon the minimization of
t h e number of power semicon- ductors and capac i to r s i ze in
the con t ro l l e r which is the goal of present development
work.
One l imi t a t ion of the brushless system is a rel- a t i v e
l y low sta r t ing t o rque as compared to t ha t ava i l - ab l e
from a DC commutator motor. For most blower ap- p l i c a t i o n s
t h i s is not a problem, although longer acc- e lerat ing t imes
must be accepted. The s t a r t i ng t o rque can be increased to
values almost comparable with DC Commutator motor values, but only
with a la rge pena l ty in con t ro l l e r cos t and
complexity.
Audible noise generation has been a problem with some of the
larger , t ract ion-type disc motors(3) , but has not been
noticeable in t h e blower motor. Noise r e s u l t s mainly from s
t eep wave f r o n t s in the con t ro l l e r current wave form,
and t h i s must be considered in the control ler design.
14
-
F i n a l l y , e f f o r t is s t i l l r equ i r ed t o i n su
re that the con t ro l l e r is designed for maxb r e l i a b i l i
t y and s e r v i c e a b i l i t y in the automotive
environment.
CONCLUSIONS
There is much interest within the automotive industry today in
improving efficiency, reducing war- r an ty cos t s , and reducing
packaging size and complex- i t y f o r t h e many auxiliary
components used in auto- mobiles and trucks. With the gradual
acceptance of e lec t ronic c i rcu i t s for emiss ions cont ro l
, engine control, speed control, etc., it is appropr i a t e t o
eva lua te t he po ten t i a l bene f i t s that e l ec t ron ic
con- t r o l s might have i n some of t h e v e h i c l e
auxiliaries. This paper descr ibes an e lectronical ly control led,
brushless motor f o r u s e i n AC and heater blowers. Compared t o
p r e s e n t l y used DC conmutator motors, the brushless motor h
a s p o t e n t i a l f o r improving operating efficiency, reduced
warranty problems, reduced weight and ove ra l l blower package s i
z e , and continuously- var iable speed control . This
motor/controller system is proposed as a v i ab le cand ida te fo r
fu tu re use in vehicular blower and fan applications.
REFERENCES
"Rel iab i l i ty Pred ic t ion of Electronic Equipment",
MIL-HDBK-217B, 20 September, 1974.
L. R. Foote, e t a l , "Electr ic Vehicle Systems Study", Ford
SRS r epor t 173-132, October, 1973.
L. E. Unnewehr and W. Koch, "An Axial Air-Gap Reluctance Motor
for Var iab le Speed Application", IEEE Transactions on Power
Apparatus and Systems, January, 1974.
L. E. Unnevehr, "Series-Commutated SCR Controll- e rs for Var
iab lespeed Reluc tance Motor Drives", IEEE Power Electronics
Specialists Conference Record, June 1973.
Sidney A. Davis, "Stepper Motors", Electromechan- ican Design,
Ju ly , 1964.
U. S. Patent #3,560,820, "Reluctance Motor Power Circui t Con
taining Series Capacitance", Febru- ary, 1971.
U. S. Patent 83,697,839, "Bridge Circuit Con- t r o l l e r " ,
October, 1972.
U. S. Patent #3,697,840, "Controller for Variable Reluctance
Motor", October, 1972.
U. S. Patent 83,560,817, "Reluctance Motor Power Circuit",
February, 1971.
U. S. Patent #3,560,817, "Reluctance Motor Power Circui t" ,
FebruaT, 1971.
U. S. Patent #3,714,533, "Sine Pulse Controller for .Variable
Reluctance Motor", January, 1973.
L. E. Unnewehr, "Magnetic Analysis of an Axial- Gap Reluctance
Motor", IEEE Applied Magnetics Workshop Record; June, 1975.
15