Brushless 1

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BRUSHLESS DC MOTOR


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The motors in Row 2 are derived from those in Row 1 by replacing field

windings with permanent magnets. The synchronous motor immediately

becomes brushless, but the d.c. motor must go through an additionaltransformation, from Row 2 to Row 3 with the inversion of the stator and

rotor, before the brushless version is achieved.


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Commutator speed is a limitation, and noise, wear, RFI, and

environmental compatibility can be troublesome. The space required

for the commutator and brushgear is considerable, and the cooling of

the rotor, which carries the torque-producing winding, is not always

easy.

The PM d.c. commutator motor

In small d.c. commutator motors, replacing the field winding and pole

structure with permanent magnets usually permits a considerablereduction in stator diameter, because of the efficient use of radial

space by the magnet and the elimination of field losses. Armature

reaction is usually reduced and commutation is improved, owing to the

low permeability of the magnet


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The PM d.c. motor is usually fed from an adjustable voltage supply, either linear or pulse-width

modulated.

In automotive applications the PM d.c. motor is well entrenched because of its lowcost and because of the low-voltage d.c. supply. Of course it is usually applied as a

fixed-speed motor or with series-resistance control

The a.c. induction motor drive

In very large drives a.c. induction or synchronous motors are preferred because of

the limitations of commutation and rotor speed in d.c. motors.

Slip is essential for torque production in the induction motor, and it is impossible,even in theory, to achieve zero rotor losses. This is one of the chief limitations of the

induction motor, since rotor losses are more difficult to remove than stator losses.

By providing flux without copper losses, they directly alleviate the excitation penalty.


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In the fractional and low integral-horsepower range the complexity of the a.c. drive is a drawback, especially when

dynamic performance, high efficiency, and a wide speed range are among the design requirements. These

requirements cannot be met adequately with series- or triac-controlled induction motors, which are therefore

restricted to applications where low cost is the only criterion

Together these factors favour the use of brushless PM drives in the low power range

The brushless d.c. PM motor

The smaller the motor, the more sense it makes to use permanent magnets for excitation. There is no single

'breakpoint' below which PM brushless motors outperform induction motors, but it is in the 1-10 kW range. Above

this size the induction motor improves rapidly, while the cost of magnets works against the PM motor. Below it, the

PM motor has better efficiency, torque per ampere, and effective power factor. Moreover, the power winding is on

the stator where its heat can be removed more easily, while the rotor losses are extremely small. These factors

combine to keep the torque/inertia ratio high in small motors.

The brushless d.c. motor is also easier to control, especially in its 'squarewave' configuration . Although the inverter

is similar to that required for induction motors, usually with six transistors for a three-phase system, the control

algorithms are simpler.

The brushless PM a.c. synchronous motor

The brushless synchronous machine has permanent magnets instead of a field winding. Field control is againsacrificed for the elimination of brushes, sliprings, and field copper losses. This motor is a 'classical' salient-pole

synchronous a.c. motor with approximately sine-distributed windings, and it can therefore run from a sinewave

supply without electronic commutation.

The PM synchronous motor operates as a synchronous reluctance motor if the magnets are left out or

demagnetized. This provides a measure of fault-tolerance in the event of partial or total demagnetization through

abnormal operating conditions.


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Why brushless d.c.?

The brushless d.c. motor is shown in its most usual form in Figure 4.1

alongside the PM d.c. commutator motor. The stator structure is similar to thatof 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.

(a) Brushless d.c. motor and (b) PM d.c. commutator motor.


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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.

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

arma ure w n ng. e same me e con uc on o ea roug e

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.


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The absence of commutator and brushgear 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-typedrives 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.

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.


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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.


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PM excitations 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.


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Magnetic circuit analysis on open-circuit

(a) Motor cross section and flux pattern,


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(b) Magnet demagnetization curve, (c) Magnetic equivalent circuit.


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In the following, the steel core of the stator and the rotor shaft are

assumed to be infinitely permeable.

Each magnet is represented by a 'Norton' equivalent circuitconsisting of a flux generator in parallel with an internal leakage

permeance

whereAm is the pole area of the magnet; /m is the magnet length in the

direction of magnetization (in this case its radial thickness); and Br is the

remanent flux-density. the relative recoil permeability, is, the slope of thedemagnetization curve divided by absolute permeability. In this case the

outer pole area is larger than the inner pole area, but to keep the analysis

simple we will take the average. With a magnet arc of 120 degrees,


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where g is the equivalent airgap length allowing for slotting. The

slotting can be taken into account by means of Carter's coefficient, in

which case

the rotor leakage permeance is quite small, typically 5-20 per

cent of the magnet internal permeance, and it is convenient to

include it in a modified magnet internal permeance by writing


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Squarewave brushless motor: torque

and e.m.f. equations

Note that the two-pole magnet has a pole arc of 180 degrees, instead of the 120 degrees analysed in the previous

section. The airgap flux-density waveform is ideally a square wave as shown in Fig. 4.5(b). In practice, fringing

causes the corners to be somewhat rounded. The coordinate axes have been chosen so that the centre of a north

pole of the magnet is aligned with the x-axis, i.e. at 6 = 0.

The stator has 12 slots and a three-phase winding. Thus there are two slots per pole per phase. Each phasewinding consists of two adjacent full-pitch coils of Nx turns each, whose axes are displaced from one another by

30 degrees. The winding is a single-layer winding, and any slot contains A^ conductors from only one phase

winding. This winding is equivalent, in the active length, to a degenerate concentric winding with only one coil per

pole per phase, having a fractional pitch of 5/6. This is a more practical winding than the one analysed because it

has less bulky endwind

Consider the flux-linkage of coil a1A1 as the rotor rotates.). The flux-linkage varies linearly with rotor positionbecause the airgap flux-density set up by the magnet is constant over each pole-pitch of the rotor. Maximum

positive flux-linkage occurs at 0 and maximum negative flux-linkage at 180. By integrating the flux-density around

the airgap, the maximum flux-linkage of the coil can be found as ings and is generally easier to assemble. For the

same reason, its copper losses are lower.


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Brushless 1

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