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

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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.

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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 .

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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 .

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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:

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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.

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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

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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.

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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.

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