Squarewave permanent-magnet brushless motor drives

 SQUAREWAVE PERMANENT-MAGNET BRUSHLESS MOTOR DRIVES.

The brushless d.c. motor is shown in its most usual form in figure below alongside the PM d.c. commutator motor. The stator structure is similar to that of a polyphase a.c. induction motor. The function of the magnet is the same in both the brushless motor and the d.c. commutator motor. In both cases the airgap flux is ideally fixed by the magnet and little affected by armature current.

The most obvious advantage of the brushless configuration is the removal of the brushes. Brush maintenance is no longer required, and many problems associated with brushes are eliminated. For example, brushes tend to produce RFI (radio-frequency interference) and the sparking associated with them is a potential source of ignition in inflammable atmospheres. These problems should not be overstated, however. RFI at least has the advantage of high frequency, so that filter components need not be very large. This is not necessarily the case with the lower-order harmonics associated with the commutation of the brushless motor. Commutator motors are quite commonly immersed in automobile petrol tanks to drive the fuel pump. This shows that they are not automatically excluded from 'hazardous' environments.
The problems that arise with commutator motors are sometimes not so obvious. In some applications the accumulation of brush debris or dust is a problem,  particularly  if  it  gets  into  the  bearings  or  if  it  forms  a  conducting track  that  leads  to flashover.  The  operation  and  life  of  brushes  depend  on factors   such  as  atmospheric  conditions,  which  may  necessitate  the  use  of different  brush  grades in  the same motor  operating  in different  climates. An advantage of  the brushless configuration  in which the rotor is inside the stator is that more cross-sectional area is available for  the power or 'armature' winding.  At  the  same  time  the  conduction  of  heat  through  the  frame   is improved.  Generally  an  increase  in  the  electric  loading  is  possible providing  a greater specific  torque. The efficiency  is likely to be higher than that of  a commutator motor of  equal size, and the absence of  brush friction  helps further  in  this  regard. 
The absence of  commutator and brush gear reduces the motor length. This is useful  not only as a simple space saving, but also as a reduction  in the length between  bearings, so that  for  a given  stack  length  the  lateral  stiffness  of  the rotor  is greater,  permitting  higher speeds  or a longer  active  length/diameter ratio. This is important  in servo-type drives where a high torque/inertia  ratio is required. The removal  of  the commutator  reduces the  inertia  still further.
Commutators  are  subject  to  fairly  restrictive  limits  on  peripheral  speed, voltage  between  segments,  and  current  density. The  maximum  speed  of  the brushless motor  is limited by the retention  of  the magnets against centrifugal force.  In small motors with low rotor speeds, the magnets may  be bonded  to the  rotor  core,  which  is  usually  solid  (unlaminated).  The  bonding  must obviously  have  a  wide  temperature  range  and  good  ageing  properties.  For high  rotor  peripheral  speeds  it  is necessary  to  provide  a  retaining  structure such as a stainless-steel can or a kevlar or wire wrap. This may necessitate an increase  in  the  mechanical  airgap,  but  fortunately   the  performance   is  not unduly  sensitive  to  the  airgap,  which  is often  twice  as large  as in  induction motors  or switched  reluctance  motors.

The  brushless  configuration   does  not  come  without  some  disadvantages. The two main disadvantages relative to the commutator motor are (i) the need for   shaft   position  sensing  and  (ii)  increased  complexity  in  the  electronic controller.  Also,  the  brushless  motor  is  not  necessarily  less  expensive  to manufacture   than  the  commutator  motor,  which  is  perhaps  slightly  more amenable  to automated  manufacture. It is important  to weigh the advantages and disadvantages  of  the brushless d.c. motor  relative to induction  motor  drives, which  are not  only  'brushless' but make use of'standard'  motors. In the same frame,  with the same cooling, the  brushless  PM  motor  will  have  better  efficiency  and  power  factor,   and therefore   a  greater  output  power;  the  difference   may  be  in  the  order  of 20-50 per  cent,  which  is  by  no  means  negligible.  The  power  electronic converter  required  with  the  brushless  motor  is  similar  in  topology  to the p.w.m. inverters  used  in  induction  motor  drives. The  device ratings  may be lower,  especially  if  only  a  'constant  torque'  characteristic  is  required.  Of course,  the  induction  motor  can  be  inexpensively  controlled  with  triacs  or series SCRs, but the performance  so obtained,

is inferior  to that of  the brushless d.c. system  in  efficiency,  stability,  response,  and  controlled  speed  range.  To obtain  comparable  performance   in  the  control  sense,  the  induction  motor must be fed  from  a p.w.m. inverter, which is arguably more complex than  the brushless  PM  motor  drive.  However,  the  induction  motor  is  capable  of operation  in  the  'field   weakening'  mode,  providing  a  constant-power capability  at high speed. This is difficult  to achieve with brushless d.c. motors with  surface-mounted   rotor  magnets. 

Something should  be said  here about  the effects  of  scale. PM  excitation viable only in smaller motors, usually well below 20 kW, and is also subject to certain  constraints  on  the  speed  range.  In  very  large  motors  PM  excitation does not make sense because the magnet weight (and cost) becomes excessive, while  the  alternative  of  electromagnetic  excitation  either  directly  (as  in  the synchronous  machine) or by induction  (as in the induction  motor)  becomes relatively  more cost-effective.